Abstract

Reversal of unified polarization of low-frequency magnetic field variation was found by Kaguya at equator crossings in the lunar wake under the condition of dawn-dusk directed background magnetic field. The polarization was left-handed over a frequency range from 0.01 to 0.3 Hz in the southern hemisphere of the lunar wake in the dawnward-directed magnetic field. It turned to be right-handed when the spacecraft entered the northern hemisphere of the wake. In the duskward-directed magnetic field the polarization reversed. The same configuration by 90 degrees rotated was observed in northward-directed magnetic field, too. The sense of rotation was alike that of surface waves generated by Kelvin-Helmhlotz instability, but it was not likely because the waves of unified polarization were not always accompanied by velocity shear. They penetrated into the very center of the wake, suggesting that they were not propagating along a surface of wake boundary. The polarization was highly elliptic except for high frequency component. The magnetic field variation was predominantly perpendicular to the background magnetic field, suggesting that they were shear Alfven waves. The mechanism of unifying the polarization is not yet understood.