Title: Heliospheric Magnetic Field Modulation in the Last Centuries: Validation from Measurements of the Cosmogenic Ti-44 in Meteorites
Author: G. Bonino
Affil: Dipartimento di Fisica Generale, Universita' di Torino, Italy and Istituto di Cosmogeofisica, CNR, Torino
Email: bonino@ph.unito.it
Authors: G. Cini Castagnoli (1), D. Cane (1), C. Taricco (1) and N. Bhandari (2)
Affils: (1) Dipartimento di Fisica Generale, Universita' di Torino, Italy and Istituto di Cosmogeofisica, CNR, Torino; (2) Physical Research Laboratory, Ahmedabad, India
Abstract: Knowledge of the galactic cosmic ray (GCR) modulation in the past centuries is important for understanding the century scale heliospheric behaviour and in particular for the investigation of the solar modulation during prolonged solar quiet periods like the Gleissberg minima and the Maunder minimum of solar activity. We inferred the galactic cosmic ray annual mean spectra on the basis of the following data: primary spectra of cosmic rays obtained from balloon and spacecraft measurements during different phases of the solar cycles # 20-23; the Climax neutron monitor time series available since 1953; variations of the annual mean of the coronal source magnetic flux as derived from the aa index(1) available since 1868 and of the evolution of the Sun's large-scale magnetic field(2); the sunspot number time series from 1600. The relations among these data sets were extrapolated back to 1700. The differential flux of the galactic cosmic ray J(T,M) (particles/m2 s sr MeV) has been determined as a function of M (MeV) (the energy lost by particles in traversing the heliosphere) which depends on the solar magnetic field variations(3). The obtained results on J(T,M) have been used for the calculation of the production rate as a function of time of the cosmogenic Ti-44 (T1/2 = 59.2 y) in meteorites and have been validated by comparison with our measurements of this radioisotope in different chondrites which fell in the last 160 years(4). We deduce that during prolonged solar quiet periods the GCR flux in the heliosphere is higher (HMF lower) than during the short lasting recent decadal minima. Since magnetic and bolometric solar outputs are related, a similar behaviour (deeper minimum) of the TSI variations may be expected.
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