1. ABSTRACT This work concerns with the analysis and modelling of possible magnetohydrodynamic response of plasma of the solar low atmosphere (upper chromosphere, lower corona) on a change of current system, accompanying propagation of fluxes of fast charged particles during an impulsive phase of a solar flare. 1. BASIC IDEAS Among different effects of the primary energy release of a solar flare in the corona is the rapid acceleration of charged particles, which propagate along magnetic field lines toward the chromosphere and photosphere and interact with the surrounding atmosphere (Brown, 1973; Emslie, 1996). Observations of coronal hard X-ray sources (> 30 keV) in the cusp region above a flare loop (Masuda et al., 1994) and the consistency of their heights with the electron time-of-flight distance to the foot points of the loop (Aschwanden et al., 1996) provide strong evidence that accelerated particles in solar flares are electrons, and their acceleration occurs in the region above the flare loop. 1.1 Standart scheme of a flare Usually Coulomb collisions of non-thermal electrons of the beam with particles of a background plasma are considered as a main mechanism of energy deposition in the chromospheric part of a flare. The standard scenario of a flare supposes a certain order of events: hard X-ray bursts (thick-target bremsstrahlung by high energy electrons in lower chromosphere) heating of the chromospheric plasma to soft X-ray emitting temperatures evaporated chromospheric plasma flows According to observations this order is not universal. temporal discorrelations between the predicted classical stages location of the sources of emission and their evolution are different for different events 1.2 Further development of the model of a flare. MHD response of a magnetic tube to an injected beam of fast electrons Without putting into question the existence of the direct beam-plasma collisional mechanism of heating, we pay more attention to the effects connected with the classical Ohmic dissipation of currents. We study the magnetohydrodynamic response of a plasma in the low solar atmosphere to a changing current system of a flaring magnetic tube, which contains a beam of fast non-thermal electrons (Fig. 1). This is a special aspect of the process of a beam injection and propagation in a magnetic tube not yet considered in the literature. We suppose: The local disturbances of a current system are estimated using the classical idea of a return current. ( Emslie, 1983; Van den Oord, 1990 ). According to this idea the total current density in a magnetic tube after injection of the beam j = j' + j b should not change compared to the initial current density in the tube j. This means that the current density of the magnetic tube j' in fact changes: j' = j + j r.c. This change is due to return current j r.c = - j b, which compensates the current density j b of the injected beam of fast electrons. MHD effects triggered by beams of fast particles in magnetic tubes and their possible manifestation in microwave and X-ray emissions from solar flares M. L. Khodachenko, and H. O. Rucker Space Research Institute, Austrian Academy of Sciences, Schmiedlstr.6, A-8042 Graz, Austria E-mail: Maxim.Khodachenko@oeaw.ac.at Fig.1 Scheme of a flaring event The impulsive character of a beam injection causes two stages in the evolution of the tube: First stage is 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. In the second stage 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. The beam of fast electrons here plays a role of a disturbing factor and appears to be a kind of trigger for dynamical processes which can influence the observational features of a flaring event. The energy which is released during the MHD evolution and heating of the chromospheric plasma in a flaring magnetic tube comes not from the energy of the beam, but is stored in an initial preflaring magnetic field. The Ohmic effects, accompanying the change of the initial equilibrium current density play only the role of a destabilizing factor. The destabilization of the magnetic tube by a beam is effective if duration of the beam t b is at least of the order of the characteristic time t of temperature variation in response to changed current density. For a 20 keV beam with n b /n = 10 -4, propagating in a plasma with n = 10 10 cm -3, the characteristic time. Thus the plasma temperature T = 10 5 K yields t = = t 20 keV = 7.3 s, whereas for T = 10 6 K, t = t 20 keV = 2300 s. This means that the temporal change of the current density in the regions of non-thermal electron beam propagation will be able to disturb the plasma thermodynamical equilibrium only if the plasma temperature is low enough. Considered mechanism of plasma disturbing and heating works on the heights where the direct collisional energy deposition from the beam to the background plasma is still not efficient ( Emslie, 1983 ). SP31A-05 Particle acceleration Collision of fast electrons with particles of a background plasma Heating of the lower chromosphere to soft X-ray emitting temperatures Creation of steep pressure gradients resulting in intensive upflows of evaporated chromospheric plasma An equilibrium preflaring magnetic loop Balance of: pressure gradient and Ampere force heating and cooling mechanisms in the magnetic tube Penetration of a beam of energetic electrons into the magnetic tube Disturbance of the initial equilibrium current density in the magnetic tube Change of the current den- sity in the magnetic tube Change the Joule heating j 2 / s Disturbance of the thermodyna- mical equilibrium of the system Heating (cooling) of the plasma Brake of the force balance Complex MHD dynamics of the whole plasma-magnetic structure Energy equation The value of t increases when the plasma parameters change from the chromospheric to coronal ones. The optimal range of heights (upper chromosphere, lower corona) exists, where the MHD disturbance of a magnetic tube by a propagating beam is the most efficient