Lots of fascinating science is performed during eclipse, and Dr. Druckmüller's photography has enabled at least one leading scientist to make some wonderful discoveries about the Sun's Corona. Dr. Shadia Habbal of the University of Hawaii has worked closely with him on many eclipse expeditions, gathering data for her research. Let's talk to them both about it now:
Yes, it was perfect. Unfortunately, part of the eclipse was obscured by clouds, but because the eclipse was rather long - over four minutes - so finally, the clear period was long enough to make everything which was planned. So it means [that] not only the white light photography, but [also] with the team of Shadia Habbal, I was making for Shadia these narrow-band filter images of iron lines. So about three minutes of unobscured Sun was enough.
Of course, it is not possible to study the coronal structures [visually], because these structures are in principle in the image - but the human vision is not able to see these structures, because the contrast of the computer display or printed image is something like 1:300, 1:500 at its maximum. But the contrast of the eclipse in reality is something like 1:1,000,000, or even more! It is necessary to have some stretch of this extreme dynamic range to the 8-bit representation, so some high-dynamic-range procedure; and without this stretch, is full of details which are completely unusable for human vision. It is necessary to remove the information which is not important, or not usable for human vision, typically for example, absolute brightness. Because human vision has no ability to measure the absolute brightness, we are able only to compare; the human vision is some sort of comparator which is comparing the pixel with the neighborhood of the pixel. And if we try to simulate these properties of human vision, so we are able to create something which is near to the sensation during the eclipse. So it was my first idea, and I spent let's say, 10 years to make programs which are able to make something like this. So, it is 36,000 [code] lines, written in Borland Delphi - so, in Pascal.
That's amazing! Let's now see how these images figure into Dr. Habbal's work!
The breakthrough came in 2006, when we were observing the corona from Libya. And there was one of these emission lines that nobody had imaged the sun in, but I knew that people had seen it in the spectrum of the corona, from records dating almost to the beginning of the 20th century. And I was curious to see if we could image the corona in this spectral line, and that was a line formed by Iron that had lost 10 of its electrons [Iron-11]. So, the other spectral lines that we had observed in '95, and [that] other people had been observing for a decade, were what we call Iron-10 lines - which means it's Iron that has been stripped of 9 of its electrons, and Iron-14 - which is Iron stripped of 13 electrons. So those, we managed to image in - but this Iron-11 line was a little tricky, because the wavelength was right at the edge of the visible: at 7892Å. We tried it at somewhat toward the end of the eclipse; we literally had 14 seconds (not a lot [of] observation)! So we captured the image, which was pretty remarkable because the emission was quite extended; that means it showed up quite far from the Sun. And it took us a while to understand why, so that was really the beginning of getting a deeper physical insight into processes going on in the corona, and it was also the beginning of my collaboration with Miloš.
We actually limit our observations to [a] 5Å band, where we can isolate the line. In his [Dr. Druckmüller's] observations he has a higher spatial resolution, so he sees much finer details than we do. But it doesn't really matter, because from our images compared to his, we get an idea of how the heavy elements are behaving, and they also tell us something about the temperature in the corona. So, through these observations we were able to more or less produce a map of the distribution of the temperature in the corona - which nobody had done before.
What we found was very curious. In this Iron-11 picture, there were regions of the corona that appeared dimmer in his pictures than white light, but they were brighter than their surroundings in this Iron line. Very strange, because you would think, well, everything you see in the corona should appear in this continuous image, but it turns out that was not the case when you came down to the Iron emission. Because there were regions of the corona where these heavy elements were behaving differently than the electrons. What we finally figured out [was that] it was due to the fact [that] the Sun connects to its heliosphere and all the planets through the solar wind. And the solar wind is made primarily of electrons and protons. But in addition, you also have some traces of Alpha particles - about 5% are Alpha particles. And the rest are [a] trace of heavy elements, such as Iron.
Ah, because it's so much more tenuous. So, it's like for example, let's say you take an iron rod, and you heat it up to some temperature. So you see it glowing. And the reason you see it glowing is because you have a lot of Iron in front of you. Now you can have the same temperature in an oven. You only feel the heat, but you don't see it glowing, because the particles inside the oven are - there are much, much fewer. So you can think of the corona in the same way, in the sense that its temperature is very high, but there are so few particles that the intensity of the light that's coming from the corona is a million times dimmer than the surface of the Sun. So, the only way you can see it is when you totally block the disk of the Sun; it's always there, but you cannot see it except if you dim the surface of the Sun. It's like you have to wait till nighttime to see the stars, but they're always there.
What a great explanation - thanks!
OK, now we're going to have a bit of fun, with a fond eclipse tradition. Not everyone does this, but for those who do - it's a necessary part of any eclipse experience! Please join us...for an egg cream!