Anti-critical Temperature for Spacecraft Charging

In recent years, evidence has been found for the existence of a critical temperature for the onset of spacecraft charging to high voltages. High-voltage charging affects scientific instruments on board and is related to spacecraft anomalies. However, less attention has been given to low-voltage charging which can also affect scientific experiments on board and is relevant to surface chemistry. There also can exist an anticritical temperature for low-voltage spacecraft surface charging. Ambient electrons at very low temperatures tend to cause negative surface charging, albeit at low voltages, and as the electron temperature increases, the charging ceases at a critical value depending on the surface material. We present the theory and numerical results of anticritical temperatures for typical surface materials in Maxwellian space plasmas. The change in anticritical temperature due to a low-incident-energy enhancement of the electron backscatter yield, consistent with recent measurements, is discussed. Approximate expressions for the anticritical temperature upper limits are given on the basis of Taylor expansions at low temperature of the charging onset equation. It is shown that the existence of the anticritical temperature slightly modifies the possible triple-root configurations in the flux-voltage characteristic curve for a material. The surface charging effect of a Maxwellian plasma with flux components spanning the anticritical and critical temperatures is considered. A comparison with an empirical low-voltage charging curve is given.