H ₃ ⁺ , the ideal probe for in situ measurement of the Galactic cosmic rays

The reason why H 3 + 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 H 2 since the theoretical prediction by Spitzer & Tomasko (Spitzer & Tomasko 1968 Astrophys. J. 152 , 971–986) is reviewed. How the measurements by H 3 + 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: H 3 + , H 5 + and beyond’.