  Diffuse interstellar clouds show large abundances of H_3^+ which can be
maintained only by a high ionization rate of H_2. Cosmic rays are the dominant
ionization mechanism in this environment, so the large ionization rate implies
a high cosmic-ray flux, and a large amount of energy residing in cosmic rays.
In this paper we find that the standard propagated cosmic-ray spectrum predicts
an ionization rate much lower than that inferred from H_3^+. Low-energy (~10
MeV) cosmic rays are the most efficient at ionizing hydrogen, but cannot be
directly detected; consequently, an otherwise unobservable enhancement of the
low-energy cosmic-ray flux offers a plausible explanation for the H_3^+
results. Beyond ionization, cosmic rays also interact with the interstellar
medium by spalling atomic nuclei and exciting atomic nuclear states. These
processes produce the light elements Li, Be, and B, as well as gamma-ray lines.
To test the consequences of an enhanced low-energy cosmic-ray flux, we adopt
two physically-motivated cosmic-ray spectra which by construction reproduce the
ionization rate inferred in diffuse clouds, and investigate the implications of
these spectra on dense cloud ionization rates, light element abundances,
gamma-ray fluxes, and energetics. One spectrum proposed here provides an
explanation for the high ionization rate seen in diffuse clouds while still
appearing to be broadly consistent with other observables, but the shape of
this spectrum suggests that supernovae remnants may not be the predominant
accelerators of low-energy cosmic rays.
