Solar Spectrum

By Richard Harrison

The Solar spectrumThe Sun radiates light across a broad range of the electromagnetic spectrum. The greatest intensity is found in the visible range, which we can see with our eyes. However, the Sun emits what we call a 'continuum', a broad band of electromagnetic radiation running through the ultraviolet to the infrared. Every object, be it a star or even a person, radiates forms of light that are dependent on the temperature of the object. The hotter the object the more it will radiate towards the X-ray/gamma ray end of the spectrum. The light is emitted by the collision of particles, which make up the object. The Sun's spectrum peaks in the visible part of the electromagetic spectrum because the photosphere of the Sun has a temperature of about 5,800 kelvin.

However, for the Sun, this is not the full story. One of the amazing features about it is the fact that its outer atmosphere is much hotter than the photosphere. The tenuous gases, which make up the solar atmosphere, radiate in X-rays because they are at millions of degrees.

The radiation from the Sun also contains patterns, called spectral lines, which are due to atoms changing energy by absorbing or emitting electrons.

As far back as 1814, Josef von Fraunhofer detected hundreds of dark lines in the Sun's spectrum. It was deduced that these were lines from atoms in the solar atmosphere. Studies of these patterns can allow scientists to identify the different gases that make up the Sun and, in the late 1800s it even led to the discovery of helium by the Englishman Sir Norman Lockyer and Frenchman P.J.C. Janssen.

Today, we can measure and analyse the spectral lines from all layers of the Sun's atmosphere, using observations - in ultraviolet and X-rays in particular - from spacecraft. This is an area in which UK solar scientists have forged a world-class research programme with experiments on missions such as NASA's Solar Maximum Mission (SMM), the Japanese Yohkoh mission and the ESA/NASA Solar and Heliospheric Observatory ( SOHO). Studies of the intensities of the lines and their shape and location allow us to determine much about solar material, such as densities, temperatures and wind speeds, and to find out what the material is made of. We can map the properties of the solar atmosphere from a distance; this is effectively solar meteorology!