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Ion acceleration by Alfven waves on auroral field lines

Bingham, Robert and Eliasson, Bengt and Mendonca, Jose Tito and Stenflo, Lennart (2013) Ion acceleration by Alfven waves on auroral field lines. Physica Scripta, 87 (5). ISSN 0031-8949

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Abstract

Observations of ion acceleration along auroral field lines at the boundary of the plasma sheet and tail lobe of the Earth show that the energy of the ions increases with decreasing density. The observations can be explained by ion acceleration through Landau resonance with kinetic Alfvén waves (KAWs) such that k_A.v_i = ω_A, where k_A is the wave vector, v_i is the ion resonance velocity and ω_A is the Alfvén wave frequency. The ion resonance velocities are proportional to the Alfvén velocity which increases with decreasing density. This is in agreement with the data if the process is occurring at the plasma sheet tail lobe boundary. A quasi-linear theory of ion acceleration by KAWs is presented. These ions propagate both down towards and away from the Earth. The paths of the Freja and Polar satellites indicate that the acceleration takes place between the two satellites, between 1Re and 5Re. The downward propagating ions develop a horseshoe-type of distribution which has a positive slope in the perpendicular direction. This type of distribution can produce intense lower hybrid wave activity, which is also observed. Finally, the filamentation of shear Alfvén waves is considered. It may be responsible for large-scale density striations.