GLOBAL IONOSPHERE PERTURBATIONS MONITORED BY THE WORLDWIDE GPS NETWORK

For the first time, measurements from the Global Positioning System (G PS) worldwide network are employed to study the global ionospheric tot al electron content (TEC) changes during a magnetic storm (November 26 , 1994). These measurements are obtained from more than 60 world-wide GPS stations which continuously receive dual-frequency signals. Based on the delays of the signals, we have generated high resolution global ionospheric maps (GIM) of TEC at 15 minute intervals. Using a differe ntial method comparing storm time maps with quiet time maps, we find t hat significant TEC increases (the positive effect) are the major feat ure in the winter hemisphere during this storm (the maximum percent ch ange relative to quiet times is about 150%). During this particular st orm, there is almost no negative phase. A traveling ionospheric distur bance (TID) event is identified that propagates from the northern suba uroral region to lower latitudes (down to about 30 degrees N) at a spe ed of similar to 460 m/s. This TID is coincident with significant incr eases in the TEC around the noon sector. We also find that another str ong TEC enhancement occurs in the pre-dawn sector in the northern hemi spheric subauroral latitudes, in the beginning of the storm main phase . This enhancement then spreads into almost the entire nightside. The nighttime TEC increase in the subauroral region is also noted in the s outhern hemisphere, but is less significant. These preliminary results indicate that the differential mapping method, which is based on GPS network measurements, appears to be a powerful tool for studying the g lobal pattern and evolution process of the entire ionospheric perturba tion.