The characteristics of an anomalous ionization layer appearing during winter daytime at 69.6 ¿N, 19.2 ¿E are described. The layer is located at about 100 km altitude, is about 15 km thick and appears not to result from energetic particle precipitation. It has been observed on 80% of winter days when measurements have been made and was usually present for several hours on each of those days, whenever the solar zenith angle was less than 93¿. The layer cannot be explained by ionization of the major atmospheric constituents by solar EUV or X-ray radiation. Possible explanations in terms of the minor constituents NO and metallic atoms are considered but much larger number densities of those constituents would be required than has been observed. ¿ American Geophysical Union 1991