Cause of the mid-latitude evening increase in ƒoF₂

The behavior of the electron density and the electron temperature in the F2 region above Millstone Hill radar observatory (42.6°N, 71.5°W) has been explored throughout 1963 by means of radar backscatter experiments. In this paper results are presented for April, July, and November 1963. It is shown that the evening increase in ƒ0F2, observable at mid-latitude stations in summer, is caused by downward diffusion of ionization from above hmaxF2. This motion appears to be induced by the large fall in the electron temperature Te that occurs at sunset. On the basis of this explanation it is possible to show why the evening increase is absent in winter and is less pronounced at sunspot maximum, and also to account for why the effect is particularly pronounced for North American stations.