Cosmic ray ionization of the interstellar medium

We consider a cosmic ray spectrum that is a power law in momentum down to acutoff and derive a lower cutoff corresponding to $E_{kin} \sim (30-60)$ MeVfrom the observed ionization rates in nearby diffuse clouds. While the realspectra of cosmic rays may not be so simple, we argue that one expects asubstantial change in the spectra at such energies and that, at firstapproximation, a power law spectra with a lower cutoff is appropriate. Such adescription of the cosmic rays in the interstellar medium is not onlytheoretically more attractive than the spectra used in the literature, but isalso supported by recent observations.Comment: (to appear in MNRAS) 8 pages, Plain TeX, 2 figures available upon request, Max Planck Institut f. Radioastronomie preprint # 56