Sunspot Number Version 2.0: new data and conventions

On July 1st, 2015, the sunspot number series has been replaced by a new improved version (version 2.0) that includes several corrections of past inhomogeneities in the time series. The details of the corrections are given in several publications listed below.

New conventions and extensions:

Next to the recalibration of the data series, three main innovations come with the new sunspot number series:

  • New conventional scale: The most prominent change in the Sunspot Number values is the choice of a new reference observer, A.Wolfer (pilot observer from 1876 to 1928) instead of R. Wolf himself. This means dropping the conventional 0.6 Zürich scale factor, thus raising the scale of the entire Sunspot Number time series to the level of modern sunspot counts. The 0.6 factor has always led to some confusion and now has lost its sense more than 130 years after Wolf's observations. This change is equivalent to a change of unit and thus raises the scale of the entire sunspot number time series by a factor 1/0.6, which may significantly affect software using the sunspot number as input. This scale change, when combined with the recalibration, leads to a net increase of about 45% (correction variable with time) of the most recent part of the series, after 1947.
  • Error values: The new numbers now come with the value of the standard deviation of the base counts included in the calculations, as well as the number of data included in the calculation. This allows to assess the precision of the value (statistical uncertainty). The absolute accuracy itself is not given as part of the data files, as it is determined only over long periods of time and not for single observations. This aspect is still being developed in the scientific litterature. 
  • Version numbers: As future updates of the series can be expected based on new recovered data and progresses in the methods, we initiated a version serial number that will allow to cleanly keep track of the incremental changes. The first number indicates a new version of the series where the sunspot numbers were collectively modified by any kind of correction. The second sub-version number is used to indicate secondary modifications that do not change the numbers themselves (e.g. new error estimates) or that involve only isolated modifications of the values (e.g. a typo found in a single daily value). Each incremental change will be documented in our archive section. The past version of the sunspot number (now Version 1.0) remains accessible for reference and is now archived in a specific "Archive" section. It will not be maintained and extended anymore, as the master series now becomes version 2.

New upgraded data products and formats:

In order to accomodate the new data series, all our standard data files have been modified. Here is an overview of the new files, which now fully replace the original array of files that was introduced back in 1981. The main changes include a new file naming convention with version numbers and the addition of columns giving standard deviations and the number of input data used in the calculation. We now also separate in distinct files the monthly mean sunspot number from the 13-month smoothed sunspot number, which is considered as a secondary data set.

Daily total sunspot number (1818-now):
Old files: dayssn.dat, dayssnV0.dat, dayssn_import.dat,ISSN_D_tot.csv
replaced by: 
SN_d_tot_V2.0.txt (column format), SN_d_tot_V2.0.csv (CSV format)
Contents: year, month, day, decimal year, SNvalue , SNerror, Nb observations

Monthly mean total sunspot number (1749-now):
Old files: monthssn.dat, ISSN_M_tot.csv
replaced by: 
SN_m_tot_V2.0.txt (column format), SN_m_tot_V2.0.csv (CSV format)
Contents: year, month, decimal year, SNvalue , SNerror, Nb observations

13-month smoothed monthly total sunspot number (1749-now):
Old files: monthssn.dat, ISSN_M_tot.csv
New file: 
SN_ms_tot_V2.0.txt (column format), SN_ms_tot_V2.0.csv (CSV format)
Contents: year, month, decimal year, SNvalue , SNerror, Nb observations

Yearly mean total sunspot number (1700-now):
Old files: yearssn.dat, ISSN_Y_tot.csv
replaced by: 
SN_y_tot_V2.0.txt (column format), SN_y_tot_V2.0.csv (CSV format)
Contents: decimal year, SNvalue , SNerror, Nb observations

Daily hemispheric sunspot number (1992-now):
Old files: yearly files dssnYYYY.dat,ISSN_D.hem.txt, ISSN_D_hem.txt
replaced by a single file:
SN_d_hem_V2.0.txt (column format), SN_d_hem_V2.0.csv (CSV format)
Contents: year, month, day, decimal year, SNvalue(tot), SNvalue(N), SNvalue(S), SNerror(tot), SNerror(N), SNerror(S), Nb observations
Comment: The yearly files are replaced by a single file. Total numbers are included next to the North and South numbers.

Monthly mean hemispheric sunspot number (1992-now):
Old files: monssnns.dat
replaced by: 
SN_m_hem_V2.0.txt (column format), SN_m_hem_V2.0.csv (CSV format)
Contents: year, month, decimal year,  SNvalue(tot), SNvalue(N), SNvalue(S), SNerror(tot), SNerror(N), SNerror(S), Nb observations
Comment: the 13-month smoothed numbers are moved to a separate file.
Total numbers are included next to the North and South numbers.

13-month smoothed monthly hemispheric sunspot number (1992-now):
Old files: monsnns.dat
New file: 
SN_ms_hem_V2.0.txt (column format), SN_ms_hem_V2.0.csv (CSV format)
Contents: year, month, decimal year, SNvalue(tot), SNvalue(N), SNvalue(S), SNerror(tot), SNerror(N), SNerror(S), Nb observations
Comment: This smoothed series is not a primary product and is thus moved to a separate file. Total numbers are included next to the North and South numbers

Yearly mean total sunspot number (1610-now):
New file: GN_y_tot_V2.0.txt, GN_y_tot_V2.0.csv (CSV format)
Contents: decimal year, GNvalue , GNerror, Nb observations

Our other data products (EISN: daily estimated sunspot number, 12-month forecasts) have been adapted to match the scale of the main sunspot time series, but the file names and formats remain unchanged.

Next to the Sunspot Number data, we now also distribute the Group Number time series, which extends futher back to 1610. A new dedicated section includes the original series (Hoyt and Schatten 1998) and new revised versions. As these are research products, those series cover a fixed time interval, and are not updated and extended on a monthly basis like the Sunspot Number.

Documentation and scientific publications:

Detailed information about the various diagnostics and corrections applied to the Sunspot Number in version 2.0 can be found in:  

Clette, F., Svalgaard, L., Vaquero, J.M., Cliver, E. W., 2014: Revisiting the Sunspot Number. A 400-Year Perspective on the Solar Cycle, Space Science Reviews, Volume 186, Issue 1-4, pp. 35-103. DOI: 10.1007/s11214-014-0074-2
or as an ISSI book:
The Solar Activity Cycle, Space Sciences Series of ISSI, Volume 53. ISBN 978-1-4939-2583-4, Springer, pp. 35-103. DOI: 10.1007/978-1-4939-2584-1_3

Clette, F., Lefèvre, L., 2016, The New Sunspot Number: assembling all corrections, Solar Physics, 291, DOI: 10.1007/s11207-016-1014-y

Clette, F., Lefèvre, L., Cagnotti, M., Cortesi, S., Bulling, A., 2016: The Revised Brussels-Locarno Sunspot Number (1981 - 2015), Solar Physics, 291, DOI: 10.1007/s11207-016-0875-4

An entire issue of the Solar Physics Journal is dedicated to the recent recalibration of the Sunspot number:
Clette, F., Cliver, E.W., Lefèvre, L, Svalgaard, L., Vaquero, J.M., Leibacher, J.W., 2016: Preface to Topical Issue: Recalibration of the Sunspot Number, Solar Phys., 291, DOI: 10.1007/s11207-016-1017-8

All of the presentations given at the occasion of four dedicated Sunspot Number Workshops are also accessible at: http://ssnworkshop.wikia.com/wiki/Home

For specific technical questions, in particular, if you need to adapt automated data import software used for operational purposes, please contact us by e-mail at silso.info@oma.be