The present study investigates the seasonal dependence of the geomagnetic activity influence on the diurnal variability of the maximum electron concentration of the ionosphere over Bulgaria. Data from the ionosonde station Sofia for the period of 1995–2014 are used. The geomagnetic activity is described by the planetary Kp-index. The ionospheric response to the geomagnetic storms is studied by considering the deviation of the diurnal variability from its steady (median) course. It is found that the ionospheric reaction in summer is stronger than that in winter. Using the correlation analysis the seasonal dependence of the ionospheric response time delay for two ionospheric characteristics, foF2 and MUF3000, was determined. It is found that the time delay of the negative ionoispheric response in winter is longer than that in summer; an explanation of this result is offered.


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