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The effects of magnetic fields on photoelectron-mediated spacecraft potential fluctuations

Previously, we have experimentally studied photoelectron-mediated spacecraft potential fluctuations associated with time-dependent external electric fields. In this paper, we investigate the effects of magnetic fields on such spacecraft potential fluctuations. A magnetic field is created above the UV-illuminated surface of a spacecraft model to alter the escape rate of photoelectrons. The packet of the observed potential oscillations becomes less positive with increasing magnetic field strength because more of the emitted photoelectrons are returned to the surface. As a result, the photoelectric charging time is increased, corresponding to a decrease in the response frequency of the photoemitting surface. The amplitude of the potential oscillations decreases when the response frequency becomes lower than the electric field oscillation frequency. A test particle simulation is validated with the laboratory experiments and applied to estimate the photoelectron escape rate from the Van Allen Probes spacecraft, showing that the photoelectron current is reduced by as much as 30\% when magnetic field strength is 1200 nT. Based on our laboratory results and computer simulations, we discuss the effects of magnetic fields on the spacecraft potential fluctuations observed by the Van Allen Probes.

Wang, X.; Malaspina, D.; Hsu, H.-W.; Ergun, R.; M., Hor\;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2014

YEAR: 2014     DOI: 10.1002/jgra.v119.910.1002/2014JA019923

chorus waves; magnetic fields; photoelectrons; spacecraft potential fluctuations; Van Allen Probes

Photoelectron-mediated spacecraft potential fluctuations

Electric field fluctuations such as those due to plasma waves in Earth\textquoterights magnetosphere may modulate photoelectrons emitted from spacecraft surface, causing fluctuations in spacecraft potential. We experimentally investigate such photoelectron-mediated spacecraft potential fluctuations. The photoelectric charge of a spacecraft model is found to increase with increasing applied electric field as more photoelectrons escape the spacecraft model surface and dissipates with a decrease in the electric field through collection of ambient plasma electrons. When the applied electric field is driven to oscillate at a frequency lower than the response frequency of the spacecraft model, the surface potential follows the electric field oscillations. The spacecraft model maintains an approximately constant potential if the electric field oscillations are driven at a much higher frequency. When a high-frequency electric field modulated by a low-frequency envelope is applied, rectified oscillations in the potential of the spacecraft model are observed. Our experimental results indicate that photoelectron-mediated wave rectifications must be taken into account when spacecraft potential fluctuations are used to infer plasma density structures.

Wang, X.; Malaspina, D.; Ergun, R.; M., Hor\;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2014

YEAR: 2014     DOI: 10.1002/2013JA019502

chorus waves; electric field; Magnetosphere; photoelectrons; plasma density; spacecraft potential fluctuations