Solar wind‐magnetosphere coupling during intense magnetic storms (1978‐1979)

Abstract

The solar wind‐magnetosphere coupling problem is investigated for the ten intense magnetic storms (Dst<−100 nT) that occurred during the 500 days (August 16, 1978 to December 28, 1979) studied by Gonzalez and Tsurutani [1987]. This investigation concentrates on the ring current energization in terms of solar wind parameters, in order to explain the |−Dst| growth observed during these storms. Thus several coupling functions are tested as energy input and several sets of the ring current decay time‐constant τ are searched to find best correlations with the Dst response. From the fairly large correlation coefficients found in this study, there is strong evidence that large scale magnetopause reconnection operates during such intense storm events and that the solar wind ram pressure plays an important role in the ring current energization. Thus a ram pressure correction factor is suggested for expressions concerning the reconnection power during time intervals with large ram pressure variations. The best set of values found from the present study is in accord with recent similar and independent suggestions. With respect to the ring current energy injection rates during intense storm events, typical values of 150±50 nT/hour are obtained. These are considerably larger than values extrapolated from previous studies restricted to moderate storms. Such rates of energy injection are observed to get transmitted from the magnetopause to the inner magnetosphere with an average time delay of about 1 hour, although this delay can become shorter as the storm events get more intense. It is also found that AE does not respond as well as Dst to the coupling functions, except for the events that have Dst values only >−100 nT (less intense energization). This point suggests that there is a decoupling between the ring current and auroral processes during very intense storms with respect to their dependence on solar wind energization. Finally, a discussion is presented on associations of the coupling functions with the solar wind features described by Tsurutani et al. [1988] for the causes of these storm events.