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 (M_A ), 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)