Reconciling Group and Wolf Sunspot Numbers Using Backbones

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Reconciling Group and Wolf Sunspot Numbers Using Backbones

Leif Svalgaard

Stanford University5th Space Climate Symposium, Oulu, 2013

The ratio between Group SSN and Wolf [Zürich, International] SSN has a marked discontinuity ~1882:

Reflecting the well-known secular increase of the Group SSN

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Why a Backbone? And What is it?

• Daisy-chaining: successively joining observers to the ‘end’ of the series, based on overlap with the series as it extends so far [accumulates errors]

• Back-boning: find a primary observer for a certain [long] interval and normalize all other observers individually to the primary based on overlap with only the primary [no accumulation of errors]

Building a long time series from observations made over time by several observers can be done in two ways:

Chinese Whispers

When several backbones have been constructed we can join [daisy-chain] the backbones. Each backbone can be improved individually without impacting other backbones

Carbon Backbone

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The Wolfer Backbone

1876 1928

Alfred Wolfer observed 1876-1928 with the ‘standard’ 80 mm telescope

80 mm X64 37 mm X20

Rudolf Wolf from 1860 on mainly used smaller 37 mm telescope(s) so those observations are used for the Wolfer Backbone

Years of overlap

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Normalization Procedure

Wolfer = 1.653±0.047 Wolf

R2 = 0.9868

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Yearly Means 1876-1893

Wolf

Wolfer

Number of Groups: Wolfer vs. Wolf

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1860 1865 1870 1875 1880 1885 1890 1895

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Wolfer

Wolf*1.653

Number of Groups

For each Backbone we regress each observers group counts for each year against those of the primary observer, and plot the result [left panel]. Experience shows that the regression line almost always very nearly goes through the origin, so we force it to do that and calculate the slope and various statistics, such as 1-σ uncertainty. The slope gives us what factor to multiply the observer’s count by to match the primary’s. The right panel shows a result for the Wolfer Backbone: blue is Wolf’s count [with his small telescope], pink is Wolfer’s count [with the larger telescope], and the orange curve is the blue curve multiplied by the slope. It is clear that the harmonization works well [at least for Wolf vs. Wolfer].

F = 1202

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Regress More Observers Against Wolfer…

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The Wolfer Group Backbone

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1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940

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14Wolfer Group Backbone

Number of ObserversStandard Deviation

Wolfer Backbone Groups Weighted by F-value

The Wolfer Backbone straddles the interval around 1882 with good coverage (~9 observers) and with reasonable coverage 1869-1925 (~6 observers). Note that we do not use the Greenwich [RGO] data for the Wolfer Backbone.

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Hoyt & Schatten used the Group Count from RGO [Royal Greenwich Observatory] as their Normalization Backbone. Why don’t we?

José Vaquero found a similar result which he reported at the 2nd Workshop in Brussels.

Because there are strong indications that the RGO data is drifting before ~1900

Could this be caused by Wolfer’s count drifting? His k-factor for RZ was, in fact, declining slightly the first several years as assistant (seeing fewer spots early on – wrong direction). The group count is less sensitive than the Spot count and there are also the other observers…

Sarychev & Roshchina report in Solar Sys. Res. 2009, 43: “There is evidence that the Greenwich values obtained before 1880 and the Hoyt–Schatten series of Rg before 1908 are incorrect”.

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The Schwabe BackboneSchwabe received a 50 mm telescope from Fraunhofer in 1826 Jan 22. This telescope was used for the vast majority of full-disk drawings made 1826–1867.

For this backbone we compare with Wolf’s observations with the large 80mm standard telescopeSchwabe’s House?

Years of overlap

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Regressions for Schwabe Backbone

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1790 1795 1800 1805 1810 1815 1820 1825 1830 1835 1840 1845 1850 1855 1860 1865 1870 1875 1880 1885

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10Schwabe Group Backbone

Schwabe Backbone Groups

Standard Deviation

Number of Observers

The Schwabe Group Backbone

Weighted by F-value

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10Comparing Schwabe Backbone with Hoyt & Schatten Group Number

Groups

Schwabe Backbone

H&S Groups

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Joining the Two Backbones

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1860 1865 1870 1875 1880 1885 1890 1895 1900

Comparing Overlapping Backbones

WolferSchwabe

Wolfer = 1.55±0.05 Schwabe

R2 = 0.9771

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1860-1883Wolfer

Schwabe

Reducing Schwabe Backbone to Wolfer Backbone

1.55

Comparing Schwabe with Wolfer backbones over 1860-1883 we find a normalizing factor of 1.55

And can thus join the two backbones covering ~1825-1946

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Comparison Backbone with GSN and WSN

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Backbone

Sunspot Group Numbers 1790-1945

Groups = GSN/13.149Groups = WSN/12.174

Scaling all curves to match for 1912-1946 shows that the combined backbone matches the scaled Wolf Number

The discrepancy around 1860 might be resolved using the newly digitized Schwabe data supplied by Arlt et al. (This meeting).

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Conclusions

• Using the ‘Backbone’ technique it is possible to reconstruct a Group Sunspot Number 1825-1945 that does not exhibit any systematic difference from the standard Wolf [Zürich, Intnl.] Sunspot Number

• This removes the strong secular variation found in the Hoyt & Schatten GSN

• And also removes the notion of a Modern Grand Maximum