Cold Clouds as Cosmic-Ray Detectors
  Low energy cosmic-rays (CRs) are responsible for gas heating and ionization
of interstellar clouds, which in turn introduces coupling to Galactic magnetic
fields. So far the CR ionization rate (CRIR) has been estimated using indirect
methods, such as its effect on the abundances of various rare molecular
species. Here we show that the CRIR may be constrained from line emission of
H$_2$ rovibrational transitions, excited by CRs. We derive the required
conditions for CRs to dominate line excitation, and show that CR-excited lines
may be detected with the Very Large Telescope (VLT) over 8 hours integration.
Our method, if successfully applied to a variety of clouds at different
Galactic locations will provide improved constraints on the spectrum of low
energy CRs and their origins.
