Particle propagation, wave growth and energy dissipation in a flaring flux tube

Abstract

We investigate wave amplification by downgoing particles in a common flare model. The flare is assumed to occur at the top of a coronal magnetic flux loop, and results in the heating of plasma in the flaring region. The hot electrons propagate down the legs of the flux tube towards increasing magnetic field. It is simple to demonstrate that the velocity distributions which result in this model are unstable to both beam instabilities and cyclotron maser action. We present an explanation for the propagation effects on the distribution, and explore the properties of the resulting amplified waves. We concentrate on cyclotron maser action, which has properties (emission in the z mode below the local gyrofrequency) quite different from maser action by other distributions considered in the context of solar flares. The z mode waves will be damped in the coronal plasma surrounding the flaring flux tube, and lead to heating there. This process may be important in the overall energy budget of the flare. We compare the downgoing maser with the loss cone maser, which is more likely to produce observable bursts.