Frequency and duration of disturbances in the mid‐latitude F region of the ionosphere

Abstract

Disturbances of the ionospheric F region at two well‐separated mid‐latitude stations were identified on rapid run ionograms by spread echoes, blackouts, and anomalies in structure or critical frequency. The diurnal variation in the probability of disturbance consistently shows minima around sunrise and sunset. Details such as a brief postsunset enhancement in winter are revealed by the fine time resolution of the data. Disturbance probabilities were lowest in autumn and spring (8% and 9%), while that for winter (33%) exceeded the summer level (20%). Spread F constituted 32% of the events, and disturbances producing sharply defined ionogram signatures 77%, with some overlapping of types. The majority of disturbances were not correlated with geomagnetic phenomena. Disturbances at the two stations were essentially independent (spatial correlation of 0.15). Spread F events were of longer duration (25 min) than sharp disturbances (20 min). TIDs were the exception, with a most probable duration of 80 min, but accounted for only 8.3% of the sharp disturbances. The absence of disturbances in the F region at dusk and especially at dawn coincides with periods of instability in the E region.