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In this reseach, the changes in the low latitude critical frequencies (foF2) was investigated during the geomagnetically active hours (geomagnetic storms). The critical frequency (foF2) data obtained for 1991 over Manila that is low latitude ionosonde station and the planetary geomagnetic indices (K p ) were used. The superposed epoch analysis was used as a statistical method to investigate the change in the critical frequencies during geomagnetically active hours. The analyzes were conducted separately for the night hours, day hours and all hours of 1991 and they were compared with each other. The results from this research show that the highest change (increase or decrease) in foF2 values during geomagnetically active hours occurs at the event time (zero time) and the local time (day or night) has a significant impact on this change of the foF2 values. The foF2 values increase at local day hours, while the foF2 values decrease at local night hours.

The Effect of Geomagnetic Storms on foF2 Values over Low Latitude Ionosonde Station