ELECTRON BEAM-PLASMA INTERACTION AND THE RETURN-CURRENT FORMATION

Using a 3D electromagnetic particle-in-cell model, the plasma systems with electron beams are studied. The model parameters are chosen relevant to solar flares. To show the complexity of the problem studied, in the first model, we present an evolution of two oppositely propagating electron beams that escape from the localized acceleration region. Then due to the limitations of this model, in other models we consider a plasma system with one spatially homogeneous beam and a neutralizing return current. The models without the initial magnetic field revealed an importance of the Weibel instability that very efficiently transferred the beam energy to the heating of plasma electrons in the direction perpendicular to that of the beam propagation. In this case, the return current evolved from the initially shifted Maxwell distribution to the distribution with a broad flat maximum. However, in the models with a sufficiently strong magnetic field or those with shorter sizes of the numerical box (effectively the 1D case), the Weibel instability was reduced, and the electron distribution function mainly evolved in the beam-propagation direction and the return current was given by a shift of some bulk electrons and an extended tail.