It has been suggested recently that early sunspot numbers should be re-calibrated and significantly corrected using the observed daily range of the geomagnetic declination (so-called rY values). The suggested “correction” method makes an a priori detrending of the rY series and then extends the linear regression between rY and sunspot numbers established for the last 25 years to earlier times. The suggested “correction” of sunspot numbers by roughly 30% goes far beyond the traditional estimates of observational uncertainties of sunspots. Concentrating here on Zürich sunspot numbers (Rz), we demonstrate that the rY values do not actually imply that the observed Rz values in the 19th century are systematically underestimated. Rather, we find that the Rz numbers are fairly uniform after mid-19th century. The suggested “correction” is largely induced by the detrending of the rY series, which enhances the rY-based sunspot activity in the 19th century relative to later times. We also show that while the annually averaged declinations have a rough relation between sunspots and other related solar parameters, this relation is strongly seasonally dependent and, therefore, not sufficiently accurate or uniform to allow annually averaged rY values to be used as a very reliable proxy of solar activity in early times.
The paper can be purchased here: http://www.sciencedirect.com/science/article/pii/S136468260800117X
If anyone knows of another location where it can be downloaded legally let me know.
H/T to Vuk for the paper. My comment below the break.
As this month’s conference leading up to the big one in September in Sunspot New Mexico (yes, really!) approaches, Leif Svalgaard is hard at work readying his presentation trying to convince solar physics community of the need to revise old sunspot numbers along the lines that this paper explains. The ‘adjustments’ are substantial, and in the case of the Dalton Minimum cycles, more than doubles the original group sunspot numbers in the pdf version I saw.
Considering the lack of geomagnetic data for the period due to the Napoleonic wars, this is a questionable thing to do, especially considering the unusually low solar activity we are currently experiencing, with monthly sunspot numbers in the low twenties. While I agree with Leif that the group sunspot numbers may be on the low side for the C19th, his proposed revisions go far beyond the revisions to earlier data applied by Wolf in the latter half of the 1800′s, which the modern SIDC dataset is based on.
This all ties in with the recent counting of sunspecks as spots which would never have been counted in the early epoch of solar observation. The need of the scientific consensus on climate change to maintain the idea that solar variation has only a minor impact on Earth’s surface temperature has over-ridden the sound practice of the scientific method. I am not accusing Leif of anything, but I can see why the consensus will happily support his theory. One of the most egregious examples of the continued attempts to ‘flatten’ solar variation and thereby misrepresent its effects on the Earth’s climates was the paper produced by Benestad and Schmidt, which was debunked by Nicola Scafetta.
My own investigations on the correlation of solar activity with the motion of the Sun with respect to the changing centre of mass in the solar system caused by the various planetary orbits lends support to the SIDC dataset and the three hundred and fifty years of solar observations they represent. However, our work here at the talkshop is by no means ready for publication, and we can only sit on the sidelines awaiting developments.
Geomagnetic indices have several components other than solar activity which can confound interpretation. I don’t deny that geomagnetic indicators are useful, Wolf himself used them, but the back extrapolation of data to derive a trend used to make large and wholesale adjustment to sunspot numbers seems to me to be too focussed on a geomagnetism, and does not take actual observation of the Sun into sufficient account. Geomagnetism is itself affected by the strength of the heliomagnetic field, and there is a danger of circularity in the argumentation.
I have no idea if the criticisms I made of Leif’s pdf made any impact, but it looks like the adjustments may have been ‘toned down a bit’ in the shiny powerpoint which now sits on his website. I’ll say here that I think Leif’s adjustments to Waldmeier’s period as sunspot spotter in chief may well be correct, and are consistent with my own findings.
Leif’s new presentation can be found here: