Low density anomalies and sub-Alfvenic events in the solar wind

A. V. Usmanov and M. L. Goldstein, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

Spacecraft observations show that solar wind near Earth orbit is usually highly super-Alfvénic with an average Alfvén Mach number of 8.4. Gosling et al. (JGR, 87, 239, 1982) showed that during the low density events of July 4, July 31, and November 22, 1979 the solar wind became sub-Alfvénic. We use the recently released OMNI 2 spacecraft data compilation for 1963-2003 to select 23 intervals with abnormally low densities of 0.3 cm-3 or less and among them 9 events with Alfvén Mach number less than unity. We discuss a correlation between minimum Alfvén Mach numbers and minimum densities for the selected events. Using a time-dependent MHD simulation, we show that a very strong rarefaction with an embedded sub-Alfvénic region can develop on the trailing edge of a fast flow.