Europa's Alfv́n wing: Shrinkage and displacement influenced by an induced magnetic field

Abstract

The Galileo magnetometer data are used to investigate the structure of the Alfv́n wing during three flybys of Europa. The presence of an induced magnetic field is shown to shrink the cross section of the Alfv́n wing and offset it along the direction radial to Jupiter. Both the shrinkage and the offset depend on the strength of the induced field. The entry and exit points of the spacecraft into and out of the Alfv́n wings are modeled to determine the angle between the wings and the background magnetic field. Tracing of the Alfv́n characteristics in a model magnetic field consisting of Jupiter's background field and an induced field in Europa produces an offset and shrinking of the Alfv́n wing consistent with the geometric modeling. Thus we believe that the Alfv́n wing properties have been determined correctly. The Alfv́n wing angle is directly proportional to the local Alfv́n velocity, and is thus a probe for the local plasma density. We show that the inferred plasma density can be understood in terms of the electron density measured by the plasma wave experiment. When Europa is located in the Jovian plasma sheet the derived mass-per-charge exceeds the previous estimates, which is a result of increased pickup of sputtered ions near the moon. The estimated rate of O2+ pickup agrees well with the results from numerical models.