STUDY OF THE IONOSPHERIC VARIABILITY WITHIN THE EURO-ASIAN SECTOR DURING THE SUNDIAL ATLAS-1 MISSION/

In order to quantify, and to identify possible origins of, subauroral ionospheric variability during periods of moderate geomagnetic activit y, ionospheric observations taken during the SUNDIAL/ATLAS - 1 campaig n (March 24 to April 2) from 10 stations were analyzed in conjunction with observations from EISCAT, geomagnetic observations from magnetome ter networks in Scandinavia and the United Kingdom, and auroral partic le energy input observations from the NOAA - 12 satellite. The network of ionospheric stations spanned longitudes from 13 degrees E to 90 de grees E but were relatively confined in geomagnetic latitudes so that longitudinal and local time dependencies in ionospheric variability ar e more clearly exposed. The ionospheric observations were analyzed in terms of both Delta f(o)F2, the difference between the hourly f(o)F2 a t a given station and the monthly median f(o)F2 for that hour, and a n ew daily variability index Af(o)F2. The analysis using both parameters demonstrated an apparent longitudinal variation in ionospheric variab ility with a reversal at about 55 degrees E from a negative to a posit ive phase in the departure of ionospheric renditions from their median values. An analysis of these ionospheric data in conjunction with the NOAA/TIROS estimates of power deposition by auroral particles demonst rated a Significant local time dependence in midlatitude ionospheric r esponses to amoral activity. This dependence may arise from the premid night to postmidnight asymmetry in high-latitude convection electric f ields.