Observation of a double-onset substorm during northward interplanetary magnetic field

Abstract

For the first time, a substorm event with double onsets is shown observationally under northward IMF condition in this study. Magnetic field data from ground stations and from geosynchronous satellite, and aurora data from IMAGE satellite are examined to study the substorm activity. The results show that the intensity and the spatial extent of the event are as large as those of typical substorms. Another interesting finding is that two expansion onsets seem to occur during the event. A possible mechanism for the two onsets was proposed. The energy source for the event was also discussed.

Introduction

The supply of energy into the magnetosphere has been found to be strongly dependent on the direction of the interplanetary magnetic field (IMF), especially the north–south component, which has a direct influence on the occurrence of substorms (Kamide et al., 1977a). Energy build-up in the magnetotail prior to expansion onsets of substorms begins when enhanced magnetic merging is initiated on the dayside magnetopause during predominantly southward IMF (Dungey, 1961). It is thus usually the case that a substorm occurs following a long period of southward IMF.

However, it has been pointed out that substorms can occur even during period of northward IMF. Early in the 1970s, there existed reports of substorms under northward IMF conditions (Akasofu et al., 1973, Kamide et al., 1977a, Lui et al., 1976, Nishida, 1971). These events have been shown not to be different qualitatively from typical substorms, except that their intensity is weaker and their observed features are more localized in MLT, compared to normal substorms (Kamide, 1974, Kamide and Akasofu, 1974, Lui et al., 1976, Petrukovich et al., 2000).

Henderson et al. (1996) showed several examples of spontaneous substorms, which occurred without apparent triggers in the solar wind. They also showed one example (November 12, 1991) which occurred while the IMF was continually northward-directed. Recently, reports of such northward, weak substorms continued. Different from previous studies with regard to the intensity of the events, Lee et al. (2009) reported several substorms observed under northward IMF conditions, the intensity of which was at least as significant as that of typical substorms under moderately southward IMF condition. All these support the existence of substorms during northward IMF.

Meanwhile, the energy source for such events was discussed. Two main candidate mechanisms have been proposed. One likely candidate is the excess energy stored in the magnetosphere during southward IMF preceding the corresponding northward IMF period (Akasofu, 1975, Akasofu and Lepping, 1977). The other ascribes the energy of northward IMF substorms to the relative increase of energy input from the solar wind during northward IMF period, which could be caused by dayside reconnection by IMF by (Park et al., 2006, Senior et al., 2002), ULF waves (Kim et al., 2009; Lyons et al., 2009), or viscous-like process in the magnetosphere.

This paper presents one example of intense substorm event of December 13, 2004, which occurred during a period of steady northward IMF. The event showed well-defined magnetic, optical aurora, and depolarization signatures observed by the Canadian magnetometer array, the IMAGE satellite, and GOES 10 at geosynchronous orbit, respectively. What makes our event special is that two expansion onsets seemed to occur successively, with a time lag of less than 4 min.