Diffusive Acceleration at the Blunt Termination Shock

Details Diffusive Acceleration at the Blunt Termination Shock Diffusive Acceleration at the Blunt Termination Shock

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsh14a1694s&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SH14A-1694

Physics

2104 Cosmic Rays, 2114 Energetic Particles (7514), 2124 Heliopause And Solar Wind Termination, 2126 Heliosphere/Interstellar Medium Interactions, 7845 Particle Acceleration

Scientific paper

Voyager 1's recent and long anticipated passage into the heliosheath contradicted the prediction that we would observe the source of anomalous cosmic rays accelerated by the termination shock. The observed energetic protons reveal a power-law spectrum below several MeV, but above several MeV the spectrum falls more sharply, and we observe the familiar bump caused by modulation of anomalous cosmic rays. Here, we develop the theoretical framework to include the motions and drift of particles during diffusive shock acceleration at a 3-D termination shock, including cross-field diffusion, and curvature and gradient drifts. Our model supports the concept of McComas and Schwadron that because of the termination shock's blunt structure, there should be a strong deficit of locally accelerated anomalous cosmic rays near the nose. With reasonable parameters for the scattering mean free path parallel to the magnetic field of ~3 AU, and a ratio of perpendicular to parallel diffusion of ~0.03, the model produces an energy spectrum that agrees well with Voyager 1 observations near the termination shock. These parameters also lead to an acceleration time to 10 MeV of about 1 year, which is comparable to previous estimates derived from the observed charge states of ACRs. Thus, we provide a theory for diffusive acceleration at the blunt termination shock. The predictions of this theory are consistent with Voyager 1's observations showing a lack of ACRs accelerated near the nose of the termination shock and the ACR acceleration timescale derived from ACR charge-states.

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