The heliospheric magnetic field from 850 to 2000 AD inferred from 10 Be records

10 Be found in ice cores is an indicator of cosmic ray intensity in the past. We use this isotope to study cosmic ray transport and the heliospheric magnetic field before the advent of instrumental cosmic‐ray measurements in the modern space era. The galactic cosmic ray intensity is governed by scattering, convection, and drift of the charged particles in the heliospheric magnetic field, which leads to a modulation in their intensity. We model these cosmic ray intensity changes observed at Earth during the space era with solutions of the cosmic ray transport equation. This gives a set of diffusion mean free paths during the past few solar activity cycles. A relationship is then determined between these diffusion mean free paths and satellite observations of the heliospheric magnetic field during the same period, yielding a relationship between the observed cosmic ray intensity and the heliospheric magnetic field. We then calculate the diffusion mean free paths that explain the variations in the 10 Be concentration during the last millennium and use the space‐era calibration to infer heliospheric magnetic field since 850 AD. It is shown how this inversion of the 10 Be data depends on the strength of the heliospheric magnetic field and variations in its turbulence, both of which are quite uncertain. Nevertheless, the results show that for a wide range of parameters, there was a significant heliospheric magnetic field with a strength of 2 to 5 nT at Earth during the so‐called Grand Minima of solar activity. It is also shown that the strength of this field has attained six maxima in the past 1150 years, all approximating the present‐day field strength, and we speculate that a limiting mechanism may be in operation. On several occasions the strength of the field has switched rapidly from ≈2 nT to ≈6 nT within 40 years. During the Grand Minima the total field derived from the 10 Be data differs significantly from the open solar magnetic field calculated from the models of Solanki et al. [2002] and Schrijver et al. [2002].