Energetic storm particle events in the outer heliosphere

The evolution of energetic particle events with increasing heliocentric distance is studied through events of Pioneers 10 and 11. Beyond 12 AU the events become the dominant type of solar particle event at 1 AU, and the combined effects of adiabatic cooling and volume expansion rule out the possibility that the particles represent the confinement of the original particle population behind the shock. It is not established whether the particles originate from the solar wind by injection via post-shock enhancements or are energetic solar particles further energized by the shock, although their very long lifetime favors the solar wind origin.