Role of the mean electric field in diffusive shock acceleration

Diffusive shock acceleration in the mean electric field produced during the development of Bell’s nonresonant streaming instability is considered. It is shown that the electric field lowers the efficiency of acceleration, for particles producing the instability (protons and nuclei) and raising the efficiency of acceleration for particles with the opposite electric charge (electrons). This effect is important for shock waves propagating with speeds above 10000 km s−1 in the interstellar medium or for slower shock waves propagating in a medium with weak magnetic field.