11:00 〜 13:00
[PEM13-P06] Propagation characteristics of MSTIDs at different altitudes obtained by HF Doppler sounding and GPS-TEC observations
キーワード：中規模伝搬性電離圏擾乱、HFドップラー観測、GPS-TEC観測
Medium-Scale Traveling Ionospheric disturbances (MSTIDs) are frequently observed in the Northern Hemisphere on summer nights and winter days. The generation of MSTIDs is caused by the Perkins instability in summer and atmospheric gravity waves in winter (Otsuka et al., 2021). Although the horizontal propagations of MSTIDs have been examined by many researchers, their three-dimensional structure has not yet been fully investigated. Since the HF Doppler sounding in this study utilize radio waves at multiple frequencies, the ionospheric disturbances at the different altitude can be obtained. Therefore, we used HF Doppler sounding and GPS-TEC observations to examine the three-dimensional structures and propagation characteristics of MSTIDs. In addition, the direction of the horizontal propagation can be investigated by using data from multiple observation points. The Doppler-shift data were obtained by the radio waves of 5.006 MHz and 8.006 MHz with a temporal resolution of 10 seconds. The temporal resolution of GPS-TEC data was 30 seconds. The altitude of the ionospheric pierce points was set at an altitude of 300 km, which is higher than the HF Doppler sounding, and thus allows for analysis over a vertically wider altitude.
MSTID that occurred on 2014/11/10 00:00 UT ~ 07:00 UT was analyzed using the Doppler-shift data observed at Sugadaira, Oarai, and Fujisawa. The bandpass filter of 0.2-1.5 mHz was utilized to the Doppler data. In the horizontal direction, MSTIDs observed by 5.006 MHz (the altitude of the reflection point was 140 km) propagated south-east ward at 146 m/s. On the other hand, those by 8.006 MHz (185 km) also propagated south-east ward, but their speeds were 179 m/s. The direction of propagation was almost the same at both altitudes (140 km/185 km) and was also consistent with those obtained by TEC fluctuation. However, the propagation velocity of the MSTIDs at higher altitude was about 30 m/s faster than that at lower altitude. This means that, the wavefront of MSTIDs observed at lower altitude was in lead over that at higher altitude, indicating that the MSTIDs were propagating with a backward tilt to the propagation direction. With the passage of time, the inclination of the wavefront became smaller. This is because the speed of MSTID at higher altitude were faster than those at lower altitude.
MSTID that occurred on 2014/11/10 00:00 UT ~ 07:00 UT was analyzed using the Doppler-shift data observed at Sugadaira, Oarai, and Fujisawa. The bandpass filter of 0.2-1.5 mHz was utilized to the Doppler data. In the horizontal direction, MSTIDs observed by 5.006 MHz (the altitude of the reflection point was 140 km) propagated south-east ward at 146 m/s. On the other hand, those by 8.006 MHz (185 km) also propagated south-east ward, but their speeds were 179 m/s. The direction of propagation was almost the same at both altitudes (140 km/185 km) and was also consistent with those obtained by TEC fluctuation. However, the propagation velocity of the MSTIDs at higher altitude was about 30 m/s faster than that at lower altitude. This means that, the wavefront of MSTIDs observed at lower altitude was in lead over that at higher altitude, indicating that the MSTIDs were propagating with a backward tilt to the propagation direction. With the passage of time, the inclination of the wavefront became smaller. This is because the speed of MSTID at higher altitude were faster than those at lower altitude.