GEOMAGNETIC DISTURBANCES AT F1-LAYER HEIGHTS UNDER DIFFERENT SOLAR ACTIVITY CONDITIONS OVER NORILSK

We analyze the influence of geomagnetic disturbances on the electron density N-e at Norilsk ionospheric station (69 degrees N; 88 degrees E) at Fl-layer heights (120-200 km). For the analysis, we have selected 25 moderate and weak geomagnetic disturbances for two seasons - spring and fall - of 2003-2014. Using the N-e values obtained from measurements made with the Norilsk digisonde during this period, we analyze N-e variations during geomagnetic disturbances in spring and fall for a long period of time. We determine the effect of spring-fall asymmetry occurring in all solar activity phases and manifesting itself in a significant decrease in the electron density during the main phase of fall storms at all heights in comparison with quiet days: up to 2.6 times at a height of 200 km and slightly less at lower heights. This phenomenon is not observed during spring disturbances: Ne variations are much weaker.