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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 19, issue 1
Ann. Geophys., 19, 17–23, 2001
https://doi.org/10.5194/angeo-19-17-2001
© Author(s) 2001. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 19, 17–23, 2001
https://doi.org/10.5194/angeo-19-17-2001
© Author(s) 2001. This work is distributed under
the Creative Commons Attribution 3.0 License.

  31 Jan 2001

31 Jan 2001

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

S. Watari1, T. Watanabe2, and K. Marubashi1 S. Watari et al.
  • 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)

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