An Introduction to Helioseismology.

Imagine I give you an egg, now, without breaking it would you be able to determine whether this egg is raw, soft or hard boiled, or indeed just a hollow shell? Now imagine this egg is a million kilometres distant. Observing the interior of the sun is subject to similar difficulties hidden as it is within the shell of the 'photosphere'. As William Shakespeare rather more poetically puts it: 'The heaven's glorious Sun, // That will not be deep - searched with saucy looks.'

The technique by which we can learn about the internal structure of the Sun is comparable to tapping upon the egg. Like a hollow area behind a wall, a hollow egg will sound different to a hard boiled egg, and each will sound different to the egg when raw. Looking in more detail one will find that the reflected sound consists of a series of individual frequencies which reflect different possible ways in which the egg can vibrate; for instance all parts of the egg may oscillate inwards at outwards together, resulting in an oscillation like breathing into, and out from a balloon, or one half of a shell may move inwards while the other half moves outwards. Studying these oscillations (or 'modes') allows one to construct a profile of how the sound travel speed varies across the interior of the egg (or sun).

Of course we cannot actively 'tap' upon the sun, however it is fortunate that Nature has already provided us with a source of sound waves. Within the outer third of the Sun's radius there are 'convection cells' - cyclical oscillations of material which act to redistribute heat from interior to exterior layers. When uplifted material reaches the top layer of these it is moving rapidly, and will chaotically excite sound-waves. Waves of all frequencies are excited, but most will cancel out. Only those frequencies which correspond to the coherent global oscillations (described above) will have a large enough amplitude to be observed on the surface.

I work with a group here at Birmingham who observe these sound waves by looking at Doppler (velocity) induced shifts in the position of spectral lines. I find it amazing that we're able to observe these oscillations each of which has an amplitude of only about 25 cm, over a distance of 149 million kilometres - that is about a 600 billion times greater distance.



Next Time: Why bother learning about the Sun's interior? (An introduction to the Solar Neutrino Problem)