Ionospheric Effects During a Geomagnetic Storm

GPS measurements from the EPN (EUREF Permanent Network) were used to monitor the ionospheric effects of a geomagnetic storm. A series of ionosphere maps were produced to present the spatial distribution of TEC. The dense network of GPS stations in Europe allows to obtain high spatial and temporal resolution of TEC. Our estimation technique provided TEC maps with 15min interval and with a spatial resolution of 150–350 km. As an example we analysed geomagnetic disturbances, which occurred during the month of September 1999. It was found that the storm essentially modified the ionosphere.Large and medium-scale structures were detected in auroral and subauroral ionosphere as well as at middle latitudes. The deep spatial TEC changes were caused by the occurrence of the main ionospheric trough, which during the storm moved in the direction of the Equator. During most of the disturbed period the ionospheric trough moved down to a latitude of about 50°N. Under these conditions horizontal gradients in the ionosphere essentially increased, what we attribute to the occurrence of the trough. Also we detected medium-scale structures, which decreased the spatial correlation of the ionosphere.