Interplanetary magnetic field during the past 9300 yearsinferred from cosmogenic radionuclides

摘要

[1] We have reconstructed the interplanetary magnetic field (IMF), its radial component,and the open solar magnetic flux using the solar modulation potential derived fromcosmogenic ~(10)Be radionuclide data for a period covering the past 9300 years.Reconstructions using the assumption of both constant and variable solar wind speedsyielded closely similar results. During the Maunder Minimum, the strength of the IMF wasapproximately 2 nT compared to a mean value of 6.6 nT for the past 40 years,corresponding to an increase of the open solar magnetic flux of about 350%. We examinefour cycles of the Hallstatt periodicity in the IMF with a mean period of —2250 yearsand an amplitude of0.75 nT. Grand solar minima have largely occurred in clustersduring the Hallstatt cycle minima around the years —5300, —3400, —1100, and +1500 A.D.The last cluster includes the Dalton, Maunder, and Sporer minima. We predict that thenext such cluster will occur in about 1500 years. The long-term IMF has variedbetweennT and —8 nT and does not confirm a proposed floor (lower limit). There is aslowly changing long-term trend of amplitude 1.5 nT, with a minimum around the year—4600 and a maximum around 0 A.D. that may be of solar origin but which also may bedue to unknown long-term changes in the atmospheric effects or geomagnetic fieldintensity.