7. The Violent Sun
Predicting explosions on the Sun
The low solar corona is in a constant state of agitation and metamorphosis. Coronal loops are magnetically reconfigured as they twist and writhe in response to internal differential rotation and convection motions (Fig. 7.24). Yet, the coiled magnetic fields hold their energy in place, and remain without substantial change for days, weeks and even months at a time, like a rattlesnake waiting to strike. Then they suddenly and unpredictably go out of control, igniting an explosion that rips the magnetic cage open and breaks its grip apart.
Scientists may have discovered how to predict the sudden and unexpected outbursts. When the bright, X-ray emitting coronal loops are distorted into a large, twisted sigmoid (S or inverted S) configuration, a coronal mass ejection from that region becomes more likely (Fig. 7.25). In some instances, a coronal mass ejection occurs just a few hours after the magnetic fields have snaked past each other in a sinuous S-shaped feature. The mass ejection arrives at the Earth three or four days later. In the meantime, just after the mass had been expelled from the Sun, the X-ray emitting region dramatically changes shape, exhibiting the tell-tale, cusp-like signature of magnetic reconnection and a X-ray fading or dimming due to the mass removal. In other words, the magnetism gets stirred up into a complex, stressed and twisted situation before it explodes.
Now scientists can use sound waves to see right through the Sun to its hidden, normally-invisible, back side, enabling them to monitor active regions before they rotate to face the Earth. The new technique, dubbed helioseismic holography, examines a wide ring of sound waves that emanate from a region on the side of the Sun facing away from the Earth (the far side) and reach the near side that faces the Earth. When a large active region is present on the back side of the Sun, its intense magnetic fields compress the gases there, making them slightly lower and more dense than the surrounding material. A sound wave that would ordinarily take 6 or 7 hours to travel from the near side to the far side of the Sun and back again takes approximately 12 seconds less when it bounces off the compressed active region on the far side. When near-side, photosphere oscillations are examined by SOHO’s Michelson Doppler Imager, or MDI, they can detect the quick return of these sound waves.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University