Enhanced Escape of Spacecraft Photoelectrons Caused by Langmuir and Upper Hybrid Waves

Abstract

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 high‐resolution 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 whether nonlinear processes associated with Langmuir and upper hybrid waves, such as the ponderomotive force and three‐wave decay, are occurring.

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

“…This could cause more or fewer electrons to escape the potential well of the spacecraft (e.g., Malaspina et al, 2014;Wang, Malaspina, Hsu, et al, 2014). Numerical simulations by Graham, Vaivads, Khotyaintsev, Eriksson, et al (2018) and a combination of numerical simulation and laboratory experiments (Wang, Malaspina, Hsu, et al, 2014) found that this effect becomes important for very large magnetic fields B ∼ 10 3 nT which are much larger than the typical magnetic fields at the magnetopause. Effectively, the larger magnetic field causes the electron gyroradius to be smaller.…”

“…In the work of Torkar et al (2017) and Graham, Vaivads, Khotyaintsev, Eriksson, et al (2018), a clear effect of the electric field on the spacecraft potential was demonstrated, causing enhanced photoelectron escape from the spacecraft. This can lead to inaccuracies in determining the electron number density from the spacecraft potential.…”

“…The effect of aliasing can also be seen here in the DCE data where there is an aliased peak near 2000 Hz. It is important to note that the photoelectron emission due to electric field waves is a function of both the wave frequency and amplitude (Graham, Vaivads, Khotyaintsev, Eriksson, et al, 2018). Lower‐frequency waves cause more photoemission than higher‐amplitude waves.…”

“…Figure 1a shows the high-frequency AC coupled electric field measurements from the Spin Plane Double Probes instrument (Lindqvist et al, 2016) and the Axial Double Probes (Ergun et al, 2016). These are presented in a coordinate system which is aligned with the mean magnetic field direction defined as the average measured from the fluxgate magnetometer (Russell et al, 2016) This interval was investigated by Graham, Vaivads, Khotyaintsev, Eriksson, et al (2018) and and identified as an example of a Langmuir wave with E || ≫ E ⟂ which causes the electron density estimation from spacecraft potential to become inaccurate. Figure 1b shows the envelope of the total fluctuations.…”

“…More recently, another limitation has been discovered, where the electric field can have an effect on the spacecraft potential in modifying the photoelectron escape from the spacecraft. It was shown that strong DC and AC electric fields can have an influence on the spacecraft potential (Graham, Vaivads, Khotyaintsev, Eriksson, et al, 2018; Malaspina et al, 2014; McFadden et al, 2003; Torkar et al, 2017). Graham, Vaivads, Khotyaintsev, Eriksson, et al (2018) used data from MMS to show that when high‐frequency upper hybrid or Langmuir waves are present (such as are common near the Earth's magnetopause, Graham, Vaivads, Khotyaintsev, André, et al, 2018), they enhance photoelectron escape from the spacecraft leading to an increase in the spacecraft potential.…”