Abstract / Description of output
For a particle to be accelerated diffusively at a shock by the
first-order Fermi acceleration mechanism, the particle must be
sufficiently energetic that it can scatter across all the micro- and
macrostructure of the shock, experiencing compression between the
converging upstream and downstream states. This is the well-known
"injection problem." Here the interaction of ions with the ramp of a
quasiparallel shock is investigated. Some ions incident on the shock
experience specular reflection, caused either by the cross-shock
electrostatic potential or by mirroring as the magnetic field is bent
and compressed through the ramp. Scattering of reflected ions by
self-generated and pre-existing turbulence in the region upstream of the
shock then acts to trap backstreaming ions and return them to the ramp,
where some experience further reflections. Such repeated reflections and
scattering energize a subpopulation of ions up to energies sufficiently
large that they can be diffusively shock accelerated. Two ion
distributions are considered: pickup ions which are assumed to be
described by a shell distribution, are thermal solar wind ions which may
be described by a kappa distribution. Injection efficiencies are found
analytically to be very high for pickup ions and much lower for thermal
solar wind ions, suggesting that this injection mechanism, stochastic
reflected ion or SRI acceleration, is a natural precursor for the
acceleration of the anomalous cosmic ray component at a quasiparallel
shock. While significantly less efficient, SRI acceleration is also
viable for thermal solar wind ions described by a kappa distribution.
Original language | English |
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Pages (from-to) | 4560-4576 |
Journal | Physics of Plasmas |
Volume | 8 |
Publication status | Published - 1 Oct 2001 |
Keywords / Materials (for Non-textual outputs)
- Shock waves and discontinuities
- Plasma turbulence