Sun struck



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Roast in peace? Not when the sun's in a stormy mood, as it is nowadays.

























Pressure waves bounce around. The fewer the bounces, the more info they bring to the surface. In two weeks, the magnetic disturbance at the focus may rotate around in a position to bombard Earth with sun trash.

Courtesy Douglas Braun, Colorado Research Associates.


When objects vibrate, the motion tells us what's going on inside.Good vibrations
The greenest pasture in sun studies is helioseismology, the study of seismic vibrations. Just as seismic waves are the basic source of information about the Earth's interior, pressure waves in the sun tell us about the forbidding interior of our local star.

When objects vibrate, their motions carry clues about what's going on inside. A sea horse's whine (if it had one) would pass faster through water than a horse's whinny passes through air. Sound waves, and all other pressure waves, reflect the pressure, temperature and composition of whatever they pass through. Ultrasound images show a heart or developing fetus. In seismology, the study of the Earth's interior, the velocity and direction of pressure waves caused by earthquakes all reflect conditions inside the Earth.

photo of sun: Orange disk with white magnetic storms and loops of stuff on the edge.The notion of using pressure waves to study the sun was broached in 1970, but recording those waves proved a major technical challenge since they rise and fall only about 20 to 40 kilometers. That's the thickness of a straw compared to the sun's diameter of 1.5 million kilometers.

You read it here -- it's red-shifted
Detecting these waves depends on the Doppler effect -- which changes the tone of sound and light waves depending on whether the source is approaching or receding. Doppler -- the "train whistle" effect -- also tells us that the universe is expanding. We know that distant stars and galaxies are receding from us because their light is "red-shifted," meaning the waves are stretched out so they seem redder than they otherwise would.

Doppler images, taken from the Solar and Heliospheric Observatory, tell us whether the parts of the sun's surface are moving closer or further, explains Lindsey. "If you look at any point, all spectral lines are red or blue shifted, depending on whether it's moving toward or away from you."

SOHO Doppler instrument tells that the sun's surface tends to rise for a few minutes at a velocity of half-a-kilometer per second, then it falls again. A full stroke moves the surface roughly 40 kilometers.

The global perspective on helioseismology looks at resonant frequencies extracted from the millions of vibration patterns in the sun. Only a few dozen really matter for determining what's deep inside -- the global perspective. It's like an organ pipe, Lindsey adds. "You don't hear the local details, but rather the whole organ pipe, which rings at a certain frequency."

Waves from a focus on one side of the sun enter the interior, and bounce before appearing on the surface on the opposite side.Then comes the gruesome part -- using "spherical harmonics" (just the name sends us Why Filers scurrying for our math texts!) to distinguish and analyze these waves. It sounds simple when Sylvain Korzennik, a solar physicist at the Smithsonian Center for Astrophysics, describes the process. "You take the image and multiply it by special functions and get amplitudes of normal modes, the basic waves." By keying on these frequencies, or modes, scientists can determine the sun's large-scale internal structure, just as seismologists used pressure waves to locate Earth's core and mantle.

Now, Lindsey and colleague Douglas Braun have driven the process one furlong further, using a more local perspective. They are using pressure waves not just to infer internal conditions, but also to figure out what's going on the far side of the sun.

Why worry about the far side? Recall that mass ejections can damage satellites and power grids. Factor in the sun's habit of rotating every 27 days or so, and the fact that mass ejections near the center of the sun's disk -- as we see it -- are most dangerous to us.

Add it up, and you'll see that magnetic storms on the far side may rotate into a threatening position two weeks later. Presently, storms can only be detected when they are one-quarter rotation -- one week away from disk center -- from a dangerous position. Detecting magnetic storms on the far side would double the warning time, and. for astronomers, that's about as practical as things get!

In March, Lindsey and Braun reported that, after 10 years' effort, they had finally tracked the pulsations back to the other side of the sun (see "Helioseismology..." in the bibliography). And while they can only detect magnetic anomalies at least 2 trillion square kilometers in size, the whole point is to warn of big mass ejections.

What powers these giant mass ejections? Rubber band engines?


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