Wavelike Structures of E-Region Irregularities Observed by All-Sky Radar at Low Latitude

We report for the first time a case of wavelike structures of E-region irregularities observed by an all-sky radar at Ledong (18.44 degrees N, 108.97 degrees E), China. The wavelike structures covered up to 250 km zonally, fluctuated in the vertical and horizontal planes, and were observed in the directions perpendicular and significantly off-perpendicular to the geomagnetic field. The wavelength, and vertical and horizontal oscillating amplitudes were estimated at about 20-70, 5, and 2 km, respectively. Eventually, the large-scale wavelike structure broke into spatially separated small-scale patches and disappeared. The case provides observational evidence for the wave modulating sporadic E (Es) mechanism, which suggests that the wavelike structures of E-region irregularity were primarily generated by wind shears and modulated by gravity waves. The wavelike structure was possibly an intermediate state between the continuous-type and quasiperiodic-type E-region irregularities. The mechanism responsible for the echoing structure observed in off-perpendicular directions is unclear. The possible contributions from refraction and reflection by intense Es layer/structure were discussed. Plain Language Summary The E-region irregularities were usually observed using narrow-beam very high frequency radars. Due to the limited field-of-view (FOV) of the narrow-beam radars, the spatial morphology of large-scale irregularity structures could not be obtained. Over low latitudes, the E-region irregularities were traditionally classified into two main types: the continuous type linked with the tidal sporadic E (E-s) layer and the quasiperiodic (QP) type due to the drift of small irregularity patches. Facilitating from the very large FOV of the all-sky radar at Ledong, we report a case of E-region irregularities with different echo patterns from the continuous and the QP types. These E-region irregularities showed wavelike structures covering up to 250 km in the zonal direction and were observed in directions both perpendicular and significantly off-perpendicular to the geomagnetic field. In combination with the collocated Es and neutral wind measurements, the possible mechanisms responsible for the wavelike irregularity structures are discussed.