2

Science Discoveries and Technical Advances

2.1 BRIEF INTRODUCTION OF SCIENTIFIC PROGRESS

The first half of the period covered by the 2013 solar and space physics decadal survey (NRC, 2013) has witnessed a series of major scientific advances in solar and space physics, spurred by a vibrant community with the robust support of the relevant federal agencies. Many of these advances are sparked by new instrumentation and the new perspectives that result from advances in theory, modeling, and computation.

Among the new instrumentation launched over the past 6 years are the Parker Solar Probe (PSP), which has set the record for closest approach to the Sun; the Interface Region Imaging Spectrograph Small Explorer (IRIS SMEX), which resolves the solar atmosphere with unprecedented resolution; the Magnetospheric Multiscale Mission (MMS), which has probed the fine-scale processes in Earth’s magnetosphere; and Global-Scale Observations of the Limb and Disk Mission of Opportunity (GOLD MoO), NASA’s first scientific payload hosted on a commercial spacecraft. The twin Van Allen Probes were launched in August 2012, just before the decadal survey was published, and recently completed their mission to unlock the secrets of particle acceleration in Earth’s Van Allen radiation belts. Most recently, the Space Environment Testbeds (SET-1) mission and Ionospheric Connection Explorer (ICON) were launched in 2019, growing NASA’s solar and space physics fleet.

Missions in their extended phase continue to provide new perspectives and the continuous measurements necessary for studying and monitoring the Sun-Earth system. Notably, the two Voyager spacecraft have now both reached interstellar space (Box 2.1). Missions working in tandem also stimulate advances in understanding. For example, combining data from spacecraft near Mercury, Venus, and Earth, along with remote observations of distant comets, enables scientists to track how solar disturbances change as they propagate through the solar system.

New ground-based observatories and operational assets have also been deployed. The National Science Foundation (NSF) has completed the Jansky Very Large Array (JVLA) and is projected to complete the Daniel K. Inouye Solar Telescope (DKIST) observatory in 2020. Moreover, the National Oceanic and Atmospheric Adminisration (NOAA) has launched the Deep Space Climate Observatory (DSCOVR) and the Geostationary Operational Environmental Satellite (GOES-16 (R) and GOES-17 (S)) missions. All of these

resources contribute to a system-spanning observational network—the Heliospheric System Observatory (HSO). The HSO is the fundamental workhorse within which an evolving fleet of individual observatories from different agencies and international partners contribute to understanding the workings of the local cosmos as a system-of-systems and thereby uncover, and ultimately mitigate, its impacts on society.

Ongoing investigations combining observations, models, and theory have provided new views of the workings of the solar atmosphere—from small-scale heating to vast eruptions; how couplings in the solar corona, the magnetosphere, and high in Earth’s atmosphere are guided by waves; the details of the radiation belts; how magnetic reconnection converts magnetic energy into particle acceleration; and how heating processes and cooling aerosols affect the chemistry in the upper reaches of Earth’s atmosphere. The multitude of perspectives and measurements are increasingly combined in system-wide numerical models, while machine learning is used with rapidly increasing frequency and skill to sift through the growing volumes of data. Learning about solar and space physics is also helped by the Sun itself: the unusually weak recent solar cycle gives us a better view of background processes that were until now generally masked by strong solar variability. In turn, these background processes help researchers differentiate anthropogenic from solar-induced effects.

Solar and space physics has reached a level of sophistication where more comprehensive physics-based numerical modeling, which relies much less on simplifying approximations, is possible. Such numerical sophistication enables both the interpretation of observations and the exploration of new scientific questions. This advance is especially apparent in areas where there are complex couplings¹ between regions, such as the various layers of the solar atmosphere, and also between the solar wind and Earth’s magnetic cocoon and upper atmosphere. The importance of advancing all branches of heliophysics to further understand the entire Sun-Earth chain (and much more broadly to understand the diversity of star-planet chains) was stressed in the decadal survey, and that insight is paying off.

Over the past several decades, a number of computational models have been developed. However, the ability of the community at large to take advantage of these had been limited by both access and user capability. The Community Coordinated Modeling Center (CCMC) at NASA Goddard Space Flight Center (GSFC) was developed over 19 years ago in part to address this need, by working with model developers to transition selected modeling resources to available tools for research. The 6-year period since publication of the 2013 decadal survey has seen a dramatic rise in the use of these models, due in part to an active program of information dissemination and user training through programs such as NASA’s Living With a Star (LWS) and the NSF workshops for Solar Heliospheric and Interplanetary Environment (SHINE), Geospace Environment Modeling (GEM), and Coupling, Energetics and Dynamics of Atmospheric Regions (CEDAR). NASA has also led the continuation of several summer schools focused on the fundamentals of heliophysics and on its space weather aspects (including model-based forecasting). In addition, model user support through those programs and through the broader heliophysics research opportunities in NASA’s Research Opportunities in Space and Earth Science (ROSES) has resulted in cross-community partnerships of both intellectual and practical value.

The period since publication of the decadal survey has also seen the emergence of new areas of scientific exploration. With the recent explosion of interest in exoplanets and astrobiology, space physicists are applying the knowledge gained about “universal processes” from studying our local cosmos to these other systems (Box 2.2). Other exciting developments have their origin in comparisons of different environments, including (1) the exploration of space-weather conditions in other planetary systems using models based on our local cosmos, (2) the comparison of the planetary environments of Earth and Mars to better

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¹ These couplings may involve different combinations of matter, energy and charge transfer, chemical or composition alterations, and mechanical and electromagnetic forces.

constrain atmospheric losses induced by space weather, and (3) the ion-neutral couplings that occur in settings as different as the solar chromosphere, Earth’s ionosphere, and the interaction of the solar wind with the partially ionized interstellar medium.

The importance of space weather continues to grow as illustrated by the recent development of the National Space Weather Strategy and Action Plan. The challenges of forecasting space weather are rooted in the complexity of the Sun-Earth system, but significant progress has been made in organizing a national effort to improve our predictive capabilities (Box 2.3). Finally, a chapter reviewing significant events since the decadal survey would not be complete without mention of the “Great American Eclipse” of 2017 (Box 2.4).

The 2013 decadal survey identified four key science goals and the science challenges that flow from these goals from the three primary branches of heliophysics—atmosphere-ionosphere-magnetosphere interactions (AIMI), solar wind-magnetosphere interactions (SWMI), and solar and heliospheric physics

(SHP) (see Table E.1). The sections below provide a brief overview of science highlights from the first half of the decadal period in each of these subdisciplines. Appendix E provides a more in-depth look at selected scientific discoveries and their relationship to the 12 science challenges outlined in the decadal survey. Substantial scientific and technical progress has been made in each of the science challenge areas, but for brevity, only a few examples of progress are presented in this chapter and in Appendix E. Additionally, this chapter discusses in Section 2.5 some recent advances in research tools important for heliophysics science.

2.2 SOLAR AND HELIOSPHERIC PHYSICS

Over the first half of the decade covered by the decadal survey, scientists have made great advances in understanding the dynamic solar magnetic field and how it shapes the whole of the space environment, ranging from the Sun to far beyond the planets. Some highlights are given here in brief, with more detail on selected findings mapped back to the decadal survey challenges in Appendix E.

• Close to the Sun, the Parker Solar Probe has set the record for closest approach to the Sun; its ongoing mission is to sample solar coronal particles and the solar electromagnetic field to understand coronal heating, solar wind acceleration, and the formation and transport of solar energetic particles.
• Far from the Sun, measurements by the Voyager spacecraft (that exited the heliosphere in 2012 and 2018, respectively), combined with Interstellar Boundary Explorer (IBEX) and Cassini data, transformed our knowledge of the outer boundary of the heliosphere, placing outer-heliospheric science solidly among the other fast-developing branches of heliophysics.
• Fine-scale High Resolution Coronal Imager (Hi-C) rocket imagery combined with global-scale Solar Dynamics Observatory (SDO) images of the solar corona revealed the small-scale signatures of the reconfiguration of the magnetic field and of Alfvén waves running through that magnetic field. Combined SDO, IRIS, and Hinode observations have furthered our understanding of the mechanisms that extract energy from the magnetic field either in the form of heat or through explosive eruptions, and also of the mechanisms that transport energy between different wave types and physical domains.
• Technical advances in computational methods and infrastructure provide critically needed insights into both the source of the solar magnetism and the formation of, and explosive instabilities in, the globally connected solar atmosphere as discovered with SDO observations. These newly developed computer models can be applied to new data, but also to archival data to efficiently learn from decades-long historical archives.
• Machine learning and big data techniques are helping us move toward improved multi-day predictions of space weather, while increasingly realistic computer models in regular use at the CCMC are now able to work across multiple physical regimes as they simulate quantities that can be directly compared to real-world observations.
• Detailed observations of space-weather throughout the solar system helped planetary scientists and stellar astrophysicists understand the range of possibilities for stellar wind conditions, and thus how these conditions influence exoplanets.

These exciting new results from both space-based missions and ground-based instruments demonstrate the importance of diverse and complementary observing capabilities, both remote and in situ, in order to understand fundamental physical processes. They also emphasize the importance of accessible, standardized archives and the need to develop community-usable numerical tools for modeling and analysis, sometimes using artificial intelligence, including data-driven full-system models that include the space-weather forecasting systems.

Selected findings are discussed in detail in Appendix E for the four SHP science challenges that include understanding the solar dynamo, the solar dynamic atmosphere, solar eruptions (e.g., magnetic reconnection), and interactions with the local interstellar medium. As one example here, the key drivers for solar activity are magnetic field emergence, transport, and energy release on the Sun. The solar dynamo (Section 1.2) is the underlying fundamental process inside the Sun. Measurements and models of these subsurface movements are critical to understand the dynamo and to forecast the 11-year solar cycle. Results from two methods are combined to infer plasma flows in the solar interior. One method is called helioseismology

whereby the properties of sound waves are measured as they run through much of the solar interior in order to deduce the conditions of the gas that they traverse. Another method is to track magnetic elements as they move across the solar surface based on high-resolution magnetographs and (for sunspots) direct imaging. In a recent result from using helioseismology, Zhao et al. (2013) detected an equatorward flow in the Sun’s interior between 0.83 and 0.91 solar radii, sandwiched between poleward flows below and above. This flow (illustrated in Figure 2.1) may be a key ingredient of the dynamo as it can transport magnetic field lines both in the deep interior and near the surface. However, the rise of bundles of magnetic field to the solar surface was found to be much slower than expected from some models (Birch et al., 2016), meaning more theoretical work is needed to better understand how flux tubes rise through the surrounding gas.

2.3 SOLAR WIND-MAGNETOSPHERE INTERACTIONS

Solar extreme ultraviolet radiation and the solar wind, including transients related to stream structure and coronal mass ejections, impinge on Earth’s protective magnetic shield, producing a variety of conditions in Earth’s space environment. Since the publication of the decadal survey, significant progress has been made in understanding how these conditions come about, involving both the externally driven and the internally shaped processes by which the solar radiation and solar wind couple to a planetary magnetosphere, and how these processes transport mass and energy into and within the magnetosphere. Selected discoveries for the SWMI challenges are discussed in some detail in Appendix E. Some of the SWMI highlights include the following:

• MMS has observed how electrons are accelerated and heated even as they slip across the magnetic field in the process of magnetic reconnection.
• Waves excited by the solar wind flowing along Earth’s magnetosphere in the interface layer called the magnetopause have been discovered to play a substantive role in controlling how efficiently the magnetic fields of the solar wind and of Earth reconnect. Where such waves are strongest depends both on the solar wind conditions and plasma and field conditions close to the magnetopause.
• The Van Allen Probes mission has changed our understanding of the structure of Earth’s Van Allen belts and of the processes that accelerate charged particles to ultra-high speeds. At times we observe three or more radiation belts. The inner edge of the outer belt for ultra-high energy electrons is unexpectedly sharp, and the inner belt is nearly void of high-energy electrons most of the time.
• The unusually weak recent solar cycle is providing important insights into how the inner-heliospheric conditions affect the propagation of coronal mass ejections, thereby changing the magnetic field and the dynamic pressure of heliospheric storm fronts as they reach Earth. The weak cycle has also given new insights into how the ionosphere responds to levels of lower-energy ionizing radiation.
• With spacecraft near Mercury, Venus, Earth, Jupiter, and Saturn, the evolution of solar eruptions traveling through the heliosphere could be observed and compared with simulation results. The analysis of many tails of comets over their observable trajectories is helping us understand solar-wind variability, specifically its turbulence, and how that evolves from near the Sun outward.
• Space-weather conditions at Mars were studied with particular emphasis on the solar wind coupled to that planet’s atmosphere with its weak, local magnetism.

Selected discoveries discussed in detail in Appendix E for the four SWMI science challenges include understanding magnetic reconnection in the magnetosphere; energetic particle processes; coupling between the magnetosphere, ionosphere, and thermosphere; and different magnetospheric systems (e.g., other planets). As one example, our understanding of the physics of magnetic reconnection has undergone a revolutionary change since publication of the decadal survey. Specifically, the role of electrons in the process of magnetic reconnection was revealed using new measurements from MMS.

MMS uses four identically instrumented spacecraft, flying in the tightest-ever pyramid formation for a satellite constellation, to measure the electromagnetic field with unprecedented accuracy, sampling 100 times faster than previous missions. MMS observed that the reconnection process is highly localized at the magnetopause, the boundary between the solar wind and Earth’s magnetosphere. Moreover, the reconnection process can strongly energize electrons (Figure 2.2). Reconnection briefly decouples electrons from the magnetic field and then accelerates them in the electric field aligned with the magnetic field as a

consequence of the strong gradients in the reconfiguring field (Burch et al., 2016; Burch and Phan, 2017; Chen et al., 2016; Graham et al., 2016, 2017; Torbert et al., 2018). The extraordinary time resolution of the electron instruments on MMS captured this acceleration while the unprecedented accuracy of the electric field measurements captured the reconnection electric fields responsible for the acceleration. However, because the electron diffusion regions cover only a small volume, they are insufficient to explain the overall observed energization. The details of the cross-scale coupling facilitated by reconnection remains a challenge.

2.4 ATMOSPHERE-IONOSPHERE-MAGNETOSPHERE INTERACTIONS

The AIMI region starts roughly just above Earth’s stratosphere (50 km) and extends up to several thousand kilometers above ground. The AIMI region is impacted by the Sun, interactions with the magnetosphere, and also by processes occurring in the atmosphere below. Discovering the processes that govern the conditions at this interface between Earth and space is fundamental to understanding planetary atmospheres and exospheres, as well as for operational needs, including the protection of astronauts and spacecraft, the protection of humans on the ground, and for radio and navigation signal situational awareness. Distributed observational capabilities increasingly enforce the realization that the geospace system, from below the ionosphere to the outer reaches of the magnetosphere, is a single connected system. Among the many advances in understanding the AIMI are the following:

• The weak recent solar cycle simplified the separation of solar influences from effects from the lower atmosphere, enabling improved understanding of the coupling processes between the AIMI region and the atmosphere below. Models are bridging the knowledge gap on the coupling between larger-scale instabilities and smaller-scale turbulence that is important in regulating the dynamics of geospace.
• The energy of precipitating particles and heat from solar radiation enhance the concentration of nitric oxide (NO) in the thermosphere, which in turn has brighter infrared emissions to cool the thermosphere back down efficiently.
• Joint analyses and comparisons of plasma-neutral interactions in the solar chromosphere and in the terrestrial ionosphere stimulated by the NASA LWS Research and Analysis program has provided deeper insights into the similarities and differences between these environments and is leading to sharing of insights between two communities previously working largely in isolation.
• Atmospheric waves generated by tides, terrain, and atmospheric instabilities have been observed and modeled as they travel upward, strengthening in the process. Waves are also generated in the dynamics of the polar vortex at stratospheric altitudes. All these wave phenomena can modify high-atmospheric properties, including ionospheric properties, far from the latitudes where they originally formed, which, in turn, couple to space weather phenomena further out.
• Drivers of long-term trends in upper atmospheric properties are better clarified using ever more sophisticated global circulation models (GCMs) to reveal the dynamics effects from solar variability, the cooling influence of anthropogenic methane and carbon dioxide, and even the top-down coupling of atmospheric changes resulting from the long-term change of the terrestrial magnetic field, of which the shift of the magnetic poles is one consequence.

Selected discoveries are discussed in detail in Appendix E for the four AIMI science challenges, including understanding how the ionosphere-thermosphere system responds to magnetosphere forcing, plasma-neutral coupling processes, lower-atmosphere forcing effects in the ionosphere-thermosphere, and the causes for long-term changes in the AIM system. The discovery of a new, different auroral phenomenon

is noted here as one example of the new AIMI findings. Bright visible auroras are produced by energetic particles flowing along magnetic field lines into the upper atmosphere. Auroras have long been exploited as diagnostic of less accessible magnetospheric processes, but there are still surprises. Recently a new, very different, auroral phenomenon has been discovered by a large ad hoc network of citizen auroral watchers. This faint feature, known as STEVE (Strong Thermal Emission Velocity Enhancement) and shown in Figure 2.3, appears to be caused by a high-speed plasma jet flowing perpendicular to Earth’s magnetic field, exciting optical emissions through pathways not yet identified. The discovery of STEVE (MacDonald et al., 2018; Gallardo-Lacourt et al., 2018) highlights the discovery potential of geospace facilities that may be realized in creative and cost-effective ways.

2.5 ADVANCES IN RESEARCH TOOLS FOR HELIOPHYSICS SCIENCE

The past 5 years have seen significant advances in computational, data science, and observational tools. Some highlights include the following:

• Ultra-high-resolution magneto-hydrodynamic (MHD) simulations of Earth’s magnetosphere have been pushed to the limits of the physics that MHD can capture, revealing highly structured features in dipolarization fronts.
• A new generation of whole-Earth atmosphere models now span from Earth’s surface to the upper thermosphere and ionosphere and can capture the atmospheric driving of space weather effects.

• The heliophysics community is embracing advances in data science, with broad applications ranging from forecasting of solar flares to scintillation in the ionosphere.
• Machine learning and other new data analysis tools are becoming increasingly important as missions and simulations produce increasingly large amounts of data.
• The rapid development of the small satellite industry, including the plans for large commercial satellite constellations, provides new opportunities for heliophysics science.
• Increased rideshares, opportunities for hosted payloads, and new manufacturing methods offer new and low-cost access to space.

2.5.1 Computational Tools

Simulations have been a critical tool in heliophysics for a while, but new models have unprecedented spatial resolution (global MHD), include detailed physics (hybrid and particle-in-cell simulations), and/or make use of vast observational data (empirical models, and data assimilation), to name just a few advancements. Computer power has increased to the level that three-dimensional simulations of different heliophysical regions can have higher spatial and temporal resolution than the corresponding observations. For example, simulation of the response of the chromosphere, transition region, and corona can have higher spatial and temporal resolution than is presently observable with IRIS. The new generation of whole-Earth atmosphere models such as WACCM-X (Liu et al., 2018) now span the altitude range from Earth’s surface to the upper thermosphere with an interactive ionosphere and electric wind dynamo to connect tropospheric weather with space weather at resolutions capable of resolving mesoscale processes. While the simulations are more detailed than what can be currently observed, that does not mean the simulations are correctly emulating the observed physical processes. Each improvement of the simulations, whether that be by increased spatial or temporal resolution of more sophisticated physical models, must be validated by real observations. Nevertheless, these tools are providing important context for observations, and they are necessary for the system-level science approach that is critical for advancing heliophysics research.

The benefits of model development and CCMC support to make them accessible can be seen in the many applications for both observational and theoretical studies. For example, the WSA-ENLIL-cone heliospheric model and MAS/CORHEL (Corona-Heliosphere) are widely applied to space weather event interpretation (e.g., Jian et al., 2011; Moestl et al., 2015; Colaninno et al., 2013; Rouillard et al., 2016; Gibson et al., 2016), and to further modeling (e.g., interplanetary shocks used for solar energetic particle event modeling) (Lario et al., 2017; Schwadron et al., 2017; Luhmann et al., 2017). The Block-Adaptive-TreeSolarwind-Roe-Upwind-Scheme, Lyon-Fedder-Mobarry, and Open Geospace General Circulation Model magnetosphere models have been used to investigate topological and phenomenological characteristics and responses to both nominal external conditions and solar events (e.g., Haiducek et al., 2017; Samsonov et al., 2016; Ilie et al., 2015). The Thermosphere-Ionosphere-Electrodynamics General Circulation Model and Global Ionosphere Thermosphere Model have been applied to interpretations of measurements of airglow and total electron content, toward understanding ion-neutral coupling across species and altitudes, vertical momentum transport (Zhu et al., 2017), and to studying the effects of waves and instabilities (e.g., Maute and Richmond, 2017; Greer et al., 2018; Li et al., 2018; Lin et al., 2018). Processes previously excluded, such as polar outflow, are now also being incorporated into models (Welling et al., 2016). These models are only a handful of the many models used in heliophysics research, and this discussion is intended to just provide examples of science usages of a few models. Applications of the broad range of models that have been made available so far have scarcely touched the breadth and scope of investigations that can be pursued as a result of this greater accessibility.

2.5.2 The Increasing Role of Data Science in Heliophysics

The solar and space physics community is also increasingly employing data science in their research, in particular to analyze the large and complex data sets that our facilities now produce. Data science tools encompass three main areas: modern statistical techniques, modern computational techniques, and knowledge of a specific scientific domain—in this case, solar and space physics.

These techniques have recently been used to forecast and characterize solar flares (e.g., Bobra and Couvidat., 2015; Nishizuka et al., 2018; Panos et al., 2018) and solar energetic particle events (e.g., Winter et al., 2015), to study electron densities in the Van Allen belts (e.g., Zhelavskaya et al., 2016), scintillation in the ionosphere (e.g., McGranaghan et al., 2018) and particles and waves in the inner magnetosphere (e.g., Bortnik et al., 2016) and solar wind (e.g., Camporeale et al., 2017).

One especially useful class of techniques, called machine learning and data mining, unearth patterns and behaviors in data sets that are difficult to discover via simple statistical relationships or by eye in a way that is scalable and reproducible (Ivezic et al., 2014; LeCun et al., 2015). Machine learning algorithms may be easiest to understand via an example. Identifying features on the solar surface, such as prominences, flare ribbons, and sunspots in terabyte- and petabyte-scale data sets are important for solar physics and space weather applications, but it is not feasible to manually identify these features in millions of images. Further, manually identified features prove difficult to reproduce. Learning algorithms can identify features in a way that is reproducible, scalable, and fast.

2.5.3 The Small Satellite Revolution

In addition to advances in theoretical and analysis tools, there has been rapid development in the capabilities of small satellites (<180 kg) as an observational tool. Tiny satellites of less than 10 kg, often called CubeSats because of their commonly used 10 cm × 10 cm × 10 cm unit (1 U) form factor, have been in development for more than 20 years, but their scientific potential has been broadly recognized only recently. Although the decadal survey recognized the growing potential of CubeSats, the extent to which small satellites would be embraced in the commercial sector was not fully anticipated. Commercial satellite constellations of hundreds of small (approximately 100 kg) satellites are now being deployed to create an internet in space. Additionally, commercial satellite constellations of dozens of nano (approximately 10 kg) satellites are now being deployed to regularly image Earth from space and monitor Earth’s weather, Earth’s climate, and space weather using radio occultation measurements. These efforts in turn are leading to a growing industry of commercial off-the-shelf parts, launch opportunities, and new ways of manufacturing small satellites.

The rapid growth of low-cost access to space is one of the key components for this small satellite revolution. Rideshares on large-lift launch vehicles are now common place, and rideshare costs continue to drop as the space industry is positioning itself for more customers. Small-lift launch vehicles, such as the Rocket Lab Electron, are also competing for their share of launches of small satellites. Opportunities for hosted payloads, such as the flight of the NASA GOLD instrument on a commercial communication satellite in geostationary Earth orbit, are also growing, but less so than rideshares and small-lift launch vehicles.

These advances present a real opportunity for all areas of science, including heliophysics. Moreover, the large constellation class missions that have been discussed for decades may soon be within reach. The Committee on Space Research roadmap Small Satellites for Space Science was recently published (Millan et al., 2019) and outlines some pathways by which the science community can leverage these developments and form international partnerships to pursue ambitious goals using small satellites.

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