Composition of Anomalous Cosmic Rays and Implications for the Heliosphere

Abstract

We use energy spectra of anomalous cosmic rays (ACRs) measured with the Cosmic Ray instrument on the Voyager 1 and 2 spacecraft during the period 1994/157-313 to determine several parameters of interest to heliospheric studies. We estimate that the strength of the solar wind termination shock is 2.42 (-0.08, +0.04). We determine the composition of ACRs by estimating their differential energy spectra at the shock and find the following abundance ratios: H/He = 5.6 (-0.5, +0.6), C/He = 0.00048 ± 0.00011, N/He = 0.011 ± 0.001, O/He = 0.075 ± 0.006, and Ne/He = 0.0050 ± 0.0004. We correlate our observations with those of pickup ions to deduce that the long-term ionization rate of neutral nitrogen at 1 AU is ~8.3 × 10⁻⁷ s⁻¹ and that the charge-exchange cross section for neutral N and solar wind protons is ~1.0 × 10⁻¹⁵ cm² at 1.1 keV. We estimate that the neutral C/He ratio in the outer heliosphere is 1.8(-0.7, +0.9) × 10⁻⁵. We also find that heavy ions are preferentially injected into the acceleration process at the termination shock.

Additional details