Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsm32a1141c&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #SM32A-1141
Physics
2708 Current Systems (2409), 2764 Plasma Sheet, 2788 Storms And Substorms, 7827 Kinetic And Mhd Theory
Scientific paper
Our 3D quasi-static solutions of magnetosphere show that as the plasma pressure gradient increases in the near-Earth plasma sheet region during the substorm growth phase, a current sheet with enhanced cross-tail current density is formed around the local midnight in a finite radial (7 - 10 RE) and azimuthal domain. In the ionosphere the Birkeland current moves equatorward, the current density is enhanced, and its latitudinal width shrinks. This is consistent with the observed ionospheric growth phase signatures that the polar cap region expands equatorward, both the electron and proton aurora emissions move equatorward and brighten, and the auroral oval shrinks in width. In the current sheet the plasma beta becomes about 50 or larger and the magnetic field curvature is enhanced, and we find that the kinetic ballooning instability (KBI) can be excited with amplitude localized at the maximum plasma beta region. The KBI explains the low frequency (in Pi 2 frequency range and about 1 min period) instability observed by AMPTE/CCE excited at about 2-3 minutes before the substorm onset. The KBI is responsible for substorm onset because as it grows to a large amplitude with δ B/B >= 0.2 - 0.5, it causes enhanced ion drift in the westward or eastward direction. During the explosive growth phase that lasts about 30 sec before onset, the enhanced ion drift is in the westward direction. The eastward-westward oscillating ion drift caused by KBI then excites higher frequency instabilities in Pi 1 frequency range during the expansion phase, and the plasma and magnetic field become strongly turbulent. The plasma transport in both radial and azimuthal direction caused by the turbulence relaxes the steep plasma pressure profile during the expansion phase. As the plasma pressure profile relaxes in the plasma sheet, the magnetic field configuration dipolarizes and eventually returns to the pre-substorm more dipole-like geometry. Theories of growth phase magnetospheric configuration, KBI mechanism and dipolarization will be presented.
Cheng C. Z.
Zaharia Sorin
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