HR: 1340h
AN: A43C-0117    [Abstracts]
TI: Interstellar environment change: effects on heliospheric structure, galactic cosmic ray modulation and cosmogenic isotope production.
AU: Mueller, H R
EM: hans.mueller@dartmouth.edu
AF: Dartmouth College, Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755 United States
AU: * Florinski, V
EM: vflorins@ucr.edu
AF: University of California, Riverside, Institute of Geophysics and Planetary Physics, University of California, Riverside, CA 92521 United States
AU: Zank, G P
EM: zank@ucr.edu
AF: University of California, Riverside, Institute of Geophysics and Planetary Physics, University of California, Riverside, CA 92521 United States
AB: Galactic cosmic ray (GCR) intensity levels in the inner heliosphere over the past million years, preserved in cosmogenic isotope records, display significant variability on virtually all timescales. Here we focus on the variability caused by changes in the interstellar environment of the Sun as it encounters interstellar clouds or low-density regions (supernova bubbles) during its journey through the Galaxy. Three possible environments are compared and the resulting structure of the heliosphere investigated: 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 evolution and particle diffusion we investigate the dependence of the cosmic-ray mean free paths and intensities on the size of the modulation region and the pickup ion (PUI) intensities. We show that, while denser clouds usually yield smaller diffusion coefficients due to enhanced PUI turbulence, GCR radiation levels in the inner heliosphere are actually increased due to a reduction in the size of the modulation region. Our results indicate that GCR intensities at Earth can vary by a factor 2 to 7 between 300 MeV and 1 GeV compared to the present intensity. Interestingly, most of the changes are due to a variation in the thickness of the modulation wall in the inner heliosheath. Finally, we calculate cosmogenic isotope production rates in the Earth's atmosphere for the three environments and show that Beryllium-10 concentration could vary between 25% declines in low-density environments to increases in excess of 300% in high density interstellar clouds.
DE: 1115 Radioisotope geochronology
DE: 2104 Cosmic rays
DE: 2124 Heliopause and solar wind termination
DE: 2126 Heliosphere/interstellar medium interactions
DE: 2151 Neutral particles (7837)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005