In order to quantify, and to identify possible origins of, subauroral ionospheric variability during periods of moderate geomagnetic activity, ionospheric observations taken during the SUNDIAL/ATLAS-1 campaign (March 24 to April 2) from 10 stations were analyzed in conjunction with observations from EISCAT, geomagnetic observations from magnetometer networks in Scandinavia and the United Kingdom, and auroral particle energy input observations from the NOAA-12 satellite. The network of ionospheric stations spanned longitudes from 13 ¿E to 90 ¿E but were relatively confined in geomagnetic latitudes so that longitudinal and local time dependencies in ionospheric variability are more clearly exposed. The ionospheric observations were analyzed in terms of both Δf0F2, the difference between the hourly f0F2 at a given station and the monthly median f0F2 for that hour, and a new daily variability index Af0F2. The analysis using both parameters demonstrated an apparent longitudinal variation in ionospheric variability with a reversal at about 55 ¿E from a negative to a positive phase in the departure of ionospheric conditions from their median values. An analysis of these ionospheric data in conjunction with the NOAA/TIROS estimates of power deposition by auroral particles demonstrated a significant local time dependence in midlatitude ionospheric responses to auroral activity. This dependence may arise from the premidnight to postmidnight asymmetry in high-latitude convection electric fields. ¿ American Geophysical Union 1996