DIRECT EVIDENCE OF THE INTERPLANETARY MAGNETIC-FIELD EFFECT ON THE COSMIC-RAY SHADOW BY THE SUN

The Tibet air-shower array, being successively operated at an altitude of 4300 m above sea level since 1990, has detected the shadows of the Sun and Moon in the 10 TeV cosmic-ray flux. In particular, the shadow of the Sun was observed in the direction significantly away from the apparent solar position, under the influence of the solar and interplanetary magnetic fields. A further analysis shows that the shadows by cosmic rays coming from the away and toward field sectors in the interplanetary space are shifted in opposite directions to one another according to the polarity of the fields. This is the first direct observation of the influence of the interplanetary magnetic field on the shadow's displacement. The results imply that observation of the cosmic-ray shadow by the Sun can produce new information on the relation between a time variation of the large-scale structure of the solar and interplanetary magnetic fields and the cycle of solar activity.