Solar-cycle variation of low density solar wind during more than three solar cycles

The May, 1999 low density (< 1 cm(-3)) solar wind interval is one of a seri es of intervals of low density solar wind which have been detected since in -situ, near-Earth observations began. Examining the NSSDC OMNI database sin ce 1965, covering solar Cycles 20-23, we show that such intervals, which ar e also periods of unusually low mass flux and low dynamic pressure, occur m ost frequently around sunspot maximum and are rarer at solar minimum. The o ccurrence rate of low-density plasma may be higher in weaker sunspot cycles (Cycle 20 and the current Cycle 23). Around two-thirds of periods with den sities less than or equal to 1 cm(-3) are associated with transient solar w ind structures, in particular with ejecta and post-shock flows. The majorit y of other events are associated with corotating streams. The May 1999 even t is unusual because it is not associated with an ejecta or stream. A simil ar period was observed in July August 1979.