Spread‐ F /sporadic E coupling at Chung‐Li, especially for postsunset periods of sunspot maximum years

Using ionogram‐recorded data, the coupling between spread‐ F occurrence and enhanced E s blanketing frequencies ( f b E s ) (indicative of f o E s changes), has been investigated for an equatorial‐ionospheric‐anomaly crest station, Chung‐Li, for the postsunset periods of sunspot‐maximum years. Apart from the evidence for coupling from inspections of the f ‐plots alone, for the years analyzed (1981, 1982, 1988, and 1989), statistical results of more significance were found from superposed‐epoch analyses. Isolated spread‐ F events of duration of 2 hours or less which started at or before 2200 LT were used as controls on time series. These series were made up of comparisons between 15‐min f ‐plot f b E s values and interpolated monthly median f b E s values. Evidence for coupling was found for the analyses which involved 97 controls. The evidence in the f ‐plots for three events involving E s patches, which investigators have examined in some detail using the VHF radar at Chung‐Li, suggests that these events are also coupled to isolated spread‐ F occurrence.