Physics – Plasma Physics
Scientific paper
Feb 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004jgra..10902211m&link_type=abstract
Journal of Geophysical Research, Volume 109, Issue A2, CiteID A02211
Physics
Plasma Physics
6
Magnetospheric Physics: Mhd Waves And Instabilities, Space Plasma Physics: Kinetic And Mhd Theory, Magnetospheric Physics: Magnetospheric Configuration And Dynamics, Magnetospheric Physics: Magnetotail, Magnetospheric Physics: Storms And Substorms
Scientific paper
For a realistic, highly stretched, two-dimensional tail configuration, in which the pressure gradient force is balanced with the curved field line tension force at the equator, the growth rates and the real frequencies of the ideal magnetohydrodynamic (MHD) and two component fluid (nonideal MHD) ballooning modes, in which the phrase ``two component fluid'' means that the Hall and the electron pressure gradient terms are included in the generalized Ohm's law, the ion bounce frequency ωbi, the ion magnetic drift frequency ωdi, the ion diamagnetic drift frequency ω*i, and the ion cyclotron frequency ωci are calculated numerically at the equator as a function of X from the near-Earth tail (X = -15 RE) to the midtail (X = -30 RE). Contrary to the well-known dipole field case, in which the bounce frequency decreases with increasing |X|, the ion bounce frequency increases with |X| for the tail configuration. The ion magnetic drift frequency dominated by the curvature drift frequency also increases with increasing |X|. The exact growth rates of the ideal and nonideal ballooning modes, γe and γne, which are nearly equal, are given by 1.22VA/Rc, where VA is the Alfvén velocity and Rc is the field line curvature radius at the equator, and satisfy ωbi, ωdi, ω*i < γe < ωci on average in the near-Earth tail at X ~ -15 RE. Also, the ion motion remains adiabatic in the near-Earth tail at X ~ -15 RE. Therefore it is a posteriori verified that the fluid or MHD description of the linear stability of the ballooning instability is valid, and the critical β and the Alfvén time scale τA ~ Rc/VA of the ballooning instability onset obtained by the fluid theory are validated in the near-Earth tail as close as 15 RE from the Earth. Despite the plasma being collisionless and high-β in the near-Earth tail, the subtle collisionless kinetic effects due to the field line curvature in high-β collisionless plasma are not significant enough to invalidate the fluid description in the near-Earth tail. The Alfvén time scale of an e-folding growth of the Alfvén wave trapped within Rc in the equatorial region is of the order of a few tens of seconds or less in the near-Earth tail. It is faster than the bounce time of the bulk of ions and can explain the observed rapid time scale of a substorm onset. There is excellent agreement between the critical β and the Alfvén time scale obtained analytically for the two component fluid plasma and those obtained by a three-dimensional particle simulation.
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