Shock Acceleration with a Focused Transport Approach

Details Shock Acceleration with a Focused Transport Approach Shock Acceleration with a Focused Transport Approach

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusmsh53a..06l&link_type=abstract

American Geophysical Union, Spring Meeting 2007, abstract #SH53A-06

Physics

2104 Cosmic Rays, 2124 Heliopause And Solar Wind Termination, 2139 Interplanetary Shocks, 2152 Pickup Ions, 7845 Particle Acceleration

Scientific paper

Observations by Voyager 1 reveal accelerated energetic ion spectra with large, magnetic-field-aligned particle anisotropies upstream of the termination shock, as well as highly anisotropic intensity spikes when the spacecraft crossed the shock. Similar features are observed at nearly-perpendicular interplanetary shocks. These features do not agree with simple diffusive shock acceleration theory based on the standard cosmic-ray transport equation which is limited to near isotropic particle distributions. It will be shown that shock acceleration with the focused kinetic transport equation, which is not limited to small anisotropies, is a viable alternative, while still retaining many of the familiar transport mechanisms associated with standard cosmic-ray transport theory (the focused transport equation is essentially the zero gyro-radius limit of the drift kinetic equation including pitch- angle scattering). We discuss how the focused transport equation contains all the physics associated with "scatter-free" shock drift acceleration theory, but since it also contains pitch-angle scattering of particles by small- scale turbulence, it can describe both shock drift acceleration with scattering and first order Fermi shock acceleration without requiring small anisotropies. On this basis we will show with simulations that the above- mentioned observations can be explained naturally. It will also be illustrated (i) that the lack of cross-field transport inherent in the focused transport equation does not exclude the acceleration of unaccelerated pickup ions at a nearly perpendicular shock when random variations in the field angle (shock obliquity) are included in the focused transport model, and (ii) that time-dependent shocks inevitably lead to accelerated spectra with more than a single power law. We argue that anomalous cosmic-ray intensity peak beyond the shock is due to further heating in the heliosheath, either by stochastic acceleration from turbulence or by adiabatic compression of the solar wind by the local interstellar flow.

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