Global pattern of large-scale ionospheric disturbances during the magnetic storm of September 25, 1998, as inferred from GPS network data

The results of studying ionospheric effects of the large magnetic storm of September 25, 1998, based on an analysis of the global maps of total electron content (TEC) constructed using GPS network data, have been presented. The most pronounced consequence of the magnetic storm shown in the TEC maps includes prolonged negative disturbances extending to geographic latitudes of lower than 30degrees N, disappearance of the equatorial minimum, and displacement of the main maximum to morning hours at the phase of storm recovery. The TEC response to the magnetic storm depends on the local time of its commencement, which results in a clearly defined longitudinal effect. In the European sector, the deltaTEC variations change their sign with a period of about a day, and the positive and negative phases are observed in daytime and at night, respectively. At the same time, over North America, the positive phase appears only at night, the negative phase is observed in daytime, and the negative disturbances are observed during the entire period of observation at latitude from 30degrees to 60degrees N. The latitudinal effect is also significant. The maximal positive variations (to 100% and more) were observed in the equatorial region and at high latitudes. The deltaTEC variations have a quasi-wavy character with a period of about one day, which is possibly related to the propagation of large-scale ionospheric disturbances.