Review of Better Space Weather Proxies for Spacecraft Surface Charging

The National Oceanic and Atmospheric Administration provides four space weather data products targeted at satellite operators: the energetic proton flux, energetic electron flux and ambient magnetic field at the location of a Geostationary Operational Environmental Satellite spacecraft, and the global Kp index. Kp is a global blended measure of the disturbance of the geomagnetic field at mid-latitude ground stations. Elevated levels of the Kp index are associated with magnetic storms and substorms, which have been correlated with spacecraft surface charging. However, a review of the Kp index during recurring solar array circuit failures showed no evidence that they might be caused by surface charging and discharging. Kp in this case was absolutely misleading. Two other sources of data, ground station magnetograms from high-latitude observatories near the auroral arc and X-ray emissions from the atmosphere in the auroral regions, demonstrate that severe substorm activity was occurring at the affected spacecraft immediately preceding nearly every instance of circuit failure. Based on these results, new space weather data products to show the risk of surface charging in geostationary earth orbit are proposed.