Does the Surface of the Moon Really Charge to Extreme Positive Potentials in the Magnetotail Lobes? A Re-analysis of Apollo/CPLEE observations

Details Does the Surface of the Moon Really Charge to Extreme Positive Potentials in the Magnetotail Lobes? A Re-analysis of Apollo/CPLEE observations Does the Surface of the Moon Really Charge to Extreme Positive Potentials in the Magnetotail Lobes? A Re-analysis of Apollo/CPLEE observations

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p51c1458s&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P51C-1458

Physics

[2732] Magnetospheric Physics / Magnetosphere Interactions With Satellites And Rings, [2744] Magnetospheric Physics / Magnetotail, [5421] Planetary Sciences: Solid Surface Planets / Interactions With Particles And Fields, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

Electron measurements made from the lunar surface by the Apollo 14 Charged Particle and Lunar Environment Experiment (CPLEE) have been used to infer dayside surface potentials of >200 V positive when in the tail lobes. Based on typical plasma conditions in the tail lobes at lunar orbit, and studies of the photoemission of electrons from lunar regolith samples, the surface is only expected to charge ~10 V positive. So this appears to suggest a factor ~20 discrepancy between observation and theory - a substantial difference which was quite puzzling. The earlier analyses had assumed the tail lobes were so tenuous that CPLEE would not be sensitive enough to detect any ambient plasma electrons, and so would only be able to measure photoelectrons attracted back to the surface (i.e., those unable to escape the lunar surface potential). Therefore, when electrons with energies up to ~200 eV were detected in the tail lobes during the lunar daytime, they were interpreted as photoelectrons attracted back by a surface potential of at least 200 V positive. A preliminary re-analysis of these CPLEE observations indicates that they can be well fitted by a bi-Maxwellian distribution (with a hot and a cold component). The tenuous hot component, which included the electrons with energies of ~200 eV, was almost certainly from the tail lobes. The source of the denser cold component is less clear since it has a temperature of ≈6.5 eV, which is over four times hotter than expected for electrons photo-emitted from the lunar surface. Possible sources for this cold component will be discussed, as well as what lunar surface potentials are consistent with the re-analysis of the CPLEE measurements.

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