CHAMP/SWARM Floating Potential Modelling
Swarm is a 3-satellite mission that will make precise high-resolution magnetic field measurements in low Earth orbit (SWARM mission). Each satellite carries an Electric Field Instrument (EFI) which is used to make corrections to the magnetic field measurements. However, the satellite interacts with the local plasma and may induce local electric fields that disrupt these measurements. To be certain that this will not significantly interfere with EFI, modelling of the spacecraft-plasma interaction has been performed both analytically and numerically with the Spacecraft Plasma Interaction Software (SPIS).
It was found that the spacecraft potential could be kept between -0.2 and -0.6V by placing a conductive plate on the ram face of the spacecraft. This is low enough to minimize electric fields due to the spacecraft potential. Placing the electric field instrument on this same ram face avoids electric fields due to the wake structure around the spacecraft. Without the conductive plate, the spacecraft potential would be more variable and its value would depend on the detailed layout of thousands of tiny metallic interconnects on the solar panel.
The Swarm satellite is based on the design of the earlier CHAMP (CHAllenging Mini-satellite Payload) satellite (see the CHAMP mission homepage). One significant difference between the two missions is the satellite orientation with respect to their direction of travel. For Swarm the boom will be in the wake while for CHAMP it was in the ram direction. A Langmuir probe on CHAMP was used to determine the electron temperature, density, and plasma potential with respect to the satellite floating potential. As a validation exercise the CHAMP floating potential was also modelled using SPIS. The modelled and observed potentials both lay in the region of 0V to -1V. However, a detailed correlation between the model and Langmuir probe results was not successfully found. In fact, it was determined, from Langmuir probe theory, that the potential and temperature measurements were inconsistent. This pointed to a problem with the data processing. At present, the raw data have not been re-processed to correct this problem and so the validation has not been completed.