The direct penetration of the high‐latitude electric field to lower latitudes, and the disturbance dynamo, both play a significant role in restructuring the storm‐time equatorial ionosphere and thermosphere. Although the fundamental mechanisms generating each component of the disturbance electric field are well understood, it is difficult to identify the contribution from each source in a particular observation. In order to investigate the relative contributions of the two processes, their interactions, and their impact on the equatorial ionosphere and thermosphere, the response to the March 31, 2001, storm has been modeled using the Rice Convection Model (RCM) and the Coupled Thermosphere‐Ionosphere‐Plasmasphere‐Electrodynamics (CTIPe) model. The mid‐ and low‐latitude electric fields from RCM have been imposed as a driver of CTIPe, in addition to the high latitude magnetospheric sources of ion convection and auroral precipitation. The high latitude sources force the global storm‐time wind fields, which act as the driver of the disturbance dynamo electric fields. The magnitudes of the two sources of storm‐time equatorial electric field are compared for the March 2001 storm period. During daytime, and at the early stage of the storm, the penetration electric field is dominant; while at night, the penetration and disturbance dynamo effects are comparable. Both sources are sufficient to cause significant restructuring of the low latitude ionosphere. Our results also demonstrate that the mid‐ and low‐latitude conductivity and neutral wind changes initiated by the direct penetration electric field preferentially at night are sufficient to alter the subsequent development of the disturbance dynamo.

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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.