The UV/EUV irradiance is the major energy source for the Earth's atmosphere and the variations cause changes in the photochemistry, dynamics, and the energy balance of the atmosphere and plays an important role in the formation and destruction of ozone. The variations in the short wavelength (UV and EUV) solar spectral irradiance are much larger than in the total solar irradiance. In X-rays the variations over a solar cycle can be up to a factor of 20. How the changes in the upper atmosphere can affect the thermosphere is yet unknown.
The image from the satellite Yohkoh shows a very active X-ray Sun (1991) on the left. The Sun on the far right (1995) is from the time of the solar minimum.
