Anisotropy of Cosmic Rays and Chaotic Trajectories in the Heliosphere

TL;DR

This paper investigates how chaotic trajectories of cosmic rays in the heliosphere influence their anisotropic arrival directions, using a novel FTLE-based method to analyze trapping effects and predict time-variable anisotropy patterns.

Contribution

It introduces a new application of FTLE to study chaos in cosmic ray trajectories within the heliosphere, linking chaos levels to anisotropy and temporal variations in arrival maps.

Findings

Chaotic trapping affects cosmic ray anisotropy patterns.

Heliospheric structures induce sector-dependent chaos levels.

Time variability in arrival maps is linked to chaos dynamics.

Abstract

As cosmic rays (CRs) propagate in the Galaxy, they can be affected by magnetic structures that temporarily trap them and cause their trajectories to display chaotic behavior, therefore modifying the simple diffusion scenario. When CRs arrive at the Earth, they do so anisotropically. These chaotic effects can be a fundamental contributor to this anisotropy. Accordingly, this requires a comprehensive description of chaos in trapping conditions since assessing their repercussions on the CR arrival directions is necessary. This study utilizes a new method described in L\'opez-Barquero and Desiati (2021) to characterize chaotic trajectories in bound systems. This method is based on the Finite-Time Lyapunov Exponent (FTLE), a quantity that determines the levels of chaos based on the trajectories' divergence rate. The FTLE is useful since it adapts to trapping conditions in magnetic structures or even propagating media changes. Here, we explore the effects that chaos and trapping can have on the TeV CR anisotropy. Concretely, we apply this method to study the behavior of CRs entering the heliosphere. Specifically, how the distinct heliospheric structures and CR impinging directions from the ISM can affect chaos levels. The heliosphere has an intrinsic directionality that affects CRs differently depending on where they enter it. This feature causes preferential directions from which particles tend to be more chaotic than others. This eventually translates into changes in the arrival maps which are not uniformly distributed. Instead, we expect sectors in the map to change separately from others, creating a time variation that could be detected. Consequently, this result points to the idea that time-variability in the maps is essential to understanding the CR anisotropy's overall processes.