Effect of the solar wind proton entry into the deepest lunar wake

We study effect of the solar wind (SW) proton entry deep into the near-Moon wake that was recently discovered by the SELENE mission. Because previous lunar-wake models are based on electron dominance, no effect of SW proton entry has been taken into account. We show that the type-II entry of SW protons forms proton-governed region (PGR) to drastically change the electromagnetic environment of the lunar wake. Broadband electrostatic noise found in the PGR is manifestation of electron two-stream instability, which is attributed to the counter-streaming electrons attracted from the ambient SW to maintain the quasineutrality. Acceleration of the absorbed electrons up to ∼1 keV means a superabundance of positive charges of 10^-510^-7 cm ^-3 in the near-Moon wake, which should be immediately canceled out by the incoming high-speed electrons. This is a general phenomenon in the lunar wake, because PGR does not necessarily require peculiar SW conditions for its formation.