‘Polar’ Substorms and the Harang Discontinuity

We analyzed 214 events of ‘polar’ substorms on the Scandinavian meridian IMAGE, i.e., substorms recorded by magnetometers located at geomagnetic latitudes above ~70° MLAT at 1900−0200 MLT during a magnetically quiet time in the absence of negative magnetic bays at lower latitudes. The Harang discontinuity, which separates the westward and eastward electrojets by latitude, is a typical structure for the indicated MLT sector of the high-latitude ionosphere. The global distribution of ionospheric electrojets and the location of the Harang discontinuity during development of ‘polar’ substorms were studied using the maps compiled from the results of spherical harmonic analysis of magnetic measurements on 66 simultaneous ionospheric communications satellites of the AMPERE project. Based on analysis of these maps, it is shown that the instantaneous location of the equatorial boundary of the ionospheric current of a ‘polar’ substorm determines the instantaneous location of the polar boundary of the Harang discontinuity, and the polar boundary of the eastward electrojet determines its equatorial boundary. It has been established that the appearance of 90% of ‘polar’ substorms is observed simultaneously with increasing planetary substorm activity according to the AL-index and development of a magnetospheric substorm in the postmidnight sector. At the same time, the development of evening ‘polar’ substorms is associated with the formation of near-midnight magnetic vortices at geomagnetic latitudes of ~70° MLAT (near the “nose” of the Harang discontinuity), indicating a sharp local enhancement of the field-aligned currents. This leads to the formation of a new substorm in the evening sector of near-polar latitudes, called a ‘polar’ substorm with typical features of the onset of a substorm (Pi2 geomagnetic pulsation bursts, sudden onset of the substorm close to the equatorial boundary of the contracted oval (the development of a “substorm current wedge”, etc.)