Sheath and Magnetic field Effects on Swarm Electric Field Instrument (EFI)
Marchand, Richard1; Burchill, Johnathan2; Knudsen, David2
1University of Alberta, CANADA; 2University of Calgary, CANADA

Results from a combination of Particle In Cell (PIC) and test particle simulations are presented to characterize and calibrate the effect of magnetic fields on the Thermal Ion Imager (TII), which is part of the Swarm Electric Field Instrument (EFI). Each of the three Swarm satellites, presently scheduled for launch in the summer 2013, will determine ionospheric electric fields with a pair of TIIs mounted on the ram face, with which three dimensional ion distribution functions will be measured. These distribution in turn will serve to infer the ambient electric field from the relation E = -v xB, where v is the plasma drift velocity measured in the satellite rest frame, and B is the local magnetic field. The relation between v and the distribution of ion fluxes on the pixel array in the TII is straightforward in the absence of a sheath. It has been shown, however, that the ambient electric field, combined with the sheath electric field, leads to significant distortions in the particle distribution functions at the aperture of the TII sensors. These in turn result in shifts in the ion flux profiles on the pixel arrays of the TIIs. A systematic characterization of these effects is obtained from simulations of the interaction between the ram face of Swarm and space plasma for representative latitudes, longitudes, ionospheric plasma, diurnal and seasonal conditions. Plasma parameters are estimated from the IRI ionospheric model and, in each case, the magnetic field is obtained from the IGRF model for actual spacecraft orbital parameters. By parametrizing the shifts resulting from the combination of the sheath and ambient E = -v xB electric field, it is then possible to correct for the associated aberration in the measured velocity and, hence, improve the sensitivity of the TII sensor in the measurement of the plasma flow velocities.