Characteristics and implications of Doppler spectra of E region quasi‐periodic echoes observed by the multibeam middle and upper atmosphere radar

Abstract

We present multibeam middle and upper atmosphere (MU) radar observations of quasi‐periodic (QP) echoes from 3.2‐m field‐aligned irregularities associated with plasma patches in the nighttime midlatitude sporadic E (Es) layers over Shigaraki, Japan, to give a cohesive view of the QP echo characteristics. Echo regions with zonal scales less than 50 km moved westward at altitudes near 105 km with trace velocities of 110–120 m s−1 and crossed the radar beams successively. Doppler spectra of the echoes changed in time, range, and azimuth and exhibited mostly type 2 echoes with Doppler velocities between −200 and +200 m s−1 and occasionally type 1 echoes due to the two‐stream instability (“the first type 1 echo detection by the MU radar”). We find that the spectra near the middle of the drifting echo region have two type 2 peaks, one at positive Doppler velocity (motion away from the radar) and the other at negative one (motion toward the radar) and that the spectra in the west and east parts of the echo region have single type 2 peak at positive and negative Doppler velocities, respectively. Interestingly, the type 1 echoes lasting for about 4 min on a fixed beam were located near the middle of the echo region. Their Doppler shifts between 250 and 320 m s−1 are very consistent with those from previous type 1 echo observations at other midlatitude locations. Occasionally, type 1 and type 2 echoes coexisted in the same range gates. On the basis of recent observations, theories, and simulations related to QP echoes, we propose a simple model of an Es plasma patch to explain the observations. In the model, polarization electric fields with opposite polarity inside and outside the patch, elongated along the NW‐SE direction, play a predominant role in exciting type 1 and type 2 echoes, and cause a variety of Doppler spectra.