A view of the inner heliosphere during the May 10–11, 1999 low density anomaly

Abstract

On May 10 and 11, 1999, near-Earth spacecraft observed the solar wind density drop to below 0.1 particles cm⁻³. Using those data, we have mapped solar wind parameters back to the Sun from 1 AU using two techniques. The first assumed constant-velocity trajectories plus corotation, while the second employed MHD-derived magnetofluid parameters. This inverse tracing creates a view of the inner heliosphere useful for identifying the source location on the Sun of the density anomaly. We compare the two methods and show that the source location of the anomaly predicted by MHD is mapped ∼20° eastward of the constant-velocity result. The coronal magnetic field maps indicate that the low density event occurred as the polar coronal magnetic field began reversing. We suggest that the event was initiated by a latitudinal excursion of the low velocity heliospheric current sheet toward the helioequator. The emergence of this slow flow into the preexisting faster wind produced strong rarefaction and anomalously low densities.