Heliospheric magnetic field 1835–2009

We use recently acquired geomagnetic archival data to extend our long‐term reconstruction of the heliospheric magnetic field (HMF) strength. The 1835–2009 HMF series is based on an updated and substantiated InterDiurnal Variability ( IDV ) series from 1872 onwards and on Bartels' extension, by proxy, of his u ‐series from 1835 to 1871. The new IDV series, termed IDV09, has excellent agreement (R 2 = 0.98; RMS = 0.3 nT) with the earlier IDV05 series, and also with the negative component of Love's extended (to 1905) D st series (R 2 = 0.91). Of greatest importance to the community, in an area of research that has been contentious, comparison of the extended HMF series with other recent reconstructions of solar wind B for the last ∼100 years yields a strong consensus between series based on geomagnetic data. Differences exist from ∼1900–1910 but they are far smaller than the previous disagreement for this key interval of low solar wind B values which closely resembles current solar activity. Equally encouraging, a discrepancy with an HMF reconstruction based on 10 Be data for the first half of the 20th century has largely been removed by a revised 10 Be‐based reconstruction published after we submitted this paper, although a remaining discrepancy for the years ∼1885–1905 will need to be resolved.