Cosmic-ray AnisotropyVarious past observations showed a slight non-uniformity (anisotropy) in the arrival directions of cosmic rays. There are two possible causes for this phenomenon. One is the "flow" of cosmic rays that stems from the influence of the interstellar magnetic field on electrically charged cosmic rays within the space relatively near the solar system. We can study the magnetic field structure of the interstellar space near the solar system through model calculations to reproduce the observed anisotropy. The other cause for the anisotropy is the movement of the solar system inside the Galaxy. When the solar system moves in a certain direction, we observe more cosmic rays from that direction compared with other directions, just as when we drive a car on a rainy day, more raindrops fall from the direction in which we are moving.
Map of the distribution of the cosmic-ray arrival directions in the northern celestial hemisphere, observed by the Tibet ASγ experiment. The red (blue) pixels indicate directions where the arrival rate of cosmic rays is higher (lower) than the average, with the maximum difference being ~±0.2%. The white solid line shows the Galactic plane, and the white dotted line shows the celestial equator.