“POLAR” SUBSTORMS AND THE HARANG DISCONTINUITY

The substorms that we term as “polar” substorms, are recorded in the evening sector of the Earth at the geomagnetic latitudes above 70° MLAT under the absence of negative magnetic bays at the lower latitudes. Like the “classical” substorms, the “polar” substorms are accompanied by aurora arcs brightening, poleward expansion, substorm current wedge formation. The onsets of “polar” substorms are typically located near 70° MLAT at ~19-23 MLT. Other important structure, namely, the Harang discontinuity (the evening narrow latitude-zone between the westward and eastward electrojets), is often observed in the same area and at the same MLT interval. Our aim is to study a possible relationship between the location of the “polar” substorms and the Harang discontinuity (HD). Using the IMAGE geomagnetic data, we found that the “polar” substorm onsets exhibit a tendency to occur near the HD latitude. The longitudinal relationship between the “polar” substorms and the HD was studied basing on the ionospheric AMPERE measurements by the 66 simultaneous satellites. We revealed the ground-based magnetic vortex associated with FACs enhancement near the eastward edge of the Harang discontinuity region separated the evening “polar” substorm development and the after-midnight westward electrojet location. Two typical events of the “polar” substorms are discussed in detail.