The New Sunspot Number: Assembling All Corrections

Abstract

Based on various diagnostics and corrections established in the framework of several Sunspot Number Workshops and described by Clette et al. (Space Sci. Rev. 186, 35, 2014), we now assembled all separately derived corrections to produce a new standard version of the reference sunspot-number time series. We explain here the three main corrections and the criteria used to choose a final optimal version of each correction factor or function, given the available information and published analyses. We then discuss the differences between the new corrected series and the original sunspot number, including the disappearance of any significant rising secular trend in the solar-cycle amplitudes after this recalibration. We also introduce the new version management scheme now implemented at the World Data Center Sunspot Index and Long-term Solar Observations (WDC-SILSO), which reflects a major conceptual transition: beyond the rescaled numbers, this first revision of the sunspot number also transforms the former static data archive into a living observational series open to future improvements.

Appendix A: New Schemes for Version Tracking and Official Validation

To manage future upgrades of the sunspot number in a structured way, the WDC–SILSO has now implemented the following version-tracking scheme:

• Successive versions are identified by a unique version number. This number is included in the file name and directory names containing the data.

• Only the latest version (highest version number) is distributed as the master series by WDC–SILSO. Only this version is maintained and extended by appending new sunspot numbers derived from observations from the SILSO network.

• All past versions remain accessible through a dedicated entry in the dedicated “Archive” section of the SILSO web portal. These past versions are mainly provided as reference, e.g. if a past published analysis needs to be reproduced to distinguish changes associated with the use of a more recent version of the sunspot number from other factors.

• Each version is documented, with a description of the file contents and information about the corrections applied relative to the earlier versions and, of course, with the release date. This information may consist of explanatory files stored with the data files, scientific publications describing the related analyses, and if relevant, ancillary data files needed to derive the corrections.

The version number consists of two numbers $\mathrm{N.n}$, where

• “N” is the main version number: it corresponds to any major modification of the $S_{\mathrm{N}}$-series leading to systematic changes in the values over long time-intervals.

• “n” is the sub-version number. It reflects either secondary changes that do not modify the primary $S_{\mathrm{N}}$-values (updated error estimates, file formats) or minor local corrections to the time series itself, without long-range influence on the global homogeneity of the series (correction or replacement of single values, e.g. due to isolated errors).

In this scheme, the original series is now labeled as version 1.0, while the current new release is numbered 2.0. We note that although in this transitional article, we kept the former $R_{\mathrm{i}}$ notation when referring to version 1.0, only the $S_{\mathrm{N}}$ symbol should be used in the future for all versions of the sunspot number, with the distinction between different versions being given by the version number itself (e.g. $S_{\mathrm{N}}$ V1.0). To avoid confusion and an excessive adaptation work for our users, we consider that new version releases should be reasonably spaced in time. A minimum interval of one year seems to be appropriate. This must provide enough time to prepare and validate a new modification.

Regarding this validation, the WDC–SILSO initiated discussions in the framework of the International Astronomical Union (IAU) for the creation of an advisory and accreditation scheme, which could take the form of an advisory committee or of expert working groups and workshops. These expert groups will serve as links between WDC–SILSO and the scientific community and provide a more formal endorsement of new updated versions of this reference solar index. Together with solar physicists, these groups can include data-processing experts and representatives of users. The role of these expert groups will be the following.

• The approval of new corrections to the sunspot number, proposed by WDC–SILSO or other research teams, after an in-depth assessment of the data and methods.

• Submitting suggestions and advice for new corrections and updates to the sunspot number, e.g. based on newly published results.

• Proposing additional data products or upgrades to services based on the sunspot-number data (data distribution, graphics, etc.).

• The designation of the sunspot-number series as one of the reference datasets in astronomy.

The details of this validation strategy are currently being settled in the framework of the IAU Inter-Division Working Group “Coordination of Synoptic Observations of the Sun”, in view of a progressive implementation in late-2016 and in the course of 2017.