Seeing the Sun – Photosphere, Chromosphere and Corona
It is the largest and brightest object in the Solar System, but observations of the Sun are not straightforward. Ground-based solar telescopes operate during the day, when the atmosphere is warmer than at night, and combined with the Sun’s heat, the resulting air turbulence can greatly reduce these instruments’ performance.
Poor seeing can be improved by careful climate control. The largest solar telescope is the 1.6 m New Solar Telescope (NST) at the Big Bear Solar Observatory in California. Alongside a sophisticated cooling system, the NST benefits from its location on a mountain lake, with a stable atmosphere and cold water to lower the air temperature.
As in other fields of astronomy, atmospheric turbulence is overcome altogether if the telescope is in space. The Solar and Heliospheric Observatory (SOHO) has been studying the Sun from a heliocentric orbit since 1996.
Most solar telescopes detect visible light emitted from the Sun’s “surface", the photosphere. Its atmosphere – the chromosphere and corona – is visible only during a total eclipse, which astronomers can mimic using an instrument called a coronograph. When attached to a telescope, this blanks out the Sun’s disk.
The solar atmosphere can also be explored at wavelengths at which it is brighter than the photosphere. The chromosphere is clearly visible through filters for the light emitted by excited hydrogen or calcium atoms, while the SOHO is investigating the corona with an ultraviolet telescope. Radio waves, X-rays and gamma rays all contribute to the solar picture. Such observations were largely impossible before the advent of space telescopes, as most of this radiation is absorbed by Earth’s atmosphere long before it reaches the ground.
Using a spectrometer to measure the absorption and emission of radiation at these different wavelengths, the temperature, density and chemical composition of the Sun's visible layers have also been determined.