Formation of the cosmic-ray halo: The role of nonlinear Landau damping

Abstract

We present a nonlinear model of self-consistent Galactic halo, where the processes of cosmic ray (CR) propagation and excitation/damping of MHD waves are included. The MHD-turbulence, which prevents CR escape from the Galaxy, is entirely generated by the resonant streaming instability. The key mechanism controlling the halo size is the nonlinear Landau (NL) damping, which suppresses the amplitude of MHD fluctuations and, thus, makes the halo larger. The equilibrium turbulence spectrum is determined by a balance of CR excitation and NL damping, which sets the regions of diffusive and advective propagation of CRs. The boundary $z_{cr}(E)$ between the two regions is the halo size, which slowly increases with the energy. For the vertical magnetic field of $\sim 1~\mu G$, we estimate $z_{cr} \sim 1$ kpc for GeV protons. The derived proton spectrum is in a good agreement with observational data.