Enhanced photoelectron escape caused by Langmuir and upper hybrid waves : MMS observations

The spacecraft potential is often used to infer rapid changes in the thermal plasma density. The variations in spacecraft potential associated with large-amplitude Langmuir and upper hybrid waves are investigated with the Magnetospheric Multiscale (MMS) mission. When large-amplitude Langmuir and upper hybrid waves are observed the spacecraft potential increases. The changes in spacecraft potential are shown to be due to enhanced photoelectron escape from the spacecraft when the wave electric fields reach large amplitude. The fluctuations in spacecraft potential follow the envelope function of the Langmuir and upper hybrid waves. Comparison with the highresolution electron moments shows that the changes in spacecraft potential associated with the waves are not due to density perturbations. Indeed, using the spacecraft potential as a density probe leads to unphysically large density fluctuations. In addition, the changes in spacecraft potential are shown to increase as density decreases: larger spacecraft potential changes are observed in the magnetosphere, than in the magnetosheath and solar wind. These results show that external electric fields can lead to unphysical results when the spacecraft potential is used as a density probe. The results suggest that fluctuations in the spacecraft potential alone cannot be used to determine Space Research Institute, Austrian Academy of Sciences, Graz, Austria. c ©2018 American Geophysical Union. All Rights Reserved. whether nonlinear processes associated with Langmuir and upper hybrid waves, such as the ponderomotive force and three-wave decay, are occurring. Keypoints: • The electric field of Langmuir and upper hybrid waves increases the spacecraft potential due to enhanced photoelectron escape. • Increased spacecraft potentials associated with Langmuir and upper hybrid waves are not due to density depletions or ponderomotive force. • The magnitude of the changes in spacecraft potential due to Langmuir and upper hybrid waves becomes larger as density decreases. c ©2018 American Geophysical Union. All Rights Reserved.

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