Cosmic-Ray Self-Confinement in the Hot Phase of the Interstellar Medium

Extract

It is well known that, when cosmic rays stream along the field lines of the galactic magnetic field at a velocity which exceeds the Alfven velocity in the medium, they excite hydromagnetic waves of a wavelength comparable to their Larmor radius. These waves, in turn, scatter the cosmic rays, forcing them to reduce their bulk speed (Wentzel 1974, and references there-in). In a stationary situation, the bulk speed of the cosmic rays will depend on their scale height, and on the strength of the relevant damping mechanisms affecting the waves. Isotope observations imply that cosmic rays of energy ≲ 1 GeV are confined in the galaxy for a time ~ 2.10⁷ yrs (Garcia-Munoz et al. 1977); a simple interpretation of the composition data in the energy range 1–100 GeV implies that the confinement time T decreases as the rigidity ε = cp/eZ increases: Tα ε^−a, with a = 0.3 − 0.5. This energy dependence of the confinement time should hold at least up to ε ≃ 3.10⁶ GV, to be consistent with the lack of structure in the cosmic ray spectrum in the range 10–3.10⁶ GV (see Cesarsky 1980 and references therein).