Acceleration of low-energy ions at the termination shock of the solar wind

The Voyager 1 investigators have reported that the spacecraft crossed the termination shock of the solar wind at 94 AU from the Sun. The intensity of low-energy ions ( less than or similar to 3 MeV nucleon(-1)) increases abruptly at the shock. It is argued in this paper that the spectral shapes of the low-energy ions observed at the termination shock, both upstream and downstream, require that the pressure of the accelerated particles is behaving like that of a simple ideal gas, without heat flux, and that the intensity increase across the termination shock can be determined by assuming that the pressure of the accelerated particles behaves according to the Rankine-Hugoniot relationship. The approach taken in this paper is contrasted with diffusive shock acceleration. The implications of the conclusions from this work for the acceleration of suprathermal tails on the distribution of particles in the solar wind and for the origin of anomalous cosmic rays are discussed.