The ionospheric electric fields and currents and the associated ground magnetic variations, generated by the dynamo action of winds simulated with the National Center for Atmospheric Research, Boulder, Colo. (NCAR) Thermospheric General Circulation Model (TGCM), are modelled and compared with observations for equinox solar minimum conditions. The dynamo model uses a tilted dipole geomagnetic field and allows for field-aligned current flow between conjugate points, but no magnetospheric dynamo effects are included. Two TGCM wind simulations are used, one of which is driven only by in situ solar ultraviolet heating and the other of which includes lower boundary forcing that mimics the effects of upward propagating semidiurnal tides, as described by Fesen et al. (1986). Without tidal forcing, the TGCM winds produce ground magnetic variations that have the general pattern of observed Sq variations but are only about half as strong.

The addition of tidal forcing, as specified by Fesen et al., improves the agreement between calculated and observed magnetic variations and also between calculated and observed electric fields, although the nighttime electric fields are not well reproduced. The dynamo effects of the simulated semidiurnal tides appear to be somewhat overestimated, as has been similarly noted by previous modellers. The nightside disagreement between modelled and observed ionospheric electrodynamic drifts appears to be related to inaccurate simulation of F-region dynamo effects, which are sensitive to the nightside electron density distribution. The field-aligned currents between southern and northern magnetic hemispheres are substantial even at equinox, with about 290 kA total current flowing at 1700 UT. The ground magnetic effects associated with these field-aligned currents are roughly 25% the size of the magnetic effects associated with the horizontally nondivergent component of ionospheric current. The field-aligned current distribution is considerably more sensitive to the wind and conductivity patterns than is the horizontal ionospheric current distribution. ¿ American Geophysical Union