Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm43a1710n&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SM43A-1710
Physics
2721 Field-Aligned Currents And Current Systems (2409), 2753 Numerical Modeling, 2790 Substorms
Scientific paper
The substorm mechanism is investigated by analyzing the numerical results obtained from the recent more refined magnetohydrodynamic (MHD) simulation. Present simulation results reproduce the observed signatures of substorm onset including the dipolarization, the geosynchronous D deflection, formation of the near earth neutral line (NENL) and earthward flow in the plasma sheet together with the development of the nightside field aligned currents (FACs) and electrojets in the ionosphere. The sudden onset is promoted in the magnetosphere by the abrupt formation of high-pressure region in the inner magnetosphere. This high- pressure region is generated through the feeding of internal energy by the magnetic tension release from the NENL, associated with sudden movement of relatively high-pressure region gradually established around x=- 10 Re during the growth phase to the geosynchronous altitude. Such rapid movement can be realized in the magnetosphere, because magnetospheric motion can be decoupled from ionospheric convection if it is a short time. The position of resulting high-pressure region is more inside from the braking point of flow from the NENL. Primary driver of the substorm current system is the partial ring current and incidental region 2 FAC which is driven through the conversion of internal energy to electromagnetic energy by convection crossing the high-pressure region formed at the geosynchronous altitude. Associated nightside region 1 FAC is not from the traditional current wedge but from the usual cusp dynamo. The region 1 FAC path to the ionosphere is altered from dayside to nightside so as to form a short circuit in the ionosphere with the region 2 FAC from the partial ring current. In the expansion phase, convection proceeds to relax the distorted pressure distribution formed at the onset, accompanying the development in dipolarization and plasma sheet thickening in the magnetosphere and increase of westward and eastward electrojets in the ionosphere. The present model can explain the longstanding question concerning the position of first brightening arc, together with the explosive growth phase and the thinning of mid-tail plasma sheet just after the onset.
Fujita Shigeo
Hashimoto Katsumi
Kikuchi Tatsuru
Nakamizo Aoi
Shimazu Hironori
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