Earth’s neighborhood in space—the local cosmos—provides a uniquely accessible laboratory in which to study the behavior of space plasmas (ionized gases) in a wide range of environments. By taking advantage of our ability to closely scrutinize and directly sample the plasma environments of the Sun, Earth, the planets, and other solar system bodies, we can test our understanding of plasmas and extend this knowledge to the stars and galaxies that we can view only from afar.
Solar and space physics research explores a diverse range of plasma physical phenomena encountered at first hand in the solar system. Sunspots, solar flares, coronal mass ejections, the solar wind, collisionless shocks, magnetospheres, radiation belts, and auroras are just a few of the many phenomena that are unified by the common set of physical principles of plasma physics. These processes operate in other astrophysical systems as well, but because these systems can be examined only remotely, theoretical understanding of them depends to a significant degree on the knowledge gained in the studies of the local cosmos. This report, Plasma Physics of the Local Cosmos, by the Committee on Solar and Space Physics of the National Research Council’s Space Studies Board attempts to define and systematize these universal aspects of the field of solar and space physics, which are applicable elsewhere in the universe where the action is only indirectly perceived.
The plasmas of interest to solar and space physicists are magnetized—threaded through with magnetic fields that are often “frozen” in the plasma. In many cases, the magnetic field plays an essential role in organizing the plasma. An example is the structuring of the Sun’s corona by solar magnetic fields in a complex architecture of loops and arcades—as seen in the dramatic close-up views of the solar atmosphere provided by the Earth-orbiting TRACE observatory. In other cases, such as the Sun’s convection zone, the plasma organizes the magnetic field. Indeed, it is the twisting and folding of the magnetic field by the motions of the plasma in the solar convection zone that amplifies and maintains the Sun’s magnetic field. In all cases, however, the plasma and the magnetic field are intimately tied together and mutually affect each other. The theme of magnetic fields and their interaction with plasmas provides an overall framework for this report. An overview is presented in Chapter 1, introducing the chapters that follow, each of which treats a particular fundamental set of phenomena important for our understanding of solar system and astrophysical plasmas.
The question of how magnetic fields are generated, maintained, and amplified, together with the complementary question of how magnetic energy is dissipated in cosmic plasmas, is explored in the second chapter of this report, “Creation and Annihilation of Magnetic Fields.” The focus is on the dynamo and on magnetic reconnection. Chapter 2 discusses the current understanding of the workings of these processes in both solar and planetary settings and identifies several outstanding problems. For example, understanding how the differential rotation of the solar interior arises represents a significant challenge for solar dynamo theory. In the case of planetary dynamos, important open questions concern the role of physical processes other than the Coriolis force in determining the morphology and alignment of the magnetic field (e.g., of Uranus and Neptune) and the influence of effects such as fluid inertia and viscous stress on Earth’s dynamo. With respect to magnetic reconnection, a significant advance in our understanding has been achieved with the development of the kinetic picture of this process. However, what triggers and maintains the reconnection process is the subject of great debate. Moreover, how reconnection operates in three dimensions is not well understood.
Chapter 3, “Formation of Structures and Transients,” examines some of the important structures that are found in magnetized plasmas. These include collisionless shocks, which develop when the relative velocity between different plasma regimes causes them to interact, producing sharp transition regions, and current sheets, which separate plasma regions whose magnetic fields differ in orientation and/or magnitude. A transient structure that occurs in a number of different plasma environments (solar active regions, the corona, the solar wind, the magnetotail) is the flux rope, a tube of twisted magnetic fields. Scientists have learned much about the plasma structures in our solar system but still have numerous questions. Studies of Earth’s bow shock have provided basic understanding of shock dissipation and shock acceleration in collisionless plasmas, but much work remains in extending this understanding to large astrophysical shocks. This will require understanding of strong interplanetary shocks in the outer heliosphere and, ultimately, direct observation of the termination shock. Flux ropes have also been extensively observed, but many unanswered questions remain: How are flux ropes formed and how do they evolve? What determines their size? How are they destroyed? What is their relation to magnetic reconnection?
Chapter 3 also examines magnetohydrodynamic turbulence, a phenomenon that is a classic example of the way in which magnetized plasmas couple strongly across multiple spatial and temporal scales. In turbulent coupling, energy is fed into the largest scales and then progressively flows down to smaller scales, eventually reaching the “dissipation scale,” where heating of the plasma occurs. Turbulence has been most completely studied in the solar wind, but questions remain concerning the detailed structure of heliospheric turbulence and how this structure affects energetic particle scattering and acceleration. Turbulent processes also occur in the Sun’s chromosphere as well as in Earth’s magnetopause and magnetotail. Outstanding problems include the role of turbulence in transport across boundary layers, the onset of turbulence in thin current sheets, and the coupling of micro-turbulence to large-scale disturbances.
Plasmas throughout the universe interact with solid bodies, gases, magnetic fields, electromagnetic radiation, and waves. These interactions can be very local or can take place over regions as large as the size of galaxies. Chapter 4 discusses four classes of plasma interaction. Electromagnetic interaction is exemplified by the coupling of a planetary ionosphere and magnetosphere by electrical currents aligned with the planet’s magnetic field. The aurora is a familiar and dramatic manifestation of the energy transfer that results from this coupling. Electromagnetic coupling is also believed to be important in stellar formation, through the redistribution of angular momentum between the protostar and the surrounding nebular material. Flow-object interactions refer to the processes that occur when plasma flows past either a magnetized or an unmagnetized object. Typical processes include reconnection, turbulent wakes, convective flows, and pickup ions. The third class of plasma interactions are those that involve the coupling of a plasma with a neutral gas, such as the exchange of charge between ions and neutral atoms or collisions
between ions and neutrals in Earth’s auroral ionosphere, which drive strong thermospheric winds. The final category is radiation-plasma interactions, which is important for understanding the structure of the Sun’s corona: radiation-plasma interactions produce a monotonically decreasing temperature-altitude profile in the corona in great contrast to a falling-then-rising profile produced by the standard quasi-static models.
Chapter 5, “Explosive Energy Conversion,” treats the buildup of magnetic energy and its explosive release into heated and accelerated particles as observed in solar flares, coronal mass ejections, and magnetospheric substorms. Since the first observation of a solar flare in 1859 and the recognition that solar disturbances are associated with auroral displays and geomagnetic disturbances, magnetic energy release has been a central topic of solar-terrestrial studies. Because of their potentially disruptive influence on both ground-based and space-based technological systems, such explosive events are of practical concern as well as of great intrinsic scientific interest.
Both solar flares and coronal mass ejections (CMEs) result from the release of magnetic energy stored in the Sun’s corona. It is not understood, however, how energy builds up and is stored in the corona or how it is then converted into heating in flares or kinetic energy in CMEs. At Earth, magnetic energy stored in the magnetotail through the interaction of the solar wind and the magnetosphere is explosively released in substorms, periodic disturbances that convert this energy into particle kinetic energy. The details of how stored magnetic energy is transferred from the lobes of the magnetotail to the plasma sheet and ultimately dissipated remain subjects of intense debate. The storage and release of magnetic energy occur universally in astrophysical plasmas, as evidenced by the enormous flares from M-dwarfs and the stellar eruption observed in the young XZ-Tauri AB binary system. What is learned about the workings of magnetic storage-release mechanisms in our solar system is likely to contribute to our understanding of analogous processes in other, remote astrophysical systems as well.
The key mechanisms by which magnetized plasmas accelerate charged particles are reviewed in Chapter 6, “Energetic Particle Acceleration.” Shock acceleration occurs throughout the solar system, from shocks driven by solar flares and CMEs to planetary bow shocks and the termination shock near the boundary of the heliosphere. Particles are accelerated at shocks by a variety of mechanisms, and the resulting energies can be quite high, >100 MeV and even in the GeV range for solar energetic particles accelerated at CME-driven shocks. One topic of particular interest in current shock acceleration studies is the identity of the particles that form the seed population for the shock-accelerated ions. What, for example, are the sources and composition of the pickup ions that are accelerated at the termination shock to form anomalous cosmic rays?
Coherent electric field acceleration arises from electric fields aligned either perpendicular or parallel to the local magnetic field. Induced electric fields perpendicular to the geomagnetic field play a role in the radial transport and energization of charged particles in Earth’s magnetosphere and contribute to the growth of the outer radiation belt during magnetic storms. Parallel electric fields accelerate auroral electrons and accelerate plasma from reconnection sites; they are also involved in the energization of solar flare particles. Stochastic acceleration results from randomly oriented electric field perturbations associated with magnetohydrodynamic waves or turbulence. It plays a role in the acceleration of particles in solar flares, in the acceleration of interstellar pickup ions in the heliosphere, and possibly in the acceleration of relativistic electrons during geomagnetic storms.
All of these acceleration mechanisms may occur simultaneously or at different times. For example, direct energization of particles by electric fields, interactions with ultralow-frequency waves, and localized, stochastic acceleration may all contribute to the storm-time enhancement of Earth’s radiation belt. However, in this case as in others, distinguishing among the various acceleration mechanisms as well as determining the role and relative importance of each poses challenges to both the observational and the theory and modeling communities.
Plasma Physics of the Local Cosmos examines the universal properties of solar system plasmas and identifies a number of open questions illustrative of the major scientific issues expected to drive future research in solar and space physics. Recommendations regarding specific future research initiatives designed to address some of these issues are offered in another recent National Research Council report, The Sun to the Earth—and Beyond: A Decadal Research Strategy for Solar and Space Physics, which was prepared by the Solar and Space Physics Survey Committee under the auspices of the Committee on Solar and Space Physics.1 The two reports are thus complementary. The Survey Committee’s report presents a strategy for investigating plasma phenomena in a variety of solar system environments, from the Sun’s corona to Jupiter’s high-latitude magnetosphere, while Plasma Physics of the Local Cosmos describes the fundamental plasma physics common to all these environments and whose manifestations under differing boundary conditions are the focus of the observational, theoretical, and modeling initiatives recommended by the Survey Committee and its study panels.