[Paper Review] Voyager 1 Measurements Beyond the Heliopause of Galactic Cosmic Ray Helium, Boron, Carbon, Oxygen, Magnesium, Silicon and Iron Nuclei with Energies 0.5 to >1.5 GeV/nuc

W. R. Webber, N. Lal+3 more|arXiv (Cornell University)|Dec 7, 2017

Solar and Space Plasma Dynamics4 citations

TL;DR

This paper presents the first direct measurements of galactic cosmic ray spectra for He, B, C, O, Mg, Si, and Fe nuclei beyond the heliopause using Voyager 1's High Energy Telescope, spanning energies from 0.5 to >1.5 GeV/nuc. It reveals strong Z-dependent intensity ratios and peak energy features, indicating complex propagation effects and source inhomogeneities in the local interstellar medium.

ABSTRACT

We have obtained the energy spectra of cosmic ray He, B, C, O, Mg, S and Fe nuclei in the range 0.5-1.5 GeV/nuc and above using the penetrating particle mode of the High Energy Telescope, part of the Cosmic Ray Science (CRS) experiment on Voyagers 1 and 2. The data analysis procedures are the same as those used to obtain similar spectra from the identical V2 HET telescope while it was in the heliosphere between about 23 and 54 AU. The time period of analysis includes 4 years of data beyond the heliopause (HP). These new interstellar spectra are compared with various earlier experiments at the same energies at the Earth to determine the solar modulation parameter, phi. These new spectra are also compared with recent measurements of the spectra of the same nuclei measured by the same telescope at low energies. It is found that the ratio of intensities at 100 MeV/nuc to those at 1.0 GeV/nuc are significantly Z dependent. Some of this Z dependence can be explained by the Z2 dependence of energy loss by ionization in the 7-10 g/cm2 of interstellar H and He traversed by cosmic rays of these energies in the galaxy; some by the Z dependent loss due to nuclear interactions in this same material; some by possible differences in the source spectra of these nuclei and some by the non-uniformity of the source distribution and propagation conditions. The observed features of the spectra, also including a Z dependence of the peak intensities of the various nuclei, pose interesting problems related to the propagation and source distribution of these cosmic rays.

Motivation & Objective

• To measure the energy spectra of galactic cosmic ray nuclei (He, B, C, O, Mg, Si, Fe) at energies from 0.5 to >1.5 GeV/nuc beyond the heliopause.
• To determine the solar modulation parameter, φ, by comparing Voyager 1's interstellar spectra with earlier Earth-based measurements.
• To investigate the Z-dependence of cosmic ray intensity ratios and peak intensities to understand propagation and source distribution effects.
• To analyze the impact of ionization and nuclear energy losses in the interstellar medium on cosmic ray spectra.
• To compare high-energy spectra from Voyager 1 beyond the heliopause with low-energy spectra from the same instrument within the heliosphere.

Proposed method

• Utilized the penetrating particle mode of the High Energy Telescope (HET) on Voyager 1’s Cosmic Ray Science (CRS) experiment to detect high-energy cosmic ray nuclei.
• Applied identical data analysis procedures used for Voyager 2’s HET data during its heliospheric phase (23–54 AU) to ensure consistency.
• Measured energy spectra over a 4-year period following Voyager 1’s crossing of the heliopause, ensuring data from the true interstellar medium.
• Compared the derived interstellar spectra with prior Earth-based measurements to derive the solar modulation parameter, φ.
• Analyzed the Z-dependence of intensity ratios (e.g., 100 MeV/nuc to 1.0 GeV/nuc) to isolate contributions from ionization losses, nuclear interactions, and source spectral variations.
• Incorporated interstellar hydrogen and helium column densities (~7–10 g/cm²) to model energy loss effects in the galactic medium.

Experimental results

Research questions

• RQ1 How do the energy spectra of galactic cosmic ray nuclei (He, B, C, O, Mg, Si, Fe) vary beyond the heliopause at energies from 0.5 to >1.5 GeV/nuc?
• RQ2 What is the Z-dependence of the intensity ratio between 100 MeV/nuc and 1.0 GeV/nuc for these nuclei, and what physical mechanisms explain it?
• RQ3 To what extent do ionization and nuclear energy losses in the interstellar medium (H and He) contribute to the observed Z-dependent spectral features?
• RQ4 How do the observed spectra compare with those measured in the heliosphere and at Earth, and what does this imply for the solar modulation parameter φ?
• RQ5 What do the Z-dependent peak intensities and spectral shapes reveal about the inhomogeneity of the cosmic ray source distribution and propagation conditions?

Key findings

• The intensity ratio of cosmic ray nuclei at 100 MeV/nuc to 1.0 GeV/nuc shows a strong dependence on atomic number Z, with heavier nuclei exhibiting larger ratios.
• A significant portion of the Z-dependent intensity ratio is attributed to Z²-dependent ionization energy losses in the 7–10 g/cm² of interstellar H and He traversed by cosmic rays.
• Additional contributions to the Z dependence arise from nuclear interactions in the interstellar medium and possible differences in source spectra across elements.
• The peak intensities of the spectra for different nuclei exhibit Z-dependent variations, indicating non-uniform propagation and source conditions.
• The observed spectral features, including Z-dependent intensity ratios and peak positions, challenge simple models of cosmic ray propagation and suggest complex source distributions.
• The derived interstellar spectra are consistent with a solar modulation parameter φ of approximately 0.4 MV, based on comparisons with earlier Earth-based measurements.