Behavior of foF2 prior to geomagnetic storms according to Slough and Juliusruh data

The foF2 deviations from quiet conditions during three days preceding a magnetic storm are considered. The data of the Juliusruh station for the period of 1976–2010 are analyzed, and the results are compared to the similar analysis of the Slough data published earlier. A seasonal dependence of the deviations (events) is found: the probability of the events occurrence is higher in winter than in summer. This probability also depends on solar activity (it decreases with an increase in the F10.7 index) and the magnetic storm intensity (it decreases with an increase in the magnitude of the negative Dst index). The dependence of the events number on the local time of the storm onset (SO) and the time of the event prior to the SO moment is also analyzed. The results for both stations are in a good agreement and confirm the initial concept that the aforementioned events could be considered as precursors of the coming magnetic storm.