Amenomori, M (reprint author), HIROSAKI UNIV,DEPT PHYS,HIROSAKI,AOMORI 036,JAPAN.
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.