A polar magnetic substorm observed in the evening sector with a two-dimensional magnetometer array

A two-dimensional array of 18 three-component magnetometers recorded magnetic fields, on 1974 August 14, associated with a polar magnetic substorm which occurred while the array was in the evening sector. An eastward ionospheric current flowed above the array for more than six hours. Perturbation fields at six representative epochs were modelled by three-dimensional magnetospheric-ionospheric current loops of Boström Type 1, the fields at array stations being calculated by a method due to Kisabeth and fitted to the observed fields by iterative adjustment of parameters of the current loop geometry. Current density distribution across the current width was estimated by an inverse method due to Oldenburg. A satisfactory fit was obtained for the first epoch and very good fits for the other five. In addition to the eastward electrojet above the array, five of the six current models include a westward ionospheric current to the north of the eastward current. This northern current decayed rapidly during the first three hours, in contrast to the continuing eastward current. This difference in the lifetimes of the two electrojets, and perturbations recorded at North American observatories, support the Rostoker–Kisabeth hypothesis of continuation of the westward substorm electrojet into the evening sector, north of the eastward electrojet there. At the last five of the six epochs modelled, the eastern end of the eastward ionospheric current ended just east of the array, which recorded strong evidence of the upward field-aligned current there and located it within 5° of longitude absolutely, and its time displacements within 2°. The upward field-aligned current moved westward no more than 5° in the four hours 04.53–08.53 UT, very nearly rotating with the Earth and drifting eastward 55° relative to geomagnetic midnight. The paper concludes with some general conclusions from three near-midnight substorms studied with the array and reported previously, and the present evening-sector substorm study. Two-dimensional magnetometer arrays are suited to the detection and location of near field-aligned currents and this has led to the discovery that all four substorm current systems studied have drifted eastward, relative to geomagnetic midnight.