ALPACA Experiment

Solar Coronal Magnetic Field

The Sun has a powerful magnetic field, which, carried by the flow of plasma called the solar wind, affects the Earth and other distant planets. While the magnetic field on the solar surface has been measured by terrestrial optical telescopes, we know little about the magnetic field of the solar corona because of the difficulty of its measurement. We will observe the "Sun’s shadow", a phenomenon caused by the Sun’s shielding of ultra-high-energy cosmic rays, and examine the theoretical models of the solar coronal magnetic field. Since cosmic rays are electrically charged particles, their paths are bent by the solar magnetic field when they pass by the Sun. The Sun’s shadow observed at the Earth, therefore, appears differently according to the state of the solar magnetic field, and makes it possible for us to indirectly probe the structure of the solar magnetic field.

Sun's Shadow

The "Sun’s shadow", observed by the Tibet ASγ experiment yearly from 1996 to 2009 (no data for 2006). The color shows the depth of the shadow.
(Amenomori et al., PRL, 111, 011101, 2013)