Galactic cosmic ray flux decline and periodicities in the interplanetary space during the last 3 centuries revealed by ⁴⁴Ti in meteorites

Abstract

⁴⁴Ti and ²⁶Al activities and heavy nuclei tracks produced by cosmic rays have been measured in 19 stone meteorites that fell during the period 1766 to 2001. The gamma activity measurements of cosmogenic isotopes were performed using a highly specific and selective large volume Ge-NaI (Tl) spectrometer. The ⁴⁴Ti activity, corrected for the target element abundances and shielding effects, shows a decrease of about 43% over the past 235 years. Superimposed on this declining trend, the ⁴⁴Ti activity shows a centennial oscillation. The variations in ⁴⁴Ti activity are related to changes in cosmic ray intensity caused by heliospheric magnetic field modulation in the interplanetary space between heliocentric distances of 1 and 3 AU. Using the relations between solar open magnetic flux, modulation parameter, and galactic cosmic ray flux, we have calculated the variation in production rates of ⁴⁴Ti in meteorites back to about 1700 AD. We show that the measured ⁴⁴Ti activity in meteorites over the past 235 years and the calculated production rates are in agreement. Our results are consistent in phase and magnitude with doubling of the solar open magnetic field intensity over the past century. The data also imply that the centennial scale oscillation deduced by ⁴⁴Ti is in phase with Gleissberg solar cycle, but its amplitude is larger than expected from the calculated cosmic ray flux.