HR: 17:45h
AN: SM14A-08    [Abstracts]
TI: The High-Beta Interchange Mode in Magnetotaillike Configurations
AU: * Pritchett, P L
EM: pritchet@physics.ucla.edu
AF: Department of Physics and Astronomy, University of California, 405 Hilgard Avenue, Los Angeles, CA 90095-1547 United States
AU: Coroniti, F V
EM: coroniti@astro.ucla.edu
AF: Department of Physics and Astronomy, University of California, 405 Hilgard Avenue, Los Angeles, CA 90095-1547 United States
AB: In the near-Earth plasma sheet the pressure gradient and magnetic curvature are both directed earthward. This region is thus potentially unstable to the pressure-gradient ballooning mode provided that the plasma $\beta$ becomes large enough. Theoretical analysis of the critical $\beta$ for this mode has been inconclusive: the applicability of MHD (which gives a low critical $\beta$) to the near-Earth tail current sheet just prior to substorm onset is dubious, and kinetic analyses, which are exceedingly complex, have reached contradictory conclusions. Early particle-in-cell (PIC) simulations with a very low mass ratio $m_i/m_e = 16$ suggested that an alternative interchange mode, the high-$\beta$ mode, may indeed be more robust than the standard pressure gradient mode. The signature of this mode is that it occurs in regions of a tailward-directed gradient in the equatorial magnetic field; the mode is driven by the need to return magnetic flux to the dipole region. A massively-parallel 3D PIC code is used to study the properties of the high-$\beta$ mode for more realistic values of $m_i/m_e \sim 100$. The initial configuration consists of a 2D generalized Harris current sheet with a minimum $B_z$ region. The simulations are used to determine the minimal conditions required to excite the instability, to map out the unstable mode spectrum in $k_y$, and to establish the nonlinear properties of the magnetic field and plasma configurations.
DE: 7839 Nonlinear phenomena
DE: 7843 Numerical simulation studies
DE: 2744 Magnetotail
DE: 2764 Plasma sheet
SC: SPA-Magnetospheric Physics [SM]
MN: 2004 AGU Fall Meeting