Browsing by Author "Kikuchi, T."
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Item Restricted Equatorial counterelectrojets during substorms(American Geophysical Union, 2003-11-22) Kikuchi, T.; Hashimoto, K. K.; Kitamura, T.‐I.; Tachihara, H.; Fejer, B.Equatorial counterelectrojet (CEJ) events are analyzed in association with changes in the interplanetary magnetic field (IMF), polar cap potential (PCP), and electric field measured in the equatorial ionosphere. In one event on 16 July 1995, the equatorial CEJ was observed at the afternoon dip equator during the recovery phase of the substorm when the IMF turned northward. Rapid decreases in the PCP and in the auroral electrojet occurred simultaneously with the equatorial CEJ, suggesting instantaneous equatorward penetration of the rapid decrease in the electric field associated with the region 1 field‐aligned currents (R1 FACs) under the condition of a well‐developed shielding electric field due to the R2 FACs. In the other event on 8 April 1993, the equatorial CEJ associated with the northward turning of the IMF was directly related to a rapid decrease in the equatorial electric field measured by the Jicamarca incoherent scatter radar as well as to a decrease in the PCP. We confirm the scenario for the substorm‐associated equatorial CEJ as caused by the dominant R2 FACs when the R1 FACs decrease abruptly because of the northward turning of the IMF. We also suggest that the DP 1 current system is composed of the Hall currents surrounding the R2 FACs and the equatorial CEJ closing with the R2 FACs, which are superposed on the DP 2 currents caused by the R1 FACs, being dominant when the IMF turns northward. The coherent occurrence of the electric field in the F region with the electric current in the E region at the equator is explained by applying the Earth‐ionosphere waveguide model of Kikuchi and Araki [1979b] as a most promising transmission mechanism. All the conditions for the equatorial CEJ most likely occur during the substorm, but the northward turning of the IMF and the resultant decrease in the PCP play a crucial role under the condition of well‐developed R2 FACs.Item Open Access Equatorial ionospheric electric fields during the November 2004 magnetic storm(American Geophysical Union, 2007-10) Fejer, B. G.; Jensen, J. W.; Kikuchi, T.; Abdu, M. A.; Chau Chong Shing, Jorge LuisWe use radar measurements from the Jicamarca Radio Observatory, magnetometer observations from the Pacific sector and ionosonde data from Brazil to study equatorial ionospheric electric fields during the November 2004 geomagnetic storm. Our data show very large eastward and westward daytime electrojet current perturbations with lifetimes of about an hour (indicative of undershielding and overshielding prompt penetration electric fields) in the Pacific equatorial region during the November 7 main phase of the storm, when the southward IMF, the solar wind and reconnection electric fields, and the polar cap potential drops had very large and nearly steady values. This result is inconsistent with the recent suggestion that solar wind electric fields penetrate without attenuation into the equatorial ionosphere for several hours during storm main phase. The largest daytime prompt penetration electric fields (about 3 mV/m) ever observed over Jicamarca occurred during the November 9 storm main phase, when large equatorial electrojet current and drift perturbations were also present in the Pacific and Brazilian equatorial regions. The rise and decay times of these equatorial electric fields were about 20 min longer than of the corresponding solar wind electric fields. The ratios of prompt penetration electric fields and corresponding solar wind electric field changes were highly variable even during the day, and had largest values near dawn. Also, the prompt penetration electric fields did not show polar cap potential drop saturation effects. Our results clearly highlight that the relationships of prompt penetration and solar wind electric fields, and polar cap potentials are far more complex than implied by simple proportionality factors.