Pre-storm enhancements of foF2 above Europe

In spite of many years of investigations of effects of geomagnetic storms on the F region ionosphere, still many open questions remain, one of them being the pre-storm enhancements of NmF2, the maximum electron concentration in the ionosphere. All 65 strong geomagnetic storms observed over the period 1995-2005 are analyzed as for the occurrence of the pre-storm enhancements over Europe using data of eight ionosonde stations covering European area. About 20-25% of storms are accompanied by sufficiently strong pre-storm enhancements. The pre-storm enhancements occur both day and night, more often in summer half of the year, do not exhibit a systematic latitudinal dependence, and are not accompanied by a corresponding change of hmF2. Several constraints for their mechanism were established, several potential sources were excluded, but the origin of pre-storm enhancements remains to be uncovered.