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 quasi‐neutrality. Acceleration of the absorbed electrons up to ∼1 keV means a superabundance of positive charges of 10−5–10−7 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.