Alfven Wing Formation at the Magnetosphere and the Saturation of the Ionospheric Potential

During periods in which the interplanetary magnetic field (IMF) becomes large and the solar wind density becomes small, the solar wind Alfvenic Mach number can become reduced compared to nominal conditions. We present a variety of data sources that show that the ionospheric cross polar cap potential (CPCP) tends to saturate during these time periods. In addition, we show modeling results that indicate that Alfven Wings form around the magnetosphere during low Mach number periods. Alfven Wings have been shown to exist at other objects such as Ganymede and Io, in which the Mach number is quite low. One of the characteristics of an Alfven Wing is that the electric field within the Wing is significantly reduced from the ambient electric field. This implies that the open field lines in the magnetosphere map into regions in which the electric field is reduced from the expected value - hence saturated. This indicates that the saturation of the ionospheric cross polar cap potential is most likely caused by the formation of Alfven Wings during low Alfvenic Mach number time periods, such as those that occur during magnetic clouds.