Development of high latitude phase fluctuations during the January 10, April 10-11, and May 15, 1997 magnetic storms

GPS satellite transmissions have been used to study the development of moderate ionospheric phase irregularities. The use of the multi-station, multi-path observations of the GPS beacons has allowed the study of the time development of irregularities as a function of latitude and longitude of individual storms. The basic storms studied were those of January 10, April 10-11, and May 15, 1997. The results from studying these storms showed the unique nature of each storm. For the three storms, data were available from four stations near 65 degrees Corrected Geomagnetic Latitude (CGL); the stations ranged from Fairbanks to Tromso. In addition, data from higher latitude stations are analysed. For the January storm, irregularity development started at Fairbanks. Then as magnetic midnight approached longitudes to the west, the storm effects reached the Tromso-Kiruna longitudes. For the April magnetic storm, at 65 degrees CGL, irregularity development maximized at approximately the same UT at stations ranging in longitude from Fairbanks to Kiruna. For this storm, the development of irregularities was dominated by storm time. The May storm irregularities were dominated by magnetic local time once the storm commenced. With both total electron content and rate of change of total electron content (phase fluctuations) available, it was noted that over periods of minutes, clumps of irregularities were accompanied by increases in TEC. In addition total electron content increased over large areas during maximum magnetic activity in the auroral oval. During the storms, ionograms showed that the altitude of maximum electron density fluctuated; at times the dominant maximum Frequency was noted in the E layer and at other times in the F layer. This fluctuation of electron density in each layer during storms led to the conclusion that the turbulent activity within the auroral region dominated the development of irregularities. The irregularities that are noted on trans-ionospheric paths are therefore thought to be in both the E and the F layers with a combination of structured hard and soft electron precipitation and coupling initiating the turbulent activity. The general positive correlation of one period's phase scintillation data with the Ultra Violet Imager observations as shown on POLAR indicates the importance of 100-200 km precipitation. However the very high occurrence of spread F at high latitudes as shown by both ground and satellite ionosondes indicates the considerable contributions of F layer irregularities. (C) 1999 Elsevier Science Ltd. All rights reserved.