A unique electrical hardware configuration was included onboard 21 SpaceX Starlink satellites to enable the direct detection of electrical arc discharge events resulting from spacecraft charging phenomena. Given the relatively high number of independent satellites equipped with arc detection circuits, arc discharge events were observed with high spatial resolution between +/- 43°N latitude at altitudes ranging from 330 to 390 kilometers over the course of one year, from May 2023 to May 2024. Using the observed rate of arc discharge events as a proxy for local plasma parameters, a map of the Appleton Anomaly was generated. The arc detection circuit functionality was validated through on-orbit testing and ground testing. Ground tests were conducted a plasma source and representative spacecraft hardware within a high-vacuum facility. On-orbit experiments validated the correlation between arc discharge events and spacecraft charging phenomena – a plasma contactor was used to measure neutralization current from the spacecraft to its surrounding ambient plasma environment, and satellite orientation studies were conducted to examine the impact of the spacecraft’s ion wake on arc rate. The results of the on-orbit and ground tests support a hypothesis that the frequency of arc discharge events is directly related to the local plasma density. A proxy map of plasma density was generated, which shows strong agreement with the International Reference Ionosphere model (IRI-2016) and the World Magnetic Model (WMM).
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