Multiple-encounter shock-drift acceleration in nearly perpendicular shocks

A scheme to accelerate low-energy (slightly greater than thermal energy) ions by a nearly perpendicular magnetohydrodynamic shock, lacking upstream turbulence, is proposed. The acceleration relies on the vigorous particle cross-field scattering by downstream turbulence, competing with the convection downstream, to send particles back to the shock front for acceleration. Some fraction of the low-energy ions can be accelerated up to a maximal energy, determined by the compression ratio and shock angle, before they escape upstream to, possibly, undergo the diffusion shock acceleration. This acceleration mechanism is a strong candidate for solving the particle-injection problem of the diffusive shock acceleration. An application of this acceleration scheme to the prompt ion acceleration during solar flares is discussed.