Their results appear this week in Science. The study supports the idea that sunspots occur as hot gases rise to the surface of the Sun and spread out, cool and then flow back into the star.
This convective flow creates the pimple-like pattern of a sunspot, according to the study. Sunspots are only dark in contrast to the bright face of the Sun. If you could cut an average sunspot out of the Sun and place it elsewhere in the night sky, it would be about as bright as a full moon. Sunspots have a lighter outer section called the penumbra, and a darker central region named the umbra. Sunspots are caused by disturbances in the Sun's magnetic field welling up to the photosphere, the Sun's visible "surface".
Sunspots occur because the sun isn't a hunk of rock like the Earth and the inner planets, but a ball of continually circulating hot gases that doesn't move in one piece. The interior and the exterior of the sun rotate separately; the outside rotates more quickly at the equator than at the solar north and south poles. Specifically, a point on the equator takes 25 Earth days to go around, while a point near one of the poles takes 36 days to complete its rotation. Over time, all that messy and uneven movement twists and distorts the sun's main magnetic field in the same way that your bed sheets get wrinkled and bunched up when you toss and turn in your sleep.
The bunched up spots -- actually twists in the magnetic field lines -- have so much magnetic power that they push back the hot gases beneath them and prevent the heat from rising directly to the surface.
In other words, they become sunspots. Because sunspots are cooler than the rest of the sun's surface, they look darker. At the same time, the hot gases blocked by these sunspots flow into the areas around them, making those areas even hotter and brighter than normal. What Schwabe observed was that, although individual spots are short lived, the total number visible on the Sun at any one time was likely to be very much greater at certain times—the periods of sunspot maximum —than at other times—the periods of sunspot minimum.
We now know that sunspot maxima occur at an average interval of 11 years, but the intervals between successive maxima have ranged from as short as 9 years to as long as 14 years. During sunspot maxima , more than spots can often be seen at once. During sunspot minima , sometimes no spots are visible.
The solar magnetic field is measured using a property of atoms called the Zeeman effect. Recall from Radiation and Spectra that an atom has many energy levels and that spectral lines are formed when electrons shift from one level to another. If each energy level is precisely defined, then the difference between them is also quite precise. In the presence of a strong magnetic field, however, each energy level is separated into several levels very close to one another.
The separation of the levels is proportional to the strength of the field. As a result, spectral lines formed in the presence of a magnetic field are not single lines but a series of very closely spaced lines corresponding to the subdivisions of the atomic energy levels. This splitting of lines in the presence of a magnetic field is what we call the Zeeman effect after the Dutch scientist who first discovered it in Figure 3.
Zeeman Effect: These photographs show how magnetic fields in sunspots are measured by means of the Zeeman effect. Measurements of the Zeeman effect in the spectra of the light from sunspot regions show them to have strong magnetic fields Figure 3. Bear in mind that magnets always have a north pole and a south pole. Whenever sunspots are observed in pairs, or in groups containing two principal spots, one of the spots usually has the magnetic polarity of a north-seeking magnetic pole and the other has the opposite polarity.
Moreover, during a given cycle, the leading spots of pairs or leading principle spots of groups in the Northern Hemisphere all tend to have the same polarity, whereas those in the Southern Hemisphere all tend to have the opposite polarity. During the next sunspot cycle , however, the polarity of the leading spots is reversed in each hemisphere. For example, if during one cycle, the leading spots in the Northern Hemisphere all had the polarity of a north-seeking pole, then the leading spots in the Southern Hemisphere would have the polarity of a south-seeking pole.
During the next cycle, the leading spots in the Northern Hemisphere would have south-seeking polarity, whereas those in the Southern Hemisphere would have north-seeking polarity. Therefore, strictly speaking, the sunspot cycle does not repeat itself in regard to magnetic polarity until two year cycles have passed. Figure 4. Magnetogram and Solar Cycle: In the image on the left, called a magnetogram, we see the magnetic polarity of sunspots.
This dramatic sequence on the right shows the activity cycle of the Sun.
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