Heliospheric Magnetic Field 1835-2009

We use recently acquired geomagnetic archival data to extend our long-term reconstruction of the HMF strength. The 1835-2009 HMF series is based on an updated and substantiated IDV series from 1872-onwards and on Bartels' extension, by proxy, of his u-series from 1835-1871. 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) Dst 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 10Be data for the first half of the 20th century has largely been removed by a revised 10Be-based reconstruction published after we submitted this paper, although a remaining discrepancy for the years ~1885-1905 will need to be resolved

Geomagnetic Semiannual Variation Is Not Overestimated and Is Not an Artifact of Systematic Solar Hemispheric Asymmetry

Mursula et al. [2011] (MTL11) suggest that there is a 22-year variation in solar wind activity that coupled with the variation in heliographic latitude of the Earth during the year, gives rise to an apparent semiannual variation of geomagnetic activity in averages obtained over several solar cycles. They conclude that the observed semiannual variation is seriously overestimated and is largely an artifact of this inferred 22-year variation. We show: (1) that there is no systematically alternating annual variation of geomagnetic activity or of the solar driver, changing with the polarity of the solar polar fields, (2) that the universal time variation of geomagnetic activity at all times have the characteristic imprint of the equinoctial hypothesis rather than that of the axial hypothesis required by the suggestion of MTL11, and (3) that the semiannual variation is not an artifact, is not overestimated, and does not need revision.Comment: Submitted to GR

Errors in Scale Values for Magnetic Elements for Helsinki

Using several lines of evidence we show that the scale values of the geomagnetic variometers operating in Helsinki in the 19th century were not constant throughout the years of operation 1844-1897. Specifically, the adopted scale value of the Horizontal Force variometer appears to be too low by ~30% during the years 1866-1874.5 and the adopted scale value of the Declination variometer appears to be too low by a factor of ~2 during the interval 1885.8-1887.5. Reconstructing the Heliospheric Magnetic Field strength from geomagnetic data has reached a stage where a reliable reconstruction is possible using even just a single geomagnetic data set of hourly or daily values. Before such reconstructions can be accepted as reliable, the underlying data must be calibrated correctly. It is thus mandatory that the Helsinki data be corrected. Such correction has been satisfactorily carried out and the HMF strength is now well constrained back to 1845

Does Building a Relative Sunspot Number Make Sense? A Qualified 'Yes'

Recent research has demonstrated that the number of sunspots per group ('active region') has been decreasing over the last two or three solar cycles and that the classical Relative Sunspot Number (SSN) no longer is a good representation of solar magnetic activity such as revealed by e.g. the F10.7 cm microwave flux. The SSN is derived under the assumption that the number of spots per group is constant (in fact, nominally equal to 10). When this is no longer the case (the ratio is approaching 5, only half of its nominal value) the question arises how to construct a sunspot number series that takes that into account. We propose to harmonize the SSN with the sunspot Group Count that has been shown to follow F10.7 very well, but also to include the day-to-day variations of the spot count in order to preserve both long-term and short-term variability.Comment: 4 pages, 1 Figur