Particle acceleration by MHD surface wave and formation of aurora

Hydromagnetic surface waves, excited either by a MHD plasma instability or by an externally applied impulse, are shown to resonantly mode convert to the kinetic Alfvén wave, the Alfvén wave having a wavelength comparable to the ion gyroradius in the direction perpendicular to the magnetic field. The kinetic Alfvén wave has a component of its electric field in the direction of the ambient magnetic field and can accelerate plasma particles along the field line. Because of the property of the wave the acceleration occurs on a thin magnetic surface separated by the ion gyroradius. A possible relation between this type of acceleration and the formation of aurora arcs is discussed.