Very intense electron density enhancements of the F2 layer maximum of the winter night ionosphere over Irkutsk

The statistical analysis of the ionospheric data from the station Irkutsk (52.5° N, 104° E) in 1958-1992 and 2002-2009 revealed a very dramatic increase in the electron density of the F2 layer maximum N m (exceeding the doubled background) in 1-3 % of observations made during winter nights (02-04 LT). Most of increases corresponded to the periods of magnetospheric substorms. Very dramatic N m increases during the periods of low geomagnetic activity usually corresponded to low solar activity. In the course of magnetic storms, such Nm increases were observed only during high solar activity. The most striking example of the very dramatic N m increases was the event of 19 December 2008 when the nighttime N m maximum reached a value typical of the daytime N m maximum. We present qualitative interpretation of statistical properties of the very dramatic N m increases and discuss possible causes of the extreme N m increase on 19 December 2008.