Impact of Plasma Sheath on Rocket-based E-region Ion Measurements

We model the particle velocity distribution functions around the entrance window of the Suprathermal Ion Imager (SII) to assess the impact of electrostatic sheath on ion measurements in the E-region ionosphere. The SII sensor is an electrostatic analyzer that measures two dimensional slices of the distribution of the kinetic energies and arrival-angles of low energy ions. The study is concerned with the interpretation of data obtained from the sensor SII that was affixed to a 1-m NASA rocket 36.234 as part of the Joule II mission to investigate Joule heating in the E-region ionosphere. The rocket flew into quiet auroral conditions above Northern Alaska on 19 January 2007. The payload was spin-stabilitized with a period of 1.6 s, giving an apparent rotation of the ion flow velocity in the frame of reference of the SII. We numerically investigate the ram velocity effect on the ions velocity distributions in the vicinity of SII aperture at an altitudes of approximately 150km. The electrostatic sheath potential profiles surrounding the sensor and payload are calculated numerically with the PIC code PTetra. It is observed that the direction of the ion flow velocity vector modifies the plasma sheath potential profile. This in turn impacts the velocity distributions of molecular oxygen and Nitric oxideions at the aperture of the particle sensor. The velocity distribution functions are calculated by using test-particle modeling. These particle distribution functions are then used to inject the particles in the particle sensor, and to calculate the fluxes on the sensor microchannel plate (MCP).