Investigation of Plasma Composition and Small-Scale Density Irregularities on a Non-circular and Non-Sun-Synchronous Polar Low-Earth-Orbit (LEO) Satellite: Swarm-E e-POP Observations in the F-region and Topside Ionosphere-Thermosphere

The Swarm-E/Enhanced Polar Outflow Probe (e-POP) is in an elliptic (non-circular) and non-Sun-synchronous polar orbit (81° inclination, 325 km perigee × 1500 km initial apogee). This gives the satellite a unique vantage point among LEO satellites for observing plasma and related space weather processes in the topside ionosphere and thermosphere, especially the altitude variations of specific physical phenomena. The imaging and rapid-scanning ion mass spectrometer (IRM) on Swarm E combines the technique of ion time-of-flight (TOF), hemispherical electrostatic analysis, and 2D positional ion detection (imaging) to resolve the mass-per-charge (M/q), energy-per-charge (E/q), and incident direction of each detected ion, and to simultaneously measure the incident plasma current at high (1-ms) cadence. Data acquired over the 8-year period from launch (September 2013) to December 2021 has enabled the quantitative investigation of several important magnetosphere-ionosphere-thermosphere (MIT) coupling processes and the altitude distributions and variations of the resulting plasma composition, structure, and dynamics in the F-region and topside ionosphere-thermosphere. These include the effects of atmospheric photoelectrons on spacecraft charging, molecular and nitrogen (N+) ion enhancements in the active-time auroral ionosphere, and decameter-scale structures in equatorial plasma bubbles, for example. We present an overview of investigations of the long- (solar-cycle time scale) and short-term (down to substorm time scale) variations of the various observed features and associated phenomena in the context of their impact on MIT coupling.