3:00 PM - 3:15 PM
[PEM11-18] Analysis of spatial distributions of Coseismic ionospheric disturbances using HFD
The propagation characteristics of ionospheric disturbances associated with earthquakes have been studied by using GPS observations. Althogh the horizontal propagations of coseismic ionospheric disturbances, are extensively examined . Their vertical propagations are not reported much. Since HF Doppler observation(HFD), which is operated by the University at Electro-Communications(UEC), is enable to receive the four radio waves simultaneously, three-dimentional distributions of various ionospheric disturbances can be examined by this system. In this study, we examined the 3-dimentional distribution of ionospheric disturbances associated with earthquakes using the HFD observation.
The HFD observation used in this study enables us to observe variations at maximumly four altitudes using different transmission frequencies (5.006, 6.055, 8.006 and 9.595 MHz). The two transmitting stations were used; Chofu (UEC) and Radio Nikkei Nagara Transmitting Station, and the data from four receiving stations were used: Iitate, Sugadaira, Kakioka, and Kiso. For the seismometer data, we used the vertical velocity data obtained by the seismometer closest to each HFD reflection points. We examined the coseismic ionospheric disturbances associated with the earthquake occurred at Hamadori, Fukushima Prefecture on April 11, 2011.
HFD observations showed that all the four receiving points used in the analysis showed fluctuations with the periods of 2 to 4 minutes about 10 minutes after the earthquake. We also examined the arrival times of the fluctuations at each HFD reflection points. It is found that the fluctuations observed by the HFD were caused by the Rayleigh wave, not the direct wave from the epicenter. By comparing the vertical velocity of the waves obtained by HFD with those obtained by the seismometers, it was confirmed that the magnitude of the ionospheric disturbance decreased with the distance from the epicenter. However, the height variations of the amplitude of the disturbance were different each other. From this fact, we have to examine them in view of the disturbances in propagation upward.
The HFD observation used in this study enables us to observe variations at maximumly four altitudes using different transmission frequencies (5.006, 6.055, 8.006 and 9.595 MHz). The two transmitting stations were used; Chofu (UEC) and Radio Nikkei Nagara Transmitting Station, and the data from four receiving stations were used: Iitate, Sugadaira, Kakioka, and Kiso. For the seismometer data, we used the vertical velocity data obtained by the seismometer closest to each HFD reflection points. We examined the coseismic ionospheric disturbances associated with the earthquake occurred at Hamadori, Fukushima Prefecture on April 11, 2011.
HFD observations showed that all the four receiving points used in the analysis showed fluctuations with the periods of 2 to 4 minutes about 10 minutes after the earthquake. We also examined the arrival times of the fluctuations at each HFD reflection points. It is found that the fluctuations observed by the HFD were caused by the Rayleigh wave, not the direct wave from the epicenter. By comparing the vertical velocity of the waves obtained by HFD with those obtained by the seismometers, it was confirmed that the magnitude of the ionospheric disturbance decreased with the distance from the epicenter. However, the height variations of the amplitude of the disturbance were different each other. From this fact, we have to examine them in view of the disturbances in propagation upward.