  We report on a clear solar-cycle variation of the Sun's shadow in the 10 TeV
cosmic-ray flux observed by the Tibet air shower array during a full solar
cycle from 1996 to 2009. In order to clarify the physical implications of the
observed solar cycle variation, we develop numerical simulations of the Sun's
shadow, using the Potential Field Source Surface (PFSS) model and the Current
Sheet Source Surface (CSSS) model for the coronal magnetic field. We find that
the intensity deficit in the simulated Sun's shadow is very sensitive to the
coronal magnetic field structure, and the observed variation of the Sun's
shadow is better reproduced by the CSSS model. This is the first successful
attempt to evaluate the coronal magnetic field models by using the Sun's shadow
observed in the TeV cosmic-ray flux.
