Center for Space Physics, Boston University, Boston, Massachusetts

The largest density variations in the solar wind occur near the streamer belt, where frequently they are associated with interplanetary coronal mass ejections (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 profiles sometimes within ICMEs. On average, however, ICME densities are similar to ambient-wind densities. For a set of 34 ICMEs identified in Wind data as magnetic clouds, the average density was 11 cm^-3 in both the clouds and all slow wind during the same period. A set of low-density structures observed earlier by ISEE 3 recurred for three solar rotations, possibly owing 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.

Geophys. Res. Lett., Vol. 27, No. 23, 3769 - 3772, 2000