Dense molecular clouds play important role in cosmic rays studies. Due
to their high mass, low temperature and very high density as compared
to the interstellar plasma, they are often referred to as "space
calorimeters". Multi-wavelengths observations of these objects can
help us determine spectra of cosmic rays in the distant parts of the
Galaxy. This information may be crucial for the understanding on how
cosmic rays are accelerated and how are they being confined in the
Galaxy. On the other hand, cosmic ray studies also important for
understanding star formation. Indeed, cosmic rays can penetrate deep
into the dense clouds, far beyond the ultra-violet photons, and
therefore they can ionize medium in the darkest regions of the clouds.
Therefore, they can trigger and enhance star formation in the
molecular clouds cores.
To carefully study these two problems, one should know how charged particles penetrate the dense gas. Since cosmic rays experience intense energy losses inside, the non-zero flux of particles directed towards the molecular cloud will be formed. And as a result of streaming instability zone of high turbulence will be formed in the clouds envelope, preventing particles from propagating into the cloud. Therefore, the interaction of particles with clouds is non-trivial and is described by a system of non-linear equations. I will show that this effect can be described by a simple analytic solution, allowing easily to estimate the flux of particles into the cloud and the spectra they form inside.