3:30 PM - 5:00 PM
[PEM15-P13] Spatial structure of sporadic E: simultaneous observations with HF Doppler sounding and MU radar in Japan
Keywords:Sporadic E layer
Sporadic E (Es) layer is a layer of extremely high electron density that occurs mainly in summer at mid-latitudes around 100 km altitude. Es layer has been known to cause long-range anomalous propagation of radio waves in the VHF band because of the reflection due to the increased electron density associated with Es layer. Es layer has been studied for more than half a century using various instruments because of its influence on aeronautical navigation systems and radio broadcasts that use the VHF band. However, its spatio-temporal characteristics are still not well understood, especially we have not yet derived its three-dimensional spatial structure by combining observations with multiple instruments.
The HF Doppler (HFD) observation, operative in Japan, can detect vertical motion of ionosphere and movement of events in ionosphere. The HFD observation is known to observe characteristic (quasi-periodic) Doppler spectral variations associated with Es layer, mainly at night during the summer season. Comparing the characteristics of such quasi-periodic Doppler traces with the structure of Medium-Scale Traveling Ionospheric Disturbances (MSTIDs) visualized by GPS-TEC, it was found that they (i.e., Es layer and MSTIDs) propagate in the same direction at the same speed. This suggests that Es layer and MSTIDs propagate in tandem due to the electrical coupling between the E and F layers (Matsushima et al., 2022).
However, it is still unclarified what kind of spatial structure of Es layer is reflected in the quasi-periodic Doppler traces observed in HFD. In particular, the relationship with the quasi-periodic coherent radar echoes (QP echoes: Yamamoto et al., 1991) observed during summer nighttime Es layer cases is still unclear. Therefore, in this study, we conducted simultaneous observations of Es layer using HFD and MU radar on eight nights (May 23-26 and June 6-9, 2022) in order to directly compare nighttime Es layer observed by two different methods. During three nights in May, the MU radar detected clear signatures of QP echoes. In addition, we confirmed that quasi-periodic Doppler traces were observed in the HFD data obtained in Awaji station.
The two observation methods would have detected same Es layer because the difference in the timing of detection was small, about 10–20 minutes. The time lag would have been due to the location of the reflection point of the HFD observation in Awaji, which is about 200 km away from the MU radar observation area. When the MU radar clearly detected Es layer, the difference in speed between MSTIDs and Es layer was less than 20 %. This indicates that Es layer and MSTIDs propagated in tandem. Based on this fact, the time needed for propagation of MSTIDs between observation areas of MU radar and HFD was calculated by cross-correlation analysis of GPS-TEC data. The estimated travel time was almost equal to the time lag in the Es layer observation between the MU radar and HFD. Therefore, we conclude that the time lag of Es layer observation was due to the difference of the observation areas, which further confirms that the quasi-periodic structures in the Doppler spectra of HFD measurements are manifestations of QP echoes seen in data from coherent radars such as the MU radar.
To evaluate the identity of the two fine structures of Es layer, we plan to reconstruct 2D images of the QP echoes from the imaging observations made by the MU radar and compare them with the quasi-periodic Doppler traces seen in the HFD measurements.
The HF Doppler (HFD) observation, operative in Japan, can detect vertical motion of ionosphere and movement of events in ionosphere. The HFD observation is known to observe characteristic (quasi-periodic) Doppler spectral variations associated with Es layer, mainly at night during the summer season. Comparing the characteristics of such quasi-periodic Doppler traces with the structure of Medium-Scale Traveling Ionospheric Disturbances (MSTIDs) visualized by GPS-TEC, it was found that they (i.e., Es layer and MSTIDs) propagate in the same direction at the same speed. This suggests that Es layer and MSTIDs propagate in tandem due to the electrical coupling between the E and F layers (Matsushima et al., 2022).
However, it is still unclarified what kind of spatial structure of Es layer is reflected in the quasi-periodic Doppler traces observed in HFD. In particular, the relationship with the quasi-periodic coherent radar echoes (QP echoes: Yamamoto et al., 1991) observed during summer nighttime Es layer cases is still unclear. Therefore, in this study, we conducted simultaneous observations of Es layer using HFD and MU radar on eight nights (May 23-26 and June 6-9, 2022) in order to directly compare nighttime Es layer observed by two different methods. During three nights in May, the MU radar detected clear signatures of QP echoes. In addition, we confirmed that quasi-periodic Doppler traces were observed in the HFD data obtained in Awaji station.
The two observation methods would have detected same Es layer because the difference in the timing of detection was small, about 10–20 minutes. The time lag would have been due to the location of the reflection point of the HFD observation in Awaji, which is about 200 km away from the MU radar observation area. When the MU radar clearly detected Es layer, the difference in speed between MSTIDs and Es layer was less than 20 %. This indicates that Es layer and MSTIDs propagated in tandem. Based on this fact, the time needed for propagation of MSTIDs between observation areas of MU radar and HFD was calculated by cross-correlation analysis of GPS-TEC data. The estimated travel time was almost equal to the time lag in the Es layer observation between the MU radar and HFD. Therefore, we conclude that the time lag of Es layer observation was due to the difference of the observation areas, which further confirms that the quasi-periodic structures in the Doppler spectra of HFD measurements are manifestations of QP echoes seen in data from coherent radars such as the MU radar.
To evaluate the identity of the two fine structures of Es layer, we plan to reconstruct 2D images of the QP echoes from the imaging observations made by the MU radar and compare them with the quasi-periodic Doppler traces seen in the HFD measurements.