Observations of cosmic-ray Sun’s shadow with LHAASO-WCDA

Abstract

During the propagation of cosmic rays (CRs) in the solar system, they are blocked by the Sun and deflected by the magnetic field, resulting in a shadow on the celestial map. As CRs travel from the vicinity of the Sun to the Earth, they encounter deflection from the coronal magnetic field (CMF), the interplanetary magnetic field (IMF), and finally the geomagnetic field (GMF). The extent of deflection is determined by the magnetic field’s intensity, direction, and energy of the CRs. Variations in the magnetic field cause corresponding changes in the position and size of the Sun’s shadow. By observing the Sun’s shadow of CRs, we can investigate the magnetic fields between the Sun and the Earth, complementing other measurements on these magnetic fields. Furthermore, it serves as a means to validate magnetic field models. The Large High Altitude Air Shower Observatory (LHAASO) is a new generation cosmic-ray and gamma-ray experiment in Daocheng, western China. With a vast Water Cherenkov Detector Array (WCDA) spanning 78,000 m2, LHAASO can successfully observe the significant Sun’s shadow within just a few days. Our study uses the WCDA to measure the Sun’s shadow effect caused by CRs over multiple Carrington rotation periods. Subsequently, we compare these results with simulations based on different magnetic field models.