The overpowering brilliance of the photosphere - the Suns surface-normally prevents us from seeing the faint, thin solar atmosphere. Only during total eclipses, when the Moon passes directly in front of the Sun, is the atmosphere clearly visible from Earth. The solar atmosphere consists of two main regions, the chromosphere and the corona. Enormous eruptions and explosions called prominences and flares often rock these regions. For reasons astronomers do not fully understand, the corona is hundreds of times hotter than the photosphere. As a result, the Suns atmosphere is evaporating into space at the rate of a million tonnes every second.
Just above the photosphere lies the chromosphere a less dense layer of hydrogen and helium gas, mostly about 5,000 km thick. Nearest to the photosphere, the temperature is about 4,000°C, but it rises to more than 500,000°C at the top, where the chromosphere merges with the corona. Brush-like jets of gas, spicules, project from the corona. They rise from the edges of huge convection cells, where hot gas from the Suns interior rises and then sinks back beneath the surface.
Huge clouds and sheets of gas, or prominences, can extend upwards from the chromosphere stretching hundreds of thousands of kilometers into the corona. They are sculpted into vast loops of arches by magnetic fields over sunspot group. The gas may splatter down into the photosphere as coronal rain or erupt into space.
Streaming out from the corona into space is the solar wind. It consists of particles, such as electrons and protons, and the magnetic fields and electric currents that they generate. The strength of the solar wind varies with solar activity. It affects a region called heliosphere, which extends 15 billion km from the Sun. The solar wind passes the Earth at speed of between 300 and 800 km/s. The Earths magnetic field deflects most of the solar wind, but in the process the field is squeezed and drawn out into a long tail.
AURORA SEEN FROM SPACE
Auroras are striking displays of coloured lights that are sometimes seen over the Earths magnetic poles. They occur when solar wind particles are trapped by the Earths magnetic field and collide with molecules of air in the upper atmosphere.
Above the chromosphere and extending millions of kilometers into space is the corona the outermost region of the Sun's atmosphere. Even though temperatures can rise to more than
3 million °C, the corona is very faint, because the gas is extremely thin. Bubbles containing billions of tonnes of gas sometimes erupt from the corona, sending shock waves out into the solar wind.
Solar flares, violent explosions in the chomosphere above sunspot groups, are caused by a release of magnetic energy. They send out bursts of high-energy particles and radiation that can interface with radio communications on Earth when they strike the ionosphere the electrically charged layer of Earths atmosphere. Flares can endanger astronauts in space.