Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsa41b1731t&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SA41B-1731
Physics
[2409] Ionosphere / Current Systems, [2411] Ionosphere / Electric Fields, [2431] Ionosphere / Ionosphere/Magnetosphere Interactions, [2788] Magnetospheric Physics / Magnetic Storms And Substorms
Scientific paper
During the strong southward interplanetary magnetic field (IMF), the convection electric field originating from the region-1 field-aligned currents (R1 FACs) causes a two-cell ionospheric Hall current at high latitudes. The convection electric field penetrates to the magnetic equator, and drives the eastward equatorial electrojet (EEJ). Subsequently, when the southward IMF weakens, the reversed ionospheric current in the equatorial ionosphere, called equatorial counter electrojet (CEJ), is driven by the developed shielding electric field originating from the region-2 field-aligned currents (R2 FACs). Its state is called ‘overshielding’ because the shielding electric field overcomes the reduced convection field in lower latitudes of the R2 FACs [e.g., Kelley et al., 1979]. However, the temporal and spatial relationship of the global ionospheric current system has not been established during the storm-time overshielding yet. In this paper, we investigated magnetic field variations in high-low latitudes in the 21 CEJ events (overshielding events) during storms occurred during a period from 2001 to 2002. In high-middle latitudes during the period, the developed auroral electrojet (AEJ) moved poleward with ~3-8 degrees in magnetic latitude, maintaining the strength. This indicates a contraction of the auroral oval. Subsequently, the strength rapidly decreased without such a poleward shift. This result supports that the overshielding electric field is strengthened by the poleward shift of the R1 FACs [Kikuchi et al., 2008]. Moreover, in the 8 of 21 CEJ events, the magnetic field variation in lower latitudes than the AEJ region changed the direction with magnitudes one order smaller than the AEJ. This variation was generated by the ionospheric Hall currents associated with the overshielding electric field. We also found that the observed life time of CEJ, about 15-470 min, was either equal to or up to 3.8 times longer than that of the overshielding current at the middle latitudes (about 15-200 min). This fact implies that the CEJ at the equator is driven by the ionospheric disturbance dynamo with the life time of several to dozen hours [Blanc and Richmond, 1980] as well as the overshielding electric field with that of several tens of minute [e.g., Peymirat et al., 2000]. Thus, it is possible that we identify global ionospheric current associated with the overshielding and derive the life time of the disturbance dynamo.
Kikuchi Tatsuru
Nagatsuma Tadao
Nishimura Yasuhiro
Shinbori Atsuki
Tsuji Yoshinobu
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