What measurements in space weather are needed for predictingspacecraft charging?

摘要

Summary form only given. By solving the current balance equations,the author obtains solutions for various space plasma conditions underwhich spacecraft surface charging may occur. When the space plasma isquiet, the photoemission from surfaces in sunlight often exceeds theambient plasma current collected by the surfaces. In that situation,positive charging likely occurs but the charging level must be low (afew volts). Without sunlight or when the space plasma becomes energetic,negative charging is more likely to occur. A Maxwellian distribution isoften a good approximation for modeling the space plasma at equilibrium.Without sunlight, the current balance equation in the Maxwellian modelgives a unique solution: the plasma temperature. When the plasmatemperature exceeds a critical value, negative charging occurs. Thecritical temperature depends on the surface property. The authorpresents a table of critical temperatures for common surface materials.In a Maxwellian plasma without sunlight, the plasma temperature is theonly parameter needed for predicting charging. During magneticdisturbances, energetic plasma may arrive and, as a result, thedistribution sometimes resembles a double Maxwellian. In such asituation, the interplay between the two Maxwellian components is oftencrucial in determining charging. While the low energy (few hundred eV)Maxwellian component may favour outgoing secondary electrons, the highenergy (keVs) component does not. The author presents examples showingthat when the first Maxwellian density (and the average plasma density)is decreasing, negative charging appears not only rapidly but also tohigh levels during eclipse. In such a situation, measurements of theaverage plasma density or temperature are insufficient for predictingcharging. Instead, one needs to measure the Maxwellian densities andtemperatures. When the space plasma distribution cannot be fitted wellwith a single and double Maxwellian, one can still use the distributionfor spacecraft charging calculation. Although one can input into acomputer model any given distributions as they appear and outputnumerical results, the author advocates decomposing the distributioninto a sum of several Maxwellians in a Fourier fashion. Measurements ofthe several Maxwellian densities and temperatures would be more useful.The interplay between the various Maxwellian components gives deeperphysical insight, better prediction of the evolution of the componentsand hence better prediction of spacecraft surface charging