Acceleration of low‐energy protons and alpha particles at interplanetary shock waves

Abstract

We have investigated low‐energy protons and alpha particles in the energy range 30 keV/charge to 150 keV/charge associated with three different interplanetary shock waves in the immediate preshock and postshock region. The data were obtained with the Max‐Planck‐Institut/University of Maryland sensor system on ISEE 3. In particular, we present spatial distributions in the preshock and postshock medium as measured in the spacecraft frame and after transformation into the solar wind and the shock frame of reference, respectively, the dependence of the phase space density at different energies on distance from the shock, and the form of the distribution function of both species immediately at the shock. In the preshock region particles are flowing in the solar wind frame of reference away from the shock and in the postshock medium the distribution is more or less isotropic in this frame of reference. This is similar to findings at the earth's bow shock during times when diffuse particles are present. The distribution function in the postshock region can be represented by a power law in energy with the same spectral exponent for both protons and alpha particles. In the preshock medium the phase space densities falls off exponentially (after subtraction of a background value) with distance from the shock and the e‐folding distance scale of the proton phase space densities increases approximately proportional to the square root of energy. In addition, there is a much slower intensity increase further upstream over a time period of several hours. Although the spectra of diffuse bow shock associated particles are different from the spectra of the interplanetary shock‐associated particles in the immediate vicinity of the shock, it is concluded that the first‐order Fermi acceleration process can consistently explain the data. It is shown that independent of the acceleration mechanisms invoked, the mean free path of the low energy ions in the preshock medium is considerably smaller than the mean free path determined by the turbulence of the background interplanetary medium. This suggests that these particles generate a wave field of their own making, which in turn scatters the particles.