Spacecraft Discharge Time Constants Determined from Electron-Flux
Suppression during Sounding-Radar Operation at Mars
Abstract
Spacecraft discharge time constants are calculated from measurements of
electron differential flux before and during operation of an ionospheric
sounding radar. Determining these time constants provides insight into
how the operation of a sounding radar affects the surrounding plasma’s
interaction with the spacecraft. The analysis is enabled by the
fixed-frequency operation mode of a sounding radar which enhances
resonant interaction with the ambient plasma. This mode’s effect on
measured energy spectra of ion and electron fluxes is described.
Measurements of electron fluxes disturbed by radar operation serve as
input to a model of spacecraft discharge for calculating capacitive
discharge time constants. A case study using electron fluxes measured at
Mars yields discharge time constants in the range 0.6-0.8 ms and reveals
that a residual potential around −4 V remains on the spacecraft long
after radar operation ceases. The minimum spacecraft potential cannot be
determined with these data and model due to the narrow energy range of
electrons in the ambient plasma.