Solar variations and climate on planets

The Sun is a variable star on different time scales. During its evolution it considerably changed its luminosity and activity. The early Sun was faint but very active and this had important consequences for the evolution of the early planetary atmospheres. The importance of enhanced early solar activity at those evolutionary phases will be demonstrated by a short analysis of two recently observed solar enhanced activity events. From these events one can infer the importance of sunspots as indicator of solar activity and the solar variation in UV during times of enhanced sunspot activity was measured. The solar input is the driver for climate on planets.. It is extremely important to have accurate estimates of a variation of this input. At present, the variations are low in the visible but still high in the UV influencing e.g. higher layers in the Earth's atmosphere.