Articles | Volume 19, issue 1
https://doi.org/10.5194/angeo-19-17-2001
https://doi.org/10.5194/angeo-19-17-2001
31 Jan 2001
 | 31 Jan 2001

Statistical analysis of long-duration low-density solar wind events

S. Watari, T. Watanabe, and K. Marubashi

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)

Download