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

This paper attempts to demonstrate the changes in the F1 layer ionization during geomagnetic storm. To analyze the behavior of F1 region, we have selected eight rather strong geomagnetic storms that occurred in different seasons in 1994–1997. Their course was similar and there were at least three quiet days before each event. The electron density profiles for these events, derived from all the available ionograms of the Pruhonice station (50°N, 14.6°E), were analyzed in order to investigate electron density variability at heights of 160–190 km. Spring/autumn asymmetry of the effects in F1 region is found. We observed no significant effect of an ionospheric storm in electron density in the F1 region during spring geomagnetic storms, while there is a substantial effect in autumn at 180 and 190 km heights. 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. Substantial intra-hour variability was found in NmF2 during geomagnetic storms in daytime, while it was very weak on the storm maximum day in F1 layer.