SOHO's 'X-Ray Vision'

The team of SOHO's Michelson Doppler Imager (MDI) has refined the art of 'inverting' the Sun's oscillations and is now presenting the first set of images from the Sun's entire farside.

The sun is filled with many kinds of sound waves which are caused by the boiling motions of gas in its surface layers. SOHO's Michelson Doppler Imager (MDI) traces these sound waves and so opens up the Sun's interior to inspection. The surface oscillations generated by the sound waves can be interpreted in much the same way as earthquake waves, which reveal the secrets of the interior of our own planet.

Most spectacularly, SOHO can see right through the Sun to detect sunspots on the farside. This works as follows: The sunspots' strong magnetic fields speed up sound waves. As a result, sound waves from a group of sunspots on the farside arrive faster at the visible surface than equivalent waves coming from sunspot-free regions. Analyzing this information with a technique called helioseismology , one can build up a picture of the Sun's farside. This is a form of 'acoustic X-ray vision', similar to ultrasound scans in medicine.

MPEG movie (2.1 MB)

High-res JPEG image (70 KB)
High-res TIFF image (1.8 MB)
Small MPEG movie (1 MB)
Large MPEG movie (3 MB)

Upper left: This movie shows a sequence of "Dopplergrams" - maps of motion made by observing the Doppler effect. Light areas are moving up and darler areas are moving down. The region shown is about 1/6 the diameter of the Sun wide. The motions oscillate with a period of about 5 minutes. (2.1 MB MPEG movie)

Lower left: This diagram shows some sample sound wave paths connecting the front or Earth side of the Sun with the back or far side. Sound waves are generated by small convection cells (typically the size of France) at the visible solar surface. When these so-called "granules" cool and fall back into the interior they cause sound waves to move into the Sun in all directions. There are about 7000 such wave sources going off each second. This diagram shows the path of some of the waves from one of these starting on the front side. As the waves move into the Sun they get refracted (bent out) back to the surface because the sound speed is higher deeper into the Sun. When they get to the surface (visible layer, photosphere) they are reflected back into the Sun. When they get reflected the visible layer recoils (moves) and we are able to measure that movement.

On the right: This image shows two active regions crossing the solar east limb in November 2003. The right side, in yellow, are white light images showing sunspots. The left side (blue) shows the prediction of sunpots on the farside. Since the white light observations are images made "straight on", they are stretched into blurry lines when they are projected to show the view over the limb. This is simply because we do not have a camera above the east limb (yet).


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