[P-EM15] Dynamics of Magnetosphere and Ionosphere

[PEM15-06] Geomagnetic activity dependence of the auroral electron precipitation spectrum derived from 22-year EISCAT dataset

*大山 伸一郎^1,2、Virtanen Ilkka³、Tesfaw Habtamu³、Raita Tero⁴、Holappa Lauri³、三好 由純¹、Cai Lei³、Vanhamaki Heikki³、Aikio Anita³、小川 泰信²、細川 敬祐⁵ (1.名古屋大学宇宙地球環境研究所、2.国立極地研究所、3.オウル大学、4.オウル大学・ソダンキュラ地球物理観測所、5.電気通信大学)

キーワード：オーロラ、電子降下、EISCAT、ELSPEC

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 (CNA) in Finland, sorted by the SuperMAG 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.