Electric currents above Saint‐Santin: 2. Model

We use Ohm's law and a combination of various local models of the electrodynamic parameters of the ionsphere derived from previous incoherent scatter studies (electric fields, neutral winds, ion composition, and ion neutral collision frequencies) to generate an empirical model of the local time and seasonal variations of the horizontal electric current flow above Saint‐Santin (geographic latitude 44.1°N, geographic longitude 2.3°E). The various contributions to the total current (Hall and Pedersen contributions, neutral wind driven contributions, and electric field driven contributions) are explicitly obtained in this model. The local time variations of the two components of the total height‐integrated horizontal current are compared with the S r magnetic variation at our location. For all seasons the zonal component is weak, mainly as the result of equal and opposite contributions of the electric field and neutral winds. This result, which is consistent with the weak observed S r variation of H , is expected for a station such as ours close to the latitude of the S q focus where the current flow is essentially meridional. In contrast to the usual picture of the S q system, which suggests a southward current in the morning and a northward current in the afternoon, our model meridional current is found to be always northward during the daytime. This surprising feature of the model is supported by the direct calculation of the current from the ion drifts on a series of individual days. Examination of the S R variation of the D component of ground magnetic variations shows that a north‐south current asymmetry, though of smaller intensity, is also present in the magnetic variations. Both the net northward flow of charges revealed by the ionospheric data, and the observed discrepancy between calculated ionospheric currents and magnetic data, suggest that field‐aligned currents must be flowing from the northern to the southern hemisphere in our longitude sector during a least part of the day. This situation seems to be quite different from the American sector, where no similar asymmetry has been detected in the average current flow over Millstone Hill. Explanations of these features will have to involve the possible effects of the inclination of the magnetic dipole of the earth with respect to its rotation axis, as well as of antisymmetric tidal modes, which are likely to produce, interhemispheric asymmetries and longitude variations.