Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session NM7: Miniconference on the Plasma Physics of the Solar Wind: From Parker (1958) to the Present I |
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Chair: William Matthaeus, University of Delaware Room: Pegasus |
Wednesday, November 19, 2008 9:45AM - 10:15AM |
NM7.00001: The Last Half Century of Solar Wind Plasma Eugene Parker Over the last half century the solar wind has developed from the theoretical gross features of the expanding corona and heliosphere to recognition of a new world of plasma dynamics. It is curious, then, that along side this gratifying progress there is an ongoing and obdurate field of misunderstanding that flourishes around the periphery of the main stream. For instance, it is sometimes remarked, and seldom contradicted, that (a) hydrodynamics does not apply to the large-scale bulk motion of a collisionless plasma, (b) MHD applies to neither a collisionless plasma nor to a partially ionized gas, (c) the electric polarization field, \textbf{E=}-\textbf{v}x\textbf{B/}c, plays an active role in the dynamics, (d) an electric circuit analog can be constructed to represent an active MHD system, (e) laboratory plasmas driven by application of electric fields actually model some of the remarkable activity observed on the Sun. It is shown that Newton and Maxwell would have it otherwise. [Preview Abstract] |
Wednesday, November 19, 2008 10:15AM - 10:45AM |
NM7.00002: Observed Properties of the Solar Wind Marcia Neugebauer The earliest measurements of the solar wind fully supported Gene Parker's theory. The wind was persistent and nearly radial, its speed was hundreds of km/s, the density was as predicted, and, on average, the interplanetary magnetic field was consistent with an Archimedian spiral. The fastest wind, with speed $>700$ km/s, traced back to Bartel's unipolar M regions rather than to the hotter active regions, and the highest densities could be explained by compression where the fast wind plowed into the slower wind in its path. But, even in the early data, there were mysteries, some of which are not yet completely resolved. Understanding the alpha particles has been a challenge. Their abundance is highly variable, in the fast wind their temperature is generally $> 4$ times the proton temperature, and, despite their greater mass, they flow away from the Sun faster than the protons. To complicate the picture further, the protons, alphas, and electrons all have complex, anisotropic distribution functions, often with double peaks. The expanding wind cools more slowly than adiabatically, suggesting a zoo of wave-particle interactions probably responsible for marginal stabilities of the particle distributions. The study of interplanetary waves and turbulence is an active field of research. Recent decades have also seen the study of ions heavier than alphas, including particles in the wind that did not originate at the Sun. Fifty years after Parker's landmark paper, solar-wind physics is still an active area of research. [Preview Abstract] |
Wednesday, November 19, 2008 10:45AM - 11:15AM |
NM7.00003: Stationary spherically symmetric supersonic winds and accretion: from Parker to Bondi and back Marco Velli Although we have known the solar wind is supersonic for almost 50 years now, it is little known that the structure of the stationary spherically symmetric solar wind solutions found by Parker is fundamentally connected to the Bondi solutions for spherically symmetric accretion. In this talk I will describe how, for the simpler case of isothermal flows, changes in the relative pressure jump between the coronal base and distant medium produce changes in the resulting stationary flow. The pressure jump between coronal base and interstellar medium (ISM) functions as a control parameter in terms of which stationary flows display a hysteresis-type cycle with two catastrophy points: as the pressure of the ISM increases, the termination shock moves closer towards the coronal base, but when the shock position reaches the critical point, the flow collapses into supersonic accretion with a shock below the critical point. If the pressure of the ISM then decreases again, the flow can evolve continuously into subsonic breeze accretion, but the flow evolves back into a state characterized by a supersonic shocked wind, once the pressure difference corresponding to a static coronal stratification is exceeded. Numerical simulations are shown which confirm this scenario and illustrate the important role boundary conditions play in fluid flows around astrophysical objects. [Preview Abstract] |
Wednesday, November 19, 2008 11:15AM - 11:45AM |
NM7.00004: Velocity Filtration (VF), Coronae and Winds Jack Scudder The approach of VF for coronal winds is \textit{not} built on a presumption of an equation of state for the underlying coronal plasma; all moments are retained as VF addresses the classes of velocity space access of assumed non-thermal boundary distributions in the coherent forces of gravity, magnetic field, and electric field. The principal virtues of velocity filtration are: 1) Coronal inversion of to millions of degrees above 5000K chromosphere of scale height 180km - \textit{without ad hoc} wave damping or momentum addition; 2) Heating of coronal loops organized by altitude; temperature and density anti-correlated; 3) Sustained increase of temperature with height beyond the sonic point required to produce fast winds; 4) Recovers Parker's (1958) range of slopes of temperature profiles at the sonic point that make supersonic wind possible; 5) Predicts asymptotic wind speeds in terms of the suprathermal tail index at the inner boundary condition; 6) Parallel electric field at Parker's critical point is essentially the Dreicer limit, undercutting a Chapman-Enskog closure; 7) Minor ions are heated proportional to charge to mass ratio; 8) All stars with bound atmospheres on the ZAMS should have coronae and winds, thus accounting for their common occurrence; 9) Inhomogeneity, gravity and speed dependence of collisions are the essential seeds of VF, coronae and Parker winds; 10) VF is \textbf{f}=m\textbf{a} in the form of df/dt=0 with collisions as a correction. [Preview Abstract] |
Wednesday, November 19, 2008 11:45AM - 12:15PM |
NM7.00005: Stringent Tests of Instabilities and Heating: Microphysics of the Solar Wind in the Modern Era Justin Kasper Early observations of the solar wind established that the plasma is rich with complex distribution functions and electromagnetic fluctuations. Solar wind observed in the heliosphere possesses non-Maxwellian features such as ions with significantly different temperatures, anisotropies, and differential flow. These features are an energy source for instabilities and an artifact of heating by dissipation. The microphysics of the solar wind is the non-linear coupling between particles and fields in a medium that at times acts as a fluid, a collisionless plasma, and a collection of individual particles. The purpose of this talk is to review solar wind microphysics, from its role in the heating of the corona and expansion of the wind, through early analytic theory, to the modern era where large statistical analysis of precision distribution measurements are combined with advanced analysis and simulations to quantify the roles of heating and instabilities. In particular, I will track the development of our understanding of the mirror, cyclotron, and firehose instabilities driven by ion temperature anisotropies and the resonant heating of ions through Alfven-cyclotron dissipation. [Preview Abstract] |
Wednesday, November 19, 2008 12:15PM - 12:40PM |
NM7.00006: Fluctuations associated with anisotropy instabilities and dissipation in the solar wind Stuart Bale The proton temperature anisotropy in the solar wind is known to be constrained by the theoretical thresholds for several anisotropy-driven instabilities. Here we use approximately 1 million independent measurements of high frequency magnetic fluctuations in the solar wind to show that these fluctuations are enhanced along the temperature anisotropy thresholds of the mirror, proton firehose, and ion cyclotron instabilities. In addition, the measured magnetic compressibility is enhanced at high plasma beta ($\beta_\parallel \agt 1$) along the mirror instability threshold, consistent with expectations of the mirror mode. The power in this frequency (the `dissipation') range is often considered to be driven by the solar wind turbulent cascade, an interpretation which should be reconsidered in light of the present results. [Preview Abstract] |
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