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

Our 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.