Statistics – Computation
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmgp53a..03s&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #GP53A-03
Statistics
Computation
1150 Cosmogenic-Nuclide Exposure Dating (4918), 2104 Cosmic Rays, 4918 Cosmogenic Isotopes (1150), 5734 Magnetic Fields And Magnetism
Scientific paper
The magnetic shielding of the Earth from galactic and solar cosmic radiation is dependent on the magnitude and orientation of the Earth's magnetic field. The amount of shielding at a specific location is defined by the geomagnetic cutoff rigidity (RC), which varies on both short- and long-term time scales. On short time scales, the amount of geomagnetic shielding at any specific high-latitude or mid-latitude location is dependent on the time of day, the season of the year and space weather conditions - all of which should be considered when analyzing precise cosmic radiation data. On decadal time scales, the Earth's geomagnetic field is changing rapidly. These secular changes in the geomagnetic field are reflected in the cosmic radiation intensity data acquired by cosmic-ray monitoring instruments in locations where the geomagnetic changes are most rapid. Accurate models of the Earth's internal field magnetic field exist on annual and decadal time scales, i.e., the International Geomagnetic Reference Field. In addition, high-order models of the geomagnetic field covering time scales of centuries and even millennia are now available, allowing computation of RC over the last 7 kyr. The cosmic-ray trajectory-tracing technique, implemented on modern high-speed computers, makes it possible to compute world maps of RC for any epoch for which a higher order geomagnetic field model has been developed. We have generated global maps of millennial-scale RC values that predict surprisingly large variations in atmospheric cosmic ray intensity. The accumulation of in situ cosmogenic nuclides (CNs) in terrestrial minerals allows testing of these predictions, since they record the effects of the time-integrated cosmic- ray flux at a given location. With appropriate models for scaling in situ CN production rates with altitude and latitude, driven by these millennial-scale RC estimates, one can predict global time-integrated CN production patterns. In fact, these RC estimates predict significant longitudinal variability in time-integrated CN production, while predictions using dipolar geomagnetic approximations do not. One can test these predictions using in situ cosmogenic 14C (in situ 14C) in quartz. Due to its half-life (5730 years), in situ 14C attains secular equilibrium between production and decay after approximately 25 kyr of exposure (quite rapidly, in geologic terms), at which point its measured concentration is only a function of its integrated average production rate. Preliminary in situ 14C results from samples at secular equilibrium from 38°N and 3.5 km in Tibet and eastern California are consistent with the longitudinal variability predicted by our model.
Lifton Nathaniel
Shea Margaret Ann
Smart Don Frederick
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