Determination of Swarm Front Plate's Effective Cross Section From Kinetic Simulations

The front plates and embedded particle sensor shells that are part of the electric field instrument (EFI) on the Swarm satellites have recently been used as planar Langmuir probes, as an additional diagnostic tool to infer environment parameters. The interpretation of measured currents in terms of the plasma density or incoming flow speed, however, requires a knowledge of the front plate effective cross section A(eff). Measurements made under various space plasma conditions have led to the conclusion that this cross section is generally larger than the known geometrical cross section A(geo). Interpretations of measurements have thus been made using fixed relative enhancements of A(geo) ranging from 8% to 17%. In this paper, results from kinetic simulations are presented, from which the effective cross section can be determined over a range of plasma parameters. These are used to shed light on the physical mechanisms responsible for this enhancement and construct an empirical fit to the relative enhancement delta, where A(eff) = A(geo)(1 + delta), and, in turn, enable improvements in the accuracy of inferred plasma parameters.