10:45 AM - 12:15 PM
[PEM12-P28] Analysis of concentric ionospheric disturbances associated with typhoon Faxai in 2019 using HF Doppler sounding
Keywords:Typhoon Faxai, Concentric Traveling Ionospheric Disturbances, HF Doppler Sounding
Typhoon Faxai (No.15 in 2019) was one of the quite powerful typhoons, which came to and passed over Japan on September 8th and 9th, 2019. The minimum central pressure and the maximum wind speed of Faxai were 955 hPa and 57.5 m/s, respectively. It landed on the Boso peninsula at 20:00UT on September 9th, causing serious damage, including building corruption, wide range blackout in the south Kanto region, and water outage. Therefore, it is called the Reiwa Gan-nen Boso peninsula typhoon. Song et al.(2022) observed two Concentric Traveling Ionospheric Disturbances associated with Faxai using Total Electron Content obtained by the GNSS navigation system.
In this study, we observed the ionospheric disturbances with HF Doppler sounding system. On quiet days and after the passage of Faxai, diurnal variations in Doppler shift were confirmed. On the other hand, fluctuations around 0 Hz were observed and there were no diurnal variation on when the typhoon approached. This suggests that the ionospheric disturbances on the 8th were large enough to cancel out the diurnal fluctuations. Dynamic spectral analysis was performed to investigate the characteristics of ionospheric disturbances.It was confirmed that the spectral intensities increased on the 8th. In particular, those at 0.5 to 1.5 mHz component ware increased. The period of these fluctuations corresponds to 10 to 30 minutes. Song et al.(2022) also showed the existence of the fluctuations with a period of 18 to 22 minutes, indicating that the disturbances obtained by HF Doppler Sounding were ionospheric disturbances caused by Faxai. We calculated the time delay with a cross-correlation to determine the horizontal velocity and azimuthal angle of these disturbances. The correlation had a maximum value of 0.8, and we found that the Traveling Ionospheric Disturbances propagated from Fujisawa to the Northern and Western paths of Japan. Assuming that the disturbances were treated as plane waves, we calculated the horizontal velocity and azimuthal angle. We found that the plane wave approximation can only be applied to the local area around Fujisawa, Sugadaira, and Iitate. The horizontal velocity was calculated to be 200.35 m/s. This is consistent with the horizontal velocity of 174-200 m/s reported by Song et al.(2022). This result suggests that the ionospheric disturbances associated with Faxai propagated at the same speed at all altitudes.
In this study, we observed the ionospheric disturbances with HF Doppler sounding system. On quiet days and after the passage of Faxai, diurnal variations in Doppler shift were confirmed. On the other hand, fluctuations around 0 Hz were observed and there were no diurnal variation on when the typhoon approached. This suggests that the ionospheric disturbances on the 8th were large enough to cancel out the diurnal fluctuations. Dynamic spectral analysis was performed to investigate the characteristics of ionospheric disturbances.It was confirmed that the spectral intensities increased on the 8th. In particular, those at 0.5 to 1.5 mHz component ware increased. The period of these fluctuations corresponds to 10 to 30 minutes. Song et al.(2022) also showed the existence of the fluctuations with a period of 18 to 22 minutes, indicating that the disturbances obtained by HF Doppler Sounding were ionospheric disturbances caused by Faxai. We calculated the time delay with a cross-correlation to determine the horizontal velocity and azimuthal angle of these disturbances. The correlation had a maximum value of 0.8, and we found that the Traveling Ionospheric Disturbances propagated from Fujisawa to the Northern and Western paths of Japan. Assuming that the disturbances were treated as plane waves, we calculated the horizontal velocity and azimuthal angle. We found that the plane wave approximation can only be applied to the local area around Fujisawa, Sugadaira, and Iitate. The horizontal velocity was calculated to be 200.35 m/s. This is consistent with the horizontal velocity of 174-200 m/s reported by Song et al.(2022). This result suggests that the ionospheric disturbances associated with Faxai propagated at the same speed at all altitudes.