Statistical study of midlatitude E region echoes observed by the Hokkaido SuperDARN HF radar

摘要

Statistical characteristics of short-range coherent echoes observed by the Hokkaido Super Dual Auroral Radar Network (SuperDARN) high-frequency (HF) radar (geographic/geomagnetic latitude = 43.5°N/ 37.3°N) at midlatitudes are investigated. We show that the echo occurrence is increased during nighttime and, especially strongly, during winter and summer. During summer, the occurrence rates are still large during morning-prenoon hours; for these periods, strong sporadic E layers are observed by the Wakkanai ionosonde (geographic/geomagnetic latitude = 45.2°N/39.1°N) located in the vicinity of the radar field of view. Echo occurrence rate does not increase with the planetary magnetic activity characterized by the Kp and Dst magnetic indices and, seasonally, anticorrelates with variations of the Ap index. With respect to the magnetic L shells, echoes are seen more frequently in beams oriented almost perpendicular to L shells during summer, in beams at intermediate angles during equinoxes and winter, and sometimes in beams at the smallest available angles (~45°) during winter. Midlatitude echoes are of about the same power as at high latitudes but narrower by a factor of ~5. Power and spectral width of nighttime echoes increases with Doppler velocity, consistent with high-latitude observations. The Doppler velocity shows a clear semidiurnal variation. The local time of these maxima changes with season. It is hypothesised that many Hokkaido echoes are received at high magnetic aspect angles of several degrees, and they are coming from the lower part of the E region where the irregularity phase velocity is affected by both E × B plasma drifts and neutral winds.