The interplanetary and solar causes of geomagnetic activity

Abstract

We present a review of recent work done on the topic of interplanetary and solar causes of geomagnetic activity. During solar maximum (1978–1979), 90% of the major magnetic storms ( D ST ⩽ − 100 nT) are caused by large southward B z events associated with interplanetary shocks. Of these, roughly half of the B z events are located in the sheath and half associated with the driver gas. These two sources of southward IMFs often give magnetic storms a two-step profile. The sheath field events are generated in the interplanetary medium between the outer corona and the Earth from the “shocking” of the slow solar wind upstream of the high speed stream. In contrast, the driver gas events are fields which come from the solar source region. A correlation between the field orientation at the solar source and that at 1 a.u. was sought, but none was found. Thus, quantitative predictions of storm intensities from solar observations appear to be very difficult. Prominence eruptions are shown to be an important cause of the high speed solar wind streams that lead to magnetic storms. The other 10% of the magnetic storms arc not related to interplanetary shocks or high speed streams, but to high density “non-compressional density enhancements”. Following magnetic storms arc “high-intensity long-duration AE activity events” (HILDCAAs) that are series of continuous auroral substorms that last from days to weeks during or after the storm's recovery phase. HILDCAAs can also occur independently of magnetic storms. This continuous auroral activity is caused by the southward component of the magnetic field of interplanetary Alfvén waves, presumably through the process of magnetic reconnection with the Earth's field. These Alfvén wave trains arc often observed in the trailing portions of high speed streams. From an analysis of a year's data during solar maximum, it is found that the interplanetary medium is “Alfvénic” approx. 60% of the time. There appear to be no substantial differences in magnetusphcric response to Alfvénic or non-Alfvénic interplanetary intervals. The magnetopause boundary layer is shown to contain broad-band ELF/VLF plasma waves at least 85% of the time at all daysidc local times. These waves have sufficient amplitude to cause cross-field diffusion of magnetosheath plasma to form the low latitude boundary layer. Pitch angle scattering of the low latitude boundary layer particles is adequate to account for the dayside aurora. The only interplanetary /magnetosheath parameter that appears to affect the wave intensities is the IMF B z . Although the waves arc present at almost all times, they are intensified during southward IMF B z intervals.