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,
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.  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.