The case for a common spectrum of particles accelerated in the heliosphere: Observations and theory

In the last decade a significant discovery has been made in the heliosphere: 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; or equivalently, a differential intensity spectrum that is a power law in energy with a spectral index of -1.5. In the inner heliosphere this common spectrum occurs at quite low energies and is most evident in instruments designed to measure suprathermal particles. In the heliosheath, the common spectrum is observed over the full energy range of the Voyager energetic particle instruments, up to energies of similar to 100MeV. The remarkable discovery of a common spectrum is compounded by the realization that no traditional acceleration mechanism, i.e., diffusive shock acceleration or stochastic acceleration, can account for the common spectrum. There is thus an opportunity to once again demonstrate the relevance of heliospheric physics by developing a new acceleration mechanism that yields the common spectrum, with the expectation that such a new acceleration mechanism will find broader applications in astrophysics. In this paper, the observations of the common spectrum in the heliosphere are summarized, with emphasis on those that best reveal the conditions in which the acceleration must operate. Then, building on earlier work, a complete derivation is presented of an acceleration mechanism, a pump acceleration mechanism, that yields the common spectrum, and the various subtleties associated with this derivation are discussed.