The solar atmosphere

The solar atmosphere is generally considered to consist of three zones, known as the photosphere (light sphere), the chromosphere (colour zone) and the corona (crown). However, use of the term “atmosphere” can be misleading, because, in Earth terms, the atmosphere is understood to consist of the gassy layers that overlie the solid and liquid surface; the word “atmosphere” actually comes from the Greek for “sphere of vapour”. However, the Sun has no solid surface, as it consists entirely of gas and plasma, although the matter at the core is extremely dense.

When you see pictures of the Sun, with its sunspots, faculae and other moving features, what you are actually seeing is the top of the photosphere. This is the region from which comes most of the light that we receive on Earth, but it is relatively cool, with temperatures at around 5,500 degrees Celsius, as compared to those at the Sun’s core of something like 15 million degrees.

The photosphere is a relatively thin zone, being only about 250 miles (400 km) thick as compared to the Sun’s radius of 432,000 miles (695,500 km). The gases that comprise it are also very thin, being 10,000 times less dense than those of Earth’s atmosphere. However, it is extremely active, with its surface constantly on the move as it is driven by massive convection currents from deep within the Sun. Apart from sunspots and faculae (brighter regions), massive flares and prominences regularly erupt from the photosphere as sunspots collapse.

The chromosphere extends some 2,500 miles (4,000 km) above the photosphere. The colour in question is red, but the thinness of the gases comprising it (up to 10,000 times less dense than those of the photosphere) make it virtually transparent. Despite being further from the Sun’s core, the chromosphere is hotter than the photosphere, rising to around 8,300 degrees Celsius. Features of the chromosphere, as well as the flares and prominences mentioned above, include “spicules”, which are short-lived, fast moving jets of gas that shoot up from the Sun’s surface to a height of as much as 10,000 km.

The corona forms a thinly spread halo of gases that are even hotter than those of the chromosphere. There is a transition zone between the two in which the forces that dominate the lower levels, such as gravity, give way to the magnetic forces that determine the behaviour of the corona. In particular, the helium of the corona can become almost completely ionized (i.e. both electrons of the helium atom are lost), which enables the helium plasma to become very much hotter, to around three million degrees Celsius in fact. Although the corona starts at around 3,000 miles above the Sun’s surface, the inner corona is the second hottest part of the Sun, after the core.

The corona extends millions of kilometres into space, becoming thinner and cooler as it does so. However, the thickness of the corona varies considerably, often being almost absent at the poles.

A prominent feature of the corona are “coronal loops”, which are magnetic flux events anchored in the photosphere but forming elegant shapes that can extend high into the corona.

The corona is the source of the “solar wind”, comprising charged particles that are ejected at very high velocities that enable them to escape the Sun’s gravity and travel right through the solar system. These can cause geomagnetic storms on Earth (which affect power grids), give rise to aurorae such as the Northern Lights, and make the tails of comets point away from the Sun.

There is still much to be learned about the solar atmosphere, which is one reason why total solar eclipses are events that are highly valued by astronomers. At totality, the corona and chromosphere become visible, whereas at other times the brightness of the photosphere makes them invisible. However, the amateur observer must take very careful precautions when trying to view these phenomena for themselves during eclipses.

© John Welford