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 similar to 100 years yields a strong consensus between series based on geomagnetic data. Differences exist from similar to 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 Be-10 data for the first half of the 20th century has largely been removed by a revised Be-10-based reconstruction published after we submitted this paper, although a remaining discrepancy for the years similar to 1885-1905 will need to be resolved.