Adiabatic compression acceleration of fast charged particles

In this paper we introduce a mode of acceleration of fast, low-rigidity charged particles at shocks and compressions which does not appear to have been discussed previously. The particles propagate along the fluctuating magnetic field without scattering. The acceleration occurs when the disturbance propagates normal to a magnetic field which is turbulent on large scales. If the low-rigidity particles have speeds much greater than the speed of the disturbance, they can follow the random magnetic field lines, which meander back and forth across the compression. Because of the difference between the downstream and upstream flow speeds, the particles can be accelerated, much as in standard diffusive shock acceleration. In this picture, scattering in pitch angle is not necessary for considerable acceleration to occur. We suggest that this completely scatter-free process may accelerate low-energy superthermal electrons, for which resonant scattering may not be possible, up to energies where they can interact resonantly with longer wave-length waves generated by the ions and subsequently be accelerated by standard diffusive shock acceleration to energies comparable with the ions.