MIDNIGHT VELOCITY SHEAR ZONE AND THE CONCEPT OF HARANG DISCONTINUITY

We discuss the concept of Harang discontinuity based on ionospheric observations of electrodynamic parameters, magnetic field mapping from the ionosphere to the magneto sphere, and theoretical reasoning. We consider the Harang discontinuity ris synonymous to the midnight-premidnight sector velocity shear zone in the auroral ionosphere, arguing that its main importance is related to the dynamic development of magnetospheric conditions, especially substorms. We show that typical two-dimensional ionospheric observations are not quite consistent with the statistical Heppner-Maynard convection models and this inconsistency becomes even more apparent when mapped into the equatorial magnetotail. The upward field-aligned current from the velocity shear zone is one of the major problems in performing a realistic mapping procedure. Because the auroral arcs close to the Harang discontinuity are often found to activate and break up at the time of the substorm onset, this same upward current is probably connected with the substorm initiation mechanism in the magnetotail. We discuss the electrodynamics of all parts of this current circuit, i.e., the ionosphere, the magnetosphere, and the connecting field lines. We conclude that surprisingly few observationally proven facts are known of this circuit outside the ionosphere, and even there the great variability of the physical parameter values makes this region very difficult to study in detail.