  A flux of cosmic rays (CRs) propagating through a diffuse ionized gas can
excite MHD waves, thus generating magnetic disturbances. We propose a generic
model of CR penetration into molecular clouds through their diffuse envelopes,
and identify the leading physical processes controlling their transport on the
way 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 CRs
and their diffusive propagation, determined by the scattering on the
self-generated disturbances. A self consistent set of equations, governing the
diffusive transport regime in an envelope and the MHD turbulence generated by
the modulated CR flux, is essentially characterized by two dimensionless
numbers. We demonstrate a remarkable mutual complementarity of different
mechanisms leading to the onset of the diffusive regime, which results in a
universal energy spectrum of the modulated CRs. In conclusion, we briefly
discuss implications of our results for several fundamental astrophysical
problems, such as the spatial distribution of CRs in the Galaxy as well as the
ionization, heating, and chemistry in dense molecular clouds.
