Sunspot cycle
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Sunspot numbers rise and fall with an irregular cycle with a length of approximately 11 years. In addition to this, there are variations over longer periods. These cycles are called the Sunspot cycle. The recent trend is upward from 1900 to the 1960s, then somewhat downward.[1] The Sun was last similarly active over 8,000 years ago.
The number of sunspots has been found to correlate with the intensity of solar radiation over the period - since 1979 - when satellite measurements of radiation are available. Since sunspots are dark it might be expected that more sunspots lead to less solar radiation. However, the surrounding areas are brighter and the overall effect is that more sunspots means a brighter sun. The variation is very small (of the order of 0.1%).
During the Maunder Minimum in the 17th Century there were hardly any sunspots at all. This coincides with a period of cooling known as the Little Ice Age.
Wainwright has speculated that there may be a resonant gravitational link between a photospheric tidal force from the [planets], the dominant component by summing gravitational tidal force (75%) being Jupiter's with an 11 year cycle.
Sunspot activity has a major effect on long distance radio communications particularly on the shortwave bands although medium wave and low VHF frequencies are also affected. High levels of sunspot activity lead to improved signal propagation on higher frequency bands. Although they also increase the levels of solar noise and ionospheric disturbances. These effects are caused by impact of the increased level of solar radiation on the ionosphere.
Since the mid 18th century sunspot cycles have been numbered. As of early 2008 we are seeing the end of "cycle 23" and the beginning of "cycle 24" [1].
