Non-gyrotropic electron velocity distribution functions near the lunar surface

Although ions and electrons are usually expected to have gyrotropic (symmetric relative to the magnetic field line) velocity distribution functions (VDFs) in the plasma rest frame, they can form non-gyrotropic VDFs related to shocks, boundaries, or waves. Actually, non-gyrotropic ions have been widely observed associated with the Earth's bow shock, the interplanetary shock, the current sheet and the plasma sheet boundary layer in the Earth's mangetotail, the comets, the planets, the Moon, and so on. By contrast, only a few examples were reported on non-gyrotropic electron VDFs. A small gyroradius and short gyroperiod of electron make a non-gyrotropic electron VDF difficult to detect. A recent analysis of the high angular resolution data of low-energy electrons with the magnetic field data obtained by Kaguya (SELENE) revealed a partial loss of electrons in their VDF due to the gyro-loss effect, namely gyrating electrons being absorbed by the lunar surface. Electron VDFs produced by this effect have non-gyrotropic structure which shows an energy-dependent empty region in a certain gyrophase. One of the observed electron VDFs suggests an existence of a relatively strong electric field (~ 10 mV/m) around the Moon when it is located in the terrestrial plasma sheet. However, it remains uncertain whether this electron VDF was formed only due to such a large electric field. This study presents further theoretical considerations of the gyro-loss effect on electron VDFs conducting test particle simulations and a reanalysis of the data obtained by Kaguya. Three possible mechanisms which modify the electron gyro-loss effect are investigated, namely, perpendicular electric fields, lunar surface charging and non-uniform magnetic fields due to the diamagnetic current around the Moon. The gyro-loss effect on electron VDFs can provide a new tool to survey the electromagnetic environment in the vicinity of the Moon.