Estimation of the Electron Density From Spacecraft Potential During High-Frequency Electric Field Fluctuations

Spacecraft potential has often been used to infer electron density with much higher time resolution than is typically possible with plasma instruments. However, recently, two studies by Torkar et al. (2017, https://doi.org/10.1002/2017JA024724) and Graham, Vaivads, Khotyaintsev, Eriksson, et al. (2018, https://doi.org/10.1029/2018JA025874) have shown that external electric fields can also have an effect on the spacecraft potential by enhancing photoelectron escape from the surface. Consequently, should the electron density derived from the spacecraft potential be used during an event with a large electric field, the estimation would be contaminated and the user would see the effects of the electric field rather than density perturbations. The goal of this paper is to propose a method to remove the electric field effects to allow the density derived from spacecraft potential to be used even during large-amplitude wave events such as Langmuir waves or upper hybrid waves.

Key Points
Strong AC E-fields can perturb the spacecraft potential by enhancing photoelectron emission making the density estimation unreliable
A correction method for the spacecraft potential during large electric fields is presented so that the electron density can be inferred
This is tested for three events on the MMS spacecraft showing good agreement with different direct measurements

Plain Language Summary
Spacecraft in a plasma become charged due to a number of processes. Often the two most important processes in determining the charge are due to the ambient plasma and the photoelectron emission from the surface of a sunlit spacecraft. The potential itself is a function of the electron density, and consequently, the potential data can be used to infer the electron density if the photoelectron emission can be modeled. However, in the presence of large electric fields, the photoelectron emission can change with the electric field. This means that rather than see fluctuations of density in the spacecraft potential, the effect of the electric field is seen. Here a method is presented to remove the electric field effect on the spacecraft potential such that the density can be estimated even when there are strong electric fields present.