MHD effects triggered by beams of fast particles in magnetic tubes and their possible relation to plasma heating and dynamics during solar flares

We study the magnetohydrodynamic response of a plasma in the low solar atmosphere to a changing current system of a flaring magnetictube, which contains a beam of fast non-thermal electrons. The local disturbances of a current system of a magnetic tube when the beam is injected into it are estimated using the classical idea of a return current. According to this idea, after injection of abeam, the total current density in a magnetic tube, which includes as well the current density of the beam j_b, should not change compared to the current density in the tube before the injection. In order to keep constant the total current density j=j'+ j_b, the current density of the magnetic tube j'in fact changes. This change is due to a return current j_r.c.= –j_b, which compensates the current density of the injected beam of fast electrons. At the same time, any changes of the current density in the magnetic tube change the Joule heating and disturb the thermodynamic equilibrium of the system. Changing of the plasma temperature destroys also the force balance and starts the process of complex dynamics of the whole plasma-magnetic structure. The impulsive character of a beam injection causes two stages in the dynamic behavior of the tube. During the first stage, characterized by the presence of a beam, the preliminary equilibrium state of a magnetic tube is disturbed and complex dynamics of the plasma start in the region of the beam propagation. During the second stage, when the injection of the beam is already over, the plasma and magnetic field continue to evolve from the disturbed state and gradually relax to an equilibrium state. Various types of magnetic tube response onto injection of a beam of energetic electrons are studied using the dynamic models of the magnetic tube (Khodachenko, 1996a; 1996b) built on the basis of known self-similar solutions of plasma MHD. The model results are applied to the interpretation of observed flaring and burst phenomena.