The galactic cosmic ray intensity in the heliosphere in response to variable interstellar environments

Abstract

Indirect indices of cosmic ray intensity at Earth, preserved in cosmogenic isotope records, show strong variability on a wide range of timescales. Some of the variability may be attributed to heliospheric phenomena, such as changes in solar activity and in the geomagnetic field strength. Another possibility is changes in the factors external to the solar system, such as supernova explosions and the variability in the interstellar environment. Here we present a first systematic study of the dependence of cosmic-ray intensity in the inner and the outer heliosphere on the changing galactic environment of the Sun. Three different scenarios (tenuous Local Bubble, Local Interstellar Cloud and diffuse cold interstellar cloud) are investigated as background conditions for galactic cosmic ray propagation. Because cosmogenic isotope production is strongly biased toward higher energies of the primary particles, we compare two models with different levels of turbulence in the energy-containing part of the spectrum. It is shown that the amount of modulation that cosmic rays experience depends on the interplay between turbulence level variability due to pickup ions and the changes in the extent of the heliosheath. Our calculations show that cosmic-ray intensities at intermediate rigidities (1 GeV) vary by more than a factor of four between extreme cases. Cosmogenic isotopes show a similar response giving a range of variation between a 25% decline in low-density environments and an increase in excess of 300% in high density clouds. © 2006 Springer. All Rights Reserved.