Low-density anomalies and sub-Alfvénic solar wind

Abstract

During 10–12 May 1999, the solar wind density dropped to an anomalously low value of ∼0.1 cm⁻³. The density depletion occurred in the midst of relatively slow wind flow, in between faster flows, and was apparently associated with neither a coronal mass ejection nor a fast corotating stream. While the magnetic field intensity did not show any notable variation across the density depletion, plasma analyzers on the ACE and Wind spacecraft revealed an abnormally strong nonradial flow component with an azimuthal speed that peaked at ∼100 km s⁻¹. Usmanov et al. [2000b] suggested that the density anomaly was, in fact, a rarefaction at the trailing edge of relatively fast flow that formed as a result of suppression of coronal outflow from a region that earlier provided fast wind flow. The suppression in turn may have resulted from a rapid restructuring of solar magnetic fields during the polar field reversal. Here we show results from a two-dimensional time-dependent MHD simulation applied to the helioequatorial plane. The initially longitude-independent Parker solar wind and Archimedean spiral magnetic field are disturbed by a low-velocity/high-density jump on an inner computational boundary at 20 R⊙. We follow the development and propagation of the rarefaction to Earth orbit and compare pseudo-time series with near-Earth spacecraft measurements. We show that a strong rarefaction can develop behind the fast flow and that simulation results and spacecraft observations are generally in agreement. The simulated radial magnetic field shows a relatively small variation across the density anomaly compared with that of the density. The stream interaction generates strong azimuthal velocities in the slow flow region, as observed. The simulation shows a sub-Alfvénic flow region embedded within the low-density region that does not extend all the way back to the Sun but which has become disconnected as the depletion propagates to Earth orbit. We discuss also the correlation between low-density and sub-Alfvénic events in the solar wind as inferred from spacecraft observations using the OMNI 2 data set from 1963 to 2003.