Energetic neutral atom observations of magnetic anomalies on the lunar surface

Abstract

SARA, the Sub-KeV Atom Analyzer, on board Chandrayaan-1 recorded the first image of a minimagnetosphere above a lunar magnetic anomaly using energetic neutral atoms (ENAs). It was shown that this magnetosphere, which is located near the Gerasimovich crater, is able to reduce the solar wind ion flux impinging onto the lunar surface by more than 50%. Following this first observation, we investigated all magnetic anomalies that are in the SARA data set. We searched for a possible correlation between the solar wind plasma parameters (dynamic pressure, magnetic field), the local magnetic field, and the reduction in the reflected hydrogen ENA flux (henceforth called shielding efficiency). Having analyzed all observations by SARA, we discovered that the Gerasimovich magnetic anomaly is topologically a very simple, large-scale magnetic structure, which is favorable for this kind of investigation. Most other magnetic anomalies on the lunar surface have more small-scale features in their magnetic field structure, which complicates the interpretation of the observed data. We find a clear correlation between the plasma parameters and the shielding efficiency for the Gerasimovich case. For the other observed anomalies only about half of the cases showed such a correlation. We therefore conclude that the solar wind ions-magnetic anomaly interaction is in general more complex than in the Gerasimovich case.