Thursday, February 10, 2011

There's a hole in the sun?

And it's leaking plasma into space...

This rather dramatic photograph of the sun was taken on Feb. 1 by the Japanese-led Hinode (pronounced hi-node-ay) solar mission currently orbiting the Earth. The 5-year old spacecraft can see the sun in extreme ultraviolet (EUV) and X-ray wavelengths, so it is especially adept at seeing multi-million degree solar plasma.
In this image, two large dark "coronal holes" can be seen.

As I've mentioned in previous articles, the solar atmosphere -- or "corona" -- is hotter than the sun's surface. For want of a better analogy, it's like having a light bulb that heats the air surrounding the glass bulb hotter than the bulb itself -- quite frankly, that wouldn't make much sense. The coronal heating mystery is still not solved, although solar physicists are closing in on possible heating mechanisms.

In this image, the spacecraft has imaged the solar corona in X-ray wavelengths, so only the hottest coronal plasma can be seen. The cooler plasma appears black in comparison.

The solar surface is twisted with magnetic field lines and within those magnetic loops (known, unsurprisingly, as "coronal loops") solar plasma is trapped, accelerated, heated and pulled back to the solar surface -- producing a phenomenon called "coronal rain."

These vast areas of magnetic twisting and trapped plasma heating are known as "active regions." A couple of active regions are prominent here, one of which is active right above a sunspot (as confirmed by NASA's Solar Dynamics Observatory) -- seen slightly below center-left.

But what are those huge black regions called, where there appears to be no hot plasma?
You guessed it, these are "coronal holes" and Hinode can see two huge cool coronal hole regions, one in the top-center of the image and the other at the bottom, crowning the polar region.

These regions represent open magnetic field lines where solar plasma is being blasted into space. They act almost like a hosepipe, funneling solar plasma from the sun's interior into the fast solar wind, eventually reaching to the furthest-most reaches of the solar system.

But why are coronal holes so important?

Solar missions like Hinode, the ESA/NASA Solar and Heliospheric Observatory (SOHO), NASA's Solar Dynamics Observatory (SDO) and NASA's Transition Region and Coronal Explorer (TRACE) all keep a close eye on coronal holes as they are right at the start of the space weather chain. Space weather prediction is critical to our hi-tech civilization, so knowing where these high-energy particles and powerful magnetic fields come from is of increasing importance.


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