Pairwise energy gain-loss feature of solar wind protons in the near-Moon wake

We study solar wind (SW) intrusion into the near-Moon wake using SELENE (KAGUYA) data. It has been known that SW protons are gradually accelerated toward the wake center along magnetic field in the distant lunar wake, while SW intrusion into the near-Moon wake has never been measured. We show that the SW protons come into the lunar wake at ∼100 km altitude in the direction perpendicular to the magnetic field, as they gain kinetic energy in one hemisphere while lose in the other hemisphere. Particle trajectory calculations and theoretical treatment demonstrate that proton Larmor motions and inward electric field around the wake boundary result in energy gain and loss of the SW protons. Our result shows emergence of proton particle dynamics around the near-Moon space, and suggests that the SW protons may relatively easily access the low-latitude and low-altitude region on the lunar night side.