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Fast Alfvén Wave Heating and Acceleration of Ions in a Nonuniform Magnetoplasma

A test-particle approach is used to study the collisionless response of protons to cold plasma fast Alfvén waves propagating in a nonuniform magnetic field: specifically, a two-dimensional X-point field. The field perturbations associated with the waves, which are assumed to be azimuthally symmetric and invariant in the direction orthogonal to the X-point plane, are exact solutions of the linearized ideal magnetohydrodynamic (MHD) equations. The protons are initially Maxwellian, at temperatures that are consistent with the cold plasma approximation. Two kinds of wave solution are invoked: global perturbations, with inward- and outward-propagating components; and localized purely inward-propagating waves, the wave electric field E having a preferred direction. In both cases the protons are effectively heated in the direction parallel to the magnetic field, although the parallel velocity distribution is generally non-Maxwellian and some protons are accelerated to highly suprathermal energies. This heating and acceleration can be attributed to the fact that protons undergoing E × B drifts due to the presence of the wave are subject to a force in the direction parallel to B. The localized wave solution produces more effective proton heating than the global solution, and successive wave pulses have a synergistic effect. This process, which could play a role in both solar coronal heating and late-phase heating in solar flares, is effective for all ion species, but it has a negligible direct effect on electrons. However, both electrons and heavy ions would be expected to acquire a temperature comparable to that of the protons on collisional timescales.