THE F-LAYER AT SUNRISE

It was noticed 60 years ago that at sunrise (i) the ionospheric critic al frequency foF2 increases faster in winter than in summer and (ii) t he increase begins at a greater solar zenith angle in winter. It was l ater suggested that this ''seasonal sunrise anomaly'' is due to a seas onal change of atomic/molecular ratio in the neutral air in the F2-lay er. This paper uses the Sheffield University plasmaphere-ionosphere co mputational model (SUPIM) with the MSIS thermosphere model to examine the relative importance at sunrise of production, loss and diffusion p rocesses, and the effect of neutral air winds. The results show that b oth (i) and (ii) can be explained in terms of neutral composition chan ges.