HR: 08:45h
AN: SM31E-04    [Abstracts]
TI: Self-excited unstable behavior of the ring current in the vicinity of the geosynchronous orbit
AU: * Ebihara, Y
EM: ebihara@stelab.nagoya-u.ac.jp
AF: Institute for Advanced Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464- 8601, Japan
AU: Fok, M
EM: mei-ching.fok@gsfc.nasa.gov
AF: NASA GSFC, Code 673, Greenbelt, MD 20771, United States
AB: It has been suggested that the outer ring current is unstable to the interchange-like instability when the earthward pressure gradient is steep enough. Such a condition can be achieved from a sudden decrease in the plasma sheet density. Here we show from computational results that the interchange-like instability can occur without changing the plasma sheet density. We solved a bounce-averaged drift kinetic equation with the electric field calculated based on conservation of electric currents flowing between the magnetosphere and the ionosphere. The magnetic field in the inner magnetosphere was calculated based on requirement of the force balance between the plasma pressure and the magnetic field by using the Biot-Savart law. At the beginning of the simulation, the convection electric field was enhanced to the condition corresponding to IMF Bz=-20 nT, and kept constant in time throughout the simulation. In the early stage of the simulation, the equatorial magnetic field became highly inflated around L=5. The perpendicular plasma pressure decreases due to adiabatic cooling. The anisotropy of the plasma pressure enlarged the westward current, which further enhances in the inflation of the equatorial magnetic field. Consequently, the earthward pressure gradient became steep, and the interchange- like instability occurred spontaneously. This self-excited instability vanished when the inner edge of the plasma sheet moves to the inner region where the magnetic pressure is high in comparison with the plasma pressure. Our results imply that, in some cases, the development of the ring current does not proceed smoothly in its early stage even though the plasma sheet parameters are constant in time.
DE: 2736 Magnetosphere/ionosphere interactions (2431)
DE: 2753 Numerical modeling
DE: 2760 Plasma convection (2463)
DE: 2778 Ring current
DE: 2788 Magnetic storms and substorms (7954)
SC: SPA-Magnetospheric Physics [SM]
MN: 2007 Fall Meeting