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Svalgaard (2014) has recently pointed out that the calibration of the\udHelsinki magnetic observatory's H component variometer was probably in error\udin published data for the years 1866–1874.5 and that this makes the\udinterdiurnal variation index based on daily means, IDV(1d), (Lockwood et al., 2013a), and the interplanetary magnetic field\udstrength derived from it (Lockwood et al., 2013b), too low around the peak of\udsolar cycle 11. We use data from the modern Nurmijarvi station, relatively\udclose to the site of the original Helsinki Observatory, to confirm a 30%\udunderestimation in this interval and hence our results are fully consistent with\udthe correction derived by Svalgaard. We show that the best method for\udrecalibration uses the Helsinki Ak (H) and aa indices and\udis accurate to ±10%. This makes it preferable to recalibration\udusing either the sunspot number or the diurnal range of geomagnetic activity\udwhich we find to be accurate to ±20%. In the case of Helsinki data\udduring cycle 11, the two recalibration methods produce very similar\udcorrections which are here confirmed using newly digitised data from the\udnearby St Petersburg observatory and also using declination data from\udHelsinki. However, we show that the IDV index is, compared to later years,\udtoo similar to sunspot number before 1872, revealing independence of the two\uddata series has been lost; either because the geomagnetic data used to\udcompile IDV has been corrected using sunspot numbers, or vice versa, or both.\udWe present corrected data sequences for both the IDV(1d) index and the\udreconstructed IMF (interplanetary magnetic field). We also analyse the relationship between the derived\udnear-Earth IMF and the sunspot number and point out the relevance of the\udprior history of solar activity, in addition to the contemporaneous value, to\udestimating any "floor" value of the near-Earth interplanetary field

annales geophysicae

Reconstruction of geomagnetic activity and near-Earth interplanetary conditions over the past 167 yr – Part 3: Improved representation of solar cycle 11