Can intense substorms occur under northward IMF conditions

摘要

[1] This paper reports on several substorms observed under northward interplanetarymagnetic field (IMF) conditions, the intensity of which was at least as significant as that oftypical substorms under moderately southward IMF conditions. Such northward IMFperiods were identified during the recovery phase of three strong storms. In the case ofeach storm, two or more substorms occurred successively, being separated by —1.8-5 h,while the IMF condition continued persistently northward. The substorms are clearlyevidenced by auroral and other complementary observations. For the most intensesubstorms, the auroral breakup occurred at the magnetic latitude of —58°, and for theothers it was between 60° and 65°. The polar cap size prior to each onset was substantialdespite the northward IMF conditions. The auroral expansion following each onsetlasted from a few up to several magnetic local time hours and exhibited a clear polewardexpansion feature. For most of the events studied, geosynchronous magnetic dipolarizationspreceded by field stretching and/or energetic particle injections occurred. The occurrenceof such (intense) substorms implies that a certain (large) amount of energy remains in thetail even under northward IMF conditions. The occurrence of two or more successivesubstorms further implies that even after the release of a certain amount of energy triggeredby the substorm, the tail can still have a substantial amount of energy left, which can bereleased by a subsequent substorm(s). We conjecture that an intense substorm during anorthward IMF period can be expected when such a period belongs to the recovery phase ofan intense storm mainly because of large energy loading done by preceding southward IMFB, during the storm's main (and some early recovery) phase. In addition we argue thatsubstorm energy can also be supplied by other mechanisms of the solar wind—magnetospherecoupling under northward IMF conditions such as dayside reconnection in the presence of asubstantial IMF By component.