Galactic cosmic ray variations in the last two centuries recorded by cosmogenic ⁴⁴Ti in meteorites

Abstract

We have previously shown that the very low activity of the cosmogenic ⁴⁴Ti in meteorites records the galactic cosmic ray (GCR) flux over a century time-scale (Bonino et al., 1995). The heliospheric magnetic field (HMF) variations are the principal source of the GCR modulation in the heliosphere. We present here new measurements which extend our investigation over the last two centuries. The ⁴⁴Ti activity at the time of fall has been obtained on the basis of the recently revised value of its half-life of 59.2 years. We basically confirm our earlier findings (Bonino et al., 1997). The ⁴⁴Ti profile, with the time of fall of the meteorites, shows variations in phase in agreement with those expected. The magnitude of oscillations is ≈ 4 times higher than expected (≈ 6%) on the basis of the GCR flux over the past centuries deduced from sunspot number since 1700 and by the neutron monitor and balloon measurements in the last decades. The higher ⁴⁴Ti production rate during prolonged solar quiet periods (Gleissberg minima) may imply that during these minima the HMF was weaker than during the recent minima of the 11-year solar cycle.