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SIDC-team, World Data Center for the Sunspot Index, Royal Observatory of Belgium, Monthly Report on the International Sunspot Number, online catalogue of the sunspot index: http://www.sidc.be/sunspot-data/, 1700-2012.
"The abundance of sunspots on the sun varies on timescales from a few hours to many years. Historically, an index called the 'sunspot number' has been used to quantify the abundance of spots. This index is still in wide use today, although for some purposes it has been replaced by more readily and consistently measures indices such as the 10 centimetre solar flux. The main advantage of the sunspot number is that it is the only index for which we have a long and detailed historical record.
Sunspot Number (here denoted R) is defined as:
R = K (10G + I)
where G is the number of sunspot groups visible on the sun; I is the total number of individual spots visible; and K is an instrumental factor to take into account differences between observers and observatories.
Sunspot Number as an index can be defined on a daily basis but because of the large day-to-day variation is usually averaged over longer periods, the most common being the monthly and the yearly average. When averaged over a year, the sunspot number varies smoothly charting the progress of the solar cycle. On the other hand the daily and the monthly averages exhibit considerable variation with respect to the yearly curve. This variation is due to bursts of rapid solar region growth often associated with solar flares and other interesting events.
The most widely quoted average sunspot number is the Zurich number (Rz) which was replaced from January 1981 with the International Sunspot Number (RI). The American Sunspot Number is another series to which the IPS Culgoora Observatory contributes its observations." by Richard Thompson, Australian Government Bureau of Meteorology