What measurements in space weather are needed for predicting spacecraft charging?

Summary form only given. By solving the current balance equations, the author obtains solutions for various space plasma conditions under which spacecraft surface charging may occur. When the space plasma is quiet, the photoemission from surfaces in sunlight often exceeds the ambient plasma current collected by the surfaces. In that situation, positive charging likely occurs but the charging level must be low (a few volts). Without sunlight or when the space plasma becomes energetic, negative charging is more likely to occur. A Maxwellian distribution is often a good approximation for modeling the space plasma at equilibrium. Without sunlight, the current balance equation in the Maxwellian model gives a unique solution: the plasma temperature. When the plasma temperature exceeds a critical value, negative charging occurs. The critical temperature depends on the surface property. The author presents a table of critical temperatures for common surface materials. In a Maxwellian plasma without sunlight, the plasma temperature is the only parameter needed for predicting charging. During magnetic disturbances, energetic plasma may arrive and, as a result, the distribution sometimes resembles a double Maxwellian. In such a situation, the interplay between the two Maxwellian components is often crucial in determining charging. While the low energy (few hundred eV) Maxwellian component may favour outgoing secondary electrons, the high energy (keVs) component does not. The author presents examples showing that when the first Maxwellian density (and the average plasma density) is decreasing, negative charging appears not only rapidly but also to high levels during eclipse. In such a situation, measurements of the average plasma density or temperature are insufficient for predicting charging. Instead, one needs to measure the Maxwellian densities and temperatures. When the space plasma distribution cannot be fitted well with a single and double Maxwellian, one can still use t- e distribution for spacecraft charging calculation. Although one can input into a computer model any given distributions as they appear and output numerical results, the author advocates decomposing the distribution into a sum of several Maxwellians in a Fourier fashion. Measurements of the several Maxwellian densities and temperatures would be more useful. The interplay between the various Maxwellian components gives deeper physical insight, better prediction of the evolution of the components and hence better prediction of spacecraft surface charging.