Ionospheric shear flow situations observed by the MIRACLE network, and the concept of Harang discontinuity

Ionospheric shear flow regions connected by magnetic field lines to corresponding magnetospheric shear zones contain a substantial portion of the large-scale field-aligned currents (FACs) in the ionosphere-magnetosphere system. Therefore, these regions play a key role in the interregional coupling mechanism. The most prominent shear zone, leading to the transition from positive to negative values of the northward (X) ground magnetic disturbance component in evening sector auroral latitudes to around magnetic midnight, is named as Harang discontinuity. Using the ground-based MIRACLE network in northern Fennoscandia, we focus on two aspects of shear flow regions: First, we show that two topologically different types of Harang discontinuities often confused in the literature exist, one associated with the Earth's rotation under a stable potential structure during quiet and moderately disturbed times, and another associated with the equatorward expansion of the evening cell potential minimum during intensifications and substorms. The latter typically appears in an earlier MLT sector than the former. Secondly, for five instantaneous shear flow situations observed in summer 1998, we infer the distributions of ionospheric conductances, true ionospheric currents, and field-aligned currents using the method of characteristics. The shear flow situations studied are associated with relatively small ionospheric conductances, which show large gradients somewhat equatorward of the shear regions. Hence, two latitudinally separated layers of FACs of the same sign are present at these zones: One directly at the shear zone associated with the divergence of the electric field, and another about 1-2 degrees of latitude southward, caused by the conductance gradient. These results are discussed in terms of the magnetospheric regions possibly connected with both areas.