Magnetic Fields In The Visible Photosphere

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tte sunlight we see coming from the bright disk of the Sun originates in the photosphere, a word derived from the Greek photos for "light" and sphere for its spherical shape, tte photosphere is a thin tenuous layer, only a few hundred kilometers thick, with a temperature of 5,780 kelvin and a pressure of one ten-thousandth (0.0001) of the pressure of the Earth's atmosphere at sea level. You are looking at the photosphere when you watch the Sun rise in the morning, and continue on its dailyjourney across the sky.

Although the photosphere is the only part of the Sun we can see, an extended, invisible solar atmosphere lies above it. We look right through this overlying, rarefied gas, just as we see through the Earth's air. tte photosphere therefore does not mark the surface of the Sun, for it has no surface that divides the inside from the outside.

Observing the Sun is like looking into the distance on a foggy day. At a certain distance, the total amount of fog you are looking through mounts up to make an opaque barrier, tte fog then becomes so thick and dense that radiation can penetrate no further, and we can only see that far. When looking into the solar atmosphere, you can similarly see through only so much gas. For visible sunlight, this opaque layer is the photosphere, the level of the Sun from which we get our light and heat.

tte sharp edge of the photosphere is an illusion, caused by unusual ions that make the gas as opaque as a brick wall. In a rare collision, a hydrogen atom in the photosphere can briefly capture a free electron, temporarily becoming an ion with a negative charge, ttese negative hydrogen ions absorb radiation coming from the solar interior, re-emit visible sunlight, and account for the Sun's sharp edge. At higher levels in the solar atmosphere, there are no hydrogen ions, so the light can escape and we can see right through the overlying gas.

To most of us, the photosphere looks like a perfect, white-hot globe, round, smooth and without a blemish. It's like taking a quick, sideways glimpse of a beautiful woman or handsome man; a sustained, close-up look usually reveals some imperfection. Detailed scrutiny indicates that the Sun is pitted with dark spots called sunspots.

Large sunspots can be seen with the unaided eye through fog or haze, or sometimes at sunrise or sunset, when the Sun's usual brightness is heavily dimmed. But you normally cannot look directly at the Sun without severely damaging your eyes, and most sunspots are too small to be readily visible by naked eye observations, requiring a telescope to be detected. Ancient Chinese records nevertheless indicate that large sunspots have been observed with the unaided eye for nearly 2,000 years.

Sunspots became a lot easier to see around 1610, when not less than four men turned the newly invented telescope toward the Sun, independently and nearly simultaneously confirming the existence of sunspots. One of them, the Italian scientist Galileo Galilei (1564-1642), carried out a detailed scrutiny, announcing that sunspots are embedded in the solar atmosphere, and not in front of it. He also used their apparent motion to show that the Sun is spinning in space, and turning around once every 27 days, as viewed from the moving Earth. Because our planet orbits the Sun in the same direction that the Sun rotates, the rotation rate observed from Earth is about 2 days longer than the Sun's intrinsic rate of spin of about 25 days at the solar equator.

Galileo noticed that sunspots change in size and shape, and that they eventually fade and disappear from view, tte sunspots appear from inside the Sun, often remaining visible for only a few days. Some last just a few hours; others persist for weeks and even months. As sunspots form, they can also coalesce, and move past or even through each other.

tte dark, ephemeral spots on the apparently serene Sun therefore indicate that it is an imperfect place of constant turmoil and change, contradicting the Greek philosopher Aristotle's (384-322 BC) philosophy of cosmic perfection and immutability.

In Galileo's time it was also discovered that sunspots near the equator rotate more rapidly than those nearer to the poles; this means that different parts of the Sun's visible disk rotate about its axis at different speeds, a phenomenon now known as differential rotation, ttis effect was thoroughly studied two centuries later by Richard C. Carrington (1826-1875), a wealthy English amateur, from his private observatory at Redhill. He showed in 1863 that the Sun's apparent period of rotation increases systematically with latitude from 27 days at the equator to about 30 days halfway toward the poles, or at a latitude of30 degrees.

Because they are relatively cool, sunspots appear dark in contrast with their bright surroundings. A sunspot might have a temperature of 3,500 kelvin, for example, instead of the 5,780 kelvin of adjacent regions. Astronomers have even detected the spectral signatures of certain molecules in sunspots, while the surrounding material is so hot that molecules are broken apart. However, appearances can be deceiving, for a sunspot is

FIG. 5.1 Sunspots Thisdrawing of sunspots was made more than a century ago, in June 1861 by the Scottish engineer and astronomer James Nasmyth (1808-1890). (Adapted from Le Ciel, Librairie Hachette: Paris, 1877.)

almost 10 times hotter than the temperature of boiling water, at 373 kelvin, and although sunspots appear dark in comparison to the nearby hotter gas, they still radiate light.

Telescopic observations of large sunspots in normal white light indicate a dark center, the umbra, surrounded by a less dark penumbra, both standing in stark contrast to the rather bland and uniform background (Figs. 5.1, 5.2). Although it looks small in comparison to the solar disk, a large sunspot umbra can surpass the Earth in size.

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, tte umbra 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 20,000 and 60,000 kilometers,

FIG. 5.2 Sunspot group Intense magnetic fields emerge from the interior of the Sun through the photosphere, producing groups of sunspots. The sunspots appear dark because they are slightly cooler than the surrounding photosphere gas. This composite image shows the visible solar disk in white light, or all the colors combined (upper right) and an enlarged white-light image of the largest sunspot group (middle), which is about 12 times larger than the Earth whose size is denoted by the black spot (lower left). (Courtesy of SOHO, ESA and NASA.)

FIG. 5.2 Sunspot group Intense magnetic fields emerge from the interior of the Sun through the photosphere, producing groups of sunspots. The sunspots appear dark because they are slightly cooler than the surrounding photosphere gas. This composite image shows the visible solar disk in white light, or all the colors combined (upper right) and an enlarged white-light image of the largest sunspot group (middle), which is about 12 times larger than the Earth whose size is denoted by the black spot (lower left). (Courtesy of SOHO, ESA and NASA.)

FIG. 5.3 Detailed image of sunspots The Swedish Solar Telescope, abbreviated SST, on the Canary Island of La Palma observes the solar photosphere with unprecedented detail as small as 0.1 seconds of arc, orjust 75 kilometers on the Sun, which is about one thousand times better than the angular resolution of our unaided eyes. This image was obtained using a 1-meter adaptive mirror that can change shape 1,000 times a second to compensate for atmospheric blurring and obtain the sharpest-ever pictures of the Sun. Flowing tendrils of ionized gas, or plasma, encircle the dark sunspots; these penumbral filaments contain thin dark cores. Outside the penumbra, the solar granulation is visible. (Courtesy of SST, Goran Scharmer, Institute for Solar Physics, Royal Swedish Academy of Sciences.)

FIG. 5.3 Detailed image of sunspots The Swedish Solar Telescope, abbreviated SST, on the Canary Island of La Palma observes the solar photosphere with unprecedented detail as small as 0.1 seconds of arc, orjust 75 kilometers on the Sun, which is about one thousand times better than the angular resolution of our unaided eyes. This image was obtained using a 1-meter adaptive mirror that can change shape 1,000 times a second to compensate for atmospheric blurring and obtain the sharpest-ever pictures of the Sun. Flowing tendrils of ionized gas, or plasma, encircle the dark sunspots; these penumbral filaments contain thin dark cores. Outside the penumbra, the solar granulation is visible. (Courtesy of SST, Goran Scharmer, Institute for Solar Physics, Royal Swedish Academy of Sciences.)

which can be compared with the Earths mean diameter of 12,700 kilometers, tte penumbra consists of radial bright and dark filaments that arch and splay.

In 1908, the American astronomer George Ellery Hale (1868-1938) showed that sunspots are regions of intense magnetism, thousands of times stronger than the Earth's magnetic field. As Hale (Fig. 5.4) suggested, a subtle division and polarization of an atom's spectral lines can measure solar magnetism, ttis magnetic transformation has been named the Zeeman effect, after the Dutch physicist, Pieter Zeeman (1865-1943), who first noticed it in the terrestrial laboratory in 1896. Hendrik Lorentz (1853-1928)

FIG. 5.4 George Ellery Hale The American astronomer George Ellery Hale (1868-1938), shown here, and the French astronomer Henri Deslandres (1853-1948) independently invented the spectroheliograph in 1891. It is used to image the Sun in the light of one particular wavelength only. Hale subsequently measured the Zeeman effect on the Sun, using laboratory comparisons to establish the existence of strong magnetic fields in sunspots. In later work, Hale and his colleagues found that the majority of sunspots occur in pairs of opposite magnetic polarity, and that the preceding and following spots have opposite polarity that also reverses sign in the northern and southern hemispheres and from one 11-year activity cycle to the next. Hale developed a solar observatory at Mt. Wilson, California in the early 1900s, and even installed a solar telescope in his home in Pasadena together with a large relief of Apollo, the Sun God. (Courtesy of the Archives, California Institute of Technology.)

predicted the effect, and the two Dutch physicists shared the 1907 Nobel Prize in Physics for their investigations of the influence of magnetism on radiation.

When an atom is placed in a magnetic field, it acts like a tiny compass, adjusting the energy levels of its electrons. If the atomic compass is aligned in the direction of the magnetic field, the electron's energy increases; if it is aligned in the opposite direction, the energy decreases. Since each energy change coincides with a change in the wavelength emitted by that electron, a spectral line emitted at a single wavelength by a randomly oriented collection of atoms becomes a group of three lines of slightly different wavelengths in the presence of a magnetic field (Fig. 5.5). tte size of an atom's internal adjustments, and the extent of its spectral division, increases with the strength of the magnetic field.

Furthermore, the light at each of these divided wavelengths has a preferred orientation, or circular polarization, that depends on the direction, or polarity, of the magnetic field. Lines that are split by a magnetic field that is directed out along the line of sight have right-hand circular polarization, those pointing in the opposite, inward direction have left-hand circular polarization. So, one can measure the size of the spec-

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