H₃⁺, the ideal probe for in situ measurement of the Galactic cosmic rays

The reason why [Formula: see text] is an ideal probe for in situ measurement of cosmic ray energy density in the Galaxy is discussed. The variation of measured values of cosmic ray ionization rate ζ of H₂ since the theoretical prediction by Spitzer & Tomasko (Spitzer & Tomasko 1968 Astrophys. J. 152, 971-986) is reviewed. How the measurements by [Formula: see text] established ζ as of the order of 10^-17 s^-1 for dense clouds, 10^-16 s^-1 for diffuse clouds in the Galactic disc, and 10^-14 s^-1 for warm and diffuse gas in the Galactic centre is discussed. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H₃⁺, H₅⁺ and beyond'.