History of Solar and Galactic Cosmic Radiation Origin of Cr Flux Changes

The intensity of cosmic rays in the inner solar system is observed to vary with time over a variety of time scales. The Sun is the cause of some of these variations, but the observed longer-term variations reflect changes in the local interstellar medium. Galactic cosmic rays (GCR) dominate the average intensity of energetic particles above about 200 MeV and are affected by supernova shock waves. They are modulated by the Sun and have their lowest intensity during high solar activity. The cosmic-ray intensity has been studied using satellites for short-term variations and terrestrial and extraterrestrial materials for the longer-term variations. Studies of meteorites indicate that the average galactic flux in the inner solar system was only constant for the past few Ma. Iron meteorites are used as GCR monitors for the last 1 Ga time period. The nuclide pair K and K (1.26 Ga) shows that the flux was smaller in the distant past. The pairs KrKr, Mn-Cr and I-Xe are currently studied by the Solar Neighborhood Consortium for possible resolution of flux changes during the past 50 Ma. Significant improvements in the resolution of flux changes and for the study of the evolution of the solar environment, as a result of the galactic rotation, can be expected from the application of these techniques to suitable lunar GCR monitors. Origin of CR Flux Changes The huge energies carried by GCR (typically 100 MeV to 10 GeV) are thought to derive from supernovae (SN) explosions, which occur once every 30 to 60a in the galaxy. In order to maintain the currently observed intensity of GCR over millions of years, only a few percent of the SN energy has to be used for the GCR acceleration. There is a considerable amount of evidence that this acceleration is accomplished in the shock waves of SN explosions as they travel through the surrounding interstellar gas. Whenever the Sun was located in a region of the galactic spiral arms which show star