Mid‐latitude E region field‐aligned irregularities observed with the MU radar

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Abstract
Fine structures E region field‐aligned irregularities were observed on June 24–25, 1989, with the MU radar at Shigaraki, Japan (34.9°N, 136.1°E; geomagnetic latitude 25.0°N). The 3.2‐m scale irregularities were observed with the MU radar in five main beam directions, each of which was nearly perpendicular to the geomagnetic field at 100 km altitude. Doppler spectra were obtained every 20 s with a range resolution of 600 m. Field‐perpendicular echoes appeared from 2130 to 2330 LT and from 0400 to 1100 LT, times that correspond to postsunset and postsunrise period in the E region. A preliminary examination of the Doppler spectra indicates spectral widths of 50–120 m s−1 and the mean Doppler velocities are well below the ion acoustic speed. These spectral characteristics are consistent with those obtained in the equatorial and auroral electrojets, and have been attributed to the gradient drift instability. The echoes observed during the postsunset and postsunrise periods showed quite different morphologies in the time‐height distribution. For this reason, they are classified into two types, ‘continuous’ and ‘quasi‐periodic.’ The appearance of the ‘continuous’ echoes was mainly continuous in time and situated between 90 and 100 km altitude during the postsunrise period. The appearance of the ‘quasi‐periodic’ echoes was intermittent with periods of 5–10 min and situated above 100 km altitude during the postsunset period. The quasi‐periodic echoes showed phase propagation toward the radar, while the averaged mean Doppler velocity was away from the radar. By measuring the time delays in echo regions from five directions, an apparent westward motion (approximately 120 m s−1) of the irregularity regions was estimated.