|
|
 |
|
|
Ann. Geophys., 19, 17-23, 2001
www.ann-geophys.net/19/17/2001/
© European Geosciences Union 2001
Statistical analysis of long-duration low-density solar wind events
S. Watari1, T. Watanabe2, and K. Marubashi1
1Communications Research Laboratory, 4–2–1 Nukuikita, Koganei, Tokyo 184–8795, Japan
2Ibaraki University, 2–1–1 Bunkyo, Mito, Ibaraki 310–8512, Japan
Correspondence to: S. Watari (watari@crl.go.jp)
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)
Full Article in PDF (11509 KB) |
|
|
