Bipolar Sunspots Magnetic Loops And Active Regions

Like people, sunspots often group together, often in opposite bipolar pairs. One sunspot of each pair has positive, north, magnetic polarity, or outward-directed magnetism, and its partner has the opposite negative, south, magnetic polarity that is directed inward. Groups ofbipolar sunspots are usually oriented roughly parallel to the Sun's equator, in the east-west direction of solar rotation.

tte opposite magnetic poles are joined together, like Siamese twins, by long, thin magnetic loops that run between them, rising in arches like bridges that connect the bipolar magnetic islands, tte magnetic loops can be visualized in terms of the magnetic lines of force, or magnetic field lines, that align compass needles on Earth, tte lines of force emerge nearly radially from the sunspot with the positive north polarity and loop through the overlying solar atmosphere before re-entering the photosphere in the spot with negative south polarity, like the lines of force running between the north and south poles of the Earth or a bar magnet. It's as if a powerful magnet, aligned roughly in the east-west direction, was buried deep beneath each sunspot pair.

tte magnetized loops that arch above bipolar sunspots can be seen in images taken at the solar edge or limb (Fig. 5.12). ttey then appear bright in contrast with the dark background, sometimes extending tens of thousands of kilometers above the solar limb.

tte 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. Energized material is concentrated and enhanced within the solar active regions, where magnetic loops shape, mold and constrain the hot, electrified gas.

Each magnetic loop acts as a barrier to electrically charged particles, ttey move back and forth along the magnetic field lines but cannot move across them, tte hot material is therefore slaved to the magnetism, and becomes trapped within the closed magnetic loops where it emits bright extreme ultraviolet and X-ray radiation (Fig. 5.13).

tte ubiquitous magnetic loops are never still, but instead continually alter their shape, ttey can rise up out of the solar interior, and submerge back within it in hours or days. Sometimes you can see the hot material trying to escape its magnetic cage, in a high-velocity upwelling, or surge (Fig. 5.14).

Active regions begin their life when magnetic loops rise up from inside the Sun. tte magnetic structure of an active region then gradually changes in appearance as new magnetic loops surface within it and its sunspots move and shift about. Ms results in continued alterations of the form and intensity of their visible and invisible radiation. Eventually, the ephemeral active regions simply disappear. Over the course of weeks to months, their magnetic loops break apart, disintegrate, or move back inside the Sun where they came from.

ttus, active regions are never still, but instead continually alter their magnetic shape, ttey are the seat of profound change and unrest on the Sun! tte stressed magnetic fields build up magnetic energy that is waiting to be released, and the ongoing magnetic interaction can trigger sudden and catastrophic explosions such as powerful solar flares. Indeed, the continually evolving magnetic structure and intense radiation, as well as the eruptive solar flares, give active regions their name, tte whole range of activity varies with the 11-year solar cycle of magnetic activity, which we now focus attention on.

FIG. 5.12 Magnetic loops made visible An electrified, million-degree gas, known as plasma, is channeled by magnetic fields into bright, thin loops. The magnetized loops stretch up to 500,000 kilometers from the visible solar disk, spanning up to 40 times the diameter of planet Earth. The magnetic loops are seen in extreme ultraviolet radiation of eight and nine times ionized iron, denoted Fe IX and Fe X, at a wavelength ofl7.1 nanometers and a temperature of about 1.0 million kelvin. The hot plasma is heated at the bases of loops near the place where their legs emerge from and return to the photosphere. Bright loops with a broad range of lengths all have a fine, thread-like substructure with widths as small as the telescope resolution of 1 second of arc, or 725 kilometers at the Sun. This image was taken with the Transition Region And Coronal Explorer, abbreviated TRACE, spacecraft. (Courtesy of the TRACE consortium, LMSAL and NASA; TRACE is a mission of the Stanford-Lockheed Institute for Space Research, ajoint program of the Lockheed-Martin Solar and Astrophysics Laboratory, or LMSAL for short, and Stanford's Solar Observatories Group.)

FIG. 5.12 Magnetic loops made visible An electrified, million-degree gas, known as plasma, is channeled by magnetic fields into bright, thin loops. The magnetized loops stretch up to 500,000 kilometers from the visible solar disk, spanning up to 40 times the diameter of planet Earth. The magnetic loops are seen in extreme ultraviolet radiation of eight and nine times ionized iron, denoted Fe IX and Fe X, at a wavelength ofl7.1 nanometers and a temperature of about 1.0 million kelvin. The hot plasma is heated at the bases of loops near the place where their legs emerge from and return to the photosphere. Bright loops with a broad range of lengths all have a fine, thread-like substructure with widths as small as the telescope resolution of 1 second of arc, or 725 kilometers at the Sun. This image was taken with the Transition Region And Coronal Explorer, abbreviated TRACE, spacecraft. (Courtesy of the TRACE consortium, LMSAL and NASA; TRACE is a mission of the Stanford-Lockheed Institute for Space Research, ajoint program of the Lockheed-Martin Solar and Astrophysics Laboratory, or LMSAL for short, and Stanford's Solar Observatories Group.)

FIG. 5.13 Ubiquitous loops The looping structures of the solar magnetic field are seen in great detail with instruments aboard the Transition Region And Coronal Explorer, abbreviated TRACE, spacecraft. This TRACE image (center) was taken at the extreme ultraviolet wavelength of 17.1 nanometers, emitted by eight and nine times ionized iron, denoted Fe IX and Fe X, at a temperature of about 1.0 million kelvin. A full-disk image (upper left:) shows the ubiquitous loops across the Sun; it was taken on the same day at the same wavelength with the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, aboard the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the TRACE consortium, the SOHO EIT consortium, and NASA. SOHO is a project of international cooperation between ESA and NASA and TRACE is a mission of the StanfordLockheed Institute for Space Research, ajoint program of the Lockheed-Martin Solar and Astrophysics Laboratory, or LMSAL for short, and Stanford's Solar Observatories Group.)

FIG. 5.13 Ubiquitous loops The looping structures of the solar magnetic field are seen in great detail with instruments aboard the Transition Region And Coronal Explorer, abbreviated TRACE, spacecraft. This TRACE image (center) was taken at the extreme ultraviolet wavelength of 17.1 nanometers, emitted by eight and nine times ionized iron, denoted Fe IX and Fe X, at a temperature of about 1.0 million kelvin. A full-disk image (upper left:) shows the ubiquitous loops across the Sun; it was taken on the same day at the same wavelength with the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, aboard the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the TRACE consortium, the SOHO EIT consortium, and NASA. SOHO is a project of international cooperation between ESA and NASA and TRACE is a mission of the StanfordLockheed Institute for Space Research, ajoint program of the Lockheed-Martin Solar and Astrophysics Laboratory, or LMSAL for short, and Stanford's Solar Observatories Group.)

FIG. 5.14 Surge A sudden high-velocity upwelling tries to break free of the magnetic and gravitational constraints of the Sun. The million-degree gas can generally move only along the curved magnetic loops, but it is sometimes accelerated to such a high speed that the gas is propelled in a straight line. Nevertheless, despite the large initial speed, most of the material is pulled back down by the Sun's tremendous gravity, and the surge cools and darkens. This image was taken at the extreme ultraviolet wavelength of 17.1 nanometers, emitted by eight and nine times ionized iron, denoted Fe IX and Fe X, at a temperature of about 1.0 million kelvin, using the Transition Region And Coronal Explorer, abbreviated TRACE, spacecraft. (Courtesy of Charles Kankelborg, the TRACE consortium and NASA; TRACE is a mission of the Stanford-Lockheed Institute for Space Research, ajoint program of the Lockheed-Martin Solar and Astrophysics Laboratory, or LMSAL for short, and Stanford's Solar Observatories Group.)

FIG. 5.14 Surge A sudden high-velocity upwelling tries to break free of the magnetic and gravitational constraints of the Sun. The million-degree gas can generally move only along the curved magnetic loops, but it is sometimes accelerated to such a high speed that the gas is propelled in a straight line. Nevertheless, despite the large initial speed, most of the material is pulled back down by the Sun's tremendous gravity, and the surge cools and darkens. This image was taken at the extreme ultraviolet wavelength of 17.1 nanometers, emitted by eight and nine times ionized iron, denoted Fe IX and Fe X, at a temperature of about 1.0 million kelvin, using the Transition Region And Coronal Explorer, abbreviated TRACE, spacecraft. (Courtesy of Charles Kankelborg, the TRACE consortium and NASA; TRACE is a mission of the Stanford-Lockheed Institute for Space Research, ajoint program of the Lockheed-Martin Solar and Astrophysics Laboratory, or LMSAL for short, and Stanford's Solar Observatories Group.)

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