Aberrations in Particle Distribution Functions Near e-POP Particle Sensors

Simulation results are presented to assess the effect of the electrostatic sheath surrounding enhanced Polar Outflow Probe and its geometry on the distribution function of particles entering the two sensors. The sensors considered are the imaging and rapid-scanning ion mass spectrometer (IRM) and the suprathermal electron imager (SEI). IRM is a boom-mounted ion imager with a 360° circular aperture. It has an electrostatic toroidal deflector capable of selecting the angle of incident ions with respect to the boom axis, which effectively enables 3-D measurements of ion distribution functions over approximately half the steradians of the sphere in velocity space. SEI, also boom-mounted opposite IRM, is used to measure 2-D distribution functions in the plane of its 360° aperture perpendicular to the boom axis. Due to the strength and finite thickness of the sheath surrounding sensors, there result aberrations in the angular distribution functions at both apertures. Comparisons are made between the calculation of these aberrations using a simple thin sheath model and a fully kinetic calculation, and it is found that the thin sheath model should provide a good approximation of sheath effects over a broad range of parameters. The effects of finite electron gyroradii and the relative position of the sensor with respect to the spacecraft body are also considered for the electron velocity distribution function at different positions around the SEI sensor. Occlusion of gyrating electrons by the spacecraft is shown to lead to holes in velocity distributions at certain locations around the SEI aperture, thus leading to aberrations in measured distribution functions.