A study of the shadowing of galactic cosmic rays by the sun in a quiet phase of solar activity with the Tibet air shower array

Citation
M. Amenomori et al., A study of the shadowing of galactic cosmic rays by the sun in a quiet phase of solar activity with the Tibet air shower array, ASTROPHYS J, 541(2), 2000, pp. 1051-1058
Citations number
16
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTROPHYSICAL JOURNAL
ISSN journal
0004-637X → ACNP
Volume
541
Issue
2
Year of publication
2000
Part
1
Pages
1051 - 1058
Database
ISI
SICI code
0004-637X(20001001)541:2<1051:ASOTSO>2.0.ZU;2-N
Abstract
We have shown that the Sun's shadow by high-energy cosmic rays moves year b y year and its, behavior is correlated with a time variation of the large-s cale structure of the solar and interplanetary magnetic fields. The solar a ctivity was near minimum in the period from 1996 through 1997. Using the da ta obtained with the Tibet air shower array, we examined the shadowing of c osmic rays by the Sun in this quiet phase of the solar cycle and found that the Sun's shadow was just in the apparent direction of the Sun, though it was observed at the position considerably away from the Sun to the southwes t in the period between 1990 and 1993. It is known that the magnetic pole o f the equivalent solar dipole was reversed during the previous active phase , and near solar minimum the dipole was aligned with the rotating axis, pre serving its N-pole on the north pole side of the Sun. This causes the solar magnetic held to shift the Sun's shadow to the east. Thus, the observed re sults suggest that the shift of the Sun's shadow due to the solar magnetic field was pushed back by the effect of the geomagnetic held, since the geom agnetic field always makes the shadow shift to the west. We discuss the Sun 's shadow observed during the period near solar minimum in 1996-1997 and co mpare it with the simulation results.