Electron density estimation from measurements of spacecraft potential using Solar Orbiter

Context. Measurements of spacecraft potential (Vsc) are an important tool for plasma diagnostics. High-resolution Vsc measurements can be calibrated with low-cadence observations of electron plasma frequency to derive high-resolution electron density data. Aims. We analyze Vsc measured by the Radio and Plasma Waves (RPW) instrument suite on board Solar Orbiter. We investigate the evolution of Vsc 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 Vsc regimes, including the cases of negative Vsc. Methods. We determined Vsc by combining measurements of the probe-to-spacecraft potential, Vps, and the probe-to-plasma potential, Vpn. Then, we used the appropriate electric current balance to find a relation between electron density and Vsc, which we calibrated to the observed electron plasma frequency. With this method, we obtained the electron density at the resolution given by the Vps measurements. Results. We find that Vsc 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 Vsc 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 Vsc 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 Vsc 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 Vsc < 0 V. This approach provides reliable, high-resolution electron density estimates across both positive and negative Vsc regimes.