Diffusive Drift Acceleration at a Blunt Termination Shock

Details Diffusive Drift Acceleration at a Blunt Termination Shock Diffusive Drift Acceleration at a Blunt Termination Shock

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh52a..06s&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SH52A-06

Physics

2104 Cosmic Rays, 2114 Energetic Particles (7514), 2124 Heliopause And Solar Wind Termination, 2144 Interstellar Gas, 2152 Pickup Ions

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 energetic protons observed by Voyager 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 in the range ~50-100 MeV. This presentation develops a theoretical framework to describe drift of particles during diffusive shock acceleration, including the effects of cross-field diffusion and curvature and gradient drifts across the magnetic field. 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 of the termination shock. Azimuthal magnetic field lines frozen into the solar wind first make contact with the termination shock at its nose. Field lines then sweep accelerating particles out to the flanks of the shock where they achieve their highest energies. We show further, that diffusive shock acceleration naturally produces intermediate energy accelerated particles up to several MeV with a power-law spectrum consistent with the local properties of the shock. Above several MeV, the shock accelerated spectrum rolls over and becomes more complex due to complex drift trajectories along the shock. Latitudinal drifts are also an important factor along a non-spherical termination shock. In A-positive cycles (the previous solar magnetic cycle), latitudinal drifts move particles toward an attractor in the tail at low-latitudes. During A-negative cycles (the current solar magnetic cycle), drifts move particles toward attractors at the poles of the termination shock. Thus, the inclusion of perpendicular diffusion at a blunt termination shock suggests that diffusive shock acceleration begins a very low energies (several to 10's of keV) and explains the intermediate energy power law spectrum up to several MeV observed by the Voyager spacecraft after its passage into the heliosheath.

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