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The companion paper by Zou et al.shows that the annual and semiannual variations in the peak F2-layer electrondensity (NmF2) at midlatitudes can be reproduced by a coupledthermosphere-ionosphere computational model (CTIP), without recourse to externalinfluences such as the solar wind, or waves and tides originating in the loweratmosphere. The present work discusses the physics in greater detail. It showsthat noon NmF2 is closely related to the ambient atomic/molecularconcentration ratio, and suggests that the variations of NmF2 withgeographic and magnetic longitude are largely due to the geometry of the auroralovals. It also concludes that electric fields play no important part in thedynamics of the midlatitude thermosphere. Our modelling leads to the followingpicture of the global three-dimensional thermospheric circulation which, asenvisaged by Duncan, is the key to explaining the F2-layer variations. Atsolstice, the almost continuous solar input at high summer latitudes drives aprevailing summer-to-winter wind, with upwelling at low latitudes and throughoutmost of the summer hemisphere, and a zone of downwelling in the winterhemisphere, just equatorward of the auroral oval. These motions affectthermospheric composition more than do the alternating day/night (up-and-down)motions at equinox. As a result, the thermosphere as a whole is more molecularat solstice than at equinox. Taken in conjunction with the well-known relationof F2-layer electron density to the atomic/molecular ratio in the neutral air,this explains the F2-layer semiannual effect in NmF2 that prevails at lowand middle latitudes. At higher midlatitudes, the seasonal behaviour depends onthe geographic latitude of the winter downwelling zone, though the effect of thecomposition changes is modified by the large solar zenith angle at midwinter.The zenith angle effect is especially important in longitudes far from themagnetic poles. Here, the downwelling occurs at high geographic latitudes, wherethe zenith angle effect becomes overwhelming and causes a midwinter depressionof electron density, despite the enhanced atomic/molecular ratio. This leads toa semiannual variation of NmF2. A different situation exists in winter atlongitudes near the magnetic poles, where the downwelling occurs at relativelylow geographic latitudes so that solar radiation is strong enough to producelarge values of NmF2. This circulation-driven mechanism provides areasonably complete explanation of the observed pattern of F2 layer annual andsemiannual quiet-day variations.Key words: Atmospheric composition and structure(thermosphere-composition and chemistry) - Ionosphere (mid-latitude ionosphere;modelling and forecasting

Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion