X-RAYS FROM THE SUN

X-ray astronomy began in 1948 with the first detection of X-rays from the Sun. Astronomical X-ray observations need to be performed from hig h-altitude rockets and satellites because the Earth's atmosphere absor bs X-rays. Currently about 100,000 X-ray sources are known all over th e sky. The Sun is by far the strongest source. The outermost solar atm osphere, the corona, emits X-rays due to its high temperature of a few million K. Solar X-ray emission is highly variable. Eruptions lead to variations of the X-ray flux on time scales of minutes. The average X -ray flux varies with the 11-year sunspot cycle by a factor of about 1 000. Solar X-rays have a profound influence on the Earth's upper atmos phere.