Combined first- and second-order Fermi acceleration in cometary environments

Details Combined first- and second-order Fermi acceleration in cometary environments Combined first- and second-order Fermi acceleration in cometary environments

Nov 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989jgr....9415011g&link_type=abstract

Journal of Geophysical Research (ISSN 0148-0227), vol. 94, Nov. 1, 1989, p. 15011-15023.

Physics

25

Cometary Magnetospheres, Heavy Ions, Magnetohydrodynamics, Plasma Acceleration, Transport Properties, Two Dimensional Models, Approximation, Diffusion Coefficient, Energy Spectra, Solar Wind, Steady State, Temporal Distribution

Scientific paper

Two-dimensional (one velocity space and one spatial dimension), time-dependent model calculations indicate that an interplay between velocity and spatial diffusion may be responsible for the acceleration of implanted heavy ions in the cometary preshock region. Velocity diffusion (second-order Fermi acceleration) accelerates the pickup ions to moderate energies, thus creating a seed population for the more efficient diffusive-compressive shock acceleration. Solar wind convection limits the time available for diffusive-compressive acceleration. Therefore, the resulting energy spectrum above the pickup energy is a combination of an exponential decrease at lower energies (up to about 100 keV) and a power law spectrum at higher energies. The calculated energy spectra constitute a surprisingly good fit to the published Vega and Giotto energy spectra above the pickup energy.

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