Early morning effect on the F-region from ground station observations

Abstract

Data from continuous vertical ionospheric recordings over Athens indicate that the morning effect on the F-region starts regularly earlier than the ƒ 0F2 predawn minimum as a rise of the virtual heights at fixed frequencies. The corresponding solar zenith angle undergoes a pronounced seasonal variation and presents a winter mean maximum value of 121°. The world-wide distribution of the predawn ƒ 0F2 minimum, taken from the prediction maps, and the day to day variation over Athens are examined and the results are discussed from the point of view of a possible magnetospheric contribution to the early morning phenomenon.