The extension of magnetospheric convection toward low latitudes is investigated with the help of incoherent scatter measurements of ExB plasma drift velocities in the midlatitude F region above Saint-Santin (France). The observed relations of these drifts with auroral magnetic activity, the north-south component Bz of the interplanetary magnetic field, and the time derivative of Dst suggest that this low-latitude extension is not systematically associated with all polar magnetic substorms, but occurs in conjunction with large (5 gammas or more) southward Bz and is closely related in intensity with the negative excursions of d(Dst)/dt. We propose that plasma injection and the resulting development of an asymmetric ring current are closely linked with the low-latitude extension of magnetospheric convection.