Cosmic-ray Composition as Observed by Ulysses

Abstract

The cosmic ray isotopic composition measurements from the High Energy Telescope (HET) on the Ulysses spacecraft are reviewed. The source isotopic composition of key elements is found to be surprisingly like the Solar system abundances with the notable exception of ^22Ne. The average density of interstellar material cosmic rays traverse is found to be ~0.25 atom cm^-3, corresponding to a confinement time of ~20 Myr. Vanadium isotopic abundances are shown to be consistent with weak cosmic-ray reacceleration. The implications of these measurements are discussed.