Dramatic Influence of Temperature on Charging

Charging of cables outside of a spacecraft has been cited as the cause of multiple spacecraft anomalies based upon correlations with peaks in the energetic electron environment and because the electron flux exposure of external cables is much higher than internal cables. However, external cables are subject to much wider temperature changes than hardware located inside a thermally regulated spacecraft. Temperature affects bulk resistivity and radiation-induced conductivity, but temperature is often ignored in charging analyses and tests. In this paper, temperature effects are combined with a diurnal temperature variation and an external electron environment to simulate the charging of an external coax. The simulation shows that the coax dielectrics behave as perfect insulators during the cold portion of the 24 h cycle, but the accumulated charge leaks out during the hot portion of the cycle. There is no memory or hysteresis of charge density from one day to the next. This behavior is compared to the behavior of materials inside the spacecraft, where low flux and stable temperatures lead to charging time constants ranging from weeks to months that produce a slow, stair-step increase of trapped charge in response to multiple electron storms. These variations in charging response explain why some anomalies correlate best with peak 24 h average flux, whereas others correlate well with 14- or 21-day averaged flux. Analyses and tests used to qualify designs or investigate anomalies must account for the dramatic effects that temperature variation can have on the charging of insulators and isolated conductors.