Can intense substorms occur under northward IMF conditions?

Abstract

This paper reports on several substorms observed under northward interplanetary magnetic field (IMF) conditions, the intensity of which was at least as significant as that of typical substorms under moderately southward IMF conditions. Such northward IMF periods were identified during the recovery phase of three strong storms. In the case of each storm, two or more substorms occurred successively, being separated by ∼1.8–5 h, while the IMF condition continued persistently northward. The substorms are clearly evidenced by auroral and other complementary observations. For the most intense substorms, the auroral breakup occurred at the magnetic latitude of ∼58°, and for the others it was between 60° and 65°. The polar cap size prior to each onset was substantial despite the northward IMF conditions. The auroral expansion following each onset lasted from a few up to several magnetic local time hours and exhibited a clear poleward expansion feature. For most of the events studied, geosynchronous magnetic dipolarizations preceded by field stretching and/or energetic particle injections occurred. The occurrence of such (intense) substorms implies that a certain (large) amount of energy remains in the tail even under northward IMF conditions. The occurrence of two or more successive substorms further implies that even after the release of a certain amount of energy triggered by the substorm, the tail can still have a substantial amount of energy left, which can be released by a subsequent substorm(s). We conjecture that an intense substorm during a northward IMF period can be expected when such a period belongs to the recovery phase of an intense storm mainly because of large energy loading done by preceding southward IMF Bz during the storm's main (and some early recovery) phase. In addition we argue that substorm energy can also be supplied by other mechanisms of the solar wind–magnetosphere coupling under northward IMF conditions such as dayside reconnection in the presence of a substantial IMF By component.