Large‐scale magnetic field perturbations and particle measurements at 1400 km on the dayside

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Abstract
Particle and magnetic field measurements made on the dayside of the magnetosphere by the Isis 2 satellite are reported. Northern hemisphere magnetic perturbations perpendicular to the main geomagnetic field are obtained by subtracting a model field from the field measured at the spacecraft. When plotted as two‐dimensional vectors, these perturbations, on the average, fit a pattern very similar to the usual two‐cell convection pattern except in the noon cleft region, where the perturbation direction tends to be eastward when the IMF By component is positive and westward when it is negative. This relatively strong dependence of the perturbation direction on the sign of By is taken as further evidence for interconnection between the IMF and the geomagnetic field. Comparing various features of the magnetic perturbations with particle measurements, it is found that, on the average, the maximum perturbation in the noon sector falls within the region of cleft precipitation lying near the poleward edge of cleft electrons and just poleward of the 40‐keV electron boundary. The location of the maximum perturbation in this sector is identified with the polar cap boundary. In the dawn and dusk sectors a marked change in slope of the magnitude of the perturbation typically occurs at the poleward edge of the plasma sheet; this is also the poleward edge of the region 1 field‐aligned currents defined by Iijima and Potemra (1976a) and identified here with the polar cap boundary.

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