Ram/Wake and Surface Layer Effects on DC Electric Field Measurements in LEO

The USAF Communication/Navigation Outage Forecast System satellite, launched into an eccentric low earth orbit (401 km perigee by 867 km apogee) of 13° inclination on April 16, 2008, has a set of dc electric field probes that constitute part of the Vector Electric Field Investigation (VEFI). In order to obtain the ambient electric field, the v×B component of electric field must be subtracted from the VEFI measurements. After this subtraction and the subtraction of the ambient dc electric components, a residual dc offset directed toward the spacecraft wake is still observed, which varies somewhat within an orbit and on longer timescales. One of the interesting features of these offsets is that when the satellite is occasionally rotated, the offsets are reset to their baseline values, only to come back within a month or so. Various hypotheses have been proposed to explain the residual dc offsets. In this paper, we explore the possibilities that either the influence of the spacecraft wake on the sensors or that modified surface layers on the probe surfaces are producing the offsets. Nascap-2k and EWB models are used to show the various influences of the wake and of surface materials. Finally, a hypothesis is produced that quantitatively explains many of the salient features of the offsets. The feasibility of using dc electric field probes in space is reaffirmed. Recommendations for probe construction on future spacecraft to ameliorate spurious effects are presented.