Observation of the Sun's Shadow by High Energy Cosmic Rays in a Quiet Phase of Solar Activity

We have shown that the Sun's shadow by high energy cosmic rays moves year by year and its behavior is related to a time variation of global structure of solar and interplanetary magnetic fields. As well known, the Sun was almost in a quiet phase between 1995 and 1998. In this phase, the direction of equivalent dipole magnetic field of the Sun was almost parallel to the rotation axis of the Sun, while changing its polarity every 11 year. So, if the dipole field strongly affected the Sun's shadow, the shadow would move to westward or eastward direction according to a change of the dipole field direction. We examined this effect using the data set taken with the Tibet-II air shower array during the period from 1995 through 1998. In this paper, we show that the Sun's shadow is sensitive to the solar magnetic field and a long-term observation provides direct information about a change of the solar magnetic field according to the cycle of solar activity.