Plasma environment in the vicinity of the lunar wake: Detection of counter-streaming electrons by NOZOMI spacecraft

Details Plasma environment in the vicinity of the lunar wake: Detection of counter-streaming electrons by NOZOMI spacecraft Plasma environment in the vicinity of the lunar wake: Detection of counter-streaming electrons by NOZOMI spacecraft

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....9580f&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #9580

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Scientific paper

The plasma environment in the vicinity of the Moon is discussed based on the observation by NOZOMI spacecraft during its lunar swing-by on Dec. 18, 1998. Backstreaming electrons were detected by Particle Spectrum Analyzer/Electron Spectrum Analyzer (PSA/ESA) during NOZOMI's second lunar swing-by. These events can be categorized into two types from the characteristics of the electron distribution function: (1) backstreaming electrons having a velocity distribution similar to that of the solar wind electrons, but its phase space density ratio to the solar wind electrons decreases as a function of velocity; (2) backstreaming electrons which are thermalized and have a flux comparable to or dominating that of the solar wind electrons. We considered possible source locations as well as possible mechanisms that can produce these backstreaming electrons. After careful investigations of the velocity distribution function of the electrons and the magnetic field orientation, we concluded that the origin of backstreaming electrons observed in the first event is the lunar wake region, where the electrostatic potential drop associated with the ambipolar plasma expansion reflects the solar wind electrons. The energy of the reflected electrons reaches 480eV, so that the magnitude of ambipolar electrostatic potential is estimated to be at least 480V. The value of 480V is consistent with the value of 400V concluded by Ogilvie et al.[1996] based on the accelerated solar wind proton data from the WIND spacecraft, but is larger than the value of about 40V obtained by Birch and Chapman[2001] from the numerical calculation. As for the second event, the electron energy spectrum implies that the solar wind electrons have thermalized at the downstream region. There are two candidates for the thermalization mechanisms; the terrestrial bow shock and the interaction region of the solar wind ions in the downtail of the Moon. When solar wind protons passing both side of the terminator of the Moon interact with each other behind the void region, the two-stream instability can easily develop. After the thermalization of the solar wind electrons, the considerable amount of the solar wind electrons can be reflected back. Both cases can account for the observed counterstreaming electrons of the second event.

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