LATITUDE DEPENDENCE OF THE TIME-DELAY IN SPREAD-F OCCURRENCE FOLLOWING GEOMAGNETIC-ACTIVITY

Superposed-epoch methods have been used to investigate the occurrence of spread-F relative to control days for four separate levels of geomagnetic activity as determined by Ap-indices. These levels represent low, medium, high and very-high activity. The spread-F data were obtained from 40 ionosonde stations which were subdivided into four latitude groups. The locations of stations in these groups embraced all L-shells between 3.3 and 1.5. The analysis covered a period from July 1957 to December 1971. The results show that the peak in a period of increased occurrence for spread-F is delayed relative to a similar peak of increased occurrence for geomagnetic activity. This peak is found to be delayed more and more as lower-latitude regions are considered. The maximum delay for the lowest-latitude group is 4 days for medium activity, 2 days for high activity and 1 day for very-high activity. Thus, the magnitude of the delay is shown to be very sensitive to the level of geomagnetic activity. The results are similar for low geomagnetic activity (Ap ≤ 5) except that here the spread-F occurrence has associated decreases which also are shown to progress to lower-latitude regions. The maximum delay recorded for the maximum dip in the decreases is 3 days. All peaks and dips in occurrence are recorded at high levels of significance. The relevance of these spread-F delays to other geophysical phenomena showing similar delay characteristics is discussed. © 1979.