Heliospheric modulation of cosmic rays during prolonged solar minima deduced from cosmogenic radioisotopes in meteorites

Production of cosmogenic radioisotopes in meteorites depends on the galactic cosmic ray flux, which is controlled by the heliospheric magnetic field (HMF), which, in turn, depends on the solar activity. The radioisotopes can, therefore, serve as proxy records of the behaviour of the heliosphere in the past, in particular over prolonged solar quiet periods, such as during Gleissberg or Maunder minima, when the production rate of radioisotopes is expected to change significantly. For this purpose we have measured the Ti-44 (T-1/2 = 66.6 yr) in ten stony meteorites, which fell between 1840-1992. The results indicate that the production of Ti-44 was significantly higher when the solar activity was persistently low, indicating that the HMF was weak during such periods, enabling higher flux of cosmic rays to enter the inner heliosphere. (C) 1997 COSPAR. Published by Elsevier Science Ltd.