The sub-Alfvénic solar wind and low density anomalies

A. V. Usmanov, W. M. Farrell, K. W. Ogilvie, M. L. Goldstein, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

The solar wind flow at 1 AU is normally strongly supersonic and super-Alfvénic with average Mach and Alfvén Mach numbers to be 12 and 9 respectively. Gosling et al. (JGR, 87, 239, 1982) showed that for portions of the low density event on November 22, 1979 the Alfvén Mach number was considerably less than 1, i.e., the solar wind was sub-Alfvénic. We have scanned through the OMNI and the ACE spacecraft data bases from 1963 to 2002 and selected those events having abnormally low densities of 0.4 cm-3 or less. Among the total of 18 such events, 7 have minimum density values of 0.2 cm-3 or less. We found that during all 7 events (including in particular the events of May 10-12, 1999, May 23-24, 2002, and November 22, 1979), the solar wind became sub-Alfvénic for varying time intervals. We will discuss the correlation between the minimum Alfvén Mach numbers and the minimum densities for the selected 18 events and will discuss the origins of this correlation in terms of solar observations and MHD simulations.