The Common Spectrum in the Heliosphere: Observations and a Theory to Explain the Observations

Abstract:

A fundamental discovery has been made in the heliosphere during the last decade: the spectrum of particles accelerated in both the inner heliosphere and in the heliosheath is the same: a power law in particle speed with a spectral index of -5, when the spectrum is expressed as a distribution function. The -5 common spectrum eventually rolls over at a speed corresponding to the maximum energy to which particles are accelerated. In the inner heliosphere the rollover occurs at quite low energies and thus the -5 portion is only observed in suprathermal particles. In the heliosheath, the rollover occurs at much higher energies and the common spectrum is observed over the full energy range of the Voyager energetic particle instruments, up to energies of ~100 MeV. Here recent and previous observations of the common spectrum in the heliosphere and heliosheath are presented, with emphasis on those that best reveal the conditions in which the acceleration must operate. Finally, we describe the physics of the pump acceleration mechanism, that yields the common -5 power law spectrum, and relate the rollover speeds to conditions in the regions where particles are accelerated.