[Paper Review] On the Observation of the Cosmic Ray Anisotropy below 10$^{15}$ eV

G. Di Sciascio, R. Iuppa|arXiv (Cornell University)|Jul 8, 2014

Astrophysics and Cosmic Phenomena5 references18 citations

TL;DR

This paper reviews observational evidence for cosmic ray anisotropy below 10¹⁵ eV, identifying a persistent large-scale sidereal anisotropy with a 'tail-in' excess (40°–90° R.A.) and a 'loss cone' deficit (150°–240° R.A.) at amplitudes of 10⁻⁴ to 10⁻³. Despite expectations of isotropy due to galactic magnetic field deflections, the anisotropy persists, and no standard model of cosmic ray propagation or galactic magnetic fields fully explains the combined large- and medium-scale features, highlighting a critical gap in current astrophysical theory.

ABSTRACT

The measurement of the anisotropy in the arrival direction of cosmic rays is complementary to the study of their energy spectrum and chemical composition to understand their origin and propagation. It is also a tool to probe the structure of the magnetic fields through which cosmic rays travel. As cosmic rays are mostly charged nuclei, their trajectories are deflected by the action of galactic magnetic field they propagate through before reaching the Earth atmosphere, so that their detection carries directional information only up to distances as large as their gyro-radius. If cosmic rays below $10^{15}{ m\,eV}$ are considered and the local galactic magnetic field ($\sim3{ m\,μG}$) is accounted for, gyro-radii are so short that isotropy is expected. At most, a weak di-polar distribution may exist, reflecting the contribution of the closest CR sources. However, a number of experiments observed an energy-dependent \emph{"large scale"} anisotropy in the sidereal time frame with an amplitude of about 10$^{-4}$ - 10$^{-3}$, revealing the existence of two distinct broad regions: an excess distributed around 40$^{\circ}$ to 90$^{\circ}$ in Right Ascension (commonly referred to as "tail.in" excess) and a deficit (the "loss cone") around 150$^{\circ}$ to 240$^{\circ}$ in Right Ascension. In recent years the Milagro and ARGO-YBJ collaborations reported the of a "medium" scale anisotropy inside the tail-in region. The observation of such small features has been recently claimed by the IceCube experiment also in the Southern hemisphere. So far, no theory of cosmic rays in the Galaxy exists which is able to explain the origin of these different anisotropies leaving the standard model of cosmic rays and that of the galactic magnetic field unchanged at the same time.

Motivation & Objective

• To analyze observational evidence of cosmic ray anisotropy in the energy range below 10¹⁵ eV, where isotropy is expected due to strong magnetic field deflections.
• To investigate the origin of a large-scale sidereal anisotropy with a 'tail-in' excess and 'loss cone' deficit in right ascension, observed across multiple experiments.
• To assess whether current models of cosmic ray propagation and galactic magnetic fields can explain the coexistence of large-scale and medium-scale anisotropies.
• To highlight the lack of a unified theoretical framework that explains both the large-scale dipolar anisotropy and localized medium-scale features without modifying the standard model of cosmic rays or galactic magnetic fields.

Proposed method

• Analysis of data from ground-based cosmic ray detectors including Milagro, ARGO-YBJ, and IceCube, focusing on arrival direction distributions in sidereal time.
• Use of 2D sky maps to identify localized anisotropy features, particularly within the 'tail-in' region (40°–90° R.A.).
• Comparison of anisotropy amplitude and phase across energies from 10¹¹ eV to 10¹⁴ eV to assess energy dependence.
• Evaluation of theoretical models of cosmic ray diffusion, including dependence of diffusion coefficient D on rigidity R (D ∝ R⁰.¹⁵–⁰.⁶), to assess their ability to reproduce observed anisotropies.
• Investigation of alternative mechanisms such as magnetic reconnection in the heliospheric tail and turbulent scattering in the local interstellar magnetic field.
• Correlation of anisotropy features with interstellar energetic neutral atom distributions and heliospheric structures to test external influences.

Experimental results

Research questions

• RQ1 Why does a large-scale anisotropy with an amplitude of 10⁻⁴ to 10⁻³ persist in cosmic ray arrival directions below 10¹⁵ eV, despite strong galactic magnetic field deflections expected to isotropize the flux?
• RQ2 What physical mechanism can simultaneously explain the observed 'tail-in' excess (40°–90° R.A.) and 'loss cone' deficit (150°–240° R.A.) in the sidereal frame?
• RQ3 How can medium-scale anisotropies, reported by Milagro and ARGO-YBJ in the 'tail-in' region, be reconciled with standard diffusion models of cosmic ray propagation?
• RQ4 Can interactions with the heliospheric magnetic field or turbulent ripples in the interstellar medium account for the observed energy-dependent anisotropy features?
• RQ5 Why does no existing theory of cosmic ray propagation in the Galaxy successfully explain both large-scale and small-scale anisotropies while preserving the standard model of cosmic rays and galactic magnetic fields?

Key findings

• A large-scale anisotropy with amplitude 10⁻⁴ to 10⁻³ is consistently observed in the sidereal time frame across multiple experiments, with a 'tail-in' excess in right ascension from 40° to 90° and a 'loss cone' deficit from 150° to 240°.
• The anisotropy remains uniform in phase and amplitude across the energy range 10¹¹ eV to 10¹⁴ eV, indicating a stable directional modulation.
• Medium-scale anisotropies were reported by Milagro and ARGO-YBJ within the 'tail-in' region, suggesting localized structures in the cosmic ray flux.
• The IceCube experiment recently claimed evidence of similar small-scale anisotropies in the Southern Hemisphere, extending the spatial coverage of the phenomenon.
• No theoretical model based on the standard cosmic ray propagation framework and the standard galactic magnetic field model can simultaneously explain both the large-scale dipolar anisotropy and the localized medium-scale features.
• The coexistence of these features challenges current understanding and suggests the need for new physics or refined models of magnetic field structure and particle scattering.