As active as was the Sun in September, as quiet is it now. Both the 10.7 cm solar radio flux and the International Sunspot Number are pointing to solar minimum.
Both the 10 cm radio flux and the sunspot number are an index for solar activity. The 10 cm 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 Ri equal to 0. The 10cm flux reached a maximum of only 83 sfu (solar flux unit) on October 04 and dropped to 72 sfu on October 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 and you immediately see the steady decrease of solar activity in the month October.
