Modelling spacecraft-emitted electrons measured by SWA-EAS experiment on board Solar Orbiter mission

TL;DR

The paper addresses contamination of thermal electron measurements by spacecraft-emitted electrons and charging effects in the Solar Orbiter environment. It develops a SPIS-based numerical model with a virtual SWA-EAS detector to generate and compare 1D energy spectra for ambient, photoelectron, and secondary-electron populations under two plasma conditions at ~0.3 AU. The results show qualitative agreement with SWA-EAS data, revealing that contamination persists above the spacecraft potential threshold due to distant surface emissions and that the break location depends on ambient conditions and potential distributions. The findings imply that instrument-specific potentials may differ from spacecraft potentials and that refining potential diagnostics is crucial for accurately retrieving ambient plasma properties from in situ measurements.

Abstract

Thermal electron measurements in space plasmas typically suffer at low energies from spacecraft emissions of photo- and secondary electrons and from charging of the spacecraft body. We examine these effects by use of numerical simulations in the context of electron measurements acquired by the Electron Analyser System (SWA-EAS) on board the Solar Orbiter mission. We employed the Spacecraft Plasma Interaction Software to model the interaction of the Solar Orbiter spacecraft with solar wind plasma and we implemented a virtual detector to simulate the measured electron energy spectra as observed in situ by the SWA-EAS experiment. Numerical simulations were set according to the measured plasma conditions at 0.3~AU. We derived the simulated electron energy spectra as detected by the virtual SWA-EAS experiment for different electron populations and compared these with both the initial plasma conditions and the corresponding real SWA-EAS data samples. We found qualitative agreement between the simulated and real data observed in situ by the SWA-EAS detector. Contrary to other space missions, the contamination by cold electrons emitted from the spacecraft is seen well above the spacecraft potential energy threshold. A detailed analysis of the simulated electron energy spectra demonstrates that contamination above the threshold is a result of cold electron fluxes emitted from distant spacecraft surfaces. The relative position of the break in the simulated spectrum with respect to the spacecraft potential slightly deviates from that in the real observations. This may indicate that the real potential of the SWA-EAS detector with respect to ambient plasma differs from the spacecraft potential value measured on board. The overall contamination is shown to be composed of emissions from a number of different sources and their relative contribution varies with the ambient plasma conditions.

Figures (9)

• Figure 1: Electron phase space densities (upper panels) and differential energy flux (lower panels) as measured by SWA-EAS are shown (black crosses) as a function of the energy for the selected samples A (left) and B (right). Measured data are over-plotted by a fit with a simple model (gray line) composed from a sum of two Maxwellian distributions for the core (red) and halo (green) ambient electron populations. Displayed fitted plasma parameters are corrected to the spacecraft potential energy measured by RPW (black dashed vertical line).
• Figure 2: The computational mesh used in the simulation model. The left panel shows the whole computational volume (view from top) comprised in an ellipsoid with 30 m and 25 m long semi-axes along the main X and Y axes (and 20 m semi-axes along Z). The right panel shows the surface mesh model of the Solar Orbiter with solar panels rotated at an angle of 79$^{\circ}$ to Sun normal, reflecting the actual geometry configuration at 0.3 AU. The SWA-EAS detector is modelled as a single sphere at the tip of the payload boom.
• Figure 3: Final structure of the potential around the spacecraft body is shown for simulation run A (top row) and run B (bottom row) at time t=1.5 s. The left and middle column show the 2D cuts in the XY and XZ planes, respectively. The right column shows potential profiles along X (blue), Y (green), and Z (orange) axis as a function of the distance from the virtual SWA-EAS detector. The dashed line in the right panel displays the final surface spacecraft potential and the dotted line line shows the background (zero) plasma potential for reference.
• Figure 4: The electron an ion densities at the final simulation time t=1.5 s are show for run A (columns 1 and 2) and run B (columns 3 and 4) as 2D slices in the XY and XZ plane: row 1a-4a for ambient electrons (AE), row 1b-4b for photoelectrons (PE), row 1c-4c for secondary electrons from electron impacts (SE), row 1d-4d total electron density (AE+PE+SE+SI), and row 1e-4e for ambient ion density (AI). All densities are normalized to the initial ambient plasma density n$_{0}$ and plotted in logarithmic scale showing relative increase in densities in red and decrease in densities in blue colours.
• Figure 5: Example of photoelectron trajectory emitted from the solar panel and impacting the surface of the SWA-EAS detector. Change in the electron kinetic energy (blue) and potential (red) along the trajectory is shown in the left panel as a function of the time of flight. The sample trajectory is taken from the simulation run A.