Testing spacecraft charging predictions as Parker Solar Probe approaches the Sun

¹ Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, 3665 Discovery Drive, Boulder 80303, CO, United States
² Department of Astrophysical & Planetary Sciences, University of Colorado Boulder, 2000 Colorado Ave, Boulder 80309, CO, United States
³ Department of Physics, University of Colorado Boulder, 2000 Colorado Ave, Boulder 80309, CO, United States
⁴ Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley 94720, CA, United States

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

Received: 28 August 2025
Accepted: 6 March 2026

Abstract

Context. Surface charging must be carefully simulated and predicted for any spacecraft, but especially for those encountering highly variable plasma environments, such as spacecraft near the Sun. Parker Solar Probe (PSP) is a NASA mission that makes in situ solar wind measurements between 9.8 and 155 solar radii (R_S). Prior to launch it was predicted that the plasma and photon environment near the closest solar approach would result in large negative spacecraft voltages. Charging of the predicted magnitude (−10 V to −100 V) would significantly modify the measured electron and ion distributions and potentially disturb electric field measurements.

Aims. Multiple surface charging models of PSP that were run prior to launch agreed that PSP should charge negatively as it approaches the Sun. This work aims to compare observational voltage data against pre-launch charging model predictions to investigate the physics of spacecraft charging in near-Sun plasma and photon conditions.

Methods. PSP measurements of spacecraft floating potential are evaluated and compared against pre-launch models. Numerical models and analytic estimates are employed to evaluate how various parameters impact PSP spacecraft surface charging, including variations in the ambient plasma conditions, surface resistance changes brought on by variations in temperature and photoelectron flux, consideration of higher-fidelity spacecraft geometry, and the presence of a second higher-energy population of photoelectrons.

Results. The observed PSP spacecraft floating potential is positive for the majority of each solar encounter at close solar radial distances (R < 25 R_S). This directly disagrees with pre-launch simulation predictions. Multiple possibilities for the data-model discrepancy are explored, such as secondary electron emission, surface resistance changes, and photoelectron energy distributions. Simulations reveal that the most effective positive charging mechanism is an enhanced photoelectron yield, which may be caused by a population of higher-energy photoelectrons and lower than predicted plasma densities at perihelion.