Plasma and Potential at the Lunar Surface

Photoemission and electrostatic potentials on the dayside lunar surface in the terrestrial magnetotail lobes

Despite the need to accurately predict and assess the lunar electrostatic environment in all ambient conditions that the Moon encounters, photoemission and electrostatic potentials on the dayside lunar surface in the terrestrial magnetotail lobes remain poorly characterized. We study characteristics and variabilities of lunar photoelectron energy spectra by utilizing Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) and Apollo measurements in combination with the Flare Irradiance Spectral Model (FISM). We confirm that the photoelectron spectral shapes are consistent between ARTEMIS and Apollo and that the photoelectron flux is linearly correlated with the FISM solar photon flux. We develop an observation-based model of lunar photoelectron energy distributions, thereby deriving the current balance surface potential. The model predicts that dayside lunar surface potentials in the tail lobes (typically tens of volts) could increase by a factor of 2 - 3 during strong solar flares.

Detection of a strongly negative surface potential at Saturn's moon Hyperion

On 26 September 2005, Cassini conducted its only close targeted flyby of Saturn's small, irregularly shaped moon Hyperion. Approximately 6 min before the closest approach, the electron spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) detected a field-aligned electron population originating from the direction of the moon's surface. Plasma wave activity detected by the Radio and Plasma Wave instrument suggests electron beam activity. A dropout in energetic electrons was observed by both CAPS-ELS and the Magnetospheric Imaging Instrument Low-Energy Magnetospheric Measurement System, indicating that the moon and the spacecraft were magnetically connected when the field-aligned electron population was observed. We show that this constitutes a remote detection of a strongly negative (∼ -200 V) surface potential on Hyperion, consistent with the predicted surface potential in regions near the solar terminator.