The characteristics of solar wind magnetic field during the negative-AU and large-AU (>1200nT) events

Abstract:

The negative-AU events are relatively unusual, which have caught our attention. To understand the cause of negative AU, we first eliminate the ring-current effect by considering only the events with AU<SYM-H. Most of the events are associated with an arrival of a magnetic cloud. The interplanetary magnetic field (IMF) at 1 AU has strong negative Bz and By components during these negative-AU events. We also found that one of the negative AU events has been reported by Feldstein et al. (2006). They associated the enhancement of westward electric jet by the negative IMF-By component based on previous models obtained independently by Friis-Christensen et al. (1972) and Sumaruk & Feldstein (1973). Enhancement of electric jet in opposite direction is expected to be found when the IMF-By is positive. To verify their models we also examine the strong AU events with AU > 1200nT. We found that these large-AU events are associated with IMF-Bz<0 and IMF-By >0. Both negative-AU and large-AU events tend to occur during the beginning of the main phase of a strong magnetic storm with Kp= 7~9. The enhancement of Cowling electrojet has been proposed by Kan et al. (2011) for the triggering of substorm onset. We will discuss the possibility that a similar enhancement process might take place in the dayside auroral oval during these extreme AU events.