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Antarctic Study Probes Sun's Atmosphere

by Louise Good

Stuart Jefferies in the office on Maui (top) and at the South Pole.

Haleakala, the House of the Sun, in warm, sunny Hawaii is undoubtedly the world's best place to study the Sun, but for some experiments, it just won't do. IfA solar astronomer Stuart Jefferies goes to the frigid South Pole, where for several months each year, the Sun never sets. There he and his colleagues are able to make continuous measurements of sound waves in the solar atmosphere.

What they hope to learn more about is the surprising temperature structure of the Sun, which has been a scientific enigma for 60 years. The temperature at the center of the Sun, where nuclear fusion takes place, is about 13 million degrees Celsius (23 million F). On the Sun's visible surface (photosphere), it is a relatively cool 6,000 degrees (10,000 F). But, surprisingly, the Sun's atmosphere is hotter than the surface and reaches a sizzling 2 million (3.6 million F) degrees in the corona, the part of the Sun’s outer atmosphere that is visible from the ground only during a total solar eclipse or with a coronagraph. No one knows why the Sun's temperature pattern is that way, but Jefferies and his colleagues have been trying to find out.

Jefferies thinks the temperature pattern has its roots in a convective process that starts near the solar surface. Here hot bubbles of gas that rise to the surface of the Sun cool quickly, producing plumes of cooler gas that fall back into the Sun. As the gas falls sharply back into the surface of the Sun, it travels faster than the local sound speed and generates millions of sound waves throughout the Sun's interior. When the sound waves reach the surface, the majority of them (the ones with frequencies below 5 mHz) are normally reflected back toward the interior by the rapid change in density that occurs at the surface. However, magnetic fields on the Sun that are inclined to the surface of the solar atmosphere allow the low-frequency sound waves to escape through "holes" that open temporarily. In this way, the sound waves transfer their energy into the solar atmosphere. For a while, scientists have known that in theory this could happen in regions of strong magnetic fields such as sunspots. But even when there are very few such regions, the temperatures of the upper atmosphere and the corona remain constant, so there must be some other mechanism that allows heat to be transferred to the upper atmosphere. Jefferies thinks that this transfer also occurs at the boundaries of structures called "supergranules," large convection cells on the Sun's surface that last about a day. The convection currents crash into the magnetic fields that congregate at the boundaries of the supergranules, causing even magnetic field lines that are close to perpendicular to the surface to have occasional significant inclination. At this lesser angle, magneto-acoustic portals open briefly, allowing sound-wave energy to escape into the upper atmosphere. Though these portals are small and exist for only a short time, they are ubiquitous, so in total they allow a large amount of energy to travel outward.

An inside view of MOTH during its assembly at South Pole Solar Observatory.

To make these observations, Jefferies has designed a new telescope that will enable him to study the solar atmosphere at higher altitudes than previous observations. He has dubbed the new instrument "MOTH2" after the first version of the instrument, Magneto Optical filters at Two Heights (MOTH), even though the acronym no longer works because the new telescope will allow observations of four spectral lines (sodium, potassium, calcium, and helium) to probe four heights in the solar atmosphere instead of using two lines to look at two heights. Its resolution will be five times better, and it will be able to detect sound waves traveling twice the speed as those detected by the original MOTH. The instrument should be ready for use in December, the beginning of summer at the South Pole.

Jefferies is working with Neil Murphy of the Jet Propulsion Laboratory (Pasadena, CA) and Wayne Rodgers of the Eddy Company (Apple Valley, CA) on the telescope design. Scott McIntosh of the Southwest Research Institute and Bernard Fleck of the European Space Agency will participate in the data analysis.