6. The Extended Solar Atmosphere


Fig6_1 Eclipse corona

Fig6_1 Eclipse corona

Fig. 6.1 . The million-degree solar atmosphere, known as the corona, is seen around the shadowed disk of the Moon during the solar eclipse on 11 July 1991. The electrically charged gas is concentrated by magnetic fields into numerous fine rays as well as larger helmet streamers. (Courtesy of HAO/NCAR.)


Fig6_2 Sunspot group

Fig6_2 Sunspot group

Fig. 6.2 . Intense magnetic fields emerge from the interior of the Sun through the Sunís visible disk, the photosphere, producing groups of sunspots. The sunspots appear dark because they are slightly cooler than the surrounding photosphere gas. This composite image was taken in white light, or all the colors combined. The enlarged image shows the biggest sunspot group, which is about 12 times larger than the Earth whose size is denoted by the black spot (lower left). (Courtesy of SOHO/ESA/NASA.)


Fig6_3 Solar magnetic activity cycle

Fig6_3 Solar magnetic activity cycle

Fig. 6.3 . The 11-year solar cycle of magnetic activity plotted from 1975 to 2007. Both the positions of sunspots (top) and the numbers of sunspots (bottom) wax and wane in cycles that peak every 11 years. Similar 11-year cycles have been observed for more than a century. At the beginning of each cycle, the first sunspots appear at about 30 degrees solar latitude and then migrate to 0 degrees solar latitude, at the solar equator, when the cycle ends. This plot of the changing positions of sunspots resembles the wings of a butterfly, and has therefore been called the butterfly diagram. The cycles overlap with spots from a new cycle appearing at high latitudes when the spots from the old cycle persist in the equatorial regions. The solar latitude is the angular distance from the plane of the Sunís equator, which is very close to the plane of the Earthís orbit about the Sun, called the ecliptic. (Courtesy of David Hathaway/NASA/MSFC.)


Fig6_4 Winding up the field

Fig6_4 Winding up the field

Fig. 6.4 . A model for generating the changing location, orientation and polarity of the sunspot magnetic fields. Initially the magnetic field is supposed to be the dipolar or poloidal field seen at the poles of the Sun (left). The internal magnetic fields then run just below the photosphere from the Sunís south to north poles. As time proceeds, the highly conductive, rotating material inside the Sun carries the magnetic field along and winds it up. Because the equatorial regions rotate at a faster rate than the polar ones, the internal magnetic fields are stretched out and wrapped around the Sun's center, becoming concentrated and twisted together like ropes (middle and right). With increasing strength, the submerged magnetism becomes buoyant, rises and penetrates the visible solar disk, the photosphere, creating magnetic loops and bipolar sunspots that are formed in two belts, one each in the northern and southern hemisphere (right). [Adapted from Horace W. Babcock, The topology of the Sunís magnetic field, and the 27-year cycle, Astrophysical Journal 133, 572-587 (1961)]


Fig6_5 Magnetic Loops

Fig6_5 Magnetic Loops

Fig. 6.5 . 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 the extreme ultraviolet radiation of eight and nine times ionized iron, denoted Fe IX and Fe X, formed at a temperature of about 1.0 million K. 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, a joint program of the Lockheed-Martin Solar and Astrophysics Laboratory, or LMSAL for short, and Stanford's Solar Observatories Group.)


Fig6_6 Coronal loops

Fig6_6 Coronal loops

Fig. 6.6 . The corona is stitched together with ubiquitous coronal loops that are created when upwelling magnetic fields generated inside the Sun push through the visible solar disk, the photosphere, into the overlying, invisible chromosphere and corona. These closed magnetic structures are anchored in the photosphere at footpoints of opposite magnetic polarity (marked with + and -). Coronal loops can be filled with hot gas that shines brightly at extreme ultraviolet and x-ray wavelengths. Driven by motions in the underlying photosphere and below, the coronal loops twist, rise, shear, and interact, releasing magnetic energy that can heat the solar corona and power intense solar flares or coronal mass ejections. Large coronal loops are found in the bulb-like base of coronal steamers, whose long, thin stalks extend out into space. Magnetic fields that are anchored in the photosphere at one end can also be carried by the solar wind into interplanetary space, resulting in open magnetic fields and a channel for the fast solar wind.


Fig6_7 Magnetic and gas pressure

Fig6_7 Magnetic and gas pressure

Fig. 6.7 . The ratio of gas to magnetic pressure, denoted by the symbol b, plotted as a function of height above the photosphere. The magnetic pressure is greater than the gas pressure in the low corona, where b is less than one, and magnetic fields dictate the structure of the corona. Further out, the gas pressure can exceed the magnetic pressure, which permits the solar wind to carry the Sunís magnetic field into interplanetary space. In the photosphere, below the corona and chromosphere, the gas pressure also exceeds the magnetic pressure, and the moving gas carries the magnetic fields around. [Adapted from G. Allen Gary, Plasma beta above a solar active region: Rethinking the paradigm, Solar Physics 203, 71-86 (2001).]


Fig6_8 Sun in X-rays

Fig6_8 Sun in X-rays

Fig. 6.8 . Ionized gases at a temperature of a few million K produce the bright glow seen in this x-ray image of the Sun. It shows magnetic coronal loops that thread the corona and hold the hot gases in place. The brightest features are called active regions and correspond to the sites of the most intense magnetic field strength. This image of the Sunís corona was recorded by the Soft X-ray Telescope, abbreviated SXT, aboard the Japanese Yohkoh satellite on 1 February 1992, near a maximum of the 11-year cycle of solar magnetic activity. Subsequent SXT images, taken about five years later near activity minimum, show a remarkable dimming of the corona when the active regions associated with sunspots have almost disappeared, and the Sunís magnetic field has changed from a complex structure to a simpler configuration. (Courtesy of NASA/ISAS/LMSAL/NAO Japan, U. Tokyo.)


Fig6_9 Far Magnetic Fields

Fig6_9 Far Magnetic Fields

Fig. 6.9 . In the low solar corona, strong magnetic fields are tied to the Sun at both ends, trapping hot, dense electrified gas within magnetized loops. Far from the Sun, the magnetic fields are too weak to constrain the outward pressure of the hot gas, and the loops are opened up, allowing electrically charged particles to escape. They form the solar wind that carries the magnetic fields away from the Sun. (Courtesy of Newton Magazine, the Kyoikusha Company.)


Fig6_10 Solar flare model

Fig6_10 Solar flare model

Fig. 6.10 . A solar flare is powered by magnetic energy released from a magnetic interaction site above the top of a coronal loop. Electrons are accelerated to high speed during a solar flare, generating a burst of radio energy as well as impulsive loop-top hard x-ray emission. Some of these non-thermal electrons are channeled down the loop and strike the chromosphere at nearly the speed of light, emitting hard x-rays by electron-ion bremsstrahlung at the loop footpoints. When beams of accelerated protons enter the dense, lower atmosphere, they cause nuclear reactions that result in gamma-ray spectral lines and energetic neutrons. Material in the chromosphere is heated very quickly and rises into the coronal loop, accompanied by a slow, gradual increase in soft x-ray radiation. This upwelling of heated material is called chromospheric evaporation, and occurs in the decay phase of the flare.


Fig6_11 A huge coronal mass ejection

Fig6_11 A huge coronal mass ejection

Fig. 6.11 . A huge coronal mass ejection is seen in this coronagraph image, taken on 5 December 2003 with the Large Angle Spectrometric Coronagraph, abbreviated LASCO, on the Solar and Heliospheric Observatory, or SOHO for short.† The black area corresponds to the occulting disk of the coronagraph that blocks intense sunlight and permits the corona to be seen.† An image of the singly ionized helium, denoted He II, emission of the Sun, taken at about the same time, has been appropriately scaled and superimposed at the center of the LASCO image.† The full-disk helium image was taken at a wavelength of 30.4 nanometers, corresponding to a temperature of about 60,000 K, using the Extreme-ultraviolet Imaging Telescope, or EIT for short, aboard SOHO. (Courtesy of the SOHO LASCO and EIT consortia. SOHO is a project of international cooperation between ESA and NASA.)


Fig6_12 Magnetosphere

Fig6_12 Magnetosphere

Fig. 6.12 . The Earthís magnetic field carves out a hollow in the solar wind, creating a protective cavity, called the magnetosphere. A bow shock forms at about ten Earth radii on the sunlit side of our planet. The location of the bow shock is highly variable since it is pushed in and out by the gusty solar wind. The magnetopause marks the outer boundary of the magnetosphere, at the place where the solar wind takes control of the motions of charged particles. The solar wind is deflected around the Earth, pulling the terrestrial magnetic field into a long magnetotail on the night side. Plasma in the solar wind is deflected at the bow shock (left), flows along the magnetopause into the magnetic tail (right), and can then be injected back toward the Earth within the plasma sheet (center). The Earth, its auroras, atmosphere and ionosphere, and the two Van Allen radiation belts all lie within this magnetic cocoon.


Fig6_13 Bastille Day Flare (left)

Fig6_13 Bastille Day Flare (left)

Fig. 6.13 . A powerful solar flare (left), occurring on Bastille day 14 July 2000, unleashed high-energy protons that began striking the SOlar and Heliospheric Observatory, abbreviated SOHO, spacecraft near Earth about 8 minutes later, continuing for many hours, as shown in the image taken 12 hours after the flare began (right). Both images were taken at a wavelength of 19.5 nanometers, emitted at the Sun by eleven times ionized iron, denoted Fe XII, at a temperature of about 1.5 million K, using the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, on the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the SOHO EIT consortium. SOHO is a project of international cooperation between ESA and NASA.)


Fig6_13 Bastille Day Flare (right)

Fig6_13 Bastille Day Flare (right)

Fig. 6.13 . A powerful solar flare (left), occurring on Bastille day 14 July 2000, unleashed high-energy protons that began striking the SOlar and Heliospheric Observatory, abbreviated SOHO, spacecraft near Earth about 8 minutes later, continuing for many hours, as shown in the image taken 12 hours after the flare began (right). Both images were taken at a wavelength of 19.5 nanometers, emitted at the Sun by eleven times ionized iron, denoted Fe XII, at a temperature of about 1.5 million K, using the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, on the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the SOHO EIT consortium. SOHO is a project of international cooperation between ESA and NASA.)


Fig6_14 Magneatic spiral

Fig6_14 Magneatic spiral

Fig. 6.14 . The trajectory of flare electrons in interplanetary space as viewed from above the Sunís polar regions using the Ulysses spacecraft. The squares and crosses show Ulysses radio measurements of type III radio bursts. As the high-speed electrons move out from the Sun into progressively more tenuous plasma, they excite radiation at successively lower plasma frequencies. The numbers denote the observed frequency in kiloHertz, abbreviated kHz. Since the flaring electrons are forced to follow the interplanetary magnetic field, they do not move in a straight line out from the Sun, but instead travel along the spiral pattern of the interplanetary magnetic field, shown by the solid curved lines. The magnetic fields are drawn out into space by the radial solar wind, and attached at one end to the rotating Sun. The locations of the orbits of Mercury, Venus and the Earth are shown as circles. [Courtesy of Michael J. Reiner/Ulysses is a project of international collaboration between ESA and NASA.]


Fig6_15 Magnetic Cloud

Fig6_15 Magnetic Cloud

Fig. 6.15 . When a coronal mass ejection travels into interplanetary space, it can create a huge magnetic cloud containing beams of electrons that flow in opposite directions within the magnetic loops that are rooted at both ends in the Sun. The magnetic cloud also drives a shock ahead of it. Magnetic clouds are only present in a subset of observed interplanetary coronal mass ejections. (Courtesy of Deborah Eddy and Thomas Zurbuchen.)