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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%.
Shiokawa et al. (Wed,) studied this question.