Changes in the F1 region electron density during geomagnetic storms at lowsolar activity

This paper attempts to demonstrate the changes in the F1 layer ionization d uring geomagnetic storm. To analyze the behavior of F1 region, we have sele cted eight rather strong geomagnetic storms that occurred in different seas ons in 1994-1997. Their course was similar and there were at least three qu iet days before each event. The electron density profiles for these events, derived from all the available ionograms of the Pruhonice station (50 degr eesN, 14.6 degreesE), were analyzed in order to investigate electron densit y variability at heights of 160-190 km. Spring/autumn asymmetry of the effe cts in F1 region is found. We observed no significant effect of an ionosphe ric storm in electron density in the F1 region during spring geomagnetic st orms, while there is a substantial effect in autumn at 180 and 190 km heigh ts. We have compared our results with those obtained from ionograms of some other European ionospheric stations. In general, the F1 region appears to be much more stable than the F2 layer during ionospheric storms. Substantia l intra-hour variability was found in NmF2 during geomagnetic storms in day time, while it was very weak on the storm maximum day in F1 layer. (C) 2001 Elsevier Science Ltd. All rights reserved.