The solar cycle is a measure of the periodic variation of the level of activity on the Sun. The most obvious
feature which changes are the number of sunspots visible on the surface of the Sun. These increase to a
peak and die away and then build up to another peak over a cycle which lasts roughly 11 years. The
Yohkoh sateliite was launched in 1991 and observed the Sun in X-rays providing a different view of the
Sun. Yohkoh observed the Sun for 10 years covering almost a full solar
cycle. The figure below shows an image of the Sunīs disk in X-rays every other month from the launch
of the Yohkoh satellite until summer 1998. What we are seeing is the changes in the atmosphere of the
Sun, called the corona. The bright regions seens are known as active regions and these are the source of
the most violent explosions in nature - solar flares. You can clearly see that the cycle is increasing
again during this time.
The solar cycle has been observed since the 1600s through the measurement of the number of sunspots. From 1645 for 70 years there was a period in which very few sunspots were observed (Maunder Minimum). There has been a link between the activity on the Sun and the earthīs climate although it remains controversial. One fact which is undeniable is that during the period of the Maunder Minimum there was a period of extreme cold on Earth known as the Little Ice Age. The climate changed so much that it has been recorded that people could ice-skate on the Thames in June!