Very low-frequency recombination radio lines and a new method for estimation of soft cosmic ray intensity.

Explanations are given for why carbon recombination radio lines are observed at the highest excitation levels in the interstellar medium and why the radio lines of substantially more abundant hydrogen are absent. A new method is proposed for estimating the intensity of soft cosmic rays from the hydrogen-ionization rate measured from the relative intensities of highly excited carbon and hydrogen radio lines. A hydrogen-ionization rate of less than or equal to 1.5 x 10 to the -16th/s toward CasA is obtained.