Shadowing of Cosmic Rays by the Sun near Maximum or at the Declining Phase of Solar Activity

The shadows of the Sun and Moon have been detected in the 10 TeV cosmic-ray flux by the Tibet air shower array at an altitude of 4300 m above sea level. The observation covers the period 1990 June- 1993 October, which almost coincides with a near-maximum and decreasing phase of the latest solar activity cycle. Using the data obtained in this period, we examined a yearly variation of the Sun's shadow, and found for the first time that the position of the Sun's shadow changed considerably every year with the phase of the solar activity. A different variation of the Sun's shadow was also found between the "away" and "toward" sectors of the interplanetary magnetic field. These results seem to suggest a causal relation between the shadow's movement and the changing inclination of the heliospheric current sheet of the large-scale magnetic field. Further observation with higher statistics will provide a new clue to obtain direct information on the relation between a time variation of the large-scale structure of the solar and interplanetary magnetic fields and the phase of the solar activity cycle.