Long-term evolution of the heliospheric magnetic field inferred from cosmogenic ⁴⁴Ti activity in meteorites

¹ Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Torino, Strada Osservatorio 20, Pino Torinese 10025, Italy
e-mail: mancuso@oato.inaf.it
² Dipartimento di Fisica, Università di Torino, Via P. Giuria 1, 10125 Torino, Italy
³ Wentworth Institute of Technology, Boston, MA, USA
⁴ Physical Research Laboratory and Basic Sciences Research Institute, Navrangpura, Ahmedabad, India

Received: 4 January 2017
Accepted: 17 November 2017

Abstract

Typical reconstructions of historic heliospheric magnetic field (HMF) B_HMF are based on the analysis of the sunspot activity, geomagnetic data or on measurement of cosmogenic isotopes stored in terrestrial reservoirs like trees (¹⁴C) and ice cores (¹⁰Be). The various reconstructions of B_HMF are however discordant both in strength and trend. Cosmogenic isotopes, which are produced by galactic cosmic rays impacting on meteoroids and whose production rate is modulated by the varying HMF convected outward by the solar wind, may offer an alternative tool for the investigation of the HMF in the past centuries. In this work, we aim to evaluate the long-term evolution of B_HMF over a period covering the past twenty-two solar cycles by using measurements of the cosmogenic ⁴⁴Ti activity (τ_1∕2 = 59.2 ± 0.6 yr) measured in 20 meteorites which fell between 1766 and 2001. Within the given uncertainties, our result is compatible with a HMF increase from 4.87_-0.30^+0.24 nT in 1766 to 6.83_-0.11^+0.13 nT in 2001, thus implying an overall average increment of 1.96_-0.35^+0.43 nT over 235 years since 1766 reflecting the modern Grand maximum. The B_HMF trend thus obtained is then compared with the most recent reconstructions of the near-Earth HMF strength based on geomagnetic, sunspot number, and cosmogenic isotope data.