Cosmic-Ray Small-Scale Anisotropies and Local Turbulent Magnetic Fields

Abstract

Cosmic-ray anisotropy has been observed in a wide energy range and at different angular scales by a variety of experiments over the past decade. However, no comprehensive or satisfactory explanation has been put forth to date. The arrival distribution of cosmicrays at Earth is the convolution of the distribution of their sources and of the effects of geometry and properties of the magnetic field through which particles propagate. It is generally believed that the anisotropy topology at the largest angular scale is adiabatically shaped by diffusion in the structured interstellar magnetic field. On the contrary, the medium-and small-scale angular structure could be an effect of nondiffusive propagation of cosmicrays in perturbed magnetic fields. In particular, a possible explanation forthe observed small-scale anisotropy observed at theTeV energy scalemay be the effect of particle propagation in turbulent magnetized plasmas. We perform numerical integration of test particle trajectories in low-β compressible magnetohydrodynamic turbulence to study how the cosmicrays' arrival direction distribution is perturbed when they stream along the local turbulent magnetic field. We utilize Liouville's theorem for obtaining the anisotropy at Earth and provide the theoretical framework for the application of the theorem in the specific case of cosmic-ray arrival distribution. In this work, we discuss the effects on the anisotropy arising from propagation in this inhomogeneous and turbulent interstellar magnetic field.

Explaining Tev Cosmic-Ray Anisotropies With Non-Diffusive Cosmic-Ray Propagation

“…Giacinti & Sigl (2012) also showed that these local magnetic field structures could create such anisotropies. Similar work to our code was calculated in López-Barquero et al (2015). However, that study primarily looked at particles with PeV-scale energies, where the Larmor radius and mean-free-path are on parsec scales, similar in size to the coherence length of the magnetic turbulence.…”

“…Full MC integration of these TeV particles is intractably slow, so this was why a hybrid diffusion+MC method was needed. Additionally, López-Barquero et al (2015) assumed that the source population was diffuse, which allowed them to time-reverse incoming particles to get maps of the anisotropic sky. However, our code allows for general source distributions in which particular CR sources are dominating the anisotropy.…”

“…Due to the inclusion of anisotropic cosmic rays sources, our code cannot conveniently be time-reversed to create artificial "back-tracked" skymaps as was done in López- Barquero et al (2015). Instead, we used the method suggested in that paper of increasing the size of the target to record those particles that pass "nearby."…”

Damping of Alfvén Waves by Turbulence and Its Consequences: From Cosmic-Ray Streaming to Launching Winds

“…The value of l min can be large in partially ionized gas (see Xu et al 2015). Due to the differences of r L for protons and electrons Eq.…”

Large-scale Cosmic-Ray Anisotropy as a Probe of Interstellar Turbulence

“…In principle, this could be due to a coincidental alignment of the CR density gradient and the field direction, but we interpret it as supporting the view that CRs diffuse mainly along field lines, rather than across them, in which case the density gradient is not strongly constrained. Although explanations have been advanced for small-scale anisotropies that can arise from the configuration of the interstellar magnetic field (Giacinti & Sigl 2012;Battaner et al 2015;López-Barquero et al 2016), both the amplitude and the shape of the global anisotropy remain unexplained. Until now, the amplitude has attracted most of the attention (Blasi & Amato 2012;Pohl & Eichler 2013;Kumar & Eichler 2014;Mertsch & Funk 2015).…”