Anisotropic beams of energetic particles upstream from the termination shock of the solar wind

For similar to 2.5 yr before the crossing of the termination shock of the solar wind, Voyager 1 observed a series of highly anisotropic beams, which are believed to result from downstream particles leaking upstream across a broad range of heliographic longitudes. We show that the downstream spectrum of the accelerated particles is a power law with spectral index of - 5 when expressed as a distribution function over this same broad longitude range. We also show that the upstream beams contain too little heat flux to influence the shock structure or the processes occurring downstream. We argue that underlying the beams is an isotropic distribution of suprathermal particles, which also has a power-law spectrum with spectral index of - 5, and we find that these conditions are consistent with a simple shock-heating model for the acceleration of energetic particles at the termination shock.