Field‐aligned currents deduced from EISCAT radar observations and implications concerning the mechanism that produces region 2 currents

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Abstract
Large‐scale field‐aligned currents have been determined from electric field and conductivity measurements performed with the European Incoherent Scatter (EISCAT) radar. The result, given for a quiet and stationary day, is globally consistent with the usual picture of the large‐scale morphology of field‐aligned currents involving region 1 and region 2 current systems. A comparison between the electric potential pattern (deduced from electric field measurements) and the field‐aligned current pattern in region 2 has been performed. This involved, in particular, comparing the phases (in terms of local time) of the diurnal harmonics of the field‐aligned current and potential distributions. The results imply that the field‐aligned current tends to be distributed in phase opposition, rather than phase quadrature, with the potential. This is found to be irreconcilable with the mechanism usually put forward as the generator of field‐aligned currents in the medium and inner magnetosphere (namely, charge separation due to gradient drifts of energetic particles at the ring current inner edge). We show that the loss of convected energetic electrons by precipitation due to pitch angle scattering produces azimuthal pressure gradients sufficiently pronounced that it may constitute the dominant mechanism for driving field‐aligned currents in the medium and inner magnetosphere, thus accounting for the observed phase characteristics of region 2 currents.