A comparison of nighttime TID characteristics between equatorial‐ionospheric‐anomaly crest and midlatitude regions, related to spread F occurrence

Abstract

The occurrence characteristics of TIDs with enhanced wave amplitudes, as detected by spread traces on ionograms, has been investigated for an equatorial‐ionospheric‐anomaly crest station (Chung‐Li) for comparison with results from midlatitudes. Two distinct types of distributions are found. One set (type 1) is directly related to sunspot number, while the other (type 2) is indirectly related. Their semiannual variations are also inversely distributed. The type 2 variations are the same as those found for midlatitudes. Type 1 events occur mainly before midnight, being influenced by the significant postsunset height rises, while type 2 events predominate after midnight. The influence of large‐scale traveling ionospheric disturbances (LS‐TIDs) at both latitude regions has been considered, particularly relating to the spread F associated with their passage. The results presented for Chung‐Li have revealed characteristics similar to those found in midlatitudes. Thus an alternative explanation is available to explain, at least in terms of ionogram‐recorded spread F events, the direct relationship which exists between equatorial spread F and geomagnetic activity in the presunrise hours.