5. A Magnetic Star
The photosphere and its magnetism
The photosphere is the lowest, densest level of the solar atmosphere. It is the place where most of the Sun’s energy escapes into space. The photosphere is the source of our visible sunlight, so it is appropriately named from the Greek word photos meaning “light”. It is the only part of the Sun that our eyes can see. Travelling at the speed of light, it takes only about 8.3 minutes for sunlight to get from the photosphere to the Earth.
All that sunlight comes from a thin, bright shell, only a few hundred thousand meters thick, or less than 0.05 percent (0.0005) of the Sun’s radius. At the distance of the Sun, that thickness corresponds to an angular width of a fraction of an arc-second, too small to be resolved with any telescope on Earth. All stars have photospheres, although they are not all so thin as that of the Sun.
Sunspots and faculae
To most of us, the Sun looks like a perfect, white-hot globe, smooth and without a blemish. However, detailed scrutiny indicates that the photosphere is often pitted with dark, ephemeral spots, called sunspots (Fig. 5.1). The largest spots can be many times bigger than the Earth. They can be seen with the unaided eye through fog, haze or cloud, when the Sun’s usual brightness is heavily dimmed. (You cannot look directly at the Sun without severely damaging your eyes.) Ancient Chinese records indicate that such rare, large sunspots were seen at least 2,000 years ago.
Sunspots appear dark because they are much cooler than their bright surroundings. The temperature at the center of a sunspot is about 2,000 degrees Kelvin lower than the surrounding gas in the photosphere, at 5,780 degrees Kelvin. While sunspots are dark, they still radiate light. A typical sunspot is about ten times brighter than the full Moon. So the appearance of sunspots is as deceiving as the seemingly “solid” photosphere.
Modern telescopes have revealed the detailed features of sunspots (Fig. 5.3). A simple sunspot has a dark center, called the umbra, surrounded by a lighter penumbra. The umbral area is a constant fraction (0.17 ± 0.03) of the total area of the spot. A fully-developed sunspot has a typical penumbral diameter of between 20 and 60 million meters, which can be compared with the Earth’s mean diameter of 12.7 million meters.
Islands of magnetism
When placed in a strong external magnetic field, the electrons in an atom interact with the magnetic field and adjust their energy. This energy change produces a shift in the wavelength of a spectral line emitted during the electron’s orbital transition. In effect, the orbiting electron gives rise to a tiny electric current with its own weak magnetism that points in one direction like a tiny compass. The interaction of the tiny internal magnetic field with the stronger external one results in an increase or decrease in the electron’s energy, depending on the varying orientation of the two fields.
As a result, a spectral line is split into two components displaced to either side of the normal line wavelength seen without an external magnetic field (Fig. 5.2). This so-called Zeeman effect was predicted by Hendrick Lorentz, and measured in the terrestrial laboratory by Pieter Zeeman; the two Dutch physicists shared the 1902 Nobel Prize in Physics for these investigations of the influence of magnetism on radiation.
In 1908 the American astronomer George Ellery Hale used the Zeeman effect to show that sunspots contain magnetic fields that are thousands of times stronger than the Earth’s magnetic field. The sunspot magnetic field strengths are as large as 0.3 Tesla, or 3,000 Gauss; in comparison the Earth’s magnetic field which orients our compasses is about 0.00003 Tesla, or 0.3 Gauss, at the equator. Such an intense sunspot magnetic field, sometimes encompassing an area larger than our planet, requires an electrical current flow of a million million (1012) amperes.
So, it is powerful magnetic fields that protrude to darken the skin of the Sun, forming dark, Earth-sized sunspots. The intense sunspot magnetism acts as a filter or valve, choking off the upward convection and outward flow of heat and energy from the solar interior. This keeps a sunspot thousands of degrees colder than the surrounding gas in the photosphere, like a giant refrigerator.
Nowadays, astronomers use magnetographs to portray the Sun’s magnetism. They consist of an array of tiny detectors that measure the Zeeman effect at different locations across the photosphere. Two images are usually produced, one in each circular polarization, and the difference of these images produces a magnetogram (Fig. 5.3). Strong magnetic fields show up as bright or dark regions, depending on their polarity,; weaker ones are less bright or dark. These magnetograms display the longitudinal component of the magnetic field in the photosphere, or the component which is directed toward or away from us. They chart the magnetic fields running in and out of the photosphere.
Most of the sunspots occur in pairs or groups of opposite magnetic polarity, and they are usually oriented roughly parallel to the Sun’s equator, in the east-west direction of the Sun’s rotation. Moreover, all of the sunspot pairs in either the northern or southern hemisphere have the same orientation and polarity alignment, with an exactly opposite arrangement in the two hemispheres (Fig. 5.3).
The Sun has a general, weak dipole magnetic field with a north and south magnetic pole of perhaps 0.001 Tesla in strength. If the north pole of the Sun has a positive magnetic polarity that points out of the Sun, then the westernmost leader spot of the pair in that hemisphere – the one that is ahead in the direction of rotation – will always be positive, the follower negative (Fig. 5.3). In the southern hemisphere, the polarities will be reversed. This orderly arrangement, known as Hale’s law of polarity, is described by a simple model in which the magnetic field gets amplified, coiled, and wrapped around inside the Sun, looping through the photosphere to make the bipolar sunspot pairs (Fig. 5.4). Such a magnetic field is must be generated by a solar dynamo located at the base of the convection zone.
The Sun’s magnetic activity cycle
The highly magnetized realm in, around and above bipolar sunspot pairs or groups is a disturbed area called an active region. Neighboring sunspots of opposite polarity are joined by magnetic loops that rise into the overlying atmosphere, so an active region consists mainly of sunspots and the magnetic loops that connect them.
All of this activity varies in step with the periodic 11-year change in the total number of sunspots. The existence of this sunspot cycle was first suggested in the early 1840s by Samuel Heinrich Schwabe, a pharmacist and amateur astronomer of Dessau, Germany, who diligently and meticulously observed the Sun for more than forty years, noting a decade-long variation in their total numbers.
In 1848 Rudolf Wolf, a Swiss astronomer from Zurich, Switzerland, introduced a relative sunspot number, R, that could be compared for all observers, and initiated an international collaboration that has fully confirmed the sunspot cycle. The periodic variation in both the number and position of sunspots has now been carefully observed for more than a century (Fig. 5.5). Moreover, Wolf’s reconstruction of the historical evidence indicated that the 11-year cycle in the number of sunspots has been present since 1700.
The number of sunspots visible on the Sun varies from a maximum to a minimum and back to a maximum in a cycle that lasts about 11 years, but can vary between 10 and 12 years. At the maximum in the cycle we may find 100 or more spots on the visible disk of the Sun at one time; at sunspot minimum very few of them are seen, and for periods as long as a month none can be found. The maximum number of sunspots in the cycle has also fluctuated by as much as a factor of two over the past century (Fig. 5.5), and the spots practically disappeared from the face of the Sun for a 70-year interval ending in 1715.
The positions of active regions, with their bright magnetic loops anchored in sunspot footpoints, also vary during the cycle (Fig. 5.5). Active regions form in two belts of activity, one north and one south of the solar equator. At the beginning of the sunspot cycle, when solar activity rises to its maximum, the belts of activity appear about one-third of the way toward each pole. The active region belts gradually drift from these mid-latitudes to lower ones as the cycle progresses, reaching the Sun’s equatorial regions at activity minimum. The active regions fizzle out and gradually disappear at sunspot minimum, just before coming together at the equator. The cycle then renews itself once more, and active regions emerge again about one-third of the way toward the poles.
Magnetograms indicate that there is still plenty of magnetism at the minimum of the solar cycle of magnetic activity, when there are no large sunspots present. The magnetism then comes up in a large number of very small regions spread all over the Sun (Fig. 5.6). The magnetic field averaged over vast areas of the Sun at activity minimum is only a few ten thousandths of a Tesla, or a few Gauss, but the averaging process conceals a host of fields of small size and large strength. When the telescope resolution is increased, we find evidence of finer and finer magnetic fields with higher and higher field strengths.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University