Plasma Physics of the Local Cosmos (2004)

Our solar system, and stellar systems in general, are rich in the dynamical behaviors of plasma, gas, and dust organized and affected by magnetic fields. These dynamical processes are ubiquitous to highly evolved stellar systems, such as our own, but also play important roles in their formation and evolution. Stellar systems are born out of clumpy, rotating, primordial nebulas of gas and dust. Gravitational contraction, sometimes aided by shock waves (possibly from supernovas), passage through dense material, and other disruptions, forms condensation centers that eventually become stars, planets, and small bodies. Magnetic fields moderate early-phase contractions and may also play vital roles in generating jets and shedding angular momentum, allowing further contraction. The densest of the condensation centers become protostars surrounded by accretion disks. Dynamo action occurs within the protostars as the heat of contraction ionizes their outer gaseous layers, resulting in stellar winds. In similar fashion, rotating solid and gaseous planets form, and many of these also support dynamo action, producing magnetic fields. Ultraviolet and x-ray photons from the central stars partially ionize the upper atmospheres of the planets as well as any interstellar neutral atoms that traverse the systems. Viewed as a whole, the resulting plasma environments are called asterospheres, or in the Sun’s case, the heliosphere. In its present manifestation, the heliosphere—the local cosmos—is a fascinating corner of the universe, challenging our best scientific efforts to understand its diverse machinations. It must be appreciated at the same time that our local cosmos is a laboratory for investigating the complex dynamics of active plasmas and fields that occur throughout the universe from the smallest ionospheric scales to galactic scales. Close inspection and direct samplings within the heliosphere are essential parts of the investigations that cannot be carried out by a priori theoretical efforts alone.

This report summarizes much of what is known about the plasma physics of the local cosmos and lists many of the outstanding questions that will be driving the field for the near future. The discussions are organized around five broad themes, specifically (1) the creation and annihilation of magnetic fields, (2) the formation of structures and transients, (3) plasma interactions, (4) explosive energy conversion, and (5) energetic particle acceleration. These phenomena have been identified, and questions posed, in terms of specific observables either on the Sun or in various parts of the heliosphere and planetary systems. The

proposed solutions and experiments are also designed for the parameter ranges found in the local cosmos. Nonetheless, in every case the solutions are of universal applicability, and a challenge for solar and space physics in the future is to extend the models and theories that have been validated in the local cosmos to nonlocal astrophysical plasmas. It is obvious, for example, that the theories of magnetic dynamos and magnetic reconnection must be applicable to the generation of magnetic fields and to the explosive conversion of magnetic energy to heat and particle kinetic energy in every corner of the universe. Likewise, the acceleration of charged particles to high energies by shock waves and by both parallel and perpendicular electric fields is certainly of universal importance. Obstacles to plasma flows exist everywhere, and the interaction between two magnetized plasma populations can produce magnetic stresses that are relieved with cross-scale coupling processes that occur either gradually or explosively depending upon the capabilities of each plasma region.

As the discipline of solar and space physics has matured, the focus has become less on places to explore than on fundamental processes to investigate and understand. Understanding requires that the processes be investigated in diverse plasma environments. Important to this investigation are space missions to the magnetospheres of other planets as well as that of Earth, missions to sample the properties of the heliosphere, missions to observe the astonishing fine structure of the active Sun, and multispacecraft missions to sample the structure of magnetopauses, shock transitions, and the magnetic reconnection processes. The results must be fed into the latest theoretical models, and it is these models that provide the links to other parts of the cosmos. Thus, a plasma theory and modeling program that cuts across the disciplines of solar physics, space physics, and astrophysics is an important part of any efforts to understand the plasma physics of the cosmos.