11:00 AM - 1:00 PM
[PEM13-P15] Large solar flare effects on the D-region ionosphere using VLF/LF transmitter signals
When solar flare occur, electron density in the ionosphere (60-100 km altitude) increases because of intense X-rays. So far, relationship between Very Low Frequency (VLF, 3-30 kHz) and flare X-ray flux has been reported (e.g., Raulin et al., 2010), although there are few reports for horizontal distribution of the lower ionospheric reflection height of the VLF waves. The purpose of this study is to reveal horizontal distribution of electron density in the D-region ionosphere during large solar flares using multi-path VLF/Low Frequency (LF, 30-300 kHz) transmitter signals. 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 one of OCTAVE (Observation of CondiTion of ionized Atmosphere by VLF Experiment) network. A X2.2-class solar flare occurred at 8:57 UT on 6 September, 2017. During the solar flare, amplitudes of variations in the VLF/LF amplitude (ΔA) and phase (ΔP) were 2.65-14.73 dB and 31.0-150.25 degrees, respectively. Based on wave-hop method, we estimated reduction in reflection height (Δh) from the observed ΔA and ΔP. When the reference height was assumed to be 84.99 km, 82.59 km, 90.73 km, the Δh were estimated to be 4.8 km, 2.5 km and 0.4 km for BPC-KAG, BPC-RKB and JJY60-RKB paths, respectively. The difference in the Δh would be caused by difference in the latitudes of the VLF/LF paths and sunset effects during the solar flares. In this presentation, we will discuss the horizontal difference of the reflection height during the solar flare in detail.