The corona is the outer layer of the Sun's atmosphere. It extends far into interplanetary space.
Earlier the optical radiation of the corona could be observed only during a total solar_eclipse. Today we have special telescopes (coronographs) with minimum scattered light ratios, with which the corona can be observed outside of these times. Satellites and space stations, such as OSO and Skylab, have provided even more detailed information.
The corona has a ray-shaped form, which is probably caused by the magnetic_field. It undergoes cyclic changes that are probably associated with the eleven-year sunspot_cycle. When sunspot activity reaches its peak, the corona appears almost round. During periods of minimal sunspot activity it is elongated, stretching out mostly along the equator and, to a much lesser extent at the poles. The shape of the corona changes during the intervals between these cycles. The corona is much hotter than the photosphere or the chromosphere. Temperatures here range from one to two million degrees Kelvin. In order to maintain this heat, energy must constantly be supplied. How this takes place has not been ascertained. What is known is that there are so-called magnetic loops inside the corona, which are associated with the supply of energy.
Because of the intense temperatures, particles of the corona can reach speeds high enough to escape from the corona. These expelled gases are referred to as the solar_wind. It consists of charged particles ( electrons and protons ) moving away from the Sun at a speed of 3 million kilometers per second These create electrical currents and magnetic_fields that fill the otherwise seemingly empty space of the solar system.
According to information provided by space probes, the speed of the solar wind particles in the vicinity of the Earth is 400 kilometers per second.
