Analytic growth rates of NRSI derived for monenergetic (dotted) and power-law (dashed) momentum distributions of CRs. The blue curve represents the growth rate obtained from the fluid approach (Equation (3)), while all other curves are obtained from the kinetic approach (Equations (A2) and (A5)). The maximum growth rates and the corresponding wavenumbers remain almost the same; they depend on the bulk CR current.