[P-EM12] Coupling Processes in the Atmosphere-Ionosphere System

[PEM12-30] Horizontal inhomogeneity in the D-region ionosphere due to X-class solar flares by OCTAVE VLF/LF observations

*中山 雅晴¹、大矢 浩代⁵、土屋 史紀²、塩川 和夫³、野崎 憲朗⁴、中田 裕之⁵ (1.千葉大学大学院融合理工学府、2.東北大学大学院理学研究科、3.名古屋大学宇宙地球環境研究所、4.電気通信大学、5.千葉大学大学院工学研究院)

キーワード：太陽フレア、VLF、LF、電離圏

When solar flares occur, electron density in the ionosphere (60-100 km altitude) increases because of intense X-rays. So far, relationship between VLF (3-30 kHz) and X-ray flux has been reported (e.g., Raulin et al., 2010), although there are few reports for horizontal inhomogeneity of the reflection height in the D-region ionopshere. The purpose of this study is to reveal horizontal homogeneity of electron density in the D-region ionosphere during a X-class solar flare using multi-path VLF/LF (30-300 kHz) transmitter signals of "Observation of CondiTion of ionized Atmosphere by VLF Experiment (OCTAVE)" network. When solar flares occur, VLF/LF amplitude and phase vary with decreasing the reflection height. The transmitters used in this study were NWC (21.817S, 114.167E, 19.8 kHz), JJI (32.05N, 130.82E, 22.2 kHz), JJY (37.37N, 140.85E, 40.0 kHz; 33.47N, 130.18E, 60.0 kHz), and BPC (34.63N, 115.83E, 68.5 kHz). The receivers were located at KAG (Tarumizu, Kagoshima, Japan, 31.59N, 130.55E), PKR (USA, 65.125N, 147.488W), and RKB (Rikubetsu, Hokkaido, 43.45N, 143.77E), which are part of OCTAVE network. A X2.2-class solar flare occurred at 08:57 UT on 6 September, 2017. During the solar flare, amplitudes of variations in the VLF/LF amplitude (delta A) and phase (delta P) were 2.65-14.73 dB and 31.0-150.25 degrees, respectively. Using wave-hop method, we estimated reduction in reflection height (delta h) from the observed delta A and delta P. When the reference height was assumed to be 78.08-84.99 km, the delta h were estimated to be 1.0-4.8 km for BPC-KAG, BPC-RKB, JJY40-KAG, JJY60-RKB and JJY40-RKB paths. When c (the solar zenith angle) was small (large), the delta h was large (small). The difference in the delta h for each path would be caused by distance between the sub-solar point and each path, and sunset effects. Based on the IRI(International Reference Ionosphere)-2016 model, variations in electron density, c=64.34 was larger (5465.7 cm^-3 ) than that with c =80.18 (200.4 cm^-3) at 85 km heights. We found that there was such homogeneity for D-region ionization due to the solar flare. In this presentation, we will discuss the horizontal inhomogeneity of the reflection height and electron density during the solar flare in detail.