Lunar exosphere

Lunar exospheric ions are accelerated by the solar wind electric field into the suprathermal Ion Detector Experiment (SIDE) which was deployed on the lunar surface during the Apollo missions. Analysis of simultaneous mass and energy spectra from the SIDE indicates that the dominant component of the exosphere at the lunar surface is neon. The energy spectra of the ions indicate that neutral neon follows an exponential height distribution in the lunar terminator regions with a surface concentration of approximately 105cm-3. These spectra also indicate a surface concentration for neutral argon of approximately 104cm-3 Data from SIDE and other lunar surface ments indicate that the surface concentration of neon is consistent with the flux balance relation nT/sup 1/2 = constant. The solar wind electric field removes neon from the lunar exosphere at the rate of approximately.25 gm sec-1. This is consistent with the assumption that the solar wind is the only source of lunar exospheric neon. The ion spectra observed in the terminator regions also indicate that a surface potential on the order of --50 V exists when these spectra are observed. The effective screening length of this potential is of the order of 1 km. Some ions that are accelerated by the solar wind electric field do not exhibit an exponential energy spectrum. These events are believed to be related to the ion events observed by SIDE throughout lunar night