SOLAR-WIND AND MAGNETOSPHERE PLASMA DIAGNOSTICS BY SPACECRAFT ELECTROSTATIC POTENTIAL MEASUREMENTS

Several satellites (GEOS-1, GEOS-2, ISEE-1, Viking and CRRES) carried electric field experiments on which probes were driven by a current fr om the satellite to be close to the plasma potential. The potential di fference between an electric field probe and its spacecraft (with cond uctive surfaces) can be used to determine the ambient electron density and/or electron flux with limited accuracy but with high time resolut ion, of the order of 10-100 ms. It is necessary for the development of this diagnostic method to understand the photoemission characteristic s of probes and satellites. According to the electric field experiment s on the above-mentioned satellites, all materials develop very simila r photoemission properties when they are beyond the influence of atmos pheric oxygen. The photoelectron yield steadily increases over the fir st few months in space and reaches values well above those measured on clean surfaces in the laboratory. The method can be used for solar ra diation levels corresponding to distances from 0.4 to 5 AU from the Su n.