Four numerical simulations have been performed, at equinox, using a coupled thermosphere‐ionosphere model, to illustrate the response of the upper atmosphere to geomagnetic storms. The storms are characterized by an increase in magnetospheric energy input at high latitude for a 12‐hour period; each storm commences at a different universal time (UT). The initial response at high latitude is that Joule heating raises the temperature of the upper thermosphere and ion drag drives high‐velocity neutral winds. The heat source drives a global wind surge, from both polar regions, which propagates to low latitudes and into the opposite hemisphere. The surge has the character of a large‐scale gravity wave with a phase speed of about 600 m s−1. Behind the surge a global circulation of magnitude 100 m s −1 is established at middle latitudes, indicating that the wave and the onset of global circulation are manifestations of the same phenomena. A dominant feature of the response is the penetration of the surge into the opposite hemisphere where it drives poleward winds for a few hours. The global wind surge has a preference for the night sector and for the longitude of the magnetic pole and therefore depends on the UT start time of the storm. A second phase of the meridional circulation develops after the wave interaction but is also restricted, in this case by the buildup of zonal winds via the Coriolis interaction. Conservation of angular momentum may limit the buildup of zonal wind in extreme cases. The divergent wind field drives upwelling and composition change on both height and pressure surfaces. The composition bulge responds to both the background and the storm‐induced horizontal winds; it does not simply rotate with Earth. During the storm the disturbance wind modulates the location of the bulge; during the recovery the background winds induce a diurnal variation in its position. Equatorward winds in sunlight produce positive ionospheric changes during the main driving phase of the storm. Negative ionospheric phases are caused by increases of molecular nitrogen in regions of sunlight, the strength of which depends on longitude and the local time of the sector during the storm input. Regions of positive phase in the ionosphere persist in the recovery period due to decreases in mean molecular mass in regions of previous downwelling. Ion density changes, expressed as a ratio of disturbed to quiet values, exhibit a diurnal variation that is driven by the location of the composition bulge; this variation explains the ac component of the local time variation of the observed negative storm phase.

Ionosonde observations have provided the data to build a picture of the response of the midlatitude ionosphere to a geomagnetic storm. The particular characteristic of interest is the preference for “negative storms” (decrease in the peak electron density, NmF2) in summer and “positive storms” (increase in NmF2) in winter. A three‐dimensional, time‐dependent model of the coupled thermosphere and ionosphere is used to explain this dependence. During the driven phase of a geomagnetic storm the two main magnetospheric energy sources to the upper atmosphere (auroral precipitation and convective electric field) increase dramatically. Auroral precipitation increases the ion density and conductivity of the upper atmosphere; the electric field drives the ionosphere and, through collisions, forces the thermosphere into motion and then deposits heat via Joule dissipation. The global wind response is divergent at high latitudes in both hemispheres. Vertical winds are driven by the divergent wind field and carry molecule‐rich air to higher levels. Once created, the “composition bulge” of increased mean molecular mass is transported by both the storm‐induced and background wind fields. The storm winds imposed on the background circulation do not have a strong seasonal dependence, and this is not necessary to explain the observations. Numerical computations suggest that the prevailing summer‐to‐winter circulation at solstice transports the molecule‐rich gas to mid and low latitudes in the summer hemisphere over the day or two following the storm. In the winter hemisphere, poleward winds restrict the equatorward movement of composition. The altered neutral‐chemical environment in summer subsequently depletes the F region midlatitude ionosphere to produce a “negative storm”. In winter midlatitudes a decrease in molecular species, associated with downwelling, persists and produces the characteristic “positive storm”.

Solutions of the continuity equation for electrons in the F2 region of the earth's ionosphere are obtained for the region near the magnetic equator under noon conditions. The physical processes of photo‐ionization, recombination, diffusion, neutral winds, and electromagnetic drift are included explicitly in the equation; the presence of light ions (H+, He+) and the effects of ion drag, however, are specifically ignored. It is shown that upward plasma drift at the equator is very likely the cause of the Appleton anomaly, as originally suggested by Martyn; a drift velocity of about 10 m sec−1 is required. Other cases with downward drift or with neutral winds are presented. It is shown that a 15% interhemisphere asymmetry in the electron concentration at the Appleton peaks can be caused by a 60 m sec−1 neutral wind blowing from north to south. By using a very small drift velocity the time‐dependent behavior of the electron concentration along particular field lines is investigated for different initial conditions.

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Abstract. The companion paper by Zou et al. shows that the annual and semiannual variations in the peak F2-layer electron density (NmF2) at midlatitudes can be reproduced by a coupled thermosphere-ionosphere computational model (CTIP), without recourse to external in¯uences such as the solar wind, or waves and tides originating in the lower atmosphere. The present work discusses the physics in greater detail. It shows that noon NmF2 is closely related to the ambient atomic/molecular concentration ratio, and suggests that the variations of NmF2 with geographic and magnetic longitude are largely due to the geometry of the auroral ovals. It also concludes that electric ®elds play no important part in the dynamics of the midlatitude thermosphere. Our modelling leads to the following picture of the global three-dimensional thermospheric circulation which, as envisaged by Duncan, is the key to explaining the F2-layer variations. At solstice, the almost continuous solar input at high summer latitudes drives a prevailing summer-to-winter wind, with upwelling at low latitudes and throughout most of the summer hemisphere, and a zone of downwelling in the winter hemisphere, just equatorward of the auroral oval. These motions aect thermospheric composition more than do the alternating day/night (up-and-down) motions at equinox. As a result, the thermosphere as a whole is more molecular at solstice than at equinox. Taken in conjunction with the well-known relation of F2-layer electron density to the atomic/molecular ratio in the neutral air, this explains the F2-layer semiannual eect in NmF2 that prevails at low and middle latitudes. At higher midlatitudes, the seasonal behaviour depends on the geographic latitude of the winter downwelling zone, though the eect of the composition changes is modi®ed by the large solar zenith angle at midwinter. The zenith angle eect is especially important in longitudes far from the magnetic poles. Here, the downwelling occurs at high geographic latitudes, where the zenith angle eect becomes overwhelming and causes a midwinter depression of electron density, despite the enhanced atomic/ molecular ratio. This leads to a semiannual variation of NmF2. A dierent situation exists in winter at longitudes near the magnetic poles, where the downwelling occurs at relatively low geographic latitudes so that solar radiation is strong enough to produce large values of NmF2. This circulation-driven mechanism provides a reasonably complete explanation of the observed pattern of F2 layer annual and semiannual quiet-day variations.

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