Representation of the measured geosynchronous plasma environment in spacecraft charging calculations

[1] Historically, the characterization of the magnetospheric environment has limited our ability to determine spacecraft surface charging levels. One difficulty lies in the common practice of fitting the plasma data to a Maxwellian or Double Maxwellian distribution function, which may not represent the data well for use in spacecraft charging simulations. We use electron and ion flux spectra measured by the Los Alamos National Laboratory (LANL) Magnetospheric Plasma Analyzer (MPA) during eclipse in September 2001 to examine how the use of different spectral representations of the charged particle environment in computations of spacecraft potentials during magnetospheric substorms affects the accuracy of the results. We examine charging and noncharging flux spectra and the relationships between the density and temperature moments. We then calculate the spacecraft potential (zero net current) using both the measured fluxes and several different fits to these fluxes. The potential computed using the measured fluxes and secondary and backscattered fluxes computed for graphite carbon, with a constant fraction of 81% of secondary electrons escaping, is within a factor of three of the measured potential for 87% of the data. Potentials calculated using a Kappa function fit to the electron flux and a Maxwellian function fit to the ion flux agree with measured potentials nearly as well. Alternative spectral representations give less accurate estimates. The use of all the components of the net flux, along with spacecraft specific average material properties, gives a better estimate of the spacecraft potential than the measured flux from a single high-energy channel.