Thursday, April 27, 2006

The Solar Corona Problem

It's all been a bit abstract a bit lately. So I thought I'd bring things down to Earth a bit. But only metaphorically speaking.

I've decided to volunteer some time at the Chabot Space and Science Center. I'll mainly be manning their 20" reflector pictured here:

Basically I'll be something like a tour guide for visitors, answering any questions they may have. Only during daylight hours for now. It's a good excuse to read up a little on astronomy and if I stick at it then one day I'll be qualified enough to work the telescope at night, and maybe even catch a few photos through it with my own camera.

Anyway, one of the visitors was asking about the temperature of the Sun. As any photographer can tell you, the surface of the Sun has at a temperature of 5800K. But observations during eclipses show that the corona has some pretty weird spectral emission lines that correspond to atoms at very high temperatures, of the order of a few million K. Strangely, this is a bit of a mystery. At first you might think what's the problem? There's a gigantic fusion reactor with something like 2×1030kg of fuel burning right next to it. But the problem is this - how is that energy getting to the corona when the surface of the sun is at 5800K. As everyone knows, heat doesn't generally flow from a cooler region to a hotter one. To me this is much more interesting than the Solar neutrino problem. Neutrinos don't play a large part in anyone's lives and all of the evidence that there is even a problem rests on indirect evidence and theoretical work. But the solar corona problem is pretty immediate. You can literally see the temperature of the sun's surface and the anomalous high temperature emission lines require nothing more abstract than spectrography. When a visitor asked about this I had no answer so I decided to read up on the issue.

Turns out there are a few possible explanations, though none of the corresponding predicted physical proceses have been directly observed yet. My money is on Alfvén waves. These are essentially waves that form solutions to the combined Maxwell and Navier-Stokes equations. If you imagine wave travelling down a whip when it is cracked, all that enerfy gets to the end of the whip and gets concentrated in the tip resulting in it moving at supersonic speeds. Something similar may be happening with these waves. They proceed out from the sun but when they get to the corona there's nowhere for the energy to go except into heating it to incredible temperatures.

Anyway, next time someone asks about the corona I won't be telling them this. Most people don't want this much detail and even if they did, they have impatient kids with them. But it's good to have people prodding me to read up on interesting physics.

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