Short‐term delays (hours) of ionospheric spread F occurrence at a range of latitudes, following geomagnetic activity

The analyses have investigated the short‐term responses of spread F occurrence (as identified on ionograms) at low‐and middle‐latitude stations to geomagnetic activity changes so that comparisons can be made with results from equatorial stations reported earlier [Bowman, 1995]. Using superposed‐epoch methods, it is found that the AE index is either enhanced or depressed a few hours prior to spread F occurrence depending on whether or not low or high (respectively) spread F controls are used. The geomagnetic activity related to this inverse relationship is centered mainly around the local times of 1200 and 1800. A direct relationship is also found for geomagnetic activity which occurs in the night hours. The results for these other latitude stations are the same as those reported in the earlier paper for equatorial stations. This paper also considers for the equatorial station Huancayo extreme responses of spread F occurrence to geomagnetic activity, involving occasions of high spread F occurrence when there is a sudden drop in the level to zero for isolated days when a few hours earlier at the favored times mentioned above enhanced geomagnetic activity occurs. It is suggested that the inverse relationship involves the control of medium‐scale traveling ionospheric disturbance (MS‐TID) wave amplitudes by neutral‐density changes, produced by large‐scale traveling ionospheric disturbances (LS‐TIDs) generated by geomagnetic activity concentrated mainly around the local times of 1200 and 1800. The LS‐TIDs generated by geomagnetic activity at night behave differently. Height rises are produced, and as a result of the lower neutral‐density levels, spread F is recorded particularly in the presunrise period. Extending the comparison of spread F characteristics at different latitudes, some comments are made on recent observations using data from satellite recordings, concerning the similarities between electron‐density depletions detected for equatorial regions and those for other regions (e.g., midlatitude regions.)