University of Maryland and University of Michigan
Acceleration of particles in the heliosphere and beyondby the pumping mechanism
How particles are accelerated, often to extremely high energies, has been hotly debated since cosmic rays were discovered a century ago. Recent measurements of the energy spectra of accelerated particles, that finally extend down to solar wind energies, have revealed a surprising commonality in their solar-wind-frame spectral shapes above about the solar wind speed: power law distribution functions with a unique spectral index of -5 and an exponential roll over above some characteristic speed that depends on ambient solar wind properties. To explain these observations a new acceleration mechanism, the so called Pumping Mechanism, has been developed. After reviewing these recent observations of (mostly) proton spectra in the heliosphere and heliosheath, I will then briefly describe the Pumping Mechanism which relies on compressional turbulence to efficiently and rapidly transfer energy from a core population of particles to the accelerated particle population (tail particles). Next, I will demonstrate how the Pumping Mechanism predicts the observed energy spectra of locally accelerated particles (LAPs) in the heliosphere, termination shock particles (TSPs) and the anomalous cosmic rays (ACRs) in the heliosheath, and galactic cosmic rays (GCRs) originating in superbubbles of the Galaxy. Then, I will show that to explain the recent observations of energetic neutral atoms (ENAs) with IBEX and Cassini, and the rapid acceleration of ACRs, imposed by observations, requires strong turbulence in the outer regions of the heliosheath (not yet directly observable with Voyager 2). Finally, I will show that with this same strong turbulence, persisting beyond the heliopause, the very recent puzzling Voyager observations can be readily explained.