Polarization Reversal of Low‐Frequency Magnetic Variation in the Lunar Wake

Abstract

AbstractA structure of polarization of magnetic field variation in the frequency range from 0.01 to 0.3 Hz was found by Kaguya in the lunar wake. In a dawnward‐directed magnetic field, the polarization was left‐handed in the southern hemisphere and right‐handed in the northern hemisphere of the lunar wake. The sense of rotation was consistent with vortices behind an obstacle in a fast stream, although no such vortices were detected by Kaguya. The direction of magnetic field variation was predominantly perpendicular to the background magnetic field, suggesting that the waves were of shear Alfvén mode. The same polarization structure was found in rotated magnetic fields. In a selenocentric coordinate system with x axis antiparallel to the solar wind flow and the magnetic field in x‐z plane, the polarization was dependent only on y position of the spacecraft. The polarization structure was stable and unchanged in various solar wind conditions showing no preferences. It was observed in the magnetosheath as well as in the solar wind, but not in the terrestrial magnetosphere. The power density showed preferences to high‐temperature, high‐speed solar wind. The power density was higher at the wake boundary than in the center of the wake, suggesting that the magnetic fluctuations were generated at the wake boundary by ions refilling the wake, and propagate into the deep wake as shear Alfvén waves.