Can intense substorms occur under northward IMF conditions?

Abstract

[1] 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 B z 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 B y component.

X lines in the magnetotail for southward and northward IMF conditions

“…It appears that the X line during quiet time is attributed to the northward IMF condition. Substorms can occur for southward as well as northward IMF [Akasofu et al, 1973;Lui et al, 1976;Lee et al, 2010]. However, substorms are much weaker for northward than southward IMF conditions [Petrukovich et al, 2000;Kullen and Karlsson, 2004;Pulkkinen et al, 2007;Peng et al, 2013].…”

Investigations of the auroral luminosity distribution and the dynamics of discrete auroral forms in a historical retrospective

“…However, substorms are at times also observed during quiet periods with positive B IMF z . Lee et al (2010) report on several substorms observed under positive B IMF z conditions, the intensity of which was at least as significant as that of typical substorms under moderate negative B IMF z . These substorms occurred during the recovery phase of intense magnetic storms.…”

Studies of ionospheric F-region response in the Latin American sector during the geomagnetic storm of 21–22 January 2005

“…Delays normally have never been longer than one or at most two hours when the Dst index starts to decrease. Lee et al (2010) in their recent paper entitled "Can intense substorms occur under northward IMF conditions" mention that the occurrence of such (intense) substorms implies that a certain (large) amount of energy remains in the tail even under northward IMF conditions. Crowley et al (2010) also showed that large amounts of energy can be deposited into the high latitude regions during B z northward or strong B y conditions, driving large perturbations in the neutral density at times when they would not previously have been expected.…”

“…Du et al (2008) have investigated the storm on 21-22 January 2005 and called it an "anomalous" geomagnetic storm as the storm main phase occurs during northward interplanetary magnetic fields (IMFs). As described by Lee et al (2010), a southward IMF condition is generally considered to be the most fundamental precondition for a storm or substorm to occur. The Dst index typically starts to decrease about an hour after the IMF turns southward.…”