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.
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)