thermosphere/ionosphere

structure of the earth's atmosphere

variations over the solar activity cycle

While the intensity of solar visible radiation striking Earth varies by only 0.1% over the 11-year solar activity cycle, the shorter wavelength radiation that affects the upper atmosphere varies much more dramatically. Solar UV radiation varies by up to a factor of 4 (at the shorter UV wavelengths), while the intensity of X-rays can vary by factors of more than 100.