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On 2 December 1977, 1600-1700 UT (around 19 MLT) a section of the Harang discontinuity moved westward over northern Scandinavia with a velocity of about 1 km/s, during a disturbed time interval. The westward movement was clearly identified in the IMS Scandinavian Magnetometer Array observations and is consistent with simultaneous ST ARE electric field measurements. The magnetic measurements showed a marked increase of the overhead current density during the westward movement while the electric field amplitudes stayed at the same level, thus indicating a temporal enhancement of the ionospheric conductivity. The westward movement gave the opportunity to combine subsequently observed two-dimensional distributions of equivalent current vectors (after correction for the magnetic field intensity increase) and electric field vectors into two corresponding patterns fairly extended in the east-west direction and covering the total extent of the most interesting part of the Harang discontinuity region. The electrically defined Harang discontinuity was located 200-300 km to the north of the magnetically defined discontinuity. The combined observational data were used to determine, as far as possible, the ionospheric distributions of height-integrated conductivities, of horizontal height-integrated current density vectors and of field-aligned currents. The results show a reduction of the Hall conductivities and upward Birkeland currents at and around the electrically defined Harang discontinuity. Some horizontal ionospheric current crosses the discontinuity, being diverged northward from the eastward electrojet and joining the westward electrojet, after further counterclockwise rotation.
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