Wave excitation in the lunar wake associated with solar-wind proton entry

The near-Moon space environment is characterized by formation of a plasma cavity region on the night side along the solar wind (SW) flow, which is called the lunar wake. Because the Moon obstructs the SW plasma flow, the wake region is much more tenuous than the ambient SW region. The plasma environment of the lunar wake has been considered as follows; Because high energy SW electrons can easily access the night side region while SW protons cannot, the wake region should be negatively charged. The electron-rich status of the lunar wake yields ambipolar (inward) electric field at the wake boundary where SW protons gradually come to the wake center along the interplanetary magnetic field (IMF) in the distant wake. Meanwhile, it has been reported that the SW entry into the distant wake gives rise to plasma waves. Broadband electrostatic noise (BEN) was detected by the Wind spacecraft in the distant wake (∼10,000 km from the Moon). The excitation of the BEN was attributed to ion two-stream instability owing to the SW proton beams that were accelerated/decelerated into the wake by the ambipolar electric fields. It was also attributed to electron two-stream instability that is expected to occur around the wake boundary region. However, such instabilities were not anticipated to occur in the near-Moon wake (say, ∼100 km altitude), because it has been believed that a major part of the SW particles cannot access the deepest wake so that any plasma effects would be inoperative there. Recently, a Japanese lunar orbiter SELENE (Kaguya) performed comprehensive measurements of the plasma and electromagnetic environment around the Moon at ∼100 km altitude. Concerning the night-side region, two mechanisms of SW proton entry into the near-Moon wake were found. One of them, the type-II entry, which becomes evident when the non-radial IMF component is dominant, is closely related to ‘self-pickup’ process of protons that were once scattered on the lunar dayside surface or magnetically reflected around the magnetic anomalies. This mechanism lets protons access the deepest region of the wake, namely, 150