Response of ionosphere to the great geomagnetic storm of September 1998: Observation and modeling

We present the results derived from investigating of the ionospheric effects of the great geomagnetic storm on September 24–27, 1998 based on analyzing the data obtained at ionospheric stations located in the region of Siberia and the Far East. For the sake of comparison we have also introduced the data from some European stations. In addition to the vertical-incidence sounding data the measurements from the Irkutsk incoherent scatter radar are used. It is obtained that the intense negative disturbances during the main phase of the storm are observed at all stations under investigation. A comparison of the foF2 variations for the two meridians shows that even with a relatively small difference of longitudes (45°), the recovery phase has substantial differences of ionospheric disturbances. On September 26, the positive disturbances are observed and persist the following day at all stations except Salekhard and the Norilsk chain. At the Norilsk chain and at Salekhard the negative disturbances recurred after positive disturbances. The sign reversal of disturbance during the recovery phase extends over in the further displacement from the west to the east. An interpretation of the measurements includes their comparing with the results of calculations in terms of a numerical model of the ionosphere and plasmasphere. The modeling results are in the satisfied agreement with the measurements. According to the model calculations the sign of the ionospheric disturbance is changed from negative to positive as the result of combined effects of corpuscular and photo ionization. The sign of ionospheric disturbance is determined by a local time at which the magnetic storm has begun.