Galactic cosmic ray response to heliospheric environment changes and implications for cosmogenic isotope records

Changes in the local interstellar environment of the Sun due to its encounters with interstellar clouds of different densities are an important source of long term cosmic-ray variability as measured in cosmogenic isotope records. We present a systematic study of the dependence of galactic cosmic ray intensity in the solar system on the changing background conditions of the heliosphere. We compare three different scenarios: the tenuous fully ionized Local Bubble, the Local Interstellar Cloud, and a dense cold cloud of pure atomic hydrogen. Using several plausible models of interplanetary turbulence production and transport we investigate the dependence of the cosmic-ray mean free paths and intensities on the size of the modulation region and the pickup-ion intensities. Our calculations show that cosmic-ray intensities below 1 GV rigidity could vary by more than a factor of 4 between extreme cases resulting in a range of variation in Be rates between a 25% decline in low-density environments to an increase in excess of 300% in high density clouds compared to current levels.