The new Sunspot Number: re-calibration, re-computation and implications for the solar cycle
Abstract
Our knowledge of the long-term behaviour of the solar cycle and the occurrence of Grand Minima rest largely on the 400-year sunspot record, currently in the form of two time series: the Sunspot Number and the Group Number. Recently, a full revision of those two series allowed to identify and correct various inhomogeneities, thereby also eliminating most of the discrepancies between those two parallel series.We report here about the changes in those two series and one of the main implications: the absence of a progressive rise of solar activity from the Maunder Minimum to a modern maximum in the 20th century. We also focus in particular on the important corrections applied over the last 50 years, in particular a variable drift in the scale of the Specola Solare station (Locarno) that defined the long-term scale of the International Sunspot Number over the last 35 years, i.e. over the period when modern measurements of solar irradiance and solar wind particles can be correlated with the Sunspot Number for building long-term backward reconstructions of those physical parameters. Taking advantage of the archive of the World Data Center SILSO (270 stations, 550,000 observations), we could entirely re-compute the Sunspot Number. We describe the properties of the new resulting series and the new method developed to build a more stable multi-station reference for the Sunspot Number.We conclude on the release of the new reference Sunspot Number and the simultaneous adoption of new conventions and standards (error estimates, version tracking and documenting). We also consider the next steps that will allow future progresses in the characterisation of the solar cycle: the digitisation of historical drawings and the creation of image-based sunspot indices that will allow adding spatial information, to extend the one-dimensional information brought by the Sunspot Number.
- Publication:
-
IAU General Assembly
- Pub Date:
- August 2015
- Bibcode:
- 2015IAUGA..2256393C