Some effects of geomagnetic activity on spread-F occurrence and ionospheric height variations in equatorial regions

AE indices have been used to investigate, at times of increased geomagnetic activity, the possibility of significant changes to both spread-F occurerence and h′F values for 3 stations in equatorial latitudes. The investigation covered a sunspot minimum period. Furthermore, data for each of these parameters have been considered for both a pre-midnight period (interval A) and a post-midnight period (interval B). The use of the AE indices at 12 different times at 2 h intervals allows the measurement of the delay times, after increased geomagnetic activity, of any significant changes in the parameters being investigated.The results show that for interval A significant suppressions of spread-F occurrence are recorded at delay times of approximately 3 h and 9 h. These delays correspond to enhanced geomagnetic activity at local times of 1800 and 1200, respectively. Also, for interval A the h′F variations suggest that h′F is suppressed at times of spread-F suppression. For interval B spread-F occurrence seems to be controlled by two opposing effects. For several hours after enhanced geomagnetic activity spread-F occurrence increases significantly, followed by a sharp decline culminating in suppressed occurrence, again related to increased geomagnetic activity at 1800 local time for the maximum effect. Also, for interval B h′F values lift abruptly a few hours after enhanced geomagnetic activity, followed by a gradual decline when delays of up to 20 h are considered. Further work on these delays may allow reliable short-term forecasting of some ionospheric behaviour in equatorial regions.