Plasma heliophysics and astrophysics have been at the vanguard of extraordinary discoveries in the past decade, exciting worldwide interest from the public and scientists alike. Humanity left our solar system and took the first physical step into the interstellar medium (ISM) plasma with the venerable Voyager 1 and 2 spacecraft, the farthest human-made objects from the Sun. The pioneering Parker Solar Probe (PSP) is plunging repeatedly into the plasma of the solar corona, becoming the closest human-made object to the Sun. The first views of plasma orbiting the innermost neighborhood of a black hole, and of the plasma surrounding two merging neutron stars accompanied by detection of gravitational waves, have shed “light” on the most exotic events and objects in the plasma universe. The plasma environment’s role in shaping the habitability of the exploding number of detected exoplanetary systems, ranging from the chemistry of the atmosphere to the development of life forms, is now being explored on many fronts. These discoveries and the promise of breakthroughs to come are fascinating scientists and the general public alike. All this occurred a mere 60 years after the start of the space age. (See Figure 7.1.)
The vast majority of baryonic (non-dark) matter in the universe is in the form of plasma, spanning a stunning range of physical conditions, spatial scales, and dynamics. Space and astrophysical plasmas (SAPs) reach regimes inaccessible to earthbound laboratory experiments, enabling deep insights into fundamental plasma processes that impact observations and understanding of the formation and evolution of the universe. Some can be sampled directly by spacecraft, while others
only reveal their secrets through spectroscopy, imaging, polarimetry, and other remote-sensing techniques. SAPs are also fascinating and exotic—from solar flares to active galactic nuclei to black-hole accretion disks—giving plasma physics high visibility and importance in the quest to understand heliophysics and astrophysics.
To understand and predict how the plasma universe around us operates, the fundamental physical processes responsible for phenomena ranging from electron-scale interactions to galaxy clusters need to be explored. Magnetic fields are paramount in governing the behavior of cosmic plasmas and play a key role in determining the habitability of our planet. Progress has been made, but more is needed, in determining how magnetic fields are generated in planets, stars, and galaxies, and how this magnetic energy is stored and released impulsively in the form of eruptions, geomagnetic storms, and other explosive events. Magnetic reconnection—the breaking and reconfiguring of magnetic fields—is the primary candidate to explain impulsive energy release, yet it is barely understood why it occurs at certain locations and times and how the liberated energy is partitioned among mass motions, particle acceleration, and heating. This ubiquitous mechanism is thought to explain the onset of stellar and accretion-disk flares and mass eruptions, magnetospheric storms on Earth and other planets, and γ-ray flares from the Crab Nebula. New observations and computational methods have revealed much about reconnection in cosmic settings, particularly in collisionless plasmas, but the fundamental processes governing this important phenomenon are still not well understood.
Eruptive events often drive shocks, which alter plasma properties substantially, generate waves and turbulence, and can accelerate ions and electrons to high energies. Within our heliosphere (the volume occupied by the magnetic field produced by and enveloping the Sun), shock-accelerated particles can endanger astronauts and spacecraft; higher-energy cosmic rays generated by supernova shocks and other sources can cause even more damage and can penetrate far into our atmosphere, perhaps affecting weather and mutating genes. Reconnection, shocks, and waves both generate and are generated by turbulence, which redistributes energy from large to small scales while increasing the complexity of the ambient plasma. Turbulence also accelerates and disperses charged particles, and may heat stellar coronae. Not all plasmas in the universe are fully ionized; the solar chromosphere, planetary ionospheres, the outer astrosphere, the ISM, and protoplanetary accretion disks all contain neutral atoms and molecules, which are not tied to the magnetic field as charged particles are. Partial ionization substantially alters the dynamics and energetics of the entire plasma system, through interactions between the charged and neutral particles, different responses to radiative input, and modifying conductivity dependent processes (e.g., reconnection and turbulence).
The full range of phenomena found in SAPs has counterparts in other areas of plasma physics. This overlap is particularly valuable for advancing our understanding of the underlying fundamental mechanisms. Magnetic reconnection, particle
acceleration, turbulence, shocks, and instabilities govern energy release and transport in laboratory devices such as toroidal plasmas and inertial confinement fusion experiments, as well as in stellar eruptions, planetary and pulsar magnetospheres, and galactic winds. The complex dynamics and energetics of partially ionized plasmas and ion-neutral coupling dominate stellar chromospheres, planetary ionospheres, comets, and the ISM, and are the processes in applications of low-temperature plasmas (LTPs) for chemical conversion and materials processing (Chapter 5). Fundamental dimensionless parameters such as the ratio of the particle mean free path to the system size, the ratio of thermal to magnetic pressure, the fluid and magnetic Reynolds numbers, and the ratio of the gyroradius to the system size enable advances in apparently unrelated environments to apply elsewhere. For example, collisionless plasmas are found in Earth’s magnetosphere, stellar winds, and the diffuse medium permeating galaxy clusters. Fusion devices contain high energy-density plasmas (Chapter 4), as do stellar interiors, black hole inflows, and the cores of super-Earths and hot Jupiters.
The plasma β (ratio of thermal to magnetic pressure) in SAPs varies widely depending on the environment. For example, β is less than 1 in the very local interstellar medium, the solar corona, and the outer regions of molecular clouds, whereas β ≥1 in the solar wind, stellar convective zones, planetary magnetosheaths, and the intracluster medium. Fusion plasmas (Chapter 6) are necessarily low β, and thus have some commonalities with and substantial differences from high-β SAPs. Because of these differences and similarities, we can develop deep insights into different plasma behaviors by studying and comparing related phenomena in the different subfields of plasma science and engineering (PSE).
Advances in observational capabilities, and extensions to new diagnostic regimes, invariably uncover new phenomena, challenge existing theories and spark new ones, and motivate cross-disciplinary collaborations. In the past decade new heliophysics missions populated geospace and our heliosphere, complementing continuing missions to form the Heliophysics System Observatory. Breakthrough advances in plasma astrophysics were brought about by data generated by space-based high-energy observatories. The advent of smallsats and cubesats has begun a new era of in situ multipoint observations, which are essential for understanding the connections between kinetic and global-scale behaviors, and has driven innovative developments in miniaturization of instruments. In parallel, new ground-based facilities have opened new windows on nearby and remote cosmic plasmas.
The high-resolution, high-cadence images of the solar corona obtained by the Interface Region Imaging Spectrograph (IRIS), Hinode, and the Solar Dynamics Observatory (SDO) have revealed distinct prerequisites and signatures of magnetic reconnection in explosive jets, flares, and coronal mass ejections (CMEs): plasma sheets surrounding current sheets, inflows, Alfvénic outflows, and plasmoids. The transition from the magnetically structured solar corona to the more isotropic, turbulent solar wind was seen for the first time in images from the Solar TErrestrial
RElations Observatory (STEREO) Heliographic Imager. Quasiperiodic plasma density structures were tracked from the inner heliosphere to their impact on Earth’s magnetosphere, by combining data from multiple spacecraft and modeling. These periodic plasma bursts were speculated to be “fossil” markers of structure generated near the Sun. Such structures in the solar wind have long been known to drive compressional oscillations in Earth’s magnetosphere, thus affecting particle energization and losses in the radiation belts. Recent investigations by PSP have identified other signatures of this solar wind driving and, most important, established that these pulses originate at the Sun. The mysteries of collisionless reconnection are being unraveled by the Magnetospheric Multiscale (MMS) mission, a 4-spacecraft cluster. By measuring electron distribution functions in a reconnection site in the magnetotail, MMS found strong support for a leading explanation for magnetic field-line breaking. Van Allen Probes discovered a third radiation belt and identified waves in the belts that accelerate particles and those that contribute to particle precipitation. The twin Voyager spacecraft, launched in 1977, have journeyed well past their nominal 5-year missions and are headed into the ISM, revolutionizing our understanding of the interaction of the solar wind with the local interstellar medium (LISM).
An impressive example of exotic plasma physics is the recently imaged supermassive black hole (BH) in the galaxy M87 by the Event Horizon Telescope. (See Figure 7.2.) This image shows the radiation of the hot, dense plasma orbiting the black hole, which emits no radiation. In conjunction with the groundbreaking detection of gravitational waves from cosmic events, electromagnetic emissions from the turbulent plasmas surrounding the BH gave vital clues about the nature and characteristics of the system—information that could not be derived from gravitational radiation alone. The detection of exoplanets (planets orbiting stars other than the Sun) in an extraordinary range of plasma environments has exploded over the past decade. These discoveries challenge our previous understanding of planetary formation and evolution, while illustrating the power, and success, of applying theories and numerical models developed to understand our solar system to investigate exoplanets in the very different environments of distant stars. These observations and insights are being leveraged to answer some of the most fundamental questions about how Earth evolved, and how the radiative output and eruptive activity earlier in our Sun’s history affected Earth’s atmosphere, oceans, and development of life. Lessons learned from these studies will enable us to look for the markers that may indicate that life is possible on other planets.
New computational techniques and computer architectures (Chapter 2) have emerged to rapidly advance our investigations of SAPs. These advances have enabled massive particle-in-cell (PIC) and gyrokinetic simulations of kinetic-scale instabilities and waves over larger system sizes than ever before. Highly detailed magnetohydrodynamic (MHD), multifluid, and hybrid simulations model the
initiation and propagation of solar eruptions from the Sun to Earth and beyond, reveal the dynamic evolution of planetary magnetospheres in stellar systems, and capture the relativistic chaos surrounding black holes. (See Figure 7.3.) Substantial progress has been made toward overcoming the challenges of assimilating and leveraging huge amounts of data and using that data to guide complex simulations. Automated feature recognition, machine learning (ML), and artificial intelligence (AI) methods are being introduced for data mining and analysis, to cope with the growth of large datasets that cannot be adequately processed by existing techniques for reducing, calibrating, cleaning, and visualizing these data.
Perhaps the most striking growth area in SAP having direct, on-the-ground societal benefit has been the science of space weather—the causes and consequences of activity generated at the Sun that propagates through the heliosphere toward planetary surfaces, and Earth in particular. Both living organisms and technology can be adversely affected by these solar disturbances that interact with our magnetosphere and ionosphere. Our worldwide dependence on reliable electricity,
global-positioning satellites (GPS), and telecommunications relies on stable and quiescent plasmas surrounding Earth and, in particular, its ionosphere. Consequently, the disruption of these plasmas by energetic particles and hard radiation from solar eruptions can lead to widespread power and communications outages, and infrastructure damage that could take years to repair. Energetic particles and radiation pose even greater threats to manned space travel and bases on the Moon and planets that lack the natural protection that magnetic fields provide. To understand and ultimately predict space weather events, we must develop deep physical insight into all plasma mechanisms that initiate, transport, and transform these energetic storms. Space weather is now recognized by the U.S. government as
a natural threat, motivating disaster preparedness as well as investments in basic and applied research. The understanding developed from our knowledge of space weather on Earth also is more broadly applicable to planetary space weather and to the issue of habitability on planets and exoplanets. Water may be necessary for all known instances of life, but it is not sufficient to ensure its survival if extreme space weather events can damage the planet’s atmosphere.
Space and astrophysical plasma research in the United States is funded predominantly by NASA, NSF, DoD, and DoE. NASA’s Science Mission Directorate funds space missions and instruments, data analysis, theory and numerical simulations, as well as providing a small amount of funding for ground-based observations and laboratory experiments in support of its missions. The research side of NASA’s Human Exploration Mission Directorate supports applied research on solar energetic particles (SEPs), with the aim of protecting astronauts from harmful exposure. NASA’s Space Technology Mission Directorate primarily supports development of the crosscutting, pioneering, new technologies and capabilities needed for current and future missions, using high-performance computing and technology demonstrations to test emerging instrument concepts. NSF funds existing and new ground-based facilities, a wide range of research in space physics, solar physics, astronomy and astrophysics, aeronomy and the plasma physics of the upper atmosphere, the National Center for Atmospheric Research (NCAR) and its subsidiary High Altitude Observatory (HAO), and some cross-disciplinary programs that bring together basic plasma and space/astrophysics research. DoD supports university and commercial SAP research through ONR, DARPA, and AFOSR, and internal research and facilities at the Naval Research Laboratory (NRL) and the Air Force Research Laboratory (AFRL). A modest program of space and astrophysical plasma research by the external community is supported by DoE, along with projects within the agency relevant to DoE needs (e.g., updated atomic physics and opacity tables). Several DoE international (e.g., Princeton Plasma Physics Laboratory and Max Plank Institute for Plasma Physics) and domestic (e.g., with NSF) collaborative programs touch on SAPs.
The highly successful but perpetually underfunded NSF/DOE Partnership in Basic Plasma Science and Engineering has been extensively utilized by members of the space plasma physics and plasma astrophysics communities since 1997, often in collaboration with plasma physicists in different subdisciplines. A significant improvement in interagency collaboration between NASA and NSF, Next Generation Software for Data-driven Models of Space Weather with Quantified Uncertainties (SWQU), recently selected six teams of researchers to develop comprehensive space-weather models from Sun to ionosphere). This exemplary collaboration addresses both fundamental science and translational research.
The last decade brought us discipline-changing discoveries in the field of plasma astrophysics. For the first time, in August 2017 the Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo collaboration detected the merger of two neutron stars in the galaxy NGC 4993 by their gravitational wave signatures. Just a few seconds later, the Fermi Gamma-ray Observatory (Fermi) observed a γ-ray burst at the same location. Within hours, intense optical emission was detected from the same region. Ultimately, more than 60 telescopes monitored the electromagnetic counterpart for weeks until its optical and infrared emission decayed. This long-awaited detection ushered in a new era in multimessenger astronomy (i.e., coordinated observations using different “messenger” signals: electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays.) The observations were consistent with a kilonova—the radioactive-decay-powered ejecta of neutron-rich material—as predicted by state-of-the-art MHD models. Emission during a kilonova can come from multiple plasma sources, and each component can reveal different information about the original neutron stars and their merger.
Much of the excitement surrounding this first detection of merging neutron stars was centered on their role in producing heavy elements. Low-mass elements are produced in low-mass stars and expelled into the ISM late in stars’ lives, while heavier elements are produced in supernova ejecta. For heavier, so-called r-process (rapid neutron capture) elements, it was postulated as early as 1957 that neutron star mergers would result in neutron-rich ejected plasma that could produce these heavy elements. The detection of a neutron star merger, combined with the first spectroscopic detection of elements heavier than Xe, open up the possibility of probing the formation site of these heavy r-process elements. However, interpretation, analysis, and modeling of the observed spectra are severely hindered by the sparsity of atomic opacity data and model predictions for these r-process elements under kilonova conditions.
In many astrophysical settings the magnetic field controls the overall dynamics of the plasma, while the dissipation of magnetic energy may power the observed high-energy emission. In the past two decades, magnetars (strongly magnetized neutron stars possessing super-strong magnetic fields), pulsars and pulsar wind nebulae, jets of active galactic nuclei, γ-Ray Bursters, and coronae of accreting black holes have been the subjects of intensive observational studies by X- and γ-ray instruments on several satellites. These objects share one important property: relativistic plasmas that are magnetically dominated—the energy density is mostly contributed by the magnetic field rather than by the rest mass.
Other major discoveries in plasma astrophysics since the Plasma 2010 report1 include: very short and intense fast radio bursts; rapid γ-ray flares at GeV photon energies in the Crab pulsar wind nebula; ultra-rapid (~10 min) TeV flares in blazar jets emanating from active galactic nuclei; giant BHs swallowing stars in tidal disruption events; and the IceCube neutrino observatory detection of a very energetic neutrino coincident with a flaring blazar. The plasma parameters in these events differ substantially from those of traditional laboratory plasmas, planetary magnetospheres, and the interplanetary medium (IPM), as they include several “exotic” physical processes. The plasma is often relativistic in terms of particle speeds, bulk motions, and characteristic wave speeds. Radiation-reaction effects (e.g., synchrotron or inverse-Compton radiative cooling), electron-positron pair creation, ultra-strong magnetic fields, quantum electrodynamics (QED), and general-relativistic effects are all important.
In the last two decades, exoplanet science has moved rapidly beyond its initial discovery phase. With thousands of exoplanets already confirmed, researchers are now investigating key issues such as planet formation, evolution, structure, and habitability. Understanding the properties of the plasmas deep in planetary interiors and their enveloping atmospheres is essential to all these studies. Understanding the interiors of gas giants and super-Earths requires understanding matter under extreme pressures, introducing new domains of plasma physics. Fortunately, exoplanet observations and interpretations that need knowledge of plasmas under Megabar pressures now can turn to direct laboratory studies of these extreme conditions (see Chapter 4).
The large number of exoplanets already discovered invites questions about whether those planets ever had (or will have) conditions that can support life. Cross-disciplinary studies are applying space-weather theories and models developed for our solar system to answer these questions. With knowledge of the evolution of our own solar system, the models are being used to investigate conditions on hot Jupiters, super Earths, and other previously unexplored exoplanet environments. For example, observations of over 1500 stars by the Kepler space telescope designed to discover exoplanets showed that young, fast-rotating, solar-type stars produce superflares. Simulations of extreme eruptions in such fast-rotator systems, however, found that the tightly wound interplanetary magnetic field complicates and extends the routes taken by the resulting accelerated particles to the exoplanets. Simulations of the possible effect of superflare-produced energetic particles on life-enabling conditions on orbiting planets suggest that the conditions that could foster Earth-like living organisms are quite narrow in terms of radiation dose, atmospheric properties and chemical composition, and presence of liquid water. Therefore, habitability studies have focused on establishing factors besides simple distance from the star that can push a planet into or out of the “Goldilocks” zone amenable to life.
Many issues in exoplanet research have been clarified by adapting heliophysical models for exoplanetary and young-Sun conditions: the presence and cyclic nature of a magnetic dynamo, the ability of stellar winds to strip away a planet’s atmosphere, the existence of stellar eruptions that are orders of magnitude more energetic than those of our Sun, and the capability of stellar eruptions to change the atmospheric chemistry and irradiate the surface. While our Sun was still “young,” for example, solar eruptions (CMEs and eruptive flares) were substantially stronger and more frequent than today, and Earth’s atmosphere was more nitrogen-dominated. Recent simulations have shown that energetic particles from those solar eruptions could have changed Earth’s atmospheric chemistry sufficiently to warm the early Earth through a greenhouse effect. (See Figure 7.4.) The possibility of life elsewhere in the universe excites both scientists and the public, and plasma science plays a key role in this quest.
All solar phenomena include plasma physics. High-resolution, high-cadence imaging of the Sun, by space missions such as the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), SDO, and IRIS, and by ground-based facilities such as
the Goode Solar Telescope and the Extended Owens Valley Solar Array (EOVSA), has revealed key small-scale features and transient events. Distinct signatures of magnetic reconnection in eruptive events, from tiny jets to huge coronal mass ejections, and the formation of plasmoids in flare current sheets (see Figure 7.5), were detected for the first time, testing long-standing theories and firmly establishing the role of reconnection in impulsive energy release.
An outstanding unsolved problem in solar physics is how the corona is heated—this outermost layer of the Sun is nearly 3 orders of magnitude hotter than the surface. The corona is believed to be heated by nanoflares (small reconnection events), ion cyclotron waves, and/or magnetic turbulence. Observations by IRIS and the rocket-borne EUNIS spectrometer find some evidence for nanoflare heating, including nonthermal electron beams generated by these tiny events, but little agreement yet exists. The committee anticipates that Parker Solar Probe might provide a definitive answer.
The MHD dynamo that generates and recycles the Sun’s magnetic field is sustained by the energy of internal plasma motions such as differential rotation (different rotational speeds at different latitudes), turbulent convection, and meridional circulation. The toroidal magnetic field is generated through the stretching of the poloidal component of the magnetic field by differential rotation and is widely believed to be stored and amplified at the tachocline, the thin, highly sheared layer between the Sun’s rigidly rotating radiative core and its differentially rotating convective zone. Strong toroidal flux tubes are unstable to magnetic buoyancy and erupt through the surface, producing sunspots that are strongly magnetized and give birth to most fast solar eruptions. The poloidal field is thought to be regenerated through a combination of convection-zone turbulence and the redistribution of the magnetic flux from tilted bipolar sunspot pairs. The dynamo-mediated evolution of solar magnetic fields governs solar irradiance variations and eruption frequency, which affect planetary atmospheres and magnetospheres. A recent breakthrough in dynamo simulations has produced cyclic, solar-like polarity reversals of the large-scale magnetic field. Although advances have been made since the Plasma 2010 report in understanding the complex, nonlinear dynamo system of our Sun, we are still unable to definitively predict whether the downward trend in the solar magnetic output during the last few sunspot cycles will be a long-term decline, as in the Maunder minimum (1645-1715).
Solar eruptions are the most energetic phenomena in the solar system. Large eruptions release about 1024-1025 J and can accelerate particles to energies of several GeV per nucleon. RHESSI has raised new questions about particle acceleration in solar flares: how can the sources of electron-associated hard X rays (HXRs) and ion-associated γ-rays be separated by large distances, and how can relativistic flare electrons persist and produce intense radiation in the corona? Two astrophysics missions also contributed to our understanding of impulsive electron and ion acceleration on the Sun. Fermi observed γ-rays from ions accelerated to at least
several GeV, as well as pion decay from accelerated ions in many large eruptions. More puzzling, Fermi detected γ-ray emission coming from various regions of the solar atmosphere, prompting heated debates about how particles gain access to and remain in these regions. At the other extreme, the Nuclear Spectroscopic Telescope Array (NuStar) made great strides in understanding electron acceleration in active-region microflares—the faintest solar flares ever observed in HXRs.
An understanding of energetic particles in SAPs, as well as laboratory plasmas, is closely related to our understanding of collisionless shock waves and turbulence. Shocks are ubiquitous in the heliosphere, ranging from those generated by CMEs (see Figure 7.6) to forward and reverse shocks created by interacting solar-wind streams (co-rotating interaction regions [CIRs]), shocks in the distant heliosphere, and the heliospheric termination shock. Collisionless shocks are observed in the heliosphere as interplanetary (IP) shocks, inferred in supernovae and accretion disks, and are often associated with energetic particles. With the increase in computational power, theory and simulation of collisionless shocks have progressed rapidly. Numerous satellites with increasingly resolved plasma and field measurements provide in situ observations of shocks. NASA’s growing Heliophysics System
Observatory (Cluster, Time History of Events and Macroscale Interactions during Substorms (THEMIS), the Advanced Composition Explorer, Wind, and STEREO, in particular) provides multipoint shock measurements, yielding global insights into shock dynamics and structure on scales of hundreds to thousands of kilometers. Laser-driven supersonic plasma flows in the laboratory have been used to investigate collisionless shocks in a controlled setting using the Omega laser at the Laboratory for Laser Energetics and the National Ignition Facility (NIF; see Chapter 4). Recent experiments on Omega and NIF indicate that, in astrophysical environments, strong shocks generate and amplify magnetic fields and accelerate cosmic rays.
The Magnetospheric MultiScale (MMS) mission, launched in 2015, is a constellation of four closely spaced satellites that can fly as close together as 5 km. The mission was designed to measure electric currents and particle properties over a range of scales. Besides investigating Earth’s bow shock, MMS has enabled new studies of IP shocks near Earth at scales that had previously been the preserve only of bow shock research. However, IP shocks are dynamically and structurally quite different from bow shocks, especially regarding energetic particles and their back reaction on shock structure. Missions such as MMS and PSP will provide unparalleled insights into IP shocks from kinetic to MHD and global scales.
Solar flares and CME-driven shocks are primary sites for accelerating electrons and ions as solar energetic particle (SEP) events, which propagate into the heliosphere along the interplanetary magnetic field (IMF). Predicting SEP impacts at multiple vantage points is of great practical importance in the development of reliable space-weather models capable, for example, of guiding astronaut safety decisions in the new “Moon to Mars” programs. In situ measurements of charged-particle properties show that impulsive SEP events are primarily associated with solar flares, whereas gradual events correspond to CME-driven shock-accelerated particles. Since the Plasma 2010 report, studies of SEP events have benefited from multipoint observations, revealing that energetic charged particles can diffuse considerable distances longitudinally and latitudinally, contrary to expectations. These investigations have revealed the complexity of energetic charged-particle transport, but many open questions remain: in particular, how particles diffuse transverse to the magnetic field, the precise acceleration mechanism for impulsive events, and the properties of the “seed” particles before acceleration required to explain the observed fluence and energy.
The most likely acceleration process for gradual SEPs is diffusive shock acceleration (DSA), first proposed to explain the acceleration of galactic cosmic rays at supernova shocks. Gradual SEP and CIR-related events typically present an extended front of particles propagating away from (CME-driven shock) or toward (CIR shock) the Sun. For particles to be efficiently energized at a collisionless quasi-parallel shock (see Figure 7.6), they must be confined so that they diffuse across the shock multiple times. Ahead of the shock, Alfvén waves excited by
a streaming instability can scatter some of the accelerated particles back toward the shock. Similarly, downstream turbulence generated and amplified at the shock traps particles in its vicinity. The transport of these energetic particles through the corona and heliosphere with their many shocks remains a difficult problem. Two competing effects influence their propagation: focusing along the IMF due to the decreasing magnetic field strength and pitch-angle scattering by IMF turbulence. Strong scattering allows SEPs to diffuse across the IMF, whereas weak scattering leads to particles streaming freely. Numerical modeling of CME shocks and SEP events reached an impressive level of sophistication in the past decade, enabling detailed predictions that compare favorably to observed spectral, energetic, and temporal characteristics of large events.
However, some interplanetary SEP events have been observed that do not fit the DSA model. For example, in situ particle and field observations by multiple spacecraft near the heliospheric current sheet and in Earth’s magnetosphere indicate that SEPs may be associated with dynamically interacting flux ropes (helical magnetic structures that consist of a twist component and an axial field; also see Chapter 2). Theory and simulations demonstrate that flux rope merging and contraction can energize particles locally in the IPM (see Figure 7.7) and in solar eruptive flares. Flux-rope contraction and coalescence in reconnecting current sheets therefore appears to be an alternative mechanism for accelerating particles in different SAPs, offering a promising avenue for further exploration in the next decade.
MMS has probed the inner core of magnetic reconnection sites, where both ions and electrons are decoupled from the magnetic field. This is the actual region where the magnetic field breaks and reconnects. The magnetosphere and many other cosmic plasmas are sufficiently rarefied that collisions between particles are infrequent. In such collisionless plasmas, identifying the mechanism for magnetic flux breaking during reconnection has been a long-standing challenge. For the first time, MMS observations verified a recent theoretical prediction of the underlying process by examining the shape of electron velocity distributions within a reconnection region in the magnetotail. This fundamental discovery illustrates the profound value of combining theory, computation, and state-of-the-art observations.
The recently decommissioned Van Allen Probes (VAP) brought new insights into the source, transport, and loss of charged particles within Earth’s radiation belts (see Figure 7.8), and the local-global coupling of the magnetosphere and solar wind. The VAP discovered a transient third radiation belt filled with ultra-relativistic electrons, which briefly appeared in September-October 2012 after a geomagnetic storm. Analysis of VAP data identified the long-sought location and source of electron acceleration in the radiation belts. Geomagnetic substorms inject electrons deep into the nightside magnetosphere where they produce chorus waves (a type of whistler wave), which in turn accelerate high-energy radiation-belt electrons to even greater energies. The causes of particle loss from the belts
remain a topic of debate, but the likely explanations have been narrowed down by theoretical studies and VAP observations to magnetopause outflow, charge exchange, and precipitation into the ionosphere. In parallel with these observational breakthroughs, laboratory experiments on the Large Area Plasma Device at the University of California, Los Angeles, have contributed to understanding radiation-belt physics by using controlled energetic-electron beams to study the generation of and interaction with whistler waves.
Ionosphere, thermosphere, and mesosphere (ITM) plasma physics has made substantial progress on several fronts. Greater appreciation for lower atmospheric forcing of the ionosphere and thermosphere has emphasized how critical gravity-wave propagation and dissipation are to the thermosphere. Multi-instrument studies by the DoD-NASA C/NOFS (Communications/Navigation Outage Forecasting System) mission, sounding rockets, and ground-based radar facilities, accompanied by numerical modeling, have revealed the complex physics of
plasma bubbles and depletions in the ionosphere. These investigations have shown the importance of primary instabilities (e.g., Kelvin-Helmholtz) and neutral wind shears in seeding the more slowly growing Rayleigh-Taylor instability. DARPA’s new Space Environment Exploitation program aims to accurately predict near-Earth space environment disturbances and perturbations (on scales as small as 100 km) in 1-hour increments extending out 72 hours, through advances in modeling and data collection. Computer simulations that leverage the power of graphics processing units are providing higher-resolution physics-based perspectives than in the recent past (Chapter 2). New sensors and missions are being sought to increase the number of measurements in the greatly undersampled near-Earth space environment, to provide learning sets for advance ML analysis.
The Global-scale Observations of the Limb and Disk (GOLD) mission, launched in January 2018, is now a key tool for ionospheric observations, providing the first day-to-day measurements of the region and its response to forcing by the Sun, the magnetosphere, and the lower atmosphere. GOLD measures the temperature and composition of neutral gases in Earth’s thermosphere through full-disk UV imaging spectroscopy of Earth from its geostationary vantage point above the Western Hemisphere. Because the ionized plasma and neutral gases interact in the thermosphere, understanding of plasma processes and computer modeling play critical roles in interpreting GOLD data.
Even lower in Earth’s atmosphere, electrical processes produce lightning flashes that lead to plasma phenomena with measurable effects throughout the atmosphere and beyond. Sensitive photometric and high-speed video records obtained during the last decade indicate that these varied phenomena are dynamically complex. The related plasma processes exhibit a large variety of visual forms, which collectively span the full range of altitudes between the tropopause and the ionosphere. The luminous optical manifestations of these observed events roughly distinguish among the various distinctive classes: sprites, elves, and blue jets (see Figure 7.9).
Powerful lightning discharges can directly disturb the lower ionosphere, disrupting very low-frequency radio waves used commonly for communications. Over the last decade, lightning has been found to induce plasma perturbations in the ionosphere that can last tens of minutes. Lightning processes can accelerate electrons to relativistic energies. Terrestrial γ-ray flashes observed from space and from the ground are thought to be bremsstrahlung emission from the deceleration of very energetic (tens of MeV) electrons by collisions with atmospheric molecules. Up to 1017 high-energy electrons and fewer positrons are present in a typical event, which can last for fractions of milliseconds and occurs about 50 times per day globally. In addition, lightning can launch strong electromagnetic pulses and create strong quasi-static electric fields, inducing gas discharges in the upper atmosphere (Chapter 5).
NASA’s Parker Solar Probe (PSP) is a revolutionary spacecraft designed to approach within 10 Rsun (solar radii) of the Sun. PSP is producing unprecedented
data on the plasma and energetic-particle properties in the solar wind. For example, PSP magnetic-field and plasma measurements during the first solar encounter at 35.7 Rsun identified a slow Alfvénic solar wind emerging from a small equatorial coronal hole, apparently escaping from above low-lying, complex magnetic structures, which exhibited a highly dynamic magnetic field with polarity reversals on timescales from seconds to hours. These varying field structures were associated with clustered radial plasma jets with enhanced energy flux and turbulence. During periods between groups of jets, the sampled solar wind was essentially steady. The combination of PSP with other solar-heliospheric missions and observatories will help answer fundamental questions about the origin of the solar wind, including how the solar corona is heated and how and where the different types of solar wind are generated.
Observations by Voyagers 1 and 2, energetic neutral atom (ENA) observations by the Interstellar Boundary Explorer (IBEX), and sophisticated theory and modeling have revolutionized our understanding of the interaction of the solar wind with the partially ionized LISM. The neutral hydrogen component of the ISM profoundly affects the supersonic wind through the creation of pickup ions (PUIs), a suprathermal (~1 keV) plasma component that removes energy and mo-
mentum from the solar wind and introduces low-frequency turbulence in the wind beyond ~10 AU (1 astronautical unit (AU) is the distance from the Sun to Earth). Remarkably, the dissipation of this interstellar-driven turbulence gradually heats the distant solar wind, rather than the expected gradual cooling. The PUI population affects the dynamics of all ambient shocks, including the termination shock (where the solar wind transitions from a supersonic to a subsonic flow). Although anomalous cosmic rays (ACRs) were observed to affect the termination shock structure, the Voyagers did not identify the termination shock as the site at which ACRs originate, leaving the debate unresolved. Surprisingly, the inner heliosheath (the region between the termination shock and the heliopause) is far narrower than expected, and is a highly turbulent mix of compressible and incompressible plasma. The heliosheath thermal plasma remains relatively cool because PUIs and ACRs carry much of the thermal energy. The heliopause structure is largely unexplained.
Both Voyager spacecraft have now begun humanity’s first in situ exploration of galactic space, and continue to transform our understanding of cosmic rays. Voyager 1 revealed that the very local interstellar medium (VLISM) is very much influenced by the solar wind. Even at 145 AU. Voyager 1 is still observing interplanetary shocks that have propagated well beyond the heliopause, introducing variations in cosmic ray anisotropies. Voyager 1 is measuring the interstellar magnetic field (ISMF) direction and strength, which determines the global shape and nature of the boundary between the solar system and the ISM. Preliminary results from the Voyager 2 crossing of the heliopause suggest that the ISMF strength exceeds the corresponding measurement made by Voyager 1 at a much higher latitude. These observations have guided 3D MHD simulations of the dynamic magnetic field and plasma structure of the heliosphere, revealing its cometary-like shape embedded in the LISM. (See Figure 7.10.)
IBEX discovered an unexpected “ribbon” of energetic neutral atom (ENA) emission originating from the VLISM. This emission is apparently associated with the orientation of the ISMF. (See Figure 7.11.) The ribbon is thought to be due to the ionization of fast and/or hot solar-wind neutral atoms that propagate from the supersonic solar wind and inner heliosheath into the VLISM, where they are reionized to become PUIs gyrating about the ISMF. Reneutralization of these PUIs by charge-exchange with interstellar hydrogen atoms yields ENAs that can be detected at 1 AU. The ribbon observations of ENAs by IBEX suggests an ISMF strength that is roughly consistent with current Voyager 1 data. Langmuir-wave measurements by Voyager 1 suggest that the heliopause might possess a depletion layer similar to that observed at Earth’s magnetopause.
Voyager 1 discovered that shock waves propagating in the low-β (dominated by magnetic rather than thermal gas pressure) interstellar space are weak, quasi-perpendicular, unusually smooth and very wide (~104 times broader than similar shocks at Earth). The VLISM shocks are IP shocks that propagate through the
supersonic solar wind, are transmitted across the heliospheric termination shock, propagate through the heliosheath, and are then partially transmitted into the VLISM at the heliopause. Unlike the heliosphere, the thermal VLISM plasma is collisional with respect to proton-proton collisions that dissipate energy through heat conduction and viscosity. Because the VLISM shocks are weak, this dissipation determines their structure.
Although kinetic approaches are now being used in studies of magnetized
plasmas, many such investigations assume that the plasmas are in equilibrium with their magnetic fields. In contrast, most heliophysical plasmas observed since the Plasma 2010 report reveal complex configurations that are not in equilibrium, with signatures of turbulence, strong nonlinearity, and cascade-type processes. High-resolution images (Goode Solar Telescope and SDO), and spectroscopic measurements (Hinode, IRIS, and rocket-borne instruments) show that the entire solar atmosphere exhibits extraordinarily complex, nonlinear magnetic activity. The solar wind contains magnetic and kinetic energy density fluctuations fit by a power law over decades of scale, in addition to persistent, intermittent, and quasi-periodic structures. The magnetosheath and plasma sheet also exhibit noisy, bursty, random structures. Ambient turbulence and its transport and coupling to large-scale flows govern the scattering and transport of energetic charged particles, while coherent structures such as flux ropes can scatter, trap and accelerate particles. Our current inability to predict magnetic fields and particle distributions at specific locations in the heliosphere, as well as uncertainties in space weather prediction, largely stem from our inability to accurately measure or model local turbulent properties. Understanding the basic plasma physics of turbulence and its role in dictating the state of SAPs remains an outstanding problem for the next decade and beyond.
Advances in understanding SAPs provide multiple benefits to society and science. The most tangible advantages come from our growing understanding of the causes and consequences of space weather. The most extreme space weather comes from solar eruptions, in particular the fast CMEs that drive shocks, produce SEPs, and smash into our magnetosphere, triggering geomagnetic storms, aurorae, and geomagnetically induced currents that can affect pipelines and the electric power grid. SEPs can directly disrupt onboard computers and other instruments on spacecraft, disable GPS and communication satellites, and induce radiation damage to unshielded humans in space. Geomagnetic storms also cause energetic particles to be ejected from the radiation belts surrounding Earth, potentially damaging spacecraft in low Earth and geostationary orbits. The entire causal chain, from solar eruptions to the impact on the magnetosphere to the space weather effects on our technology, corresponds to a chain of intimately coupled plasma processes. Understanding the entire chain from the Sun to Earth and beyond calls for a systems approach and represents a paradigm shift that has emerged in the past decade. With our critical dependence on technology, PSE is essential for understanding and ultimately predicting space weather, as well as mitigating its effects.
An associated societal and technological benefit of space and astrophysical plasma research is the development of experimental equipment and associated detectors, probes, imaging techniques, and data-analysis software. Many techniques initially developed to visualize cosmic phenomena have crossed over to other fields, such as medical imaging, defense, and industrial applications. Large-area astronomical surveys and their catalogued data over the whole electromagnetic spectrum play important roles in developing big-data techniques for new discoveries. Numerous important spectroscopic discoveries and methods with practical utility originated in astrophysical investigations, from the discovery of helium to other species in interstellar clouds. The extreme environments of, for example, planetary and stellar interiors, pulsar magnetospheres, and intergalactic filaments offer unparalleled opportunities to explore plasma conditions that cannot be duplicated in laboratories, testing the limits of our theories and models and perhaps leading to new techniques that can be applied to high energy-density investigations and fusion reactors (Chapter 4). The need to understand extreme plasma conditions also drives the development of innovative computational techniques and hardware (see Chapter 2).
Finally, space excites people of all ages, motivating the public to learn more about science and attracting the next generation of space and astrophysical plasma scientists and engineers, and scientists and engineers in nearly all STEM disciplines. Undergraduate research in SAP would expose a more diverse student population to plasma physics, comprising a valuable mechanism to diversify the broader PSE
community. Space science and astrophysics can be taught to nonscientists and physics graduate students alike. Space missions and ground-based observatories employ scientists, engineers, and technicians from the earliest design phase through construction to operations, requiring a vast range of skills across the STEM domain. Facilities dedicated to SAP research and technology are found in university departments, private industry, aerospace companies, federal laboratories, and international institutes, all employing the diverse workforce required to keep such organizations thriving. Therefore, the effects of a healthy SAP program extend well beyond the confines of those engaged in research alone.
The growing awareness of space weather and its consequences has motivated international efforts to create or enhance space-weather forecasting facilities and build physics-based and empirical models to elucidate the underlying processes. These models are enabling increasingly accurate predictions, while the establishment of new facilities and academic programs in this field is actively increasing science capabilities in both developed and developing countries. Summer schools and other educational programs are training the next generation of space-weather researchers. In the United States, agencies such as Department of Homeland Security and the Federal Energy Regulatory Commission began to consider the societal and technological impacts of space weather events, and to include them in disaster planning. Electric power companies, space plasma scientists, and government representatives collectively developed and adopted new reliability standards to mitigate the impacts of geomagnetic disturbances. Several commercial enterprises that supply space weather products, instrumentation, simulations, and predictions were founded in the past decade, creating jobs and serving a diverse base of scientific and operational customers.
The need for high-throughput, high-cadence imaging with ever-increasing spatial and spectral resolution has driven significant advances in instrumentation and image processing that also benefit the broader science community, military, and diverse industries such as commercial Earth imaging. For example, recent efforts have focused on developing space-borne photon sieves, hard X-ray spectroscopic imagers, miniaturized plasma instruments and magnetometers, ion-neutral mass spectrometers, and sodium lidar detectors, as well as ground-based adaptive optics telescopes and interferometers with extremely long baselines. Space applications of PSE range from aerospace vehicle construction and spacecraft propulsion to sensors and detectors (see Chapter 5).
Since the Plasma 2010 report, technological advances in SAPs have improved the plasma instruments and diagnostics used to detect, measure, and record the complex multiscale and multimessenger phenomena that permeate our universe. Smallsats and CubeSats have become favored platforms for obtaining multiple samples and views of localized, transient events in geospace and beyond, driving the development of miniaturized instruments capable of fitting into the highly restrictive size and mass limits of these small spacecraft. Although this rapidly developing technology is still in its infancy, it offers enormous potential for answering many scientific questions about the origins and evolution of space weather effects, from ionospheric disturbances and magnetotail substorms to SEP events. The need for greater sensitivity and higher spatial and temporal resolution motivates a new generation of detectors, materials, and manufacturing techniques.
Several laboratory experiments performed in the past decade were designed to better reproduce important solar and astrophysical conditions and enable comparisons between laboratory experiments and SAP observations (and models). The Line Tied Reconnection Experiment, a basic plasma physics experimental facility at the University of Wisconsin, Madison, was constructed to study ideal and resistive MHD instabilities under variable boundary conditions and equilibria. In particular the line-tied conditions characteristic of the solar atmosphere and the magnetic launching of astrophysical jets were emphasized. The Facility for Laboratory Reconnection Experiments (FLARE) at the Princeton Plasma Physics Laboratory (PPPL) is a new intermediate collaborative user facility constructed by a consortium of five universities (Princeton University, University of California, Berkeley, University of California, Los Angeles, University of Maryland, University of Wisconsin, Madison) and two DoE national laboratories (PPPL and Los Alamos National Laboratory). The goal of FLARE is to provide experimental access to new regimes of the magnetic reconnection process and related phenomena directly relevant to heliophysics, astrophysics, and fusion plasmas. NIF (Chapter 4) also provides new paths to investigate nuclear processes and structural effects in the time, mass and energy density domains relevant to astrophysical plasma phenomena in a controlled terrestrial environment.
The Basic Plasma Science Facility (BAPSF) at UCLA is a national user facility for fundamental plasma science sponsored by DOE and NSF. BaPSF, and its primary experimental device, the Large Plasma Device (LAPD), provide a platform for studying processes relevant to SAP parameter regimes in a reproducible, large volume, magnetized plasma. Examples include the linear and nonlinear physics of plasma waves (e.g., Alfvén waves), collisionless shocks, magnetic reconnection, interaction between energetic particles and waves, and turbulence and transport, over a wide plasma-β range.
The Naval Research Laboratory’s ground-based Space Physics Simulation Chamber (SPSC), an ONR-sponsored facility, complements theory, modeling, and in situ measurements with laboratory experiments. SPSC provides a platform to collaboratively investigate the underlying physics of space plasmas under controlled, reproducible, scaled laboratory conditions, particularly those representative of the near-Earth space plasma environment, and a realistic testbed for the development and preflight testing of space diagnostics and hardware. The device is used for the study of ionospheric, magnetospheric, and solar wind plasma phenomena; testing/calibration of space-qualified diagnostic instruments for missions; spacecraft charging; large-volume plasma generation; and other investigations requiring a low-pressure environment.
NSF’s National Solar Observatory (NSO) operates cutting-edge facilities, develops advanced instrumentation both in-house and through partnerships, conducts solar research, and creates educational and public outreach programs. The NSO Integrated Synoptic Program (NISP), established in 2011, operates two facilities dedicated to long-term full-disk coverage of the Sun: the Global Oscillation Network Group (GONG) and the Synoptic Optical Long-term Investigations of the Sun. Data from the worldwide GONG network has advanced our understanding of the Sun through helioseismology (the study of the Sun’s interior using observed oscillations, which explores the internal structure in unprecedented detail) and prominence seismology. NSO’s Community Science Program develops analysis and modeling tools that will enhance the value of data taken with NSO’s state-of-the-art observing facilities—the Daniel K. Inouye Solar Telescope (DKIST) and NISP—and to train the next generation of solar physicists in the use and development of these tools. Until 2018, NSO also operated telescopes at Kitt Peak and Sacramento Peak Observatories that have been critical to the advancement of the field. NSF’s National Center for Atmospheric Research/High Altitude Observatory operates and updates the Mauna Loa Solar Observatory and its two primary instruments, the K-COR Coronagraph and the COronal Multi-channel Polarimeter.
The Center for Solar-Terrestrial Research at New Jersey Institute of Technology operates the Big Bear Solar Observatory (BBSO) and Extended Owens Valley Solar Array (see below), the Automated Geophysical Observatories and other instruments in Antarctica, and geospace observing facilities across South America and the United States, with funding through grants from NSF, NASA, the U.S. Air Force, the Korean National Science Foundation, and other government and private sources. BBSO presently hosts the highest resolution optical solar telescope in the world, the 1.6-m Goode Solar Telescope (GST). Operating since 2009, with adaptive optics installed in 2017, the GST determines the magnetic field and plasma dynamics in the solar chromosphere and photosphere on scales as small as 70 km. This facility also served as a valuable testbed for the adaptive optics and instruments being installed at DKIST.
The Extended Owens Valley Solar Array (EOVSA) is a world-class NSF-funded facility for scientific research at microwave radio frequencies (1-18 GHz) aimed at understanding the Sun and its influence on Earth and near-Earth space environment. EOVSA focuses on studying the magnetic structure of the solar corona, transient phenomena resulting from magnetic interactions (e.g., solar flares and associated particle acceleration and heating), and space weather phenomena. The project has provided solar-dedicated observations since its completion in 2017. EOVSA has yielded groundbreaking observations of some of the most powerful solar eruptions ever measured and provides unique insights into the heating and acceleration of high-energy coronal electrons.
Progress in understanding Earth’s ionosphere, thermosphere, and mesosphere (ITM) has long relied on ground-based observations from multiple international facilities. In the ITM, neutral gas motions drive ionospheric density and dynamics, affecting currents, plasma instabilities, and vertical coupling of atmospheric regions. A new class of Incoherent Scatter Radars, including Poker Flat Incoherent Scatter Radar (built during the past decade), RISR-N, and RISR-C, is enabling leaps forward in our understanding of ITM phenomena. T-REX, a network of multispectral all-sky imagers in Canada, is a new capability for comprehensively characterizing the high-latitude thermospheric and ionospheric state and energy inputs. Next-generation scanning Doppler imagers, such as the new facility built by the University of Alaska, are starting to collect detailed ground-based measurements of thermospheric winds. At low latitudes, upgrades and new observational techniques applied in the past decade to the 50-year old Jicamarca Radio Observatory have produced unprecedented images of the electrodynamics of the equatorial ionosphere. Ground-based lidars, which primarily measure temperature, wind speed, and metallicity in the low nightside ITM, have been greatly improved since the Plasma 2010 report. These lidars have become more robust and sensitive, covering more species and greater ranges in altitude than ever before. The international proliferation of ground-based GPS receivers, which can measure the total electron content above them, is providing new insights into the temporal and spatial structure of ionospheric variations, traveling disturbances, and other transient phenomena in the upper atmosphere.
Computations and Diagnostics
SAP research has long capitalized on computational advances in hardware, algorithm development, and access to new systems to design new instruments, assimilate data, validate predictive models, probe extreme conditions in cosmic phenomena, and reach for closure between theories and observations. Increasing access to graphical processing units (GPUs) enabled substantial speed-up of select codes, but the need for machine-specific rewriting of programs now limits the po-
tential for widespread GPU usage. Some MHD codes incorporated more relevant physical processes, such as radiative transfer, ionospheric chemistry, and the effects of partial ionization. The use of adaptive mesh refinement to place the highest spatial resolution where it is most needed, such as current sheets, steep density gradients, and boundary layers, became more common for modeling phenomena covering large dynamic ranges.
Because the most interesting physical processes are complex and nonlinear in traditional plasmas, and even more so in extreme plasmas, our ability to understand them greatly benefits from numerical simulations. Computational SAP studies are increasingly important, revolutionizing our understanding of extreme plasmas in exotic astrophysical objects. Particle-in-cell (PIC) codes offer opportunities to understand how exotic/relativistic pair plasma is produced, and how coherent radio and high-energy emissions are generated. These developments are enabling, for the first time, truly ab-initio studies of important phenomena such as Crab Nebula γ-ray flares, pulsed high-energy emission from pulsar magnetospheres, the coronae of accreting BHs, and pair-production cascades in pulsar and BH magnetospheres, as well as basic processes such as magnetic reconnection and particle acceleration in astrophysical plasmas. (See Figure 7.12.) Modeling of extreme plasma physics has been enabled recently by the development of radiative 3D general-relativistic MHD codes and their application
to global simulations of accreting BHs and their jets. These pioneering studies clearly demonstrate that rigorous numerical investigation of extreme plasma processes is now feasible and realistic, opening up broad avenues for future exploration.
Developing time-dependent simulations with evolving boundary conditions derived from observations is essential to improving the predictive capability of space-weather modeling. For example, AFRL’s ADAPT code prepares a time series of magnetograms of the solar photosphere to drive the bottom boundary of coronal and solar wind models. A serious obstacle to this approach is the lack of information about the changeable photospheric magnetic field on the far side of the Sun, which would be best rectified with multiple satellites distributed around the Sun.
New, cutting-edge global modeling techniques are being developed, such as the multiscale plasma Wave-in-Cell (WIC) simulation method that self-consistently tracks wave dynamics. These capabilities can clarify and predict the global magnetospheric impact of microscopic kinetic plasma processes, including wave-particle interactions. Such frontier computational techniques may answer key questions, such as the source of plasmaspheric hiss, the factors determining whether a geomagnetic storm enhances or depletes electrons, the global conditions required to form magnetospheric resonators, and the effectiveness of different radiation-belt remediation techniques.
Novel approaches such as Bayesian analysis, automated feature recognition, and machine learning are extracting valuable science from large data sets. For example, SDO yields 1.5 TB of data daily, necessitating innovative techniques for finding key phenomena or features and interpreting their properties efficiently. Similar approaches are being introduced to interrogate the massive data output from 3D numerical simulations, particularly for ensembles of physics-based space weather simulations. Recent application of machine-learning techniques to IRIS observations has increased the computational efficiency of chromospheric diagnostics one million-fold, compared with physics-based numerical simulations and inversion codes. This approach quickly provides chromospheric researchers with the temperature, velocity, density, and unresolved motions or turbulence as a function of height in the solar atmosphere for every IRIS observation. This new machine-learning based tool, combined with the more than 25,000 IRIS observations of the solar chromosphere since its launch in 2013, will deepen our physical insight into the transfer of energy from the Sun’s interior to the corona.
The next decade brings exciting opportunities and daunting challenges for space and astrophysical plasma science. Cross-disciplinary collaborations, international cooperation, novel observing platforms, new instruments and diagnostic methods, and targeted combinations of theory, computation, experiments, and
data analysis will be crucial for answering the most fundamental questions about the plasma universe.
Observational advances over the past decade have revolutionized many aspects of astrophysical plasmas, through innovative and exciting ground-based and space-based missions. Ongoing missions such as LIGO (The Laser Interferometer Gravitational-Wave Observatory) and planned or under construction new observational facilities such as ATHENA (Advanced Telescope for High-ENergy Astrophysics, expected to launch in 2028), ETH (Event Horizon Telescope), XRISM (X-Ray Imaging and Spectroscopy Mission), LYNX (Lynx X-ray Observatory), LISA (Laser Interferometer Space Antenna), SKA (Square Kilometre Array), and CTA (Cherenkov Telescope Array) will address major challenges in several subareas of plasma astrophysics:
- Extreme plasma physics of multimessenger cosmic plasmas
- Physics of extremely rarefied/weakly collisional plasmas
- Plasma physics of the early solar system evolution, exoplanets, and the origin of life
- Plasma physics of the interstellar medium
Multimessenger astrophysics combines information from multiple extrasolar electromagnetic radiation, gravitational waves, neutrinos, and cosmic ray observations. Exploiting these four “messengers” is transforming plasma astrophysics by opening new windows on the universe. The challenges in understanding the extreme plasma physics of multimessenger cosmic plasmas lies in the fact that energetic plasmas around neutron stars and BHs are under extreme, high-energy-density physical conditions and are governed by rich physics. These plasmas are often relativistically hot, move at relativistic speeds, and engage strongly with ambient and self-produced radiation fields. The QED interaction of photons with each other and with strong magnetic fields may lead to prolific pair creation (e.g., electrons and positrons). These “exotic” relativistic, radiative, and QED aspects of the basic collective plasma phenomena are novel and do not have analogs in traditional solar, space, and most laboratory plasmas, although recent advances in laser technology are now enabling us to access and study these extreme plasma regimes in the lab. Our understanding of extreme astrophysical and laser-plasma processes (Chapter 3) is incomplete, as classical plasma theory becomes inapplicable. There is a need for a targeted study of kinetic-level collective plasma phenomena that includes laser-plasma and astrophysically relevant physical conditions, relativistic motions, strong interaction of particles with radiation, and QED effects. This
requires developing new physical insights and building a rigorous, systematic knowledge base for these extreme plasmas, then applying this new knowledge to uncover the inner workings of the most fascinating and challenging astrophysical objects, including accreting BHs and their jets, pulsars, magnetars, and multimessenger phenomena such as neutron star mergers.
The global modeling of astrophysical multimessenger sources is further complicated by the multiscale nature of the extreme plasma processes. In many systems of interest, the micro- and macro-scales affect each other in complex ways, so a simple parameterization of a microscopic process may be insufficient, and a true multiscale problem must be solved. For example, in the context of particle acceleration, the feedback of energetic particles on large scales affects the injection process at small scales, which requires special techniques for modeling. New approaches to multiscale simulations of astrophysical objects need to be developed, including “hybrid” schemes that combine kinetic particles representing the accelerated component with MHD schemes that can efficiently evolve the background state. In fact, how to connect small and large scales in meaningful ways is a pressing issue for the entire PSE community.
The challenges in understanding extremely rarefied/weakly collisional plasmas around BHs and in clusters of galaxies (inter-cluster medium, ICM) involve a complex mix of rarefied plasma and accelerated particles (and dark matter in the case of ICM). In addition, these plasmas exhibit extreme separation of micro- and macro-scales. Although these plasmas are dominated by thermal rather than magnetic pressure, even weak magnetic fields change the overall dynamics as well as dissipative and transport properties. The dynamics of such plasmas are governed by large-scale bulk motions and complex, multiscale interactions between kinetic phenomena under conditions not in local thermodynamic equilibrium. Key questions include, where is the seed population of charged particles that are accelerated to produce the observed radio emissions, what are the properties of magnetized turbulence in this plasma regime, and what is the source of the additional heating needed to account for the observed X-ray intensity of the deep core of the clusters?
Understanding these interactions presents a challenge for plasma theory, computational studies, and experiments. Numerical investigations of weakly collisional SAP plasmas require fully kinetic (PIC) studies to explore microscale physics but will benefit from hybrid-kinetic approaches (with fluid electrons) that address the dynamic range problem. Key areas for study include elucidating the most important kinetic instabilities, particularly in the presence of strong heat fluxes; understanding how kinetic instabilities interact with other plasma processes, such as magnetic reconnection, thermal conduction, and particle acceleration; and understanding the interplay between widely separated fluid macroscales and kinetic microscales.
Partially ionized interstellar plasmas, especially low-ionization plasmas in protostellar cores and protoplanetary disks, present formidable plasma-physics
challenges, with important implications for the origin of life in the universe. Effects of dust formation (and the resulting presence of nearly macroscopic charged particles; see Chapter 2), nonideal MHD (e.g., resistivity, Hall effect, and ambipolar diffusion), and multispecies plasma components (neutrals, electrons, cosmic rays, molecular and atomic ions, and charged dust grains) collectively comprise a highly complex plasma environment. The interactions among the partially ionized gas component, the charged dust grains, the microscale dynamics of dust coagulation and planetesimal formation, the mesoscale dynamics of the magnetorotational instability in protoplanetary disks, and the macroscale dynamics of outflows and long-term disk evolution are difficult plasma-physical problems. The LTP community has long addressed partially ionized, chemically reactive, and multiphase plasmas, offering possible strategic opportunities for the SAP and LTP communities to collaborate on low-ionization astrophysical plasmas.
Space weather is recognized nationally and internationally as a potential threat to our technology-dependent society and to our space programs, including human-flight to the Moon, Mars, and beyond. The complex chain of energy release, transformation, and transport from the Sun to Earth and other sites in the heliosphere encompasses a vast range of interlinked phenomena, all requiring plasma science to achieve understanding, prediction, and mitigation when relevant. One underexplored area of practical interest is extreme events—eruptions at the most energetic end of the scale, on par with or surpassing the so-called Carrington event of 1859. Extreme events challenge our strategic planning, our physical models, and our ability to respond appropriately to loss of communications, electric power, and GPS. Although the basic physical processes associated with geomagnetic storms in our planet’s magnetosphere and ITM are understood, our comprehension and ability to model extreme Carrington-scale storms are in their infancy. The “average” solar eruptions cannot be predicted with great accuracy, much less the rare but highly destructive events. By pushing robust first-principles models to their limits and adding key physical processes that are currently missing, however, the impact of weak to extreme eruptions on the near-Earth environment can be assessed to identify the most vulnerable areas and the most likely damaging effects. This will require closer coupling between models of adjacent geospace regions, including the neutral atmosphere beneath the ionosphere. A sufficiently broad dynamic range of scales is needed to capture important local-global interactions, such as large-scale magnetic connectivity changes associated with reconnection, key atmospheric chemistry processes such as nitric oxide enhancements in geomagnetic storms, and kinetic- (e.g., wave-particle interactions) and MHD-scale processes. This area would greatly
benefit from joint programs with the LTP community, which possesses expertise in atmospheric plasma chemistry.
Several aspects of space-weather ITM modeling are ripe for improvement. Currently, many global-scale geospace models treat the ionosphere simply as an input boundary condition. This problem is often exacerbated by actual physical gaps between the computational domains being coupled, complicating realistic simulation of the transfer of plasma, electromagnetic energy, and energetic particles between domains. Global models today at best rely on parameterizations of local-scale, kinetic, and/or non-MHD processes such as kinetic instabilities and particle precipitation from the ring current and radiation belts, or neglect these effects entirely. Finally, the high-latitude (open field) and low-latitude (closed field) ionospheres usually are modeled separately, ignoring processes that affect the evolving open-closed boundary. A critical goal is to obtain a global electric-field model that couples these two regions and addresses the impact of large geomagnetic storms on the low- to mid-latitude ionosphere. Efforts to fill in these missing pieces are currently underway. Different combinations of linked models for the ring current, upper ionosphere, and mesosphere are being tested, and key physical processes within this complex system are being added. For example, metal ions that affect conductivity, irregularities, and instabilities are not included in ionospheric models currently, but their effects are being implemented in NRL’s SAMI3 ionospheric model. By the next decadal survey, the committee anticipates having robust, first-principles models of geospace from the ground to the upper reaches of the exosphere and magnetosphere.
Solar opacity calculations and measurements, especially for iron (Fe), one of the most abundant minor elements by mass, have received renewed interest due to the “solar abundance” conundrum. Analyses of the photospheric composition in the early 2000s found smaller abundances than earlier estimates for many elements. Previously accepted solar abundances were in excellent agreement with helioseismology data. With the new abundances, however, the structure predicted by solar models no longer agrees with that inferred from helioseismic studies. Increased opacities for the key elements under solar conditions would lessen the discrepancies, but there is little justification at present for such an adjustment. The opacities of Fe, Cr, and other elements at the typical electron temperature (Te ~182 eV) and density (ne ~ 1022 cm−3) near the solar convective and radiative zone boundary (CZB) were recently measured and systematically studied on the Z-machine at Sandia National Laboratories, and compared to the latest theoretical calculations. The calculated and measured Fe opacities agreed reasonably well at the lowest Te and ne, but the models underestimate the opacity as Te and ne approach CZB conditions. This finding awaits resolution. Are the opacity theories missing physics, or is there something unique in the experimental iron measurement at Te > 180 eV? To confirm laboratory findings, independent experiments using different platforms, such as NIF, are currently being pursued.
The most fundamental questions concerning particle acceleration on the Sun are: under what circumstances do the relevant acceleration mechanisms trigger and operate, and how do accelerated particles propagate and interact with their environment? Imaging spectroscopy from radio to HXR (hard X rays with energies >10 keV) and γ-ray wavelengths have shed some light on these questions in the past decade, but such observations need to be continued and improved over the next decade in order to obtain definitive answers. New radio interferometers such as EOVSA and the proposed Frequency Agile Solar Radiotelescope (FASR) offer new insights into the timing and locations of electron energization and propagation into the corona and heliosphere, and the solar environment in which the eruption energy was stored and released. When the RHESSI mission was retired in 2018, the only source of solar-dedicated high-energy imaging spectroscopy was lost. Astrophysical missions such as Fermi and NuSTAR currently remain the only instruments measuring HXRs from solar flare-accelerated electrons. By ~2025, the STIX instrument on Solar Orbiter will again provide solar-dedicated HXR imaging, though available observation time will be limited by spacecraft and telemetry constraints. The next generation of instruments designed to study the physics of flare particle acceleration, with sufficient sensitivity and dynamic range to observe faint particle acceleration sites at the same time as bright flare foot-points, urgently seeks launch opportunities in time for the next solar maximum. Instruments such as a γ-ray imager/polarimeter could explain the puzzling separation between electron- and ion-emission source regions, and the existence of long-duration coronal γ-ray sources.
To date, all in situ observations of solar wind plasmas have been single point measurements or have focused on a narrow range of scales through the use of carefully controlled formations of a few spacecraft. Unfortunately, the dynamics of turbulence depends critically on the orientation of the magnetic and plasma fluctuations relative to the mean magnetic field—a 3D quantity that no single spacecraft can measure. Cluster and MMS, which both consist of four spacecraft in a tetrahedral formation, employ a limited set of spacecraft separations to sample the multidirectional structure of the plasma. Therefore, these missions cannot simultaneously resolve fluctuations on the range of scales needed to understand the nonlinear dynamics of plasma turbulence. The largest and smallest scales sampled by these missions only differ by a factor of 10, which is nowhere near the many orders of magnitude necessary for simultaneously measuring turbulent fluctuations through the inertial and dissipation ranges. Future constellation missions with many spacecraft and variable spacing are needed to unlock the mysteries of solar wind and geospace turbulence.
Two unsolved issues in SEP propagation and acceleration are particularly urgent targets for resolution in the next decade. The recognition that the longitudinal extent of SEP events can vary significantly came with multispacecraft observations
(e.g., STEREO) of impulsive and gradual SEP events. The surprising longitudinal spread of charged particles during gradual SEP events has been ascribed to cross-field transport in the IPM. Other explanations, such as diverging field lines and longitudinal particle transport in the corona, have not been thoroughly explored yet. Current SEP models typically describe cross-field transport via an ad hoc perpendicular diffusion coefficient, and therefore remains poorly understood. Shock obliquity (the angle between the shock front and the upstream magnetic field; see Figure 7.6) can affect the injection energy and efficiency for shock-accelerated particles, the excited wave intensity at the shock, and the associated parallel diffusion. Consequently, predicting the evolving spectrum of SEPs generated at a particular shock requires detailed knowledge of the shock connectivity to the heliospheric point of interest and a complete description of the competing physical processes affecting their transport. Although progress has been made in the last decade, the current state of observations and modeling falls short of meeting these requirements. In particular, temporal and spatial variations in obliquity along a CME-driven shock, from the flanks to the leading edge, have yet to be incorporated in models of particle acceleration and propagation. The societal quest for accurate prediction of impacts from individual SEP events on spacecraft and humans at specific locations throughout the solar system depends critically on answering these questions.
The 42-year-old Voyager Interstellar Mission, migrated from the Grand Tour mission exploring the outer planets, does not have a science payload tailored to exploration of the outer heliosphere and the VLISM. In a few years, this pioneering mission will end as power supplies fall below critical levels and the heaters are turned off. Voyagers 1 and 2 have transformed our knowledge of the outer heliosphere and the LISM plasmas, yet many questions remain unanswered. ACRs do not appear to be accelerated at the heliospheric termination shock, the heliospheric shock is dissipated by reflected PUIs rather than the thermal solar wind, and most of the pressure in the heliosheath apparently resides in heated PUIs. Because the Voyager instrument suite was not designed to measure PUIs, a critical component of this highly nonequilibrated collisionless plasma has been inferred rather than measured directly in situ. Voyager and IBEX observations have challenged and transformed our earlier concepts about the global structure and energetics of the heliosphere, and of the plasma processes acting at its boundary. IMAP, and eventually an Interstellar Probe, will be critical to unraveling the multiple challenges listed above in understanding the interaction of the solar wind with the LISM.
Energetic particles not only are accelerated by shocks, but also play a fundamental role in dissipating shock waves and in determining their structure. Understanding the dynamical effects of energetic and suprathermal particles on heliospheric shocks is receiving increased interest. Numerous observations of shock waves, including some IP shocks within 1 AU, have revealed that the pressure of
the energetic-particle component considerably exceeds both the thermal gas pressure and the magnetic field pressure. Because most of the thermal heliospheric and VLISM plasma is not equilibrated with the ubiquitous energetic particles, understanding the structure and dissipation of heliospheric shocks must extend well beyond simple MHD models. Instead, multifluid models with appropriate closures are needed, informed by kinetic models of the energetic-particle component and sophisticated PIC and hybrid codes.
The unprecedented detail of measurements by MMS heralds an opportunity over the coming decade to considerably deepen our understanding of Earth’s bow shock and nearby IP shocks. The committee anticipate that future MMS studies will analyze electromagnetic waves upstream of and at/within the shock front; examine the evolution of ion and electron distributions from upstream through the shock ramp and into the downstream region; discover PI signatures at the shock; and investigate periodic nonlinear structures, such as double layers and electron holes downstream of collisionless shocks.
Whistler waves are very low frequency electromagnetic waves that can be generated in geospace and other planetary magnetospheres during particle injection events associated with lightning and geomagnetic storms and substorms. Their interactions with both ambient and injected (energetic) electrons have been invoked recently to explain particle acceleration in, and precipitation from, the radiation belts. The resulting “killer” electrons are major threats to spacecraft transiting the magnetosphere. Our ability to predict such space-weather effects depends on our understanding of whistler initiation, propagation, and complex interactions with the electron population. Currently, several petabytes of data on whistler waves in the near-Earth environment exists in independent databases across the scientific community. By leveraging modern data-mining techniques, trends and features of whistler signals could be extracted on an unprecedented scale. Such techniques will have a tremendous impact on global space-weather prediction and elucidate basic physics of magnetized plasmas.
Future understanding of plasma processes in Earth’s ITM layers requires space-borne and ground-based instrumentation capable collectively of full planet coverage. Ionospheric plasmas vary on wide temporal and spatial scales, so deconvolving the intrinsic evolution requires multipoint spacecraft observations, such as constellations that can observe planetary waves without averaging over a month or more. Multiple smallsats with instrumentation similar to that onboard GOLD could determine the large-scale structure, while widely dispersed ground-based GPS receivers could map the overlying atmospheric properties on finer scales than presently achievable. To probe systematically the bottom of the ITM is particularly challenging with in situ instruments, because orbits transiting this zone are short-lived unless onboard propulsion is included. As a result, the critical “Atmosphere-Space Transition Region” is largely unexplored, and an important objective for the coming decade.
The atmospheric region where terrestrial lightning generates sprites, elves, and blue jets is a complex interface between the lower atmosphere and the space environment. This region has considerable practical as well as intellectual interest, because most UV-blocking ozone resides there and because local disturbances can disrupt technological systems through their effects on satellite drag, communications, and electrical power distribution systems. Fundamental questions remain about the underlying physics of these events. The close spatial and temporal association observed between lightning and waveguide perturbations implies that the perturbations emanate from plasmas induced by high-altitude discharges, but the mechanisms of these perturbations remain unknown. The seed electrons for these events might be secondary electrons produced by cosmic rays or from compact regions around tips of lightning leaders. The radiation dose to people inside aircraft near a terrestrial γ-ray source could be significant and should be investigated and quantified. While some upper atmospheric effects of lightning have direct societal impact, research on other effects aids in understanding fundamental properties of gas discharges by exploring regimes in which experimental studies are not feasible. Research in the next decade will focus on understanding the initiation of sprites, elves, blue jets, and terrestrial γ-ray flashes, their observable signatures and chemical effects at different altitudes in the atmosphere, the observed gas-discharge features, and their scaling relationships to other discharge forms.
Understanding plasma conditions common to both astrophysical and heliophysical phenomena poses special challenges to PSE. Although the observational techniques and system dimensions can diverge substantially, the similarities in the underlying physical processes in the examples below call for collaborative efforts to promote advances in understanding.
The origin and evolution of cosmic magnetism remain among the most important outstanding questions of SAP science. We still cannot explain the origin of the first magnetic fields in the universe (magnetogenesis), or how magnetic fields are amplified and sustained by plasma motions (dynamo; see Figure 7.13). Observations of radiation from very distant galaxies with the upcoming SKA radiotelescope will shed some light on cosmic magnetism, but a major emphasis should be to further theory, computation, and experiment. Most astrophysical dynamo studies use an MHD model, often further assuming that fields grow without back-reacting on the plasma motions (the “kinematic” approximation), while dynamos affected by kinetic physics are only beginning to be explored. Similarly, although plasma processes and instabilities that can create magnetic fields have been identified, most saturate with weak, tangled fields, and it is unclear whether such fields seeded those observed today. This major gap in basic plasma physics theory simultaneously
involves processes such as kinetic turbulence, reconnection, particle acceleration and diffusion, anomalous viscosity and resistivity. In the next decade, progress on cosmic magnetogenesis and dynamos will require the following:
- Formulate MHD-like models that are simple enough to solve in complex, astrophysically relevant geometries, yet detailed enough to capture the multiscale interplay between, for example, kinetic instabilities, bulk fluid motions, reconnection, cosmic-ray diffusion, magnetic-field amplification, and self-organization.
- Develop stable and accurate methods that reduce particle noise and/or computational expense in high plasma-β systems, and resolve important electron kinetics without being limited by the speed of light (e.g., implicit PIC).
- Increase availability of mid-range supercomputing options (10-50 M CPU-hour) for routine computational studies involving kinetic methods.
- Develop experimental studies of high-β plasmas while prioritizing a program for funding new university-level plasma experiments and dedicated time on existing devices, accompanied by training of plasma students. Although the high Reynolds number, high-β regime is difficult to produce in the laboratory, there is a large payoff for success.
Understanding how stars cyclically generate and reverse their magnetic fields will not only shed light on our own Sun, but is profoundly important for studies of heliospheric structure and evolution, exoplanets and habitability in other stellar systems, and the generation of magnetic fields in more extreme environments. Not understanding the origins of the solar cycle greatly reduces our ability to predict and mitigate adverse space weather. The outstanding questions that confront the community include: are long-term solar cycle predictions at all possible in such systems; if short-term predictions are viable, what is the window of predictability; and how best to transition our physical insights to predictive models? Successful resolution of these fundamental questions requires a combination of new observations, breakthrough theoretical concepts, and state-of-the-art numerical simulations. Analysis of model output can capitalize on machine-learning techniques. For example, observations over a wide range of spatial scales of the magnetic flux balance at the solar poles throughout an entire activity cycle, though technologically challenging, would provide much-needed information on how much flux is carried from lower latitudes, emerges in situ, submerges, and is produced by local, small-scale dynamo action. Key problems that await solutions in the next decade include:
- Some evidence points to the tachocline (see Figure 7.14) as an important agent in achieving magnetic self-organization on large spatial scales, while other research indicates that stars without tachoclines can have strong magnetic fields. What is the role of the tachocline in the dynamo process, and in determining its cyclic nature?
- Simple (e.g., kinematic) phenomenological mean-field-like dynamo models nicely reproduce the observed evolution of surface magnetic fields over decadal timescales without turbulent induction. Global MHD models can produce reasonably solar-like large-scale magnetic cycles with decadal polarity reversals, powered by turbulent induction and differential rotation, but do not reproduce the well-observed tilted bipolar pairs on the surface. How can these starkly different modeling frameworks be reconciled?
- Can dynamically consistent MHD simulations of large-scale magnetic cycles produce grand minima (multiple cycles of suppressed magnetic activity)?
- Which aspects of dynamo models are critically dependent on computational details such as subgrid models?
Collisionless shocks can now be generated in the laboratory, positioning us to answer a multitude of basic questions about these important phenomena in the next decade.
- What are the dominant plasma instabilities that mediate the stagnation of the flow in collisionless shocks, and how is the magnetic field amplified?
- What is the effective ion mean free path associated with shock formation?
- How is magnetic turbulence generated at the shock front and how does it decay?
- What is the difference between electron and ion thermalization at the shock?
- What is the injection process for particle acceleration at the shock front, and what is the efficiency?
For SAP applications, experiments capable of exploring both magnetized and relativistic shocks must be developed. To study magnetized shocks requires large volume, high magnetic-field generators, and large phase plates on laser facilities are needed to produce large-scale plasma flows. High-intensity laser systems (>1022 W cm−2) are needed to create relativistic flows, and low-energy high-resolution particle spectrometers (<500 keV electrons; <10 MeV ions) to measure the nonthermal component of the accelerated particle spectrum. Shot opportunities in large laser facilities such as NIF are essential for meeting these objectives. Massive multidimensional simulations of the laser-target interaction, employing both radiation hydrodynamics and collisionless PIC approaches, are essential for generating and interpreting collisionless shocks in SAP-relevant laboratory environments. This project would drive fundamental understanding of collisionless shock formation and evolution, shock-associated turbulence, and charged particle acceleration.
Galactic cosmic rays, anomalous cosmic rays, and solar energetic particles are thought to be accelerated by either diffusive shock acceleration (DSA) or some form of stochastic acceleration. Both mechanisms depend fundamentally on turbulence, either by energetic particles generating their own waves/fluctuations or through waves from other sources. However, heliospheric particle observations that are inconsistent with simple DSA indicate that other physical effects must be taken into account, in particular coherent structures (such as magnetic islands) and turbulent electric fields. A key goal for the coming decade is to develop models of these al-
ternative acceleration mechanisms while incorporating numerical representations that capture new understanding of plasma turbulence under relevant conditions.
Turbulence in SAPs can be investigated observationally using ground-based observational platforms, remote sensing spectroscopy, and in situ satellites. Radio observations provide unique insights into the nature of the solar wind and ISM plasmas and their turbulent properties. Radio waves scatter as they propagate through a plasma, allowing a number of plasma properties to be deduced. Observed radio wave scattering, such as interplanetary scintillations, requires suitable background sources such as quasars and pulsars to probe the foreground medium of interest. The number and sensitivity of available radio telescopes (e.g., The Jansky Very Large Array (VLA), Very Long Baseline Array (VLBA), Ooty Radio Telescope) are limited. Hence, the number and angular density of such sources that are effective probes of the solar wind and ISM are not optimal, nor is the distribution of interferometric baselines needed to measure turbulent properties of the medium or its velocity. Recommended observational solutions are discussed later in this chapter.
The exciting new frontier of exoplanetary discovery and the quest for life elsewhere in the universe will continue to advance through observations by new missions and ground-based observatories and innovative computational studies. Broader observational scope and deeper physical understanding are primary goals for the next decade. Because transit observations preferentially select those planets that are close to their stars, the focus to date has largely been on exoplanets whose “Goldilocks” zones are very close to M stars. Late-type M stars are magnetically active, with starspots 10 times larger than sunspots. That and the close proximity of exoplanets to M stars suggests they experience severe space weather as they orbit the outer stellar corona. Energy and radiative input to their atmospheres can be immense, particularly in EUV and X-ray wavelengths, and frequent stellar eruptions can produce a lethal mix of energetic particles, high-energy photons, and magnetic interactions with the planet’s magnetic field (if it has one). Moreover, it remains unclear whether planetary magnetic fields mainly shield planets from radiation and direct stripping of the atmosphere or function more like sails, enlarging the planetary cross-section to collect and funnel more stellar-wind energy into the atmosphere. Cross-disciplinary studies have proved highly successful, and should be continued in the next decade. Future studies of stellar eruptions will estimate the plasma and radiation environments of other Earth-like planets orbiting solar-like stars by using data-based MHD models of evolving solar-like stars and their plasma astrospheres, based on observations by missions such as NASA’s Transiting Exoplanets Survey Satellite, the Hubble Space Telescope, XMM-Newton, and the upcoming James Webb Space Telescope (JWST). Data-mining and ML techniques will enable efficient extraction of the key structures and properties from the resulting simulation-generated datasets.
PSE and SAP Crosscutting
Multiscale coupling is common to SAPs as well as laboratory plasmas. Bridging the gap between kinetic- and global-scale phenomena has been a long-standing obstacle to advancing our understanding of the energetics and dynamics of activity on the Sun, in planetary magnetospheres and ionospheres, the interplanetary medium, in active galactic nuclei, within accretion disks, and other cosmic plasmas. A primary approach for addressing this problem is numerical simulation, which requires taking full advantage of innovative developments in hardware, algorithms, visualization, and data-analysis tools from across the PSE community as well as computational physics.
To capitalize on the great progress and ongoing momentum in understanding reconnection in SAPs, a coordinated program of observations, theory, numerical simulations, and laboratory experiments needs to be employed. Many key topics are ripe for breakthroughs—multiscale coupling, 3D geometries, energy conversion and partitioning, onset, partial ionization effects, interplay with ideal instabilities, reconnection in relativistic extreme environments, and connections with turbulence and shocks. Dedicated DOE, NASA, and NSF programs are needed that support the development of next-generation analytical and numerical models and the application of new numerical technology and theoretical understanding to these reconnection challenges. Computational advances require fluid, kinetic, and hybrid models, and multifluid models that include kinetic effects through physics-based closure equations. Exascale computing will enable us to finally address these questions in fully 3D systems with increasingly realistic plasma parameters. Over the next decade, unprecedented remote-sensing observations from missions such as SDO, IRIS, and Fermi, as well as exquisite in situ data from missions such as MMS, PSP, Solar Orbiter, and BepiColombo, will deliver a wealth of critical new insights on magnetic reconnection. An encouraging development is the new NSF/NASA collaborative program, Next Generation Software for Data-driven Models of Space Weather with Quantified Uncertainties (SWQU).
Turbulent cascades culminate at the dissipation scale through kinetic processes that are poorly understood. The physics of dissipation is a fundamental PSE problem, since it involves the irreversible conversion of collective fluid or kinetic-scale motions into internal energy, or “heating.” Complicating matters further, intermittency in turbulence can connect dynamically coherent structures (e.g., current sheets), patchy dissipation, and the entire inertial range of turbulence. Because the dissipation scale is generally well below the resolution limit of both observations and numerical simulations, one can only infer its properties in heliophysical and astrophysical objects. For example, the solar corona and wind clearly are heated well above the photospheric temperature, but the manner in which the Sun’s magnetic energy is transformed into heat is an open question. Similarly, the cores of galaxy
clusters exhibit far less star formation than expected, indicating the presence of an additional heating source that remains a mystery. This major problem in SAPs and laboratory plasmas encompasses turbulence scales that range from fluid to kinetic. Understanding and relating such disparate scales is a challenge for the next decade.
Finite computing resources usually make it difficult to capture the turbulent cascade all the way down to the dissipation scale when simulating high Reynolds number, multiscale turbulent plasmas. Subgrid models bypass this difficulty by parametrizing dissipation at the smallest scales that can be stably resolved on the computational grid, which introduces artificially enhanced dissipation of small-scale structures. When modeling solar/stellar MHD convection, the challenge is to ensure that larger scales remain unaffected by the subgrid model. Therefore, the choice of a subgrid model can strongly influence global characteristics of large-scale dynamo solutions. MHD dynamo simulations with the same code but with different subgrid models have indeed demonstrated that the resulting magnetic self-organization differs substantially. Choosing the appropriate subgrid model is a difficult decision for simulating other turbulent plasmas in their large-scale environments, including the solar wind, astrophysical jets, and the ICM. Consequently, progress in this area will benefit a wide range of SAP investigations.
New facilities investigating the “warm universe,” such as the Stratospheric Observatory for Infrared Astronomy, Atacama Large Millimeter Array, and the upcoming JWST, will study specific plasma environments with low ionization fractions, complicated chemistry, and complex molecular radicals. These observations probe important plasma-physical and chemical processes that lead to the formation of dust and organic molecules. Observations of kilonova emission after neutron-star mergers indicate the presence of plasma dominated by the low ionization stages of heavy elements (lanthanides and actinides). Studies of thermodynamic and radiative properties of such plasmas require high-resolution spectral information that comes from atomic, molecular, and optical (AMO) theoretical and experimental methods. Collaboration with AMO and LTP researchers is vital for future progress in this area of SAP physics.
Atomic and molecular opacities are critical for understanding many cosmic phenomena, including the internal structure and evolution of stars, stellar and planetary atmospheres, pulsating stars, and the light curves arising from supernovae. Although the complexity of the problem makes accurate calculations daunting, the advancement of high-performance computing of atomic and molecular data has generated a large amount of more accurate and complete opacity data. Extensive international efforts have calculated massive databases of opacities for plasmas in LTE, but non-LTE opacity data are still sparse. Theoretical opacities can only be validated by comparison with estimates derived from observations of the solar interior, stellar pulsations, and atmospheric spectra of stars and planets, with experimental neutrino-flux measurements, and with predictions from 3D hydrodynamics simula-
tions. Achieving stellar interior conditions in the laboratory is very difficult. Only very recently have laboratory experiments been able to reproduce plasma conditions in the solar convective envelope and approaching the radiative zone.
Dedicated DOE and NSF programs are urgently sought to support the next-generation experimental facilities and diagnostic instrumentation in concert with SAP observations. Spectroscopic diagnostics remain a critical element for determining the physical conditions in SAPs. Models of optically thin radiation (e.g., CHIANTI for calculating spectra from astrophysical plasmas) rely heavily on atomic cross-sections, ionization/recombination rates, electron excitation rates, and radiative decay rates calculated theoretically and validated against laboratory data. Besides CHIANTI, the Atomic Data and Analysis Structure (ADAS) is an interconnected set of computer codes and data collections for modeling the radiative properties of ions and atoms in SAPs, laboratory fusion devices, and technological plasmas. CHIANTI and ADAS are currently the only major resources that are used for space and astrophysical applications. Although many current SAP investigations involve atomic and spectral data for very heavy elements, available databases lack essential data on these elements. Moreover, existing codes are unable to calculate these data, and there has been insufficient funding for new or updated code development. The benchmarking of atomic data models requires new, high-resolution laboratory spectrometers in the UV and X-ray wavelength regions. Furthermore, adequate funding for maintaining and updating production codes and databases, and the workforce for atomic physics calculations, is critically needed.
Research into space and astrophysical plasmas is supported by multiple funding sources, with different requirements, constituents, and goals. Although this situation provides many opportunities and fosters cross-disciplinary studies, it also leaves some crucial areas of SAP science without reliable support. One prominent example is laboratory experiments that approach or reproduce scaled-down versions of plasma conditions in cosmic phenomena. Most facilities capable of performing these experiments are supported by agencies whose missions are not SAPs, while the agencies supporting space research are unlikely or unable to invest the substantial resources that would enable such experiments. Similarly, theory and computation are not well supported in proportion to their importance in designing and utilizing instruments, missions, and observatories. Large disparities exist in this regard between the different parts of PSE, and advances in one sector are not always available to others. Targeted cost-sharing interagency programs would benefit both SAP and basic plasma research (e.g., the recent NSF/NASA SWQU), potentially revealing new paths toward the goals of national security, energy independence, and comprehending the universe.
The range of important plasma processes and physical regimes encompassed by the Sun (and other stars) makes it an ideal laboratory for plasma investigations. Similarly, the deep understanding and powerful numerical techniques developed by the basic plasma research community need to be absorbed and applied by SAP researchers. Solar and plasma science have a long history of beneficial exchanges, yet the current level of interaction is not optimal. Focused research programs that target solar phenomena from a plasma-science perspective would enhance cross-fertilization, with joint funding from all stakeholders including NASA. A successful model to emulate is NASA’s Living with a Star (LWS) Focused Science Team program, which attacks community-proposed heliophysics problems by assembling teams of small groups that each address some aspect of the problems. Teams that include members from both plasma and solar research communities would ensure effective cross-disciplinary and productive joint research and communication.
Space-weather research extends from basic science to operational forecasting. Because different sponsors focus on different aspects of space weather, it is difficult to apply the systems approach needed to make progress on key questions. Recently, a national space weather plan has been formulated that involves all agencies whose purview includes space weather. New opportunities for basic and applied space-weather research are being created in partnership with the scientific and end-user communities. This encouraging development should continue with substantial buy-in from all sponsors, with particular emphasis on establishing long-term, stable funding to tackle the most complex problems and to validate and transition the most robust, accurate models to operational use. The NSF/NASA SWQU is an important first step in this direction.
Better understanding of space-weather effects will drive technology development in several directions. Improving the electric power grid to protect against space-weather extremes is already underway, and further improvements should be made to prevent massive shut-downs, reduce damage to transformers and other vulnerable equipment, and ensure that repairs or replacements are timely. To predict the impact of space-weather events on specific satellites throughout their orbits, models of spacecraft charging and other adverse effects on our assets in space should be linked to models of the space environment. The exploding population of spacecraft and debris in Earth orbit makes it imperative to characterize atmospheric drag as accurately as possible, to better predict satellite lifetimes, avoid collisions, and estimate fuel requirements. Current models are inadequate for calculating changes in drag due to external forcing by solar-wind CIRs, CMEs, and flare radiation, and generally predict time-averaged conditions in specific orbits rather than instantaneous local conditions throughout geospace.
The sustainability and viability of Earth-based SAP observations are threatened by the unprecedented launch of massive constellations of hundreds to thousands of commercial communication and Internet-providing satellites. The electromagnetic emissions of these satellites are encroaching on frequencies that are essential to SAP observations. The ubiquitous presence of the satellites is already interfering with optical observations. To date, there has been little discussion of how SAP research and massive satellite-based commercial ventures can co-exist. The penalty for not engaging in discussions and establishing boundaries and processes will be severe limitations on our ability to observe SAPs in future.
NSF’s Daniel K. Inouye Solar Telescope (DKIST) is the largest solar telescope in the world, able to view features on the Sun as small as 70 km across. Using adaptive optics technology, DKIST’s 4.2-m primary mirror and five major instruments will provide the sharpest views ever taken of the solar surface, with the spatial, temporal, and spectral resolution and dynamic range needed to measure elemental magnetic structures at and above the photosphere. When DKIST is fully operational, there will be unprecedented data revealing the roles played by magnetic fields and the embedding plasma in generating and transporting solar activity. DKIST is ideally suited to studying the transfer of energy from the solar interior to the outer atmosphere, particularly the partially ionized, high-to-low b chromosphere and its magnetic connection to the hot, fully ionized corona. In combination with space missions such as SDO, IRIS, PSP, and Solar Orbiter, DKIST will be a powerful tool for characterizing the Sun’s magnetized plasma properties and the physical processes behind coronal heating and eruptive activity. First-light images of solar granulation in fine detail were released on January 29, 2020. Calls for observing proposals during the 1-year Operations Commissioning Phase have begun. The committee encourages the DKIST team to enable open access to DKIST observations to the greatest possible extent, consistent with U.S. space data practices.
Radio frequency observations have probed the solar wind and the ISM for many years, revealing key properties of turbulence, coherent structures, and waves. However, the lack of a ground-based radio interferometer with the appropriate combination of sensitivity, frequency coverage, and angular resolution has been a barrier to exploiting these phenomena more effectively. NSF’s next-generation Very Large Array (ngVLA) will permit pioneering studies of the solar wind and ISM, measuring the solar-wind spatial spectrum from tens of meters to 1,000 km. Intensity scintillation tomography will map fast- and slow-wind velocity fields and their fluctuations
in the critical corona-inner heliosphere transition, complementing direct sampling by PSP. Faraday rotation and dispersion-measure observations of polarized background sources and pulsars can reveal the overall density structure and magnetic field of the outer corona and inner heliosphere. By making global measurements of the solar wind properties, turbulence, and transients with radio propagation diagnostics, the ngVLA would be highly complementary to in situ measurements made by proposed next-generation heliospheric missions such as Helioswarm. For the ISM, the ngVLA will characterize interstellar turbulence, with the same instrument using the same technique (angular broadening), over a wider spectral range than ever before.
Nonetheless, to expand the renaissance that solar radio astronomy has undergone in the past decade, significant observational advances are required. High-cadence microwave imaging, as currently obtained by EOVSA and the JVLA, probes coronal reconnection sites, regions where high-energy electrons are accelerated and propagate, and the elemental structure of the quiet chromosphere. While the JVLA and EOVSA have served as platforms for developing, testing, and demonstrating the potential of broadband imaging spectropolarimetry at radio wavelengths, there is still a need for a high-performance, solar-dedicated radioheliograph designed to fully mine the rich information content of solar radio emission. The high-level requirements for such observations are: extremely broad, continuous coverage from 50 MHz to 20 GHz with high spectral resolution, time resolution as short as 10 ms, full polarimetry, full-disk microwave imaging, a field of view out to several solar radii at lower frequencies, and wide dynamic range (~104). These requirements are met by the Frequency Agile Solar Radiotelescope (FASR), a next-generation radioheliograph recommended as a priority by previous Astronomy and Astrophysics and Heliophysics Decadal Surveys. FASR is a low-cost, low-risk, and high-reward facility that would play a unique and productive role in serving the wider solar community.
Although the case for more extensive laboratory experiments and development programs related to SAP science is made forcefully in most decadal studies, little progress has resulted. LMAP, the heliophysics laboratory program recommended in the 2014 Heliophysics Decadal Survey, has not yet been implemented by NASA. A related but very modestly funded program, the Heliophysics Technology and Instrument Development for Science, includes some laboratory plasma studies of chemical, spectroscopic, and nuclear measurements supporting observations and models. U.S. federal laboratories managed by DoD and DOE could be excellent resources for the SAP community, which lacks access to these capabilities. In return, the laboratories gain expertise in previously underexplored plasma regimes and opportunities to compare laboratory results with cosmic plasmas. For example, major laboratory facilities such as UCLA’s LAPD and NRL’s SPSC currently offer limited but vital opportunities for SAP-related experiments. A comprehensive program that facilitates coordinated experiments between federal laboratories and the SAP community should be developed and implemented across NASA, NSF, DOE, and DoD.
New missions offer great opportunities for discovery SAP science. Solar Orbiter, a European Space Agency mission with some U.S.-built components, was launched on February 10, 2020. Its primary science goals are to perform close-up, high-resolution studies of the Sun and its inner heliosphere, while orbiting at 0.3-0.9 AU from the Sun. The combination of Solar Orbiter and Parker Solar Probe will provide unprecedented opportunities to investigate multiscale phenomena in the corona and solar wind from separate vantage points, through complementary in situ and remote-sensing observations. Solar Orbiter will address the following fundamental questions:
- How and where do the solar wind plasma and magnetic field originate in the corona?
- How does solar activity drive heliospheric variability?
- How do solar eruptions produce energetic particles that traverse the heliosphere?
- What drives the solar dynamo and its connections between the Sun and the heliosphere?
The Ionospheric Connection Explorer (ICON) is investigating the connections between the neutral atmosphere and ionosphere, with three instruments that measure temperature, velocity, and composition of gases remotely and a pair of identical in situ instruments that characterize the plasma around the spacecraft. Its local measurements complement the global imaging obtained by GOLD. ICON will determine, for the first time, how dynamic terrestrial weather events, such as cyclones and El Niño, affect the ionosphere. ICON was launched on October 9, 2019, and is currently providing data to the public during its primary science phase.
Geospace Dynamics Constellation (GDC) is an ambitious space-mission concept recommended by the 2013 Heliophysics Decadal Survey for implementation as the next NASA Heliophysics LWS mission. The Science and Technology Definition Team report was released in October 2019. GDC is in formulation (Phase A), with the Announcement of Opportunity expected soon. GDC will address crucial questions pertaining to the dynamic processes active in Earth’s upper atmosphere; their local, regional, and global structure; and their role in driving and modifying magnetospheric activity. GDC will be the first mission to address these questions on a global scale with a constellation of spacecraft that permit simultaneous multipoint observations of a critically undersampled region of the ionosphere and thermosphere. This investigation is central to understanding the basic physics and chemistry of the upper atmosphere and its interaction with Earth’s magnetosphere, and will provide insights into space-weather processes throughout geospace. According to the recent Science and Technology Definition Report for GDC, the primary goals are to understand:
- How the high-latitude ionosphere-thermosphere (IT) system responds to variable solar wind/magnetosphere forcing; and
- How internal processes in the IT system redistribute mass, momentum, and energy.
Because spatial scales are vast and energetic-particle kinetics and charge exchange are particularly important, no current models comprehensively describe the heliospheric system fully. Physical understanding is hampered by the very limited set of observations. To remedy this gap, the Interstellar Mapping and Acceleration Probe (IMAP), an integrated and coordinated suite of 10 instruments, was selected last year by NASA for launch in 2024. IMAP addresses two critical problems in SAP physics: the acceleration of energetic particles in interplanetary space and the interaction of the solar wind with the local interstellar medium. IMAP’s science objectives are to:
- Improve understanding of the composition and properties of the LISM;
- Advance understanding of the temporal and spatial evolution of the dynamic boundary between the solar wind and the ISM;
- Identify and advance understanding of processes governing the interactions between the magnetic field of the Sun and the LISM; and
- Identify and advance understanding of particle injection and acceleration processes near the Sun, in the distant heliosphere, and in the heliosheath.
The HelioSwarm mission concept is a constellation of small spacecraft with a wide range of spatial separations that would simultaneously sample key physical parameters in the turbulent solar wind. By measuring properties that relate directly to the cascade of energy across scales to where energy is dissipated, and into different physical regions, a constellation mission in the solar wind will enable direct tests of current conflicting models for the spectral and spatial distributions of turbulent power, thus increasing our understanding of turbulent SAPs.
An Interstellar Probe was one of the top-10 priorities recommended as new imperatives for NASA by the National Academies’ 2013 report, Solar and Space Physics: A Science for a Technological Society.2 A NASA-funded study for a “Pragmatic Interstellar Probe” showed that speeds at least three times that of Voyager 1 were possible using available/near-term technology, enabling the Probe to reach the pristine LISM within 50 years. An augmented Interstellar Probe would address not only plasma physics of the heliosphere but also fundamental planetary science and astrophysics, as outlined by the following science goals:
- Understand our heliosphere as a habitable astrosphere.
- Investigate the plasma physical processes and global nature of the outer heliosphere, the boundary regions, the VLISM and beyond to the pristine LISM.
- Understand the evolutionary history of the solar system. Explore dwarf planets and Kuiper Belt Objects through flybys observing atmospheric and surface properties. Determine the large-scale distribution of the circumsolar debris disk.
- Open the observational window to early galaxy and stellar formation. Measure the integrated diffuse extragalactic background light from redshifted stars and galaxies dating back to ~200 million years after the Big Bang.
The primary missions of the recently launched Spectrum-Roentgen-Gamma (Spekrt-RG) spacecraft are to find and map all massive galaxy clusters in the observable universe at X-ray wavelengths, and to search for other cosmic X-ray sources, including active galactic nuclei, star formation regions, and stellar activity. Several astrophysical satellites that probe hot cosmic plasmas with high-energy emissions are planned: for example, the Athena X-ray observatory, the Hard X-ray Modulation Telescope, the Imaging X-ray Polarimetry Explorer, and the X-ray Imaging and Spectroscopy Mission. In parallel, radio and millimeter-wavelength observatories that probe nonthermal emission from relativistic magnetized cosmic plasmas have been recently updated, are developing new capabilities (e.g., interferometric observations by the Event Horizon Telescope of the plasma orbiting the M87 supermassive BH; see Figure 7.2), or are in the final design stages (e.g., the upcoming multinational SKA radiotelescope scheduled for completion in 2025). These technological advances will ensure a flow of new discoveries and new physical insights from a wide range of astrophysical plasma environments.
Comparison of International PSE Community to United States
Active non-U.S. SAP research programs exist throughout the world, with the largest groups found in China, India, Russia, several European countries, the UK, and Japan. Scientists everywhere have benefited from NASA’s open data policy, utilizing observations from heliophysics and astrophysics missions. The Japan Aerospace Exploration Agency (JAXA) and European Space Agency (ESA) missions are less generous with their data, in general, although joint missions with NASA typically adopt NASA’s policies. Data from ground-based facilities such as the Nobeyama Radio Observatory (Japan), the Nançay Radio Observatory (France), and the Low Frequency Array (LOFAR; Netherlands and other European countries) can be obtained by submitting observing proposals or contacting observatory staff.
Some major plasma physics programs that emphasize SAP simulations and/or data analysis are located at Katholieke Universiteit (KU) Leuven (Belgium), Max Planck Institute (Germany), Nanjing University (China), Kyoto University (Japan), Indian Institute for Astrophysics and the Physical Research Laboratory (India), University of St. Andrews and the Imperial College London (UK), the University of Paris/Meudon Observatory (France), and the Space Research Institute (Russia). Anecdotal evidence suggests that SAP programs at non-U.S. academic institutions are attracting more students than U.S. institutions, and that faculty positions at these international institutions are more plentiful than in the United States. China in particular has sent many graduate students and postdoctoral associates to study at international universities and laboratories, and has expanded the number of SAP faculty positions substantially in the past decade.
Since the Plasma 2010 report the number of countries launching/developing SAP space missions and building or planning new ground-based facilities has grown. India is preparing to launch Aditya-L1 in January 2022, a mission that will observe the Sun’s photosphere (soft and hard X-ray), chromosphere (UV) and corona (visible and near-infrared light); study the solar particle flux, reach the L1 orbit, and measure magnetic field strength variation around L1. SMILE, a Chinese/ESA collaborative mission under development, will observe the global structure of Earth’s magnetosphere in soft X rays. Solar C is planned as a successor to Hinode by JAXA in collaboration with ESA and NASA. This mission will focus on high-resolution VUV spectroscopy and multichannel EUV solar imaging from the chromosphere to the corona. As noted above, the Russian Space Agency, in collaboration with the Max Planck Institute for Extraterrestrial Physics in Germany, launched the Spektr-RG high-energy astrophysics space observatory to investigate galaxy clusters and active galactic nuclei.
As a result of these international activities, the United States is beginning to lose its leadership in SAP studies, missions, and facilities. Other nations, such as China and India, are prioritizing science to a degree far above that in the United States. The reasons are multifaceted. The United States is building and launching spacecraft with SAP payloads too infrequently; training too few U.S. students, especially in instrumentation and computation; and placing increasing barriers to retaining foreign nationals at U.S. institutions (particularly government facilities). Current policies make it difficult for the United States to attract and retain excellent scientists from other countries. In addition, the increasing cost of large missions needed for breakthrough science are not easily affordable for a single national space agency without affecting adversely other essential programs. Resolving the dilemma of maintaining leadership while expanding international collaborations is a difficult but crucial task facing the PSE community and its sponsors. The findings and recommendations summarized at the end of this chapter point to some solutions.
Importance of United States to Current Collaborations
SAP research relies heavily on international collaborations, particularly in the era of open data policies for space missions. The United States has led the way in opening access to NASA mission data. Most space agencies in other nations have not yet fully adopted the same approach. The growth of publicly accessible virtual observatories, software repositories, broadly accepted data standards, and other community-based resources has enabled researchers worldwide to download and analyze multiwavelength and multimessenger observations more easily than ever before. Ground-based networks also are essential for global coverage of geospace and solar phenomena, as well as long-baseline interferometry.
These necessary and desired international collaborations can be difficult to establish and maintain. Most successful international programs have been enabled by professional organizations such as the United Nations, the International Council of Scientific Unions, and the International Astronomical Union, often without commensurate funding. One recent example is the International Space Weather Action Teams (ISWAT) program managed by the Committee on Space Research, a grassroots effort that has brought together self-funded working groups to tackle key problems in all facets of space-weather science. Among other international programs, NSF provides opportunities for researchers with active NSF awards to apply for supplemental funding for research visits to collaborate with PIs funded through the European Research Council. In addition, establishing an internationally accessible environment for collaborative model development would accelerate building essential tools for SAP research. Greater participation and financial support by U.S. agencies, and collaborations with their foreign counterparts, would enable more rapid advances in understanding all aspects of space and astrophysical plasmas, and lead to crucial societal benefits.
Space technology exchanges are often restricted by U.S. International Traffic in Arms Regulation (ITAR) based on technology transfer with potential defense-related applications. Given the international advances that have already been independently made, ITAR restrictions on exchange of SAP data may need to be revisited to ensure that these are not limiting exchange of nonsensitive data.
NASA funding for plasma-related research comes largely from its ability to support missions. Basic code and simulation development, theory, and novel data-analysis techniques vital to science missions are infrequently supported by NASA. New initiatives (e.g., DRIVE Science Centers) typically fund a few large research programs, while the smaller grants are too small and short-term to enable these computational developments.
Finding: A lack of support for basic code and simulation development, theory, and novel data-analysis techniques has long been a barrier to advancing our understanding of SAPs and predictive capabilities.
In addition, the growing trend toward open-source requirements would impose an unfunded mandate on developers to make their codes publicly available and user friendly, and does not provide for user support. Coordinated multiagency and multidisciplinary funding for development of theory, plasma codes with open source versions, numerical algorithms, and new data-analysis tools (e.g., machine learning) would maximize scientific return from current and future heliophysics and astrophysics observations and simulations.
Finding: Unfortunately, the level of funding for the highly successful NSF/DOE Partnership in Basic Plasma Science and Engineering has lagged that recommended by the 2000 Plasma Decadal Review, despite its very central role in discovery plasma science and its capacity to create effective multidisciplinary bridges within plasma science.
The recent NSF/NASA Next Generation Software for Data-driven Models of Space Weather with Quantified Uncertainties is an example of a focused SAP-oriented collaboration.
Recommendation: NASA should join the NSF/DOE Partnership in Basic Plasma Science and Engineering to expand interdisciplinary basic plasma research that would benefit space and astrophysical plasmas. This expanded partnership would leverage strategic contributions from each agency to enable breakthrough progress that benefits a wide range of plasma science and engineering activities (see Table 1, #3-#5).
The contributions of NSF and DOE to the partnership are approximately $4 million to $5 million per year per agency. NASA joining the partnership with an equivalent contribution by NASA’s Science Mission Directorate without impacting other programs would significantly increase its scientific impact.
Finding: Solar, heliospheric, magnetospheric, and ionospheric physics, and astrophysics have untapped synergies with laboratory plasma experiments, with the common goal of understanding ambient plasma conditions and chemistry.
SAP examples include ionospheric studies with incoherent scattering radar, chemistry in giant molecular clouds in the interstellar medium, chromospheric
and coronal plasmas, and the very local interstellar medium. Such laboratory SAP experiments are typically not supported by NASA whereas those agencies that have the capability to perform such experiments do not necessarily address the most critical SAP topics.
Finding: Strategic funding from NASA and agencies supporting experimental facilities would enable more ambitious, innovative joint projects than a single source could support.
A successful model to emulate is NASA’s Living with a Star Focused Science Team program, which investigates major heliophysics questions by assembling teams of small groups that each address selected aspects of the science.
Recommendation: NASA and NSF should lead an effort with DoD (especially ONR and AFOSR), DOE, and other stake-holders (see Table 1, #6 and #24) to develop a collaborative program that enables space and astrophysical plasma scientists to collaborate with laboratory plasma experimentalists and advance both fields by leveraging their different needs and knowledge bases.
As noted by the 2016 National Academies report Achieving Science with CubeSats,3 CubeSats and smallsats offer novel and transformational opportunities to explore geospace and the heliosphere with unprecedented spatial and temporal coverage, as needed to resolve fundamental plasma physics problems requiring multipoint observations—for example, solar wind turbulence and magnetosphere-ionosphere coupling.
Finding: Clusters of CubeSats and smallsats carrying in situ and remote-sensing instruments are the best observing platforms for tackling many basic unsolved questions of multiscale SAPs (e.g., reconnection, turbulence, and shocks), and provide essential research and training opportunities for university faculty and students.
Finding: Existing procedures for developing, building, and operating missions are traditionally geared toward large missions, and can pose unnecessary obstacles for single spacecraft and multiplatform missions employing smallsats and CubeSats.
3 National Academies of Sciences, Engineering, and Medicine, 2016, Achieving Science with CubeSats: Thinking Inside the Box, The National Academies Press, Washington, DC, https://doi.org/10.17226/23503.
NSF initiated the CubeSat program, which has become an important gateway for students into experimental heliophysics and astrophysics. NASA currently provides CubeSat launches for NSF and will launch 10 NSF CubeSats in the future. The committee lauds this level of collaboration in a program that effectively combines workforce development and exciting plasma research.
Recommendation: With NASA and NSF as lead agencies, NASA, NSF, and DoD, as the primary sources of space missions, should explore avenues, including rideshares, international partners, and partnering with commercial launch providers, for reducing costs, lowering barriers, enabling higher-risk missions, and boosting launch opportunities for these pioneering investigations using CubeSats, smallsats, and clusters of these satellites. (see Table 1, #7).
Finding: NSF, despite having a limited investment level in their Cubesat program, has broad access to universities and undergraduate and graduate students. This access may be important in providing basic training in PSE relevant to CubeSats. The recent solicitation for a cross-cutting NSF Ideas Lab Program focused on CubeSat innovations to push the envelope of space-based research capabilities is an example of one approach.
Recommendation: In view of their limited level of investment, NSF should identify a clearer role and “identity” in their CubeSat program that distinguishes it from its NASA counterpart. To ensure cost and resource efficiencies, NSF and NASA should coordinate further on funding opportunities.
Finding: Increased support is needed to meet the challenge of designing and constructing compact plasma and remote-sensing instrumentation suitable for Cubesats and smallsats.
Recommendation: Current NASA programs such as H-FORT, H-TIDES, and the Instrument Development Program should be augmented to meet the growing demand for compact plasma and remote-sensing instrumentation suitable for CubeSats and smallsats.
SAP research, as in other STEM fields, is an international endeavor that often requires efforts and funding greater than any one country can support. Open data policies maximize scientific progress by expanding international community access to SAP plasma data, which is especially important for multiwavelength/multimessenger data sets.
Finding: The SAP community needs to agree on data standards, formatting, and processing levels for publicly available data.
Recommendation: Federal agencies that support ground-based and international space facilities (NSF, NASA, DoD, NOAA) should adopt similar open data policies and minimize barriers to international collaboration except where national security is of concern.
Recommendation: NSF should convene a workshop co-sponsored by NASA and DoD to make recommendations for how to establish and maintain open data policies.
Recommendation: Once established, maintaining and updating the open data standards should be the responsibility of governing federal organization such as one of the major contributors (NASA, DoD, NOAA) or a more specialized agency for standards such as NIST.
Hiring plasma-knowledgeable scientists within universities, national laboratories, and other institutions is critical to the long-term health of SAP and the development of the future STEM workforce. Traditionally, university physics and astronomy departments have rarely hired faculty in plasma heliophysics and plasma astrophysics, despite the highly interdisciplinary and fundamental nature of the discipline. The existing NSF program that supports the hiring of space and solar physicists, in partnership with university departments, has been quite successful, sponsoring one hire every few years.
Finding: A faculty partnership program introduced by NASA, NSF, and DOE—similar to that presented in the “Findings and Recommendations” section of Chapter 1 (see Table 1.1)—is needed to strengthen SAP hiring in universities and federal research facilities.
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