HR: 1340h
AN: SA13A-1075    [Abstracts]
TI: Seasonal dependence of SC amplitude on magnetic local time
AU: * Shinbori, A
EM: shinbori@stelab.nagoya-u.ac.jp
AF: Solar-Terrestrial Environment Laboratory, Nagoya University, Furoucho, Chikusa-ku, Nagoya, 464-8601, Japan
AU: Kikuchi, T
EM: kikuchi@stelab.nagoya-u.ac.jp
AF: Solar-Terrestrial Environment Laboratory, Nagoya University, Furoucho, Chikusa-ku, Nagoya, 464-8601, Japan
AU: Araki, T
EM: arakit@obsc.mbox.media.kyoto-u.ac.jp
AF: Polar Research Institute of China, 451 Jinqiao Road, Pudong, Shanghai, 200136, China
AB: In order to clarify a seasonal dependence of sudden commencement (SC) amplitude on magnetic local time in the low-latitude region, we analyzed geomagnetic field data obtained at Kakioka Magnetic Observatory for 12138 SC events which were identified in term of the SYM-H index and SC list provided from NGDC within a period from January 1976 to June 2007. In this analysis, we defined these SC events as a rapid increase of the SYM-H value with more than 5 nT within ten minutes. Moreover, we also used solar wind data obtained from the IMP-8, Geotail, Wind and ACE satellites within the data analysis periods from 1976 to 2007, from 1993 to 2007, from 1994 to 2007 and from 1998 to 2007, respectively. The SC events including the solar wind data were 8343 events. The diurnal variation of the normalized SC amplitude by the SYM-H value showed a remarkable dependence on magnetic local time, which indicates that the two peak values appear in the dayside sector of 11-14 h (MLT) and in the midnight sector of 22-02 h (MLT), respectively. The diurnal variation curve also showed the two minimum values in the morning sector of 5-7 h (MLT) and in the evening sector of 16-18 h (MLT), respectively, with a clear morning-evening asymmetry in their values. This trend indicates the magnetic field variations created by the dayside magnetopause, region-1 field-aligned currents (FAC) and the associated ionospheric Hall and Pedersen currents. Especially, the enhancement of the nightside SC amplitude can be considered as the FAC effect due to a weak intensity of the ionospheric current. Moreover, in this study, it is shown that the diurnal variation of SC amplitude in the low-latitude region has a seasonal dependence on MLT, which represents that the amplitude of the diurnal variation both the daytime and nighttime sectors tends to be larger in summer than in winter. This seasonal variation implies that the intensities of the FAC and reactant ionospheric current may be stronger in summer due to the enhancement of the ionospheric conductivity. Therefore, from the above result, it can be concluded that the dynamo of the FAC during SCs is a voltage generator rather than a current generator. In future, we need to investigate the variations of SC amplitude for solar activity and the direction of IMF around the solar wind shock and discontinuity.
DE: 2721 Field-aligned currents and current systems (2409)
DE: 2736 Magnetosphere/ionosphere interactions (2431)
DE: 2760 Plasma convection (2463)
DE: 2784 Solar wind/magnetosphere interactions
DE: 2788 Magnetic storms and substorms (7954)
SC: SPA-Aeronomy [SA]
MN: 2007 Fall Meeting