Particle acceleration at shocks in the presence of a braided magnetic field

Details Particle acceleration at shocks in the presence of a braided magnetic field Particle acceleration at shocks in the presence of a braided magnetic field

May 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997adspr..19..707k&link_type=abstract

Advances in Space Research, Volume 19, Issue 5, p. 707-710.

Computer Science

1

Scientific paper

The theory of first order Fermi acceleration at shock fronts assumes charged particles undergo spatial diffusion in a uniform magnetic field. If, however, the magnetic field is not uniform, but has a stochastic or braided structure, the transport of charged particles across the average direction of the field is more complicated. Assuming quasi-linear behaviour of the field lines, the particles undergo sub-diffusion ( ~ t^1/2) on short time scales. We investigate this process analytically, using a propagator approach, and numerically, with a Monte-Carlo simulation. It is found that, in contrast to the diffusive case, the density of particles at the shock front is lower than it is far downstream which is a consequence of the partial trapping of particles by structures in the magnetic field. As a result, the spectrum of accelerated particles is a power-law in momentum which is steeper than in the diffusive case. For a phase-space density f ~ p^-s, we find s =s_diff [1 + 1/(2rho_c)], where rho_c is the compression ratio of the shock front and s_diff is the standard result of diffusive acceleration:s_diff = 3rho_c/(rho_c - 1).

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