IONOSPHERIC TOTAL ELECTRON-CONTENT PERTURBATIONS MONITORED BY THE GPSGLOBAL NETWORK DURING 2 NORTHERN-HEMISPHERE WINTER STORMS

The global evolution of two major ionospheric storms, occurring on Nov ember 4, 1993, and November 26, 1994, respectively, is studied using m easurements of total electron content (TEC) obtained from a worldwide network of ground-based GPS receivers. The time-dependent features of ionospheric storms are identified using TEC difference maps based on t he percent change of TEC during storm time relative to quiet time. The onset of each ionospheric storm is indicated by the appearance of aur oral/subauroral TEC enhancements which occur within 1 hour of the begi nning of the geomagnetic storm main phase. Significant TEC enhancement s (> 100%) are observed in the winter northern hemisphere. The rate at which TEC enhancements appear is found to correlate with gradients in the Dst index. The large scale ionospheric structures identified duri ng the storms are (1) nightside auroral/subauroral enhancements which surround the auroral oval, (2) dayside (around noon) high-latitude and middle-latitude enhancements associated with traveling ionospheric di sturbances, and (3) conjugate latitudinal enhancements. For the Novemb er 1993 storm, a short positive phase (about 15 hours) is followed by a long negative phase (similar to 60 hours). In the November 1994 stor m, we have identified the clear signature of a traveling ionospheric d isturbance (TID) which propagated at a speed of similar to 460 m/s fro m similar to 60 degrees N to similar to 40 degrees N. The motion of th is disturbance appears to conserve angular momentum.