The Chromosphere

The chromosphere represents the dynamic transition between the cool temperature minimum of the outer photosphere and the diffuse million-degree corona above. It derives its name and pink colour from the red Ha line of hydrogen at 6562.8 angstroms (A); 1 A = 10-10 metre. Because this line is so strong, it is the best means for studying the chromosphere. For this reason special monochromators are widely used to study the Sun in a narrow wavelength band. Because density decreases with height more rapidly than magnetic field strength, the magnetic field dominates the chro-mospheric structure, which reflects the extension of the photospheric magnetic fields. The rules for this interplay are simple: Every point in the chromosphere where the magnetic field is strong and vertical is hot and hence bright, and every place where it is horizontal is dark. Supergranulation, which concentrates

Active region toward the limb of the Sun, with spicules (right) and some sunspots (upper left). Image captured on June 16, 2003, by the Swedish Solar Telescope, La Palma, Spain. Lockheed Martin/Solar and Astrophysics Lab the magnetic field on its edges, produces a chromospheric network of bright regions of enhanced magnetic fields.

The most prominent structures in the chromosphere, especially in the limb, are the clusters of jets, or streams, of plasma called spicules, which occur at the edges of the chromospheric network, where magnetic fields are stronger. Spicules extend up to 10,000 km (6,000 miles) above the surface of the Sun. They rise from the lower chromosphere at about 20 km/sec (12 miles/sec) to a height of several thousand kilometres, and then within 10-15 minutes they disperse or collapse. About 100,000 spicules are active on the Sun's surface at any given time. Although they are invisible in white light, early observers could see them in the hydrogen alpha (Ha) emission line with a spectrograph, comparing them to a "burning prairie."

Because it strongly emits the high-excitation lines of helium, the chromosphere was originally thought to be hot. But radio measurements, a particularly accurate means of measuring the temperature, show it to be only 8,000 K (7,700°C, 13,900°F), somewhat hotter than the photosphere. Detailed radio maps show that hotter regions coincide with stronger magnetic fields. Both hot and cold regions extend much higher than one might expect, tossed high above the surface by magnetic and convective action.

When astronomers observe the Sun from space at ultraviolet wavelengths, the chromosphere is found to emit lines formed at high temperatures, spanning the range from 10,000 to 1 million K (9,700 to 999,700°C, 17,500 to 1.8 million °F). The whole range of ionization of an atom can be found. For example, oxygen I (neutral) is found in the photosphere, oxygen II through VI (one to five electrons removed) in the chromosphere, and oxygen VII and VIII in the corona. This entire series occurs in a height range of about 5,000 km (3,000 miles). An image of the corona obtained at ultraviolet wavelengths has a much more diffuse appearance as compared with lower temperature regions, suggesting that the hot material in the magnetic elements spreads outward with height to occupy the entire coronal space. Interestingly, the emission of helium, which was the original clue that the temperature increased upward, is not patchy but uniform. This occurs because the helium atoms are excited by the more diffuse and uniform X-ray emission from the hot corona.

The structure of the chromosphere changes drastically with local magnetic conditions. At the network edges, clusters of spicules project from the clumps of magnetic field lines. Around sunspots, plages occur, where there are no spicules,

Revelations in the Dark

When flash spectra (spectra of the atmosphere during an eclipse) were first obtained, astronomers found several surprising features. First, instead of absorption lines they saw emission lines (bright lines at certain wavelengths with nothing between them). This effect arises because the chromosphere is transparent between the spectrum lines, and only the dark sky is seen. Second, they discovered that the strongest lines were due to hydrogen, yet they still did not appreciate its high abundance. Finally, the next brightest lines had never been seen before. Because they came from the Sun, the unknown source element came to be called helium. Later helium was found on Earth.

but where the chromosphere is generally hotter and denser. In the areas of prominences the magnetic field lines are horizontal and spicules are absent.

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