• Journal of Astronomy and Space Sciences
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• v.26 no.4
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• pp.451-466
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• 2009

While substorms are known to generally occur under southward IMF conditions, they can sometimes occur even under northward IMF conditions. In this paper, we studied the substorms that occurred in May, 2000 to 2002 to examine some statistical characteristics of the IMF and solar wind associated with northward IMF substorms. We focused on the cases where two or more substorms occurred successively under northward IMF conditions. Also, by checking Sym-H index associated with each of the substorms we determined whether or not there is any association of such northward IMF substorm occurrence with storm times. We also examined statistical properties at geosynchronous altitude in terms of magnetic field dipolarization and energetic particle injection. The following results were obtained. (i) Most of the northward IMF substorms occurred under average solar wind conditions. The majority of them occurred within 2 hrs duration of northward IMF Bz state, but there are also a nonnegligible number of substorms that occurred after a longer duraiton of northward IMF Bz state. (ii) While most of the substorms occurred as isolated from a magnetic storm time, those that occurred in a magnetic storm time show a higher average value of IMF and solar wind than that for the isolated substorms. (iii) About 55% of the substorms were associated with the IMF clock angle that can possibly allow dayside reconnection, and the other 45% were associated with more or less pure northward IMF conditions. Therefore, for the latter cases, the energy input from the solar wind into the magnetosphere should be made by other way than the dayside reconnection. (iv) For most of the substorms, the magnetic field dipolarizations and energetic particle injections at geosynchronous altitude were identified to be generally weak. But, several events indicated strong magnetic field dipolarizations and energetic particle injections.