UBIQUITOUS NON-THERMALS IN ASTROPHYSICAL PLASMAS: RESTATING THE DIFFICULTY OF MAINTAINING MAXWELLIANS

This paper outlines the rather narrow conditions on a radiatively decoupled plasma where a Maxwell-Boltzmann (MB) distribution can be assumed with confidence. The complementary non-thermal distribution with non-perturbative kurtosis is argued to have a much broader purview than has previously been a...

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Bibliographic Details
Published in:The Astrophysical journal 2013-06, Vol.770 (1), p.1-11
Main Authors: Scudder, J D, Karimabadi, H
Format: Article
Language:English
Subjects:
APPROXIMATIONS
ASTROPHYSICS
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
BOLTZMANN STATISTICS
Closures
Computational fluid dynamics
Electron pressure
ELECTRONS
Fluid flow
FLUIDS
HEAT FLUX
KNUDSEN FLOW
Kurtosis
MAGNETIC FIELDS
MEAN FREE PATH
PARTICLES
PLASMA
Plasmas
SIMULATION
SOLAR CORONA
SOLAR WIND
STARS
STATISTICS
Stellar atmospheres
VELOCITY
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Summary:This paper outlines the rather narrow conditions on a radiatively decoupled plasma where a Maxwell-Boltzmann (MB) distribution can be assumed with confidence. The complementary non-thermal distribution with non-perturbative kurtosis is argued to have a much broader purview than has previously been accepted. These conditions are expressed in terms of the electron Knudsen number, K sub(e), the ratio of the electron mean free path to the scale length of electron pressure. Rather generally, [functionof]([upsilon] < [upsilon] sub(2)(K sub(e) )) will be Gaussian, so that MB atomic or wave particle effects controlled by speeds [upsilon] < [upsilon] sub(2) = omega (15/8K sub(e)) super(1/4) will remain defensible, where w is the most probable speed. The sufficient condition for Spitzer-Braginskii plasma fluid closure at the energy equation requires globally K sub(e)(s) [< or =, slant] 0.01; this global condition pertains to the maximum value of K sub(e) along the arc length s of the magnetic field (to its extremities) provided that contiguous plasma remains uncoupled from the radiation field. The non-thermal regime K sub(e) > 0.01 is common in all main-sequence stellar atmospheres above approximately 0.05 stellar radii from the surface. The entire solar corona and wind are included in this regime where non-thermal distributions with kurtosis are shown to be ubiquitous, heat flux is not well modeled by Spitzer-Braginskii closure, and fluid modeling is qualitative at best.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/770/1/26