Statistical study of nighttime medium‐scale traveling ionospheric disturbances using midlatitude airglow images

We have investigated statistical characteristics of the nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed in 630‐nm airglow images at two stations, Rikubetsu (43.5°N, 34.8°MLAT) and Shigaraki (34.9°N, 25.4°MLAT), in Japan for 1998–2000 near the solar maximum period. Most of the observed MSTIDs propagate southwestward in the images. The typical wavelength, velocity, period, and amplitude are 100–300 km, 50–100 m/s, 0.5–1.5 h, and 5–15%, respectively. Seasonal variations in these parameters are not clear. The occurrence rate has a major peak (50–60%) in summer that appears ∼2 months earlier at lower latitudes and a minor peak in winter. Similar occurrence characteristics are obtained from midlatitude spread‐ F signatures using multipoint ionosonde data in Japan, though the coincidence of the spread‐ F and the MSTIDs in airglow images is only 10–15%.