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The Role of the Heliosphere in Shaping the Observed Cosmic Ray Spectral Anisotropy

Vanessa López-Barquero, Andrés Marín Portuguez, Paolo Desiati, Juan Carlos Díaz-Vélez

Abstract

Experimental results by Milagro, HAWC, and ARGO-YBJ have observed variations in the energy spectrum of cosmic rays at TeV scales in different regions of the sky. These findings on the spectral anisotropy provide insights into cosmic ray behavior. This work explores the impact of galactic cosmic ray interactions with the heliosphere in creating the observed spectral anisotropy features. Specifically, the features around 1-10 TeV, where our previous studies on the heliosphere have shown the greatest effects. In this project, we integrate particle trajectories in a state-of-the-art MHD-kinetic heliosphere model that includes the effects of the solar cycle and interaction with the interstellar medium's magnetic field. With these elements, this is the first time the exact effects of the heliosphere's magnetic field are tested to determine their influence on galactic cosmic rays and their spectral anisotropy. In our results, we identified an area on the map that exhibits a distinct cosmic ray energy spectrum compared to the all-sky distribution. This area approximately coincides with Region A, where observations have found a harder energy spectrum than the isotropic spectrum.

The Role of the Heliosphere in Shaping the Observed Cosmic Ray Spectral Anisotropy

Abstract

Experimental results by Milagro, HAWC, and ARGO-YBJ have observed variations in the energy spectrum of cosmic rays at TeV scales in different regions of the sky. These findings on the spectral anisotropy provide insights into cosmic ray behavior. This work explores the impact of galactic cosmic ray interactions with the heliosphere in creating the observed spectral anisotropy features. Specifically, the features around 1-10 TeV, where our previous studies on the heliosphere have shown the greatest effects. In this project, we integrate particle trajectories in a state-of-the-art MHD-kinetic heliosphere model that includes the effects of the solar cycle and interaction with the interstellar medium's magnetic field. With these elements, this is the first time the exact effects of the heliosphere's magnetic field are tested to determine their influence on galactic cosmic rays and their spectral anisotropy. In our results, we identified an area on the map that exhibits a distinct cosmic ray energy spectrum compared to the all-sky distribution. This area approximately coincides with Region A, where observations have found a harder energy spectrum than the isotropic spectrum.
Paper Structure (5 sections, 2 equations, 2 figures)

This paper contains 5 sections, 2 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Heliospheric Magnetic Field Model
  3. Particle Trajectory Integration
  4. Results
  5. Conclusions

Figures (2)

  • Figure 1: Model of the heliotail, as described by Pogorelov et al.(2015) Pogorelov_2015, taking into account the effects of solar cycles. The units in the x and z directions are in astronomical units (AU), while the color indicates the strength of the magnetic field ($|\mathbf{B}|$) in microgauss ($\mu\text{G}$). The spatial extent of the tail along the x-axis is of a few thousand astronomical units (AU), while, in contrast, the extent in the z-direction is considerably more limited, spanning only a few hundred AU.
  • Figure 2: Cosmic Ray Spectral Anisotropy Map. This figure shows the reduced $\chi^2$ values for the spectral distribution of particles at a median energy of 7 TeV plotted in a map in equatorial coordinates. The red color in the map indicates areas where the spectral distribution differs from the overall distribution across the entire sky. This variation is attributed to interactions with the heliosphere. The red region in the Southern Hemisphere approximately corresponds to Region A in the HAWC data, where the energy spectrum is harder than the isotropic spectrum.