Sv. Chalov et Hj. Fahr, Pick-up ion acceleration at the termination shock and the post-shock pick-up ion energy distribution, ASTRON ASTR, 360(1), 2000, pp. 381-390
It is discussed since quite some time in the literature that the solar wind
termination shock may act as an efficient particle accelerator though the
underlying physics of injecting particles into this shock acceleration proc
ess was not well understood up to now. Most of the earlier work required an
ad-hoc prescription of the rate by which particles are injected into the p
rocess of diffusive shock acceleration. Here we avoid this injection proble
m studying instead the single particle fate of pickup ions arriving at the
shock and undergoing multiple adiabatic reflections at the shock. We start
out from preaccelerated pickup ions arriving with a known isotropic distrib
ution function at the shock and suffering reflections or transmissions thro
ugh the shock depending on their actual velocity space coordinates. Upstrea
m and downstream of the shock the ions in addition are subject to Fermi-2 a
cceleration processes described by means of a phase-space transport equatio
n for a pitch-angle anisotropic distribution function. As we can show the s
pectral energy distribution resulting for the downstream pick-up ions consi
sts of two parts, the low energy keV-part which is due to directly transmit
ted ions and the high energy MeV-part which is due to multiply reflected io
ns. We also show that the resulting spectrum is fairly sensitive to the loc
ation of the shock sector with respect to the upwind direction. This fact i
s best reflected in corresponding energetic neutral atom (ENA) fluxes reach
ing the Earth from different directions and thus serves as a unique diagnos
tic tool for the remote study of the 3-d properties of the termination shoc
k.