The 11-year solar cycle variation of cosmogenic isotope production rates in chondrites

Abstract

The flux of galactic cosmic rays (GCR), which induce nuclear interactions in meteorites, changes with solar cycle due to modulation by the heliospheric magnetic field. We have analyzed the data for Na-22 activity, which are now available for about 30 chondrites that fell during the past three solar cycles. In order to eliminate the variations due to different irradiation geometry of various meteoroids in interplanetary space, we consider the activity ratio of Na-22 to the long-lived Al-26, since this ratio is nearly independent of shielding and is sensitive to time variation of GCR. The measured Na-22/Al-26 ratios as a function of time of fall of various chondrites are shown. The production ratio averaged over a solar cycle, corresponding to J_G = 1.7 is expected to be 1.5 for H chondrites and 1.35 for L and LL chondrites, appropriate for their chemical composition. The high values of Na-22/Al-26 observed during the mid 1970s reflect a prolonged solar minimum of 1976, as also seen in the neutron monitor counting rates. In comparison, the 1965 and 1987 minima have been short, resulting in a smaller increase of Na-22/Al-26. Most of the measurements agree with the expected ratio, the only significant exception is Dhajala. Explanation for meteorites that do not agree with the expected ratio may lie in their complex exposure history or in their peculiar orbits, where GCR flux is different. The high ratio of Na-22/Al-26 in Dhajala is not due to complex exposure history, but to heliolatitudinal gradient of cosmic rays, as it had a highly inclined (i = (28 ± 40)deg) orbit that lay in the southern heliosphere where the GCR gradient was high, just before its fall. Contributions to isotope production by target element abundances, shielding geometry, and GCR flux variations cannot explain the observed activity of short-lived radionuclides. The data on some meteorites, specifically Torino and Mihonoseki, where precision is good, will be discussed in this context.