A Simulation Study of the May 10-11, 1999 Low Density Anomaly

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

On May 10-11, 1999 solar wind density dropped to anomalously low values of ~0.1 cm-3. The density depletion occurred on a relatively slow wind background in between of faster flows and was apparently not associated with a coronal mass ejection or a fast corotating stream. While magnetic field intensity did not show a notable variation across the depletion, the SWEPAM analyzer on the ACE spacecraft revealed an abnormally strong non-radial flow component, with the azimuthal speed peaking at 100 km/s. Usmanov et al. [2000]  suggested that the density anomaly was in fact a rarefaction on the trailing edge of a relatively faster flow and that the rarefaction formed as a result of suppression of coronal outflow from a region that earlier provided the faster solar wind. The suppression in turn was supposed to appear due to a quick restructuring of solar magnetic fields during polar field reversal. In present work, we show results of a two-dimensional time-dependent MHD simulation in the equatorial plane with initially longitude-homogeneous Parker solar wind and spiraling magnetic being disturbed by a slower velocity/higher density pulse on an inner computational boundary at 20 solar radii. We follow the development and propagation of the rarefaction out to Earth orbit and compare pseudo-time series with ACE measurements. We show that a strong rarefaction indeed can develop behind faster flow and that simulation results and ACE observations are in good agreement. The simulated radial magnetic field shows a relatively small variation across the density anomaly compared to that in density. The stream interaction generates strong azimuthal velocities in the slow flow region, as observed. The simulation shows also a sub- Alfvénic flow region embedded into the low density region. The sub-Alfvénic region does not extend all the way to the Sun, but gets disconnected as the depletion propagates to Earth orbit.