On the road to solar minimum
As active as was the Sun in September, as quiet is it now. Both the 10 cm flux and the International Sunspot Number are pointing to solar minimum.
Nederlandstalige versie /Version franaise posted on Nov 8, 2005

Both the 10 cm radio flux and the sunspot number are an index for solar activity. The 10cm solar radio flux is a measure for the noise emitted by the Sun at a wavelength of 10.7 cm (frequency around 2800 MHz) and is measured here on earth.The international Sunspot Number is a weighted mean of all Wolf numbers observed by different observatories. The International Sunspot Number is calculated at the SIDC, the World data center for the Sunspot Index. This index for solar activity has a rich history that goes back to the beginning of the 17th century when Galileo Galilei started to do regular sunspot observations.

An indicator for the extreme low activity level was the number of spotless days in the month October 2005. In the provisional International Sunspot Numbers, we have 6 days with a R i equal 0. The 10cm flux, reached a maximum of only 83 sfu (solar flux unit) on Oct 04 and dropped to 72 sfu on Oct 26 and 27. This fluctuation of only 9 sfu over the whole month is really small compared with the flux variation in September, when the maximum was 118 sfu and the minimum 72 sfu. Just have a look at the graph on the left and you immediately see the steady decrease of solar activity in the month October.