Cosmic rays as regulators of molecular cloud properties
  Cosmic rays are the main agents in controlling the chemical evolution and
setting the ambipolar diffusion time of a molecular cloud. We summarise the
processes causing the energy degradation of cosmic rays due to their
interaction with molecular hydrogen, focusing on the magnetic effects that
influence their propagation. Making use of magnetic field configurations
generated by numerical simulations, we show that the increase of the field line
density in the collapse region results in a reduction of the cosmic-ray
ionisation rate. As a consequence the ionisation fraction decreases,
facilitating the decoupling between the gas and the magnetic field.
