THE SUN - Sun's Atmosphere

SUN’S ATMOSPHERE

The overpowering brilliance of the photosphere - the Sun’s 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 Sun’s atmosphere is evaporating into space at the rate of a million tonnes every second.

CHROMOSPHERE

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 Sun’s interior rises and then sinks back beneath the surface.

SOLAR SPACECRAFT

Ulysses is a European Space Agency (ESA) craft launched by NASA in 1990 to study the solar wind. Its orbit takes it over the Sun’s polar regions, where it detects high-speed particle streams that do not usually flow past the Earth.

SOHO (Solar and Heliospheric Observatory), launched in 1995, is a joint ESA-NASA craft for observing the corona and solar oscillations. It is stationed about 1.5 million km from the Earth.

TRACE (Transition Region and Coronal Explorer) is a NASA craft launched in 1998 to study the corona and the boundary between the chromosphere and the corona.

PROMINENCES

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.

SOLAR WIND

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 Earth’s 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 Earth’s magnetic poles. They occur when solar wind particles are trapped by the Earth’s magnetic field and collide with molecules of air in the upper atmosphere.

CORONA

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.

FLARES

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 Earth’s atmosphere. Flares can endanger astronauts in space.