Astronomy

The Structure of Sunspots



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"The Structure of Sunspots"
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A sunspot is an irregular and temporary area on the sun's visible surface which appears dark against the yellow of the sun. Although they seem small in comparison to the sun, the average sunspot is twice the size of our earth. At the other end of the scale, we don't really know how small a sunspot can be, because the smallest ones we can observe are barely visible by telescopes. Their lifespan ranges from hours to months.

The part of the sunspot visible on the photosphere is now understood to be a depression in the photosphere. The remainder of the sunspot is believed to be a magnetic flux tube, created by differential rotation in the convective zone of the sun. SOHO sound wave observations have shown the existence of powerful magnetically-linked downdrafts under each sunspot, concentrating magnetic lines to form a rotating vortex. The whole could be compared to looking up the vortex of a tornado, where magnetic forces replace wind shear. Because of this magnetic link, prototypical sunspots always come in oppositely magnetised pairs. The polarity of the sun reverses with every sunspot cycle, so the relative polarities of leading and trailing sunspot change as well.

Where the sunspot punctures the photosphere, the internal stress begins to ease. With the decrease in energy flux comes also a decrease in temperature at that location, as much as 1500 Kelvin degrees (25%) cooler than the surrounding photosphere.* The visible part of the sunspot has a penumbra surrounding a fairly sharp, central umbra, which, despite the name, have nothing to do with shadows except their darkness. The umbra is the coolest and darkest part, the penumbra less dark, but still darker than the photosphere. High resolution images show thread-like structures within the penumbra, following the more horizontal alignment of the magnetic field lines relative to the vertical alignment of the centre.

One of the apparent contradictions in sunspot structure is that although the umbra is cooler, heat does not flow out beyond the magnetic field to make that area of the photosphere hotter. Then again, the immense heat energy which creates a tornado also does not leave its surroundings hotter. Using this analogy suggests that the 'missing' heat energy could be what powers the vortex.

* Because Kelvin degrees are measured from absolute zero, it allows absolute percentages such as this one. In contrast, our familiar Celsius and Fahrenheit scales don't allow percentages the same way, since the 0 point is set to an arbitrary value.

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