  Cosmic-rays constitute the main ionising and heating agent in dense,
starless, molecular cloud cores. We reexamine the physical quantities necessary
to determine the cosmic-ray ionisation rate (especially the cosmic ray spectrum
at E < 1 GeV and the ionisation cross sections), and calculate the ionisation
rate as a function of the column density of molecular hydrogen. Available data
support the existence of a low-energy component (below about 100 MeV) of
cosmic-ray electrons or protons responsible for the ionisation of diffuse and
dense clouds. We also compute the attenuation of the cosmic-ray flux rate in a
cloud core taking into account magnetic focusing and magnetic mirroring,
following the propagation of cosmic rays along flux tubes enclosing different
amount of mass and mass-to-flux ratios. We find that mirroring always dominates
over focusing, implying a reduction of the cosmic-ray ionisation rate by a
factor of 3-4 depending on the position inside the core and the magnetisation
of the core.
