Which time tag should be attached to each daily International Sunspot Number?
The daily sunspot number is based on an average of all Wolf numbers reported during the 24h time bin of a UT day, i.e. between 0h00 and 24:00 UT.
Therefore, the most logical time tag for each daily sunspot number is 12h00 UT.
Moreover, this time is also reflecting the current distribution of stations in the worldwide SILSO network. Indeed, by an historical heritage, there is a higher concentration of stations in Europe. Most of these, including the Locarno pilot station, thus actually observe around the middle of the UT day. The resulting index, which involves a time averaging over a whole day, is thus weighted in favour of the actual sunspot status near 12h00 UT.