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The Compton-Getting origin of the large-scale anisotropy of Galactic cosmic rays

Bing-qiang Qiao, Wei Liu, Huirong Yan, Yi-qing Guo

TL;DR

This paper addresses the origin and energy dependence of large-scale Galactic cosmic-ray anisotropy and proposes a unified framework that links the energy spectrum and dipole anisotropy. The method combines spatially dependent propagation (SDP) with a two-layer halo, a local Geminga SNR as the primary nearby source, and a diffusion tensor to account for anisotropic diffusion in the local regular magnetic field. It also incorporates Compton-Getting (CG) effects from the Sun’s motion relative to either the Local Standard of Rest (LSR) or the local interstellar medium (ISM), yielding a total dipole Δ_tot = Δ_grad + Δ_CG. Key findings show that CG can modestly reduce the dipole amplitude and shift the phase toward directions away from the local magnetic-field alignment at tens of TeV, with CG-dominated behavior emerging below ~200 GeV and a phase flip toward the CG direction occurring at a few hundred GeV, offering a falsifiable prediction for future 100 GeV–TeV anisotropy measurements.

Abstract

Recent studies suggest that the anisotropy in cosmic-ray arrival directions can provide insight into local acceleration sites and propagation conditions. We developed a unified framework to interpret both the observed energy spectra and the large-scale anisotropy. In this work, we explore the influence of the Sun's motion relative to the local plasma frame - the Compton-Getting (CG) effect - on the anisotropy. We find that incorporating the CG effect could slightly reduce the dipole amplitude and shift the phase away from the direction of the local regular magnetic field at tens of TeV. At lower energies, where the anisotropy from the cosmic-ray density gradient is weak, the Sun's relative motion becomes more prominent. Below $\sim 200$ GeV, the dipole amplitude increases again, approaching the value expected from the CG effect. Additionally, a phase flip is observed at a few hundred GeV, aligning with the CG direction. Future anisotropy measurements from $100$ GeV to TeV energies could serve as a critical test of this effect.

The Compton-Getting origin of the large-scale anisotropy of Galactic cosmic rays

TL;DR

This paper addresses the origin and energy dependence of large-scale Galactic cosmic-ray anisotropy and proposes a unified framework that links the energy spectrum and dipole anisotropy. The method combines spatially dependent propagation (SDP) with a two-layer halo, a local Geminga SNR as the primary nearby source, and a diffusion tensor to account for anisotropic diffusion in the local regular magnetic field. It also incorporates Compton-Getting (CG) effects from the Sun’s motion relative to either the Local Standard of Rest (LSR) or the local interstellar medium (ISM), yielding a total dipole Δ_tot = Δ_grad + Δ_CG. Key findings show that CG can modestly reduce the dipole amplitude and shift the phase toward directions away from the local magnetic-field alignment at tens of TeV, with CG-dominated behavior emerging below ~200 GeV and a phase flip toward the CG direction occurring at a few hundred GeV, offering a falsifiable prediction for future 100 GeV–TeV anisotropy measurements.

Abstract

Recent studies suggest that the anisotropy in cosmic-ray arrival directions can provide insight into local acceleration sites and propagation conditions. We developed a unified framework to interpret both the observed energy spectra and the large-scale anisotropy. In this work, we explore the influence of the Sun's motion relative to the local plasma frame - the Compton-Getting (CG) effect - on the anisotropy. We find that incorporating the CG effect could slightly reduce the dipole amplitude and shift the phase away from the direction of the local regular magnetic field at tens of TeV. At lower energies, where the anisotropy from the cosmic-ray density gradient is weak, the Sun's relative motion becomes more prominent. Below GeV, the dipole amplitude increases again, approaching the value expected from the CG effect. Additionally, a phase flip is observed at a few hundred GeV, aligning with the CG direction. Future anisotropy measurements from GeV to TeV energies could serve as a critical test of this effect.

Paper Structure

This paper contains 7 sections, 9 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. Model Description
  3. Spatially-dependent propagation
  4. Local source
  5. Anisotropic diffusion
  6. Compton-Getting effect
  7. Results and Discussion

Figures (1)

  • Figure 1: Comparison between model calculations and observations for B/C ratio. The data points are adopted from AMS-02, CALET and DAMPE measurements 2016PhRvL.117w1102A2022PhRvL.129y1103A2022SciBu..67.2162D.