[en] At the end of the sixties it became obvious that two-dimensional dynamo models can explain nearly all facts, which had been found morphologically for mean annual Sq-fields. During the recent decade new or improved methods to measure electric fields (e.g. incoherent sy.ter facilities) and to investigate great data files have been developed. New informations received with these methods about the existence of regular variations of the Sq-field in dependence on season and universal time andtabout the electric field have been summarized in Section 2. All attempts to describe also these variations with a two-dimensional dynamo mooel did not lead to any success, but showed a strong theoretical over-estimation of the asymmetries. Therefore, it must be concluded that three-dimensional plasmaspheric current systems, taking into consideration the coupling between both hemispheres along the high-conducting magnetic field lines, are needed in order to explain the regular variations of the Sq-field. The basic equations for two- and three-dimensional dynamo models, different methods for the solution of these equations and the resulting models from different authors are compiled and discussed (Section 3). Based on all morphological and theoretical results a plasmaspheric-ionospheric current system has been constructed and some properties of the plasmaspheric field-aligned current distribution, have been derived. (orig.)