Direct measurement of internal charging currents in geostationary transfer orbit

Results are reported from the first flight of SURF, a new instrument designed to measure electrostatic charging effects on satellites. Internal charging currents detected around the geostationary transfer orbit at two different shielding depths are presented in the form of profiles of deposited current against L. Data is limited to three periods: 16th-17th November 2000, 21st-29th November 2000 and 21st-23rd March 2001. Large temporal variations of (negative) charging currents within the outer zone have been observed ranging, at the peak of the belt, over two orders of magnitude. Substantial internal charging currents are also observed within the inner zone, particularly on the less well shielded collector plate. The polarity of the inner belt charging currents is also negative showing the dominance of electron deposition: for the shielding depths used, proton currents are calculated to be small, even at the peak of the inner belt. The new data suggests that, for the higher electron energies (>1 MeV), the inner belt is softer than currently modelled in AE-8.