5. A Magnetic Star

A Magnetic Star

The entire Sun is a giant mass of incandescent gas, unlike anything we know on Earth. The Sun has no surface; its gas just becomes more tenuous the farther out you go. Although we cannot see it with our eyes, very diffuse solar gas extends all the way to the Earth and beyond.

The diaphanous solar atmosphere includes, from its deepest part outward, the photosphere, chromosphere and corona. The Sun’s magnetism plays an important role in molding, shaping and heating the coronal gas.

The visible photosphere, or sphere of light, is the part of the Sun we can watch each day. It is the level of the solar atmosphere from which we get our light and heat. The photosphere contains sunspots, thousands of times more magnetic than the Earth, and the number and position of sunspots varies over the 11-year cycle of solar magnetic activity.

The visible sharp edge of the photosphere is something of an illusion. It is merely the level beyond which the gas in the solar atmosphere becomes thin enough to be transparent. The chromosphere and corona are so rarefied that we look right through them, just as we see through the Earth’s clear air.

The chromosphere is very thin, but the Sun does not stop there. Its atmosphere extends way out in the corona, to the edge of the solar system. The corona’s temperature is a searing million degrees Kelvin, so hot that the corona is forever expanding into space.

The entire solar atmosphere is permeated by intense magnetic fields generated inside the Sun, rooted in the photosphere, and extending into the chromosphere and corona.

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

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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.

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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.

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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).

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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.

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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.

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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.

The solar chromosphere

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Just above the photosphere lies a relatively thin layer, about 2.5 million meters thick, called the chromosphere, from chromos, the Greek word for “color”. The chromosphere is so faint, and the underlying photosphere so bright, that the chromosphere was first observed during a total eclipse of the Sun. It became visible a few seconds before and after the eclipse totality, creating a narrow, rose-colored band at the limb of the Sun, punctuated by extended curls of red (Fig. 5.7).

The Sun’s temperature rises to about 10,000 degrees Kelvin in the chromosphere, but the gas density in the chromosphere drops to roughly a million times less than that of the photosphere. Because of its very low density, the chromosphere does not create absorption lines. Instead of absorbing radiation, the tenuous gas is heated to incandescence and emits spectral lines. Whenever the light from the chromosphere is isolated, during a total solar eclipse or by other means, we see bright emission lines shining at precisely the same wavelengths as many dark absorption lines in the photosphere’s light.

Dark Regions And Bright Plage

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Sunspots extend from the photosphere into the chromosphere, creating dark regions in hydrogen-alpha photographs (Fig. 5.8). Bright regions, called plage from the French word for “beach”, glow in hydrogen light; they are often located near sunspots in places with intense magnetism. The plages are a chromospheric phenomena detected in monochromatic hydrogen-alpha light; they are associated with, and often confused with, bright patches in the photosphere, called faculae, that are seen near the solar limb in white light.

Spicules

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The chromosphere is jagged, irregular and by no means smooth or homogeneous. When observed in hydrogen alpha, numerous thin, luminous extensions, dubbed spicules by Father Angelo Secchi, may be seen (Fig. 5.9). They rise and fall like chopping waves on the sea or a prairie fire of burning, wind-blown grass. The needle-shaped spicules are about 2 thousand meters in width, and shoot up to heights of 15 million meters at speeds of about 20 thousand meters per second. Individual spicules persist for only five or ten minutes, but new ones continuously arise as old ones fade away. Approximately half a million of the evanescent, flame-like spicules are dancing in the chromosphere at any given moment.

Calcium Network

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We obtain a completely different view when the chromosphere is pictured in a calcium emission line (Fig. 5.10). Bright regions of calcium light correspond to places where there are strong magnetic fields, both above sunspots and all over the Sun in a network of magnetism (Fig. 5.11).

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The calcium, or magnetic, network, coincides with the pattern formed by large-scale convective cells, known as the supergranulation, each about 30 million meters in diameter or 2.5 times the size of the Earth. The giant cells move horizontally across the photosphere, carrying the magnetic fields with them. Each supergranulation cell sweeps the magnetic fields to its outer edges, where the field collects and strengthens (Fig. 5.12). Chromospheric heating is produced above these field concentrations, resulting in the bright calcium emission that outlines the magnetic network (Fig. 5.10).

Filaments And Prominences Fig. .. 

The hydrogen-alpha photographs of the chromosphere reveal massive loops of cool dense gas that arch up over the photosphere. Seen head on, they are elongated, dark features, called filaments, that stretch up to halfway across the face of the Sun (Fig. 5.13). The cool gas looks dark against the brightness of the hot Sun beneath it. When seen from the side at the edge of the solar disk, where the chromosphere extends beyond the lowest layers of the Sun’s atmosphere, these same features light up as bright loops, called prominences, against the dark background. They can be detected during a solar eclipse as large curling pink protuberances that extend beyond the Moon’s edge, hence the name prominence from the French word for “protuberance”.

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Active prominences lie along the polarity inversion line of strong magnetic fields connected to sunspots within active regions. Active prominences are dynamic structures with violent motions and have life-times of only minutes or hours. There are various types, such as surges, sprays and loop prominences (Fig. 5.14).

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Quiescent prominences are long, thin, vertical sheets of dense plasma, with a characteristic width of about 5 million meters and length of 100 million meters. They can extend tens and even hundreds of millions of meters above the edge of the Sun (Fig. 5.17). Some of them are big enough to girdle the Earth or even to stretch from the Earth to the Moon. Quiescent prominences are exceedingly stable structures that can last for many months. They lie along the magnetic neural lines of weak bipolar magnetic regions of the solar photosphere, and can form within the cavity below a coronal streamer.