Large ionospheric plasma flows along magnetic field lines: EISCAT observations and their interpretation

Abstract

EISCAT measurements of plasma velocities in the auroral ionosphere have indicated very large plasma flows along magnetic field lines. Velocities exceeding 500 ms −1 have been observed on several occasions, always in association with large electric fields. Observations with the EISCAT radar pointing along its local field line have provided height profiles of the various plasma parameters during periods when large field-aligned flows occur. The most important factor in driving the plasma is found to be the gradient in plasma pressure. Electric fields up to 50 mVm −1 cause frictional heating of the ion population, but little change in recombination coefficient so that increased plasma pressure in the F-region drives plasma upwards into the topside. However, when the electric field is even larger then the rate of recombination increases sharply, the electron concentration is depleted, F-region plasma pressure decreases and plasma is driven downwards.