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A flux of cosmic rays (CRs) propagating through a diffuse ionized gas canexcite MHD waves, thus generating magnetic disturbances. We propose a genericmodel of CR penetration into molecular clouds through their diffuse envelopes,and identify the leading physical processes controlling their transport on theway from a highly ionized interstellar medium to a dense interior of the cloud.The model allows us to describe a transition between a free streaming of CRsand their diffusive propagation, determined by the scattering on theself-generated disturbances. A self consistent set of equations, governing thediffusive transport regime in an envelope and the MHD turbulence generated bythe modulated CR flux, is essentially characterized by two dimensionlessnumbers. We demonstrate a remarkable mutual complementarity of differentmechanisms leading to the onset of the diffusive regime, which results in auniversal energy spectrum of the modulated CRs. In conclusion, we brieflydiscuss implications of our results for several fundamental astrophysicalproblems, such as the spatial distribution of CRs in the Galaxy as well as theionization, heating, and chemistry in dense molecular clouds.

Penetration of cosmic rays into dense molecular clouds: role of diffuse envelope