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

Abstract

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.

Review of the accomplishments of mid-latitude Super Dual Auroral Radar Network (SuperDARN) HF radars

“…This process is particularly important at mid-latitudes where magnetospheric electric fields are weak. Yakymenko et al (2015) found that the diurnal variation of HOK echo velocity at short ranges cannot be explained by variation in the E × B drift alone. However, by adding a neutral wind contribution to the irregularity velocity and assuming that the irregularities were not strictly magnetic field-aligned, they were able to match the measurements with theoretical expectations.…”

“…Koustov et al (2014) performed a more comprehensive analysis of E region MSTID signatures in HOK echoes and reported preferential occurrence of such echoes in summer with a secondary maximum in winter. This tendency is despite the fact that E region nighttime HOK echoes are more frequent in winter (Yakymenko et al 2015). The authors found the E region MSTIDs propagate toward the southeast or east during pre-midnight hours and toward the southwest or west during post-midnight hours with average speeds of 86 m s −1 and 56 m s −1 , respectively.…”

“…It is speculated that neutral turbulence plays a role in generating the E region backscatter during the events. Yakymenko et al (2015) studied the statistical characteristics of short-range echoes observed by HOK generally. Echo occurrence increases at night, and there is a category of backscatter that occurs in summer in the morning-prenoon sector that can be associated with sporadic E layers.…”

“…However, the Doppler velocity of the MSTID striations were smaller than the speed of the power enhancements implying HOK monitored extended plasma blobs moving across the FOV, presumably at the background wind velocity. Plasma motions inside the blobs are assumed to be controlled by a combined effect of the electric field and neutral wind, as later expanded by Yakymenko et al (2015).…”

Velocity of SuperDARN Echoes at Intermediate Radar Ranges

“…Unfortunately, the amount of high‐quality data for the case of SuperDARN is extremely limited (Davies et al, 1999; Gillies et al, 2018; Koustov et al, 2005). Several studies used the velocity of SuperDARN echoes at far ranges as a proxy for the E × B vector at shorter ranges thus assuming ionospheric flows to be uniform (Gorin et al, 2012; Makarevitch et al, 2004; Milan & Lester, 1998; Yakymenko et al, 2015). These studies did not conclude on the relationship quantitatively.…”