The Solar Chromosphere And Its Magnetism

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Just above the photosphere lies a thin gaseous layer called the chromosphere, from chromos, the Greek word for "color " and sphere for its spherical shape, tte chromosphere is so faint that it was first observed during a total eclipse of the Sun. It became visible a few seconds before and after the eclipse, creating a narrow pink, rose or ruby-colored band at the limb of the Sun. tte solar limb is the apparent edge of the photosphere disk as viewed from the Earth; during a total solar eclipse the Moon just covers the photosphere, and the edge of the Moon coincides with the solar limb.

When observing the spectrum of visible sunlight during a total solar eclipse, bright emission lines suddenly flash into view at the exact wavelengths of some of the dark absorption lines in the photosphere, ttis is because you are looking from the side or edge, seeing the energy emitted from the chromosphere without intense photosphere sunlight in the background, tte brightest emission line is the hydrogen-alpha line of hydrogen atoms, at 656.3 nanometers, which gives the chromosphere it red color.

During the eclipse of 18 August 1868, the French astronomer Pierre Jules César "P. J. C." Janssen (1824-1907) discovered a bright yellow emission line in the chromosphere at a wavelength of 587.6 nanometers, which did not seem to come from any known element. Soon thereafter, the English astronomer Joseph Norman Lock-yer (1836-1920) succeeded in seeing the chromosphere spectral lines outside the solar limb, with a spectroscope and without an eclipse, also giving the unknown substance the name helium, from the word Helios, the "God of the Sun" in Greek mythology.

Helium wasn't discovered on Earth until 1895, by heating uranium minerals; during its radioactive decay uranium emits helium nuclei, or alpha particles, that combine with electrons to make helium atoms that get trapped in the rock.

Other prominent emission lines of the chromosphere are the two violet lines of calcium ions. Designated Ca II, the calcium is singly ionized, so the calcium atoms are missing one electron, ttey emit radiation at wavelengths of 393.4 and 396.8 nanometers, and are often called the calcium K and H lines after Fraunhofer's designation of the corresponding absorption lines in the underlying photosphere.

FIG. 5.6 Magnetogram This magnetogram was taken on 12 February 1989, close to the maximum in the Sun's 11-year cycle of magnetic activity. Yellow represents positive or north polarity pointing out of the Sun, with red the strongest fields which are around sunspots; blue is negative or south polarity that points into the Sun, with green the strongest. In the northern hemisphere (top half) positive fields lead, in the southern hemisphere (bottom half) the polarities are exactly reversed and the negative fields lead. (Courtesy of William C. Livingston, NSO and NOAO.)

FIG. 5.6 Magnetogram This magnetogram was taken on 12 February 1989, close to the maximum in the Sun's 11-year cycle of magnetic activity. Yellow represents positive or north polarity pointing out of the Sun, with red the strongest fields which are around sunspots; blue is negative or south polarity that points into the Sun, with green the strongest. In the northern hemisphere (top half) positive fields lead, in the southern hemisphere (bottom half) the polarities are exactly reversed and the negative fields lead. (Courtesy of William C. Livingston, NSO and NOAO.)

In 1891 the French astronomer Henri Deslandres (1853-1948) at Meudon and the American astronomer George Ellery Hale (1868-1938) at Mount Wilson independently invented an entirely new way of observing the chromosphere. Instead of looking at all of the Sun's colors together, they devised an instrument, called the spectrohe-liograph, meaning Sun spectrum recorder. It creates an image of the Sun in just one color or wavelength, without the blinding glare of all the other visible wavelengths. In a spectroheliograph, the sunlight falls on a vertical slit, and light coming through the slit is spread out in wavelength by a diffraction grating (Fig. 5.7). Light at the wavelength of one of the bright emission lines is then directed through a second slit, tte two slits are moved together, with the first slit scanning the Sun from side to side, and an image of the Sun is obtained at the chosen wavelength.

By tuning in the red emission of hydrogen or a violet line of calcium, the spectroheliograph can be used to isolate the light of the chromosphere and produce photographs

FIG. 5.7 Spectroheliograph A small section of the Sun's image at the focal plane of a telescope is selected with a narrow entry slit, Sj, and this light passes to a diffraction grating, producing a spectrum. A second slit, S2, at the focal plane selects a specific wavelength from the spectrum. If the plate containing the two slits is moved horizontally, then the entrance slitpasses adjacent strips of the solar image. The light leaving the moving exit slit then builds up an image of the Sun at a specific wavelength.

FIG. 5.7 Spectroheliograph A small section of the Sun's image at the focal plane of a telescope is selected with a narrow entry slit, Sj, and this light passes to a diffraction grating, producing a spectrum. A second slit, S2, at the focal plane selects a specific wavelength from the spectrum. If the plate containing the two slits is moved horizontally, then the entrance slitpasses adjacent strips of the solar image. The light leaving the moving exit slit then builds up an image of the Sun at a specific wavelength.

or digital images of it without the blinding glare of all the rest of visible sunlight. In this way, the chromosphere can be observed across the entire disk whenever the Sun is in the sky, rather than just at the edge during a brief, infrequent eclipse.

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, tte plages are chromospheric phenomena detected in monochromatic hydrogen-alpha light; they are associated with, and often confused with, bright patches in the photosphere, called faculae - Latin for "little torches," seen near the solar limb in white light. Long, dark filaments also curl across the hydrogen-alpha Sun. ttey are huge regions of dense, cool gas supported by powerful magnetic

Magnetismo Solar

FIG. 5.8 The Sun in hydrogen alpha This global image of the Sun was taken in the light of hydrogen atoms, emitting at the alpha transition that occurs at a particular red wavelength of 656.3 nanometers. It shows small, dark, magnetic sunspots, long, dark, snaking filaments, and bright plage. (Courtesy of the Baikal Astrophysical Observatory, Academy of Sciences, Russia.)

FIG. 5.8 The Sun in hydrogen alpha This global image of the Sun was taken in the light of hydrogen atoms, emitting at the alpha transition that occurs at a particular red wavelength of 656.3 nanometers. It shows small, dark, magnetic sunspots, long, dark, snaking filaments, and bright plage. (Courtesy of the Baikal Astrophysical Observatory, Academy of Sciences, Russia.)

forces. Indeed, the Sun's magnetism dominates the hydrogen-alpha world and gives rise to its startling inhomogeneity.

Although the chromosphere is often described as a thin layer of gas, about 10,000 kilometers thick, it consists of a jagged, dynamic, ever-changing set of little vertical spikes, ttey were described as early as 1877 by the Italian astronomer and priest Pietro Angelo Secchi (1818-1878), and named spicules by the American astronomer Walter Orr Roberts (1915-1990) in 1945. When you look at the edge, or limb, of the Sun in hydrogen-alpha light, hundreds of thousands of the evanescent, flame-like spicules are observed dancing in the chromosphere at any given moment.

tte short-lived spicules rise and fall like chopping waves on the sea or a prairie fire of burning, wind-blown grass (Fig. 5.9). tte needle-shaped spicules are about 2 kilometers in width, and shoot up to heights of up to 15,000 kilometers in 5 minutes, moving at speeds of up to 25 kilometers per second, tte spicules then fall back down again, but new spicules continually arise as old ones fade away.

For more than a century, no one knew for certain just what causes the upward moving spicules, but the mystery now seems to have been solved, ttere were two clues

Superficie Del Sol

FIG. 5.9 Spicules Thousands of dark, long, thin spicules, or little spikes, dot this high-resolution image of a solar active region, taken with the Swedish Solar Telescope, abbreviated SST, on the Canary Island of La Palma. Layered, needle-shaped spicules (rightside), each about a kilometer wide, shoot out to more than 15,000 kilometers. The narrowjets of gas are moving out of the solar chromosphere in magnetic channels, or flux tubes, at supersonic speeds of up to 25 kilometers per second. Time-sequenced images have shown that these spicules rise and fall in about five minutes, driven by sound waves beneath them. (Courtesy of SST, Royal Swedish Academy of Sciences, and LMSAL.)

FIG. 5.9 Spicules Thousands of dark, long, thin spicules, or little spikes, dot this high-resolution image of a solar active region, taken with the Swedish Solar Telescope, abbreviated SST, on the Canary Island of La Palma. Layered, needle-shaped spicules (rightside), each about a kilometer wide, shoot out to more than 15,000 kilometers. The narrowjets of gas are moving out of the solar chromosphere in magnetic channels, or flux tubes, at supersonic speeds of up to 25 kilometers per second. Time-sequenced images have shown that these spicules rise and fall in about five minutes, driven by sound waves beneath them. (Courtesy of SST, Royal Swedish Academy of Sciences, and LMSAL.)

to the solution. First, the spicule lifetimes are comparable to the five-minute period of the photosphere oscillations, and second, the spicules consist largely of ionized material that will follow the direction of magnetic field lines. Some of the sound waves that push the photosphere in and out also move into the chromosphere, powering shocks that drive upward flows along magnetic flux tubes, forming the spicules.

A completely different view of the chromosphere is obtained when it is pictured in the calcium H or K emission lines. 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. 5.10). Regions of intense magnetism are probably

FIG. 5.10 Calcium magnetic network This global spectroheliogram of the Sun was taken in the light of the singly ionized calcium, abbreviated Ca II, at the core of the violet K line with a wavelength of 393.4 nanometers. The emission outlines the chromosphere calcium network where magnetic fields are concentrated, like the edges of the tiles in a mosaic; it liesjust above the magnetic network of concentrated magnetism in the photosphere. The brightest extended regions are called plages; they are dense places in the chromosphere found above sunspots or other active areas of the photosphere in regions of enhanced magnetic field. (Courtesy of NSO, NOAO, and NSF.)

FIG. 5.10 Calcium magnetic network This global spectroheliogram of the Sun was taken in the light of the singly ionized calcium, abbreviated Ca II, at the core of the violet K line with a wavelength of 393.4 nanometers. The emission outlines the chromosphere calcium network where magnetic fields are concentrated, like the edges of the tiles in a mosaic; it liesjust above the magnetic network of concentrated magnetism in the photosphere. The brightest extended regions are called plages; they are dense places in the chromosphere found above sunspots or other active areas of the photosphere in regions of enhanced magnetic field. (Courtesy of NSO, NOAO, and NSF.)

30,000 km

Supergranular Cotiveetive Flow

FIG. 5.11 Magnetic canopy A two-dimensional, radial cross section of the magnetic network model of the solar transition region. The motion of supergranular convective cells (bottom) concentrates magnetic fields at their boundaries in the photosphere. The magnetic fields (arrowed lines) are pushed together and amplified up to 0.1 tesla at the cell edges. Heating in the chromosphere above this magnetic network produces bright calcium emission (see Fig. 5.10). The concentrated magnetic fields expand and flare out with height in the overlying corona, producing the magnetic canopy. Temperature, T, contours between log T = 6.1 (corona) andlogT = 5.4 (uppertransitionregion)iaemarked. [Adaptedfrom Alan H. Gabriel, Philosophical Transactions ofthe Royal Society (London) A281, 339-352 (1976).]

Supergranular Cotiveetive Flow

FIG. 5.11 Magnetic canopy A two-dimensional, radial cross section of the magnetic network model of the solar transition region. The motion of supergranular convective cells (bottom) concentrates magnetic fields at their boundaries in the photosphere. The magnetic fields (arrowed lines) are pushed together and amplified up to 0.1 tesla at the cell edges. Heating in the chromosphere above this magnetic network produces bright calcium emission (see Fig. 5.10). The concentrated magnetic fields expand and flare out with height in the overlying corona, producing the magnetic canopy. Temperature, T, contours between log T = 6.1 (corona) andlogT = 5.4 (uppertransitionregion)iaemarked. [Adaptedfrom Alan H. Gabriel, Philosophical Transactions ofthe Royal Society (London) A281, 339-352 (1976).]

associated with heating of the chromosphere, resulting in the bright calcium emission lines that coincide with the supergranulation cell boundaries and the photosphere's magnetic network.

tte flux tubes in the photosphere's magnetic network exhibit an expansion with height, opening up into the overlying solar chromosphere and forming a magnetic canopy (Fig. 5.11). tte magnetic canopy is a layer of magnetic field which is directed parallel to the solar surface and located in the low chromosphere, like a canopy in a rain forest; the tree-trunks correspond to the magnetic flux tubes in the photosphere that rise in the vertical direction and spread out like branch foliage in the chromosphere.

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