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A modified ionospheric activity index AI has been developed on the basis ofionospheric <I>fo</I>F2 observations. Such index can be helpful for an interesteduser to get information about the current state of the ionosphere. Usingionosonde data of the station Juliusruh (54.6° N; 13.4° E) this indexhas been tested for the time interval from January 1996 until December 2008.This index has no diurnal and seasonal variations, only a small positivedependence on the solar activity could be found. The variability of thisindex has, however, a marked seasonal variability with maxima during theequinoxes, a clear minimum in summer, and enhanced values in winter. Theobserved variability of AI is strongly correlated with the geomagneticactivity, most markedly during the equinoxes, whereas the influence of thesolar activity is markedly smaller and mostly insignificant. Stronggeomagnetic disturbances cause in middle latitudes in general negativedisturbances in AI, mostly pronounced during equinoxes and summer and onlypartly during winter, thus in agreement with the current physical knowledgeabout ionospheric storms

A modified index for the description of the ionospheric short- and long-term activity