Counterstreaming electrons in the near vicinity of the Moon observed by plasma instruments on board NOZOMI

The NOZOMI spacecraft approached the Moon at an altitude of about 2800 km a t 0734 UT on December 18, 1998. Around the time of closest approach, a plas ma instrument Particle Spectrum Analyzer/Electron Spectrum Analyzer (PSA/ES A) detected non-solar-wind electrons in addition to the normal solar wind c omponent. aom the characteristics of the electron distribution function, we can categorize these events into two types: (1) backstreaming electrons ex hibiting a velocity distribution similar to that of the solar wind electron s, but its phase space density ratio to the solar wind electrons decreases as a function of velocity; (2) backstreaming electrons that are thermalized and have a flux comparable to or dominating that of the solar wind electro ns. We considered possible source locations as well as possible mechanisms that can produce these backstreaming electrons. After careful investigation of the velocity distribution function of the electrons and the magnetic fi eld orientation, we concluded that their origins are (1) the lunar wake reg ion, where the electrostatic potential drop associated with ambipolar plasm a expansion reflects the solar wind electrons and produces backstreaming el ectrons of category 1, and (2) the terrestrial bow shock, where the electro ns are thermalized downstream and escaping electrons are of category 2.