MAGNETIC-DRIFT-DRIVEN INSTABILITY AND COSMIC-RAY ACCELERATION

The well known mechanism for diffusive acceleration of cosmic rays (CR) in the vicinity of a shock relies on an instability driven by the directed velocity of the cosmic rays with respect to the ambient plasma, through Landau-type resonance. Another source of directed velocity is the bending of magnetic field lines which gives rise to curvature and gradient drifts. Its effect has previously been considered on the diffusion equation of CR, but not on the instability mechanism itself. Here we show that it also results in a new source of instability, which might be important in two main cases: the very high energy CR (whose drift is the largest), and the modulation of the galactic cosmic rays by the solar cavity.