Physics – Plasma Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm51c1830l&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM51C-1830
Physics
Plasma Physics
[7526] Solar Physics, Astrophysics, And Astronomy / Magnetic Reconnection, [7835] Space Plasma Physics / Magnetic Reconnection, [7839] Space Plasma Physics / Nonlinear Phenomena, [7851] Space Plasma Physics / Shock Waves
Scientific paper
Ion heating by Petschek-reconnection-associated slow shocks is one of the potential heating mechanisms for solar flares and the solar wind (e.g., Tsuneta 1995). But, the structure of these shocks in a collisionless plasma is still an open question. Therefore a detailed study of the kinetic structure of the reconnection exhaust and how ions/electrons are accelerated is necessary. Instead of performing a complete particle-in-cell reconnection simulation, whose simulation domain is strictly limited by the available computational power, a 2-D Riemann problem is designed to study the development and dynamics of the exhaust boundary. Simulations are carried out for varying ratios of normal magnetic field to upstream magnetic field (i.e., propagation angle with respect to the upstream magnetic field). Collisionless slow shocks form and we find a critical temperature anisotropy (i.e., ɛ=1-(P∥-P⊥)/μ0B2))= 0.25 around the sharp front of the downstream rotational waves at oblique propagation angles. An explanation is proposed by looking into anisotropic fluid theory, in particularly the anisotropic Derivative Nonlinear-Schrodinger-Burgers equation, with an intuitive model of an energy closure for an oblique shock with a fair amount of back-streaming ions that escape from the shock downstream region. The anisotropy value of 0.25 (independent of plasma beta and propagation angle) is significant because it is the degeneracy point of slow and intermediate modes, and the marginal stability value of modulational waves. At very oblique propagating angles (i.e.,~80 degree), a firehose-like instability develops in the downstream region, which might link to the proton temperature anisotropy distribution in the solar wind (e.g., S. D. Bale et. al 2009). The temperature anisotropy distributions for slow shocks pairs at 3 different propagation angles. A clear tendency for the downstream anisotropy values to be locked in 0.25 is seen for all cases. The case of angle 83 degree corresponds to the exhaust of fast reconnection (i.e., reconnection rate ~ 0.1), and its center is in the firehose unstable region (i.e., ɛ < 0 ).
Drake James F.
Liu Ya-Ying
Swisdak Michael
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