Density extremes in the solar wind

The largest density variations in the solar wind occur near the streamer be lt, where frequently they are associated with interplanetary coronal mass e jections (ICMEs). They tend to be embedded in larger-scale regions of high pressure: high densities in ICME sheaths and corotating interaction regions and low densities in structures with distinctive, high-magnetic-pressure p rofiles, sometimes within ICMEs. On average, however, ICME densities are si milar to ambient-wind densities. I;or a set of 34 ICMEs identified in Wind data as magnetic clouds, the average density was 11 cm(-3) in both the clou ds and all slow wind during the same period. A set of low-density structure s observed earlier by ISEE 3 recurred for three solar rotations, possibly o wing to recurrence of the streamer belt itself, with its frequent transient outflows. Density averages less than 1 cm(-3) show a possible solar cycle variation which peaks 1-2 years prior to the peak of ICME signatures.