Evaluation of the Interplanetary Magnetic Field Strength Using the Cosmic-Ray Shadow of the Sun

TL;DR

This study uses the Sun's cosmic-ray shadow observed with Tibet-III to estimate the interplanetary magnetic field strength, revealing deviations from models and demonstrating the shadow's utility in magnetic field evaluation.

Contribution

It introduces a method to quantify the solar magnetic field strength by analyzing the Sun's cosmic-ray shadow and comparing it with magnetic field models.

Findings

The Sun's shadow deviates from the optical solar disc center depending on IMF sector.

The observed IMF strength is approximately 1.5 times larger than model predictions.

The Sun's shadow can serve as a quantitative tool for solar magnetic field assessment.

Abstract

We analyze the Sun's shadow observed with the Tibet-III air shower array and find that the shadow's center deviates northward (southward) from the optical solar disc center in the "Away" ("Toward") IMF sector. By comparing with numerical simulations based on the solar magnetic field model, we find that the average IMF strength in the "Away" ("Toward") sector is $1.54 \pm 0.21_{\rm stat} \pm 0.20_{\rm syst}$ ($1.62 \pm 0.15_{\rm stat} \pm 0.22_{\rm syst}$) times larger than the model prediction. These demonstrate that the observed Sun's shadow is a useful tool for the quantitative evaluation of the average solar magnetic field.