SPRG Seminars
February 15, 2011:
"Energetic Particle Acceleration in the Heliosphere"
Len Fisk , University of Michegan
Research during the past five years has revealed some remarkable insights into how energetic ions are accelerated in the solar wind. We have observed that the accelerated ions have a common spectral shape: when expressed as a distribution function the spectrum is a power law in particle speed with spectral index of -5, and an exponential rollover. The common spectral index of -5 occurs in quiet solar wind, in disturbed conditions downstream from shocks, and throughout the heliosheath now being explored by Voyager. The common spectral index is independent of the plasma conditions; the rollover depends on the plasma conditions, with particles being accelerated to the highest energies in conditions with extensive compressive turbulence. No traditional acceleration mechanism, e.g. diffusive shock acceleration or stochastic acceleration, can account for these observations.
A new acceleration theory has been invented in which particles are pumped out of a core distribution of low-energy particles, through successive compressions and expansions in compressive turbulence. This acceleration theory can account for the observations and has important implications for acceleration in other astrophysical settings, e.g. it may be responsible for the acceleration of particles in Coronal Mass Ejections near the Sun, and, invoking vastly different scales, could accelerate Galactic Cosmic Rays in the interstellar medium. The observations of particle acceleration in the heliosphere, the state of the theory that can account for these observations, and the implications for acceleration in other astrophysical settings will be reviewed.