Geomagnetic Activity Dependence of the Auroral Electron Precipitation Spectrum at High Latitudes

Auroral electron forcing with energy greater than tens keVs impacts D-region ionization in high-latitude regions, exhibiting increased energy from midnight to dawn. This increase, known as spectrum hardening, is characterized by a higher energy at the electron-precipitation peak flux and/or a gradual power-law spectrum gradient. The former relates to the Maxwellian component of the spectrum, while the latter pertains to the Kappa distribution component. However, limited research has distinguished these two components. This study investigated the dependence of the D-region ionization on geomagnetic activity using electron density data from the European Incoherent Scatter (EISCAT) radar in Norway and cosmic noise absorption in Finland, sorted by the SuperMAG Auroral Electrojet index (SME). An inversion method derived differential energy spectra from the EISCAT-measured height-resolved electron density. Statistical spectrum analysis by fitting the Kappa distribution function revealed that spectrum hardening from midnight to dawn at auroral latitudes is mainly driven by the Kappa distribution component, characterized by a gradual power-law spectrum gradient for moderately high geomagnetic activities (SME ≥300 nT). Conversely, for SME <300 nT, the Maxwellian component primarily also contributes to spectrum hardening. This study is the first to specify the energy range contributing to spectrum hardening.