Cosmic rays are ubiquitous, existing throughout the heliosphere and the universe. Because of their mobility, speed, and responsiveness to electromagnetic fields, cosmic rays can serve as probes of otherwise inaccessible regions of the heliosphere and the Galaxy. Observations of cosmic rays and sophisticated theory have yielded many major discoveries and have significantly advanced our understanding of the Sun, heliosphere, and the Galaxy.
Galactic cosmic rays (GCRs) are energetic (from hundreds of MeV to GeV) charged particles entering from the Galaxy. The majority of GCRs are accelerated at shocks produced by supernova explosions. As they propagate through the heliosphere, their intensity and properties are modulated by the structure of the solar wind. This modulation is seen, for example, in the anticorrelation between solar activity and GCR intensity.
Anomalous cosmic rays (ACRs) were first observed in the early 1970s as a peculiar, or "anomalous," distortion of the cosmic-ray composition. Most ACRs begin as galactic neutral atoms that enter the heliosphere and are subsequently ionized. Upon ionization, they are picked up by the solar wind’s magnetic field and carried outward, toward the termination shock. Some are accelerated to higher energies by interplanetary shocks and may experience further acceleration—to energies in excess of 1 GeV—at the termination shock. Because of their mobility, the ACRs can then move throughout the heliosphere, to be observed as a significant distortion of the observed galactic cosmic-ray spectrum.
- Solar modulation of the intensity of galactic cosmic rays (Bieber)
Cosmic Rays and Earth, Edited by J.W. Bieber, E. Eroshenko, P. Evenson, E.O. Flückiger, and Reinald Kallenbach, Space Sciences Series of ISSI, Volume 10, 2000. Reprinted from Space Sci. Rev., Volume 93:1-2, 2000.