Galactic Cosmic Ray Sun Shadow during the declining phase of cycle 24 observed by HAWC

TL;DR

This study uses HAWC data to analyze the Sun Shadow of cosmic rays during the declining phase of solar cycle 24, revealing relationships between shadow intensity and solar magnetic fields, and showing that high-energy particles are deflected by a few degrees.

Contribution

It presents the first detailed analysis of the Sun Shadow variation over three years with HAWC, linking it to solar magnetic field evolution and heliospheric topology changes.

Findings

Sun Shadow intensity decreases during solar cycle 24's declining phase.

Linear relationship between shadow deficit and active region magnetic field.

Inverse relationship between shadow deficit and polar magnetic field.

Abstract

The High Altitude Water Cherenkov (HAWC) array is sensitive to high energy Cosmic Rays (CR) in the $\sim 10$ to $\sim 200$ TeV energy range, making it possible to construct maps of the so called "Sun Shadow" ($SS$), i. e. of the deficit of CR coming from the direction of the Sun. In this work, we present the variation of the Relative Intensity of the deficit ($SS_{RI}$) for three years of HAWC observations form 2016 to 2018 in which we found a clear decreasing trend of the ($SS_{RI}$) over the studied period, corresponding to the declining phase of the solar cycle 24. By comparing the $SS_{RI}$ with the photospheric magnetic field evolution, we show that there is a linear relationship between the $SS_{RI}$ and the median photospheric magnetic field of the Active Region belt (-40$^\circ \le$ lat $\le$ 40$^\circ$) and a inverse linear relationship with the polar photospheric magnetic field (lat $\ge \pm$ 60$^\circ$). The former relationship is due to the magnetic field causing a deviation of the CR, whereas the latter reflects the change of the heliospheric field topology from multipolar to dipolar configurations. These relationships are valid only when the median magnetic field is lower than 8 G, during the declining and minimum phases of the solar cycle 24. Finally, we show that relativistic charged particles, in the 10 to 200 TeV energy range, are deflected a few degrees.