Many recent researches have been devoted to investigating the connection between seismic activity and ionospheric variations. It has been shown that the behavior of characteristic parameters of the ionosphere can be used as temporal and spatial precursors of strong earthquakes ( ).
In the present work variations of critical frequency of ionospheric F2 layer as a temporal precursor for the Qom earthquake are studied. Ionospheric data measured by vertical sounding technique at the Institute of Geophysics, University of Tehran (in Iran) are used to perform the analysis. The deviation of critical frequency of the F2 layer is the key parameter which is used to do the analysis.
To discriminate ionospheric precursors due to seismic activity from other causes of ionospheric variations, we also calculated the deviation of critical frequency of the F2 layer for some magnetic storm days and quiet days. It can be seen that during the quiet days the negative phase (negative value of the deviation of critical frequency of the F2 layer) is dominant and during the magnetic storm days the positive phase (positive value of the deviation of critical frequency of the F2 layer) is dominant. But two days before the earthquake the pattern of anomalies for the deviation of critical frequency of the ionospheric F2 layer is different. In this case the negative and positive phases are both present. This finding is in agreement with existing researches and shows the possibility of using ionospheric variations (measured by vertical sounding) as a temporal precursor for strong earthquakes.