Spread-F occurrence in mid- and low-latitude regions related to various levels of geomagnetic activity

The occurrence of spread-F from seven latitude regions for ionosonde stations (78 in all) located from L-shell = 3.3 to 1.05 has been investigated (using the superposed-epoch technique) relative to four different levels of geomagnetic activity. Data for 14.5 years were used. For moderate, high and very-high geomagnetic activity a significant peak in spread-F occurrence is found for the four latitude regions closest to the auroral zone. These peaks are delayed (after the geomagnetic activity) by a matter of days, the delays being greater for the lower levels of activity and also greater for regions further from the auroral zone. Similarly, delayed dips in spread-F occurrence are found for very-low geomagnetic activity. Analyses for the remaining three regions (those closest to the equator) failed to show corresponding delayed peaks or dips in the occurrence of spread-F relative to the appropriate levels of geomagnetic activity. It is suggested that (for the three highest levels of geomagnetic activity) the mechanism which is responsible for the suppression of spread-F in equatorial regions may operate at these low latitudes and thus counterbalance the other mechanism which is responsible for the positive correlation found for the higher-latitude regions.