Abstract
Abstract. Low solar wind density with long duration was measured by in situ observation between 11 and 12 May 1999. As a result of this low-density solar wind condition, the magnetosphere of the Earth expanded considerably. We used a database of one-hour-averaged solar wind (1963–1999) near 1 AU to determine whether or not the observed low-density event was extremely abnormal. As a result it was found that this event has the longest duration in approximately 36 years of solar wind observations. There are three events with density 0.5 cm-3 or less and duration ten hours or longer. They were observed on 4 and 31 July 1979, and 11–12 May 1999. The 4 July 1979 event recurred on 31 July 1979. The events were characterized by low-beta, low Alfven Mach number (MA ), and low dynamic pressure. The occurrence rate of low-density solar wind with density 0.5 cm-3 or less shows several peaks near solar maxima. However, it is difficult to find a clear relationship between the sunspot number and the occurrence rate.Key words. Interplanetary physics (flare and stream dynamics; solar wind plasma; sources of the solar wind)
Highlights
Long-duration low-density solar wind was observed between 11 and 12 May 1999
We studied historical low-density solar wind events using a large data set to better understand the event that occurred in May 1999
20% of the low-density events in Table 1 were associated with high-speed streams and approximately half of the events were associated with transient solar wind structures
Summary
Long-duration low-density solar wind was observed between 11 and 12 May 1999. The balance between the magnetic pressure of the geomagnetic field and the dynamic pressure of the solar winds determines the size of the Earth’s magnetosphere (Fairfield, 1971; Sibeck et al, 1991). The low-density solar winds cause a large expansion of the magnetosphere and the large outward excursion of the Earth’s bow shock (Fairfield, 1971; Cairns et al, 1995; Takeuchi et al, 1998). The solar wind conditions near 1 AU have been measured since the early 1960s. This kind of long-duration low-density solar wind has not been fully studied. We studied historical low-density solar wind events using a large data set to better understand the event that occurred in May 1999
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