There are three main reasons for including the group number:
- As both the group number and sunspot numbers are derived from the same base sunspot information, those two solar indices are intimately related. Moreover, as various analyses have shown that a different solar information is contained in the Sunspot and Group numbers, it is thus logical to provide a common access to both numbers.
- The group number series was recently submitted to a full revision, like the Sunspot Number. However,this parallel process was not organized by a Data center like in the case of the Sunspot Number. Therefore, so far, the new proposed versions of the group numbers were not made easily accessible from a central place.
- In coming months, the WDC-SILSO will start routinely producing Group Numbers by the same method as the Sunspot Number and using the same base observations from our worldwide network of observers. This will thus extend the original group number series, which was limited to its production date (1995 for the original V1.0 series), eventually making the Group Number an integral product of our WDC.
Therefore, for the convenience of our users, we gather here the various existing group numbers in a single page, with all original series converted to a similar standard file format. However, as the group number is not an internal production of the WDC-SILSO, for any question about one of the series, we invite our users to consult the indicated references and to contact the authors of the series.