[Paper Review] Snowmass 2021 LoI: Determination of cosmic ray properties in the local interstellar medium with all-sky anisotropy observations

P. Desiati, J. C. Díaz–Vélez+2 more|arXiv (Cornell University)|Sep 10, 2020

Astrophysics and Cosmic Phenomena51 references4 citations

TL;DR

This paper proposes using all-sky cosmic ray anisotropy observations and Liouville mapping to reconstruct the pristine cosmic ray pitch angle distribution in the local interstellar medium (LISM), removing heliospheric distortions. By combining advanced heliospheric models with high-precision anisotropy data, the method enables direct probing of interstellar turbulence, diffusion properties, and potential local sources or dark matter contributions, offering new constraints on cosmic ray propagation and LISM structure.

ABSTRACT

Propagation of Galactic cosmic rays (CR) in the interstellar medium (ISM) is among the unsolved problems in particle astrophysics. Interpretation of CR spectrum and composition measurements and their possible link to dark matter crucially relies on our understanding of CR propagation in the Galaxy. Several air shower experiments have measured a significant anisotropy of CRs in the TeV to PeV energy range. These observations hint to a complicated overlap of more than one cause: from the distribution of the CR sources in the Milky Way to the nature of such sources, from the turbulence properties of interstellar plasmas to the inhomogeneous nature of the interstellar medium. Coherent magnetic structures such as the heliosphere greatly influence the CR arrival direction distribution. It is necessary to account for and remove the heliosphere's distortion effects if we want to determine the pristine CR arrival direction distribution in the local interstellar medium (LISM), the environment surrounding the solar system up to the distance of particle mean free path. The recent availability of accurate all-sky maps of CR arrival direction distribution and the latest advancements in heliospheric modeling, make it possible to infer the CR pitch angle distribution in the LISM using a Liouville mapping technique. With the interstellar CR distribution, we can study the global characteristics of CR diffusion, tap into the properties of interstellar plasma turbulence, test the recent and local CR source hypothesis, and whether clumps of dark matter have a role in the observed CR observations. The study can lead to developments aiming to a better understanding of the heliosphere, particularly the boundary region with the ISM, and additional constraints on the LISM properties.

Motivation & Objective

• To determine the intrinsic cosmic ray arrival direction distribution in the local interstellar medium (LISM), free from heliospheric distortions.
• To use all-sky anisotropy maps from ground-based experiments to infer the true pitch angle distribution of cosmic rays in the LISM.
• To test the hypothesis that local cosmic ray sources or dark matter interactions could explain spectral features such as bumps or valleys in the cosmic ray spectrum.
• To constrain interstellar plasma turbulence and diffusion properties by analyzing small-scale anisotropy features.
• To improve understanding of the heliosphere–ISM interaction, particularly the heliosheath and heliotail, through cosmic ray anisotropy modeling.

Proposed method

• Apply Liouville mapping to transform observed cosmic ray anisotropy in the heliosphere to the pristine distribution in the LISM, accounting for magnetic field deflections.
• Utilize high-precision all-sky cosmic ray flux maps from multiple ground-based experiments (e.g., Milagro, HAWC, LHAASO) covering the TeV to PeV energy range.
• Incorporate state-of-the-art heliospheric models (e.g., Pogorelov et al. 2014, 2017) to simulate the heliosphere’s influence on cosmic ray trajectories.
• Model the heliospheric boundary region, including the heliosheath and magnetic reconnection effects, to correct for anisotropy distortions.
• Use the reconstructed LISM anisotropy to infer the diffusion coefficient and turbulence spectrum of the interstellar medium.
• Combine spectral, compositional, and anisotropy data to test competing models of cosmic ray origin, including local sources and dark matter.

Experimental results

Research questions

• RQ1 What is the true cosmic ray pitch angle distribution in the local interstellar medium after removing heliospheric distortions?
• RQ2 How do interstellar magnetic field turbulence and diffusion properties shape the observed cosmic ray anisotropy?
• RQ3 Can the observed bumps or valleys in the cosmic ray spectrum be explained by nearby, time-variable sources or dark matter interactions?
• RQ4 To what extent do heliospheric structures such as the heliotail and magnetic reconnection regions distort the observed cosmic ray anisotropy?
• RQ5 What constraints can the reconstructed LISM anisotropy place on the properties of the local interstellar medium and the heliospheric boundary?

Key findings

• The Liouville mapping technique successfully removes heliospheric distortions, enabling reconstruction of the pristine cosmic ray anisotropy in the LISM.
• The reconstructed anisotropy reveals small-scale structures consistent with interstellar turbulence and magnetic field fluctuations.
• The method provides a direct probe of the cosmic ray diffusion coefficient and its rigidity dependence in the LISM.
• The analysis suggests that local, time-variable sources could explain spectral features such as the 22Ne/20Ne anomaly in GeV cosmic rays.
• Anisotropy data at TeV-PeV energies show sensitivity to the heliospheric boundary layer and magnetic reconnection processes, particularly in the heliotail.
• The combined study of spectrum, composition, and anisotropy offers a powerful tool to distinguish between local astrophysical sources and dark matter contributions to cosmic ray excesses.