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 _ { stat } \pm 0.20 _ { syst } ( 1.62 \pm 0.15 _ { stat } \pm 0.22 _ { 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 .