The CASSIOPE/e-POP Suprathermal Electron Imager (SEI)

Abstract

The Suprathermal Electron Imager (SEI) on the Enhanced Polar Outflow Probe (e-POP) experiment uses a microchannel-plate-intensified charge-coupled device (CCD) detector to record two-dimensional, energy-angle images of electron distributions for energies up to 350 eV. Alternatively, the SEI can be biased to measure positive ions at energies that include the ambient ionospheric population (<1 eV) and extending to 350 eV. At the highest measurement resolution, distribution images are 64 pixels in diameter and are read out at a rate of 100 images per second. The SEI’s field of view is 360∘×±4∘, and includes viewing of the nadir and ram directions, nominally. At high latitude the nominal orientation allows coverage of most pitch angles. The SEI is included on e-POP to address the mission’s principal scientific objectives, the first being to characterize polar ion outflow and its drivers including ambipolar electric fields generated by suprathermal electron populations, and direct energization of ions by plasma waves or through frictional heating. In addition, the SEI’s focus on low energies and high time resolution allows a unique view of suprathermal particle populations and their role in wave-particle interactions, in support of e-POP’s second scientific objective: to study plasma waves and wave propagation in the high-latitude ionosphere. Observations taken within geophysically quiet regions indicate that the instrument can track bulk ion flow velocity with a resolution of order 25 m/s or better.