Electron density estimation from measurements of spacecraft potential using Solar Orbiter

¹ Swedish Institute of Space Physics (IRF), Uppsala 75121, Sweden
² Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden
³ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁴ Radboud Radio Lab, Department of Astrophysics, Radboud University Nijmegen, The Netherlands
⁵ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁶ Institute of Atmospheric Physics of the Czech Academy of Sciences, Bocňí II 1a/1401, 14100 Prague, Czechia
⁷ Astronomical Institute of the Czech Academy of Sciences, Bocňí II 1a/1401, 14100 Prague, Czechia
⁸ European Space Agency, ESAC, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28692 Villanueva de la Cañada, Madrid, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 February 2026
Accepted: 16 March 2026

Abstract

Context. Measurements of spacecraft potential (V_sc) are an important tool for plasma diagnostics. High-resolution V_sc measurements can be calibrated with low-cadence observations of electron plasma frequency to derive high-resolution electron density data.

Aims. We analyze V_sc measured by the Radio and Plasma Waves (RPW) instrument suite on board Solar Orbiter. We investigate the evolution of V_sc within the framework of the heliocentric distance and mission lifetime. We aim to obtain electron density estimates that are valid across a wider range of V_sc regimes, including the cases of negative V_sc.

Methods. We determined V_sc by combining measurements of the probe-to-spacecraft potential, V_ps, and the probe-to-plasma potential, V_pn. Then, we used the appropriate electric current balance to find a relation between electron density and V_sc, which we calibrated to the observed electron plasma frequency. With this method, we obtained the electron density at the resolution given by the V_ps measurements.

Results. We find that V_sc reaches negative values for a larger amount of time than expected for a sunlit spacecraft immersed in a tenuous plasma such as the solar wind. We are now able to identify when these V_sc changes occur, enabling us to estimate the electron density accordingly.

Conclusions. Solar Orbiter charges negatively during periods of high plasma density and low photoelectron emission. However, the measured V_sc is also influenced by the electrical disconnection of the solar arrays from the spacecraft ground during fuse-blowing events. This disconnection raises the local plasma potential near the probes, causing the measured V_sc to appear negative even when Solar Orbiter remains positively charged. By distinguishing between the genuine negative spacecraft charging and the apparent negative charging induced by solar panels, we can refine previous electron density estimation methods to include cases when V_sc < 0 V. This approach provides reliable, high-resolution electron density estimates across both positive and negative V_sc regimes.