Properties of the F2-layer maximum density variability over Irkutsk under different levels of the solar and geomagnetic activity

Using hourly data (1958-1992) from Irkutsk ionosonde station, we analyzed properties of variability of the F2-layer maximum density, N m, under different levels of the solar and geomagnetic activity. The standard deviation s( x ) of N m fluctuations relative to the quiet level x =( N m/ N m0-1)·100 %, and the average shift of these fluctuations x ave were used as characteristics of this variability. For this purpose, the empirical model of the F2-layer maximum density for quiet magnetic conditions, N m0, was constructed. The N m variability was found to depend weakly on the solar activity level; in the first approximation, this dependence can be neglected. The N m variability dependence on geomagnetic activity is among the principal ones, along with the dependences on local time and season. In general, dispersion s 2( x ) under quiet conditions is smaller than that during the periods of high geomagnetic activity. During the periods of high geomagnetic activity, however, the dispersion does not increase with the further growth of this activity, while the absolute value of shift x ave (which is mainly negative under high geomagnetic activity) still increases with growing geomagnetic activity. As a result, if this activity is high enough, the condition x ave2>s 2( x ) can be fulfilled.