Substorms under northward IMF conditions and their implications on the question of energy availability in the tail

A southward IMF condition is generally considered as the most natural precondition for a substorm to occur. The condition is preferred for effective energy entry from the solar wind into the magnetosphere that is to be released via substorm occurrence. But it has been previously reported by some researchers that substorms sometimes occur under northward IMF conditions as well. Based on the IMAGE WIC aurora observations and the solar wind data from ACE and Geotail, we found that the occurrence of substorms under preceding northward IMF conditions is indeed not uncommon. In this paper, we present details of several such substorms that occurred in 2000 and 2001. In selecting these events, we have imposed a distinguishing condition that an identified substorm is preceded by another substorm, both being under northward IMF conditions, so that the later one is indeed a northward IMF substorm in the sense that it is not affected directly by any earlier southward IMF condition that can easily supply solar wind energy. Surprisingly, the substorms identified this way are major substorms, i.e., those with clear auroral breakup followed by substantial auroral expansion. For some of the events, we find that they occurred during the recovery phase of a magnetic storm (the IMF turned and remained northward). This implies the possibility that the availability of the magnetospheric energy for the northward IMF substorms of this kind is related to the preceding magnetic storm. For other northward IMF substorms, however, they indicated no obvious association with preceding magnetic storm activity. We use various data sets and models to address the fundamental question, i.e., what process can effectively transfer the solar wind energy into the magnetosphere under northward IMF conditions to lead to a major substorm, and also the question whether the tail is left with sufficient energy for a substorm occurrence even after major energy release by an earlier substorm occurrence.