Interaction of cosmic rays with molecular clouds

We analyze interaction of fast charged particles with Galactic molecular clouds. As a results of energy losses inside the clouds, a particle flux, converging to the cloud, is formed in the cloud vicinity. This flux, in its turn, excites there an MHD-turbulence by the streaming instability, which scatter the primary particles. Therefore the CR/cloud interaction is a complex problem described by a system of non-linear equations. We suggested a model of CR penetration into the dense cloud interior through their diffuse outer envelope. Three different regimes of CR propagation are realized in the envelope depending of CR energy: 1) Low energy particles with energies of tens of MeV are almost frozen into the MHD waves and propagate by convection with Alfven velocity. These particle do not reach the dense interior because of energy losses; 2) High energy particles with energies above ten GeV do not experience any scattering and propagate freely with luminal velocities. 3) In the intermediate energy range a ”mirroring shield” is formed between the diffuse envelope and the dense interior which prevents free CR penetration into the interior from outside. This effect is a function of the hydrogen density in the diffuse envelope and the spectrum of CRs in the intracloud medium.