Observations of the lunar plasma wake from the WIND spacecraft on December 27, 1994

Abstract

On December 27, 1994, the WIND spacecraft crossed the lunar wake at a distance of 6.5 lunar radii ( R L ) behind the moon. The observations made were the first employing modem instruments and a high data rate. The SWE plasma instrument on WIND observed new aspects of the interaction between the solar wind and unmagnetized dielectric bodies. The plasma density decreased exponentially from the periphery of the wake towards its center as predicted by simple theory. Behind the moon two distinct cold ion beams were observed refilling the lunar cavity. The ions were accelerated along the direction of the magnetic field by an electric field of the order 2 × 10 −4 volts/m. The region of plasma depletion was observed to extend beyond the light shadow, consistent with a rarefaction wave moving out from the wake into the undisturbed solar wind.