The results of fifteen rocket measurements of neutral wind profiles, the ionospheric electric field and electron density at mid-latitudes are compared with the predictions of the dynamo theory of mid-latitude ionospheric currents (Sq) and the measurements of the ground-level magnetic perturbation. In daytime, the computed ionospheric currents agree well with both ground-level magnetic observations and Sq models, while the electric field structure is consistent with a dynamo source of such fields, although the field amplitude appears to be about 50% of that predicted by models such as those of STENING (1973). At twilight there is a generally poorer agreement between the calculated currents and those indicated by magnetometers and by Sq models, probably due to difficulties of determining a precise magnetic baseline corresponding to zero electric current. At night, the wind system is occasionally strong enough to drive currents of 5 to 10 amps km^-1 unless these are opposed by locally-induced, polarization electric fields of the order of 5 to 10mVm^-1. Such fields may be readily induced, at mid-latitudes, by the high-speed, night-time wind systems set up by high latitude energetic processes during geomagnetic substorms, without requiring a direct penetration of magnetospheric, convective fields to low magnetic latitudes.