Spectral intensities of Anomalous Cosmic Rays derived from the injection rate at the solar wind termination shock H.-J. Fahr1 and D. Verscharen1,2 1Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany 2Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany
Received: 16 Feb 2009 – Revised: 15 May 2009 – Accepted: 25 May 2009 – Published: 06 Aug 2009
Abstract. We investigate the well accepted idea that anomalous cosmic ray (ACR) ions
originate from suprathermal halo ions by means of diffusive shock
acceleration. Usually the seed ions are taken to be the interplanetary
pick-up ions, but here we want to enlarge this idea by taking in addition as
seed candidates also into account normal solar wind ions which according to
our recent calculations are reflected from the termination shock into the
upstream solar wind flow and then are also picked-up as suprathermal ions.
We start out from an ideally planar shock approximation and fix the ACR
spectrum by absolute spectral intensities and maximum ACR energies,
taking a precalculated fraction of the suprathermal ion flow as ACR injection
rate. Comparison of our calculated spectral intensities with
ACR measurements near 94 AU, shows that
satisfying data fits only can be achieved, if about three percent of the suprathermal ions swept
into the shock structure enter into the Fermi-1 acceleration process. We
also show that the spectral slope of the ACR spectra is decreasing and the
spectral intensities are increasing with increasing shock compression
ratios. As maximum energies available from an ideally operating diffusive
Fermi-1 acceleration process we find, depending on the shock compression
ratio, ion energies ranging from a few MeV up to 103 MeV. Compared to
observations this seems to be a little on the high side and may
point to the fact that injection into Fermi-1 acceleration near the termination shock is occurring only sporadically due to variable upstream magnetic field orientations with respect to the shock normal vector, i.e. due to variations of the obliquity of the local shock surface with respect to the local upstream magnetic field.
Citation: Fahr, H.-J. and Verscharen, D.: Spectral intensities of Anomalous Cosmic Rays derived from the injection rate at the solar wind termination shock, Astrophys. Space Sci. Trans., 5, 21-30, doi:10.5194/astra-5-21-2009, 2009.