Photoelectron and Secondary Electron Dynamics Around the Solar Probe Plus Spacecraft

Abstract:

We study the Solar Probe Plus (SPP) spacecraft interactions with near-Sun plasma environment. The plasma environment is characterized by a small Debye length due to dense plasmas and large fluxes of photo-/secondary electrons emitted from the spacecraft surface. Prior studies on this issue found that such a condition produces a non-monotonic potential distribution near the spacecraft, and the resultant spacecraft potential becomes negative. The consequences also suggest the significance of photoelectron and secondary electron dynamics near the spacecraft. In this study, we apply our original electromagnetic particle-in-cell simulation code called EMSES to the problem.

In the simulations, we consider the SPP spacecraft at its perihelion (about 0.04 AU from the Sun) and important physical effects such as spacecraft charging, photoelectron and secondary electron emission, solar wind plasma flow including the effect of spacecraft orbital velocity, and the presence of a background magnetic field. Our preliminary results show that both photoelectrons and secondary electrons from the spacecraft are magnetized in the spatial scale of several meters, and make drift motion due to the presence of the background convection electric field. In addition to this, secondary electrons emitted laterally from the spacecraft chassis are strongly affected by the radially inward electric field caused by the spacecraft potential. These effects lead to non-axisymmetric distributions of electron density and the resultant electric potential around the spacecraft.

Our simulations also predict that a strong (about 100 mV/m) spurious electric field can be observed by the probe measurement on the spacecraft due to such a non-axisymmetric effect. We also confirm that the large photo-/secondary electron current alters magnetic field intensity around the spacecraft, but the field variation is much smaller than the background magnetic field magnitude (a few nT compared to thousands of nT). We will report further parameter survey results on such spurious field generation, by changing solar wind and spacecraft orbital conditions. The results will provide useful information in predicting photo-/secondary electron interference with in-situ field measurements planned in the Solar Probe Plus and Solar Orbiter missions.