A View of the Inner Heliosphere during the May 23-24, 2002 Low Density Anomaly

Arcadi V. Usmanov, Melvyn L. Goldstein, William M. Farrell, D. A. Roberts, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA

For a two day period in May 2002, the solar wind density dropped to anomalously low values, similar to those reported previously for a May 1999 solar wind density dropout. The low density event was observed by WIND and ACE on the trailing edge of a high speed stream from a coronal hole. Using the MHD inverse mapping technique, we trace the solar wind plasma and magnetic field observables for CR1990 to create a map of the inner heliosphere to a distance of ~0.3 AU. We also examine in detail the limits of such a mapping. Specifically, it is assumed that the solar wind flow is super-Alfvénic, but we find notable exceptions particularly during low density/low velocity temporally-short periods for this and other low density anomalies. Another limit to the technique is the requirement that the solar wind structures be part of the corotating plasma system. We find that this and other anomalies are parts of the corotating structures, making the model application appropriate. The physics of the anomaly formation will be discussed.