This area contains a series of occasional articles by SWS staff and their colleagues.
For general terms and definitions refer to our Solar-Terrestrial glossary.
Commencement of Solar Cycle 24
The appearance of a reverse polarity sunspot at the relatively high latitude of 30 degrees north on 4 January, 2008 has been accepted by space scientists as the first sign of solar cycle 24. At the time this indicated solar minimum was not too far off and in fact occurred in December 2008.
The Solar Cycle
The Sun exhibits a periodic cycle of activity over approximately 11 years called the solar cycle. The number of sunspots is used to gauge the extent of this activity and the sunspot number is a count of sunspots and sunspot groups on the visible disc of the Sun.
A solar cycle begins at solar minimum with little solar activity and few sunspots on the Sun. As the cycle evolves increasing numbers of sunspots appear and events such as flares and coronal mass ejections increase in number. The peak in the sunspot number is called solar maximum. Solar activity then gradually declines returning to solar minimum and the end of the cycle. The plot of sunspot number vs time shows the progression of the cycle.
Sunspots and the Butterfly Diagram
A few months prior to solar minimum there are likely to be sunspots associated with both the new and old cycles. Those associated with the old cycle appear near the solar equator while new cycle sunspots emerge at higher latitudes (30 to 50 degrees). Around solar minimum the old cycle sunspots eventually disappear leaving only the new cycle spots. As the new cycle passes through solar maximum towards the next solar minimum and its end, sunspots emerge closer to the equator. A plot of this progression in sunspot location over the solar cycle from higher to lower latitudes is called the butterfly diagram for obvious reasons.
Satellite Eclipse Season
For about 21 days either side of the March and September equinoxes, satellites in geo-stationary orbits pass through the Earth's shadow. Read more on satellite eclipses.
Geo-stationary satellite transmissions to Earth are subject to interference from the Sun around the equinoxes when the Sun, satellite and Earth receivers line up. An Earth receiver captures not only the satellite signal but also unwanted signal from the Sun (noise). The noise strength can overwhelm the satellite signal strength. Learn more and see when interference is likely with the SWS Sun-Satellite Interference Prediction Tool.