Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh41c..07j&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SH41C-07
Astronomy and Astrophysics
Astrophysics
[7514] Solar Physics, Astrophysics, And Astronomy / Energetic Particles
Scientific paper
Observed disparities between in situ observations and current theories of charged-particle acceleration suggest that the acceleration of these particles in space is more complicated than previously thought. In its simplest form, the long-favored model of diffusive shock acceleration at a steady, planar shock is clearly inadequate. Although more-realistic models show considerable promise, no consensus has arisen. In response, a number of other acceleration mechanisms have been proposed to replace diffusive shock acceleration. Examples of proposed mechanisms include stochastic or 2nd-order Fermi acceleration and reconnection events. Because of the absence of collisions, the acceleration must be by the ambient electric field, which at the scales of interest, satisfies the MHD relation in termso of the fluid velocity U, magnetic field B/ and the speed of light c: E = - U × B/c. Therefore, consideration of the observed SPATIAL AND TEMPORAL scales and the ambient fluid velocities in any given region, such as the heliosphere or inner heliosphere, constrains the possibilities. A large spatial scale or a high fluid velocity will be able to accelerate particles to higher energies than a smaller scale or lower velocity. Stochastic mechanisms appear to be too slow. Also, it may be shown, quite generally, that if the pitch-angle distribution in the fluid frame is nearly isotropic, the MHD expression for E yields the Parker energy-change rate dT/dt {∝ ∇ \cdot U}. Hence, acceleration in nearly incompressible reconnection events requires significant anisotropies. These very-general considerations, together with the observed energy spectra and composition, lead to significant constraints on the possible mechanisms in each case.
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