Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsa31b..10k&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SA31B-10
Other
2409 Current Systems (2721), 2411 Electric Fields (2712), 2415 Equatorial Ionosphere, 2435 Ionospheric Disturbances, 2441 Ionospheric Storms (7949)
Scientific paper
The convection electric field penetrates to the equatorial ionosphere with no significant shielding effects during the DP2 fluctuation event of period of 30 - 60 min (Nishida, 1968) and during the storm main phase continuing over several hours (Huang et al., 2007). On the other hand, shielding becomes effective during the substorm growth phase (Somajajulu et al., 1987; Kikuchi et al., 2000) and even during storm main phase (Kikuchi et al., 2008). The well-developed shielding electric field results in an overshielding at the beginning of the recovery phase of storm/substorms (Kikuchi et al., 2003, 2008). Thus, the electric field manifests complex features at mid-equatorial latitudes, which is not determined only by the solar wind electric field but strongly controlled by magnetospheric processes such as the ring current. To reveal comparative roles of the convection and overshielding electric fields and in what condition the overshielding occurs at mid-equatorial latitudes, we analyzed the geomagnetic storm on 14-15 December, 2006, characterized by the quasi-periodic DP2 fluctuation of 30 min period at the beginning of the storm. We used magnetometer data from mid- equatorial latitudes to detect magnetic signatures due to the electric field originating in the magnetosphere, and used the SuperDARN data to identify electric fields associated with the solar wind dynamo (Region-1 FAC) and the ring current (R2 FAC). We further calculated an electric potential pattern caused by the R1 and R2 FACs with the comprehensive ring current model (CRCM) to better understand the SuperDARN convection pattern. First we show that the DP2 fluctuation was caused by alternating eastward (e-EJ) and westward currents (w-EJ) in the equatorial ionosphere, which were caused by the southward and northward IMF, respectively. We further show that the e-EJ was associated with the large-scale two-cell convection vortices, while the w-EJ accompanied a reverse flow equatorward of the two-cell vortices. With the aid of the CRCM ring current simulation, we show that the R2 FAC develops immediately after the growth of the R1 FAC, and produces a reversed electric potential at mid latitude when the R1 FAC decreases its intensity. Thus, the reversed convection on the SuperDARN convection map must be caused by an electric potential associated with the R2 FAC. As a conclusion, both the convection and overshielding electric fields appear at mid-equatorial latitudes, and the overshielding electric field could become predominant irrespective of the period of the disturbances, when the R1 FAC decreases its intensity. This scenario well explains both the continuous penetration over several hours during storm main phase and the overshielding at the beginning of storm recovery phase.
Ebihara Yasuhiro
Hashimoto Katsumi
Hori Toshihiro
Kataoka Ryuho
Kikuchi Tatsuru
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