[P-EM09] 宇宙天気・宇宙気候

[PEM09-22] Atmospheric ionization caused by X-rays, solar protons, and energetic electrons in September 2017 space weather event

*村瀬 清華¹、片岡 龍峰^1,2、西山 尚典^1,2、佐藤 薫³、堤 雅基^1,2、田中 良昌^1,2、小川 泰信^1,2 (1.総合研究大学院大学、2.国立極地研究所、3.東京大学)

キーワード：高エネルギー粒子降下、大気電離

On September 11, 2017, when the proton flux peaked in the geostationary orbit, mesospheric echoes were observed down to ~42 km, the lowest altitude ever observed by the PANSY radar at Syowa Station. The ionization rate calculated via PHITS (Particle and Heavy Ion Transport code System) air-shower simulation using proton fluxes obtained by the GOES satellite peaked at ~40 km, suggesting that the echo power is enhanced by the increase in electron density due to proton precipitation. The estimated intensity of cosmic noise absorption (CNA) from the resultant ionization rate was ~2.8 dB, which consistently explains the maximum intensity of observed CNA of ~3.0 dB. X-ray fluxes observed by the GOES satellite were also used as input data for PHITS to estimate the observed increase in CNA due to the two X-class flares at ~9 and ~12 UT on September 6. At ~1345 UT on the same day, a transient low-altitude echo at <50 km and CNA spike (~2.0 dB) accompanied by Pc1 geomagnetic pulsations were also observed, which is likely a relativistic electron precipitation (REP) event associated with electromagnetic ion cyclotron (EMIC) wave activity. The ionization rate due to the MeV electrons is also evaluated by PHITS. This event provides a rare opportunity to cross-validate the use of PHITS simulation for variable types of inputs (X-rays, protons, and electrons) to evaluate the atmospheric ionization.