6. Perpetual Change


Gossamer Corona

Gossamer Corona

. The Sunís corona as photographed during the total solar eclipse of 26 February 1998, observed from Oranjestad, Aruba. To extract this much coronal detail, several individual images, made with different exposure times, were combined and processed electronically in a computer. The resultant composite image shows the solar corona approximately as it appears to the human eye during totality. Note the fine rays and helmet streamers that extend far from the Sun and correspond to a wide range of brightness. (Courtesy of Fred Espenak.)


Solar Eclipse

Solar Eclipse

. During a solar eclipse, the Moon casts its shadow upon the Earth. No portion of the Sunís photosphere can be seen from the umbral region of the Moonís shadow (small gray spot); but the Sunís light is only partially blocked in the penumbral region (larger half circle). A total solar eclipse, observable only from the umbral region, traces a narrow path across the Earthís surface.


Corona Brightness and Electron Density

Corona Brightness and Electron Density

. Variation of coronal surface brightness (left scale) and electron density (right scale) with distance from the Sunís center, in units of the Sunís radius, RO = 696 million (6.96 x 108) meters. The maximum and minimum of the solar magnetic activity cycle are designated by max. and min.. The F corona values (dashed line) are continuous with the zodiacal light. For comparison, the surface brightness for the full Moon and clear sky for day and night and during a total solar eclipse are indicated.


Helmet Streamers

Helmet Streamers

. The Sunís corona photographed in white light during the solar eclipse on 11 July 1991; it extended several solar radii and had numerous fine rays as well as larger helmet streamers. (Courtesy of Shigemi Numazawa, Niigata, Japan.)


Coronal structures following sunspot maximum

Coronal structures following sunspot maximum

. Miloslav Druckmuller and Peter Aniol took this image of the solar corona during the total eclipse of 29 March 2006 from Libya. At this time, the Sun was in a declining phase from a maximum in the solar activity cycle, in 2000, so polar coronal holes, with their radial magnetic fields, were well developed, but active regions were still present at a variety of solar latitudes. The bulk of the long fine rays arise from polar and low-latitude plumes that overlie small magnetic bipoles inside coronal holes, helmet streamer rays that overlie large loop arcades and separate coronal holes of opposite polarity, and “pseudostreamer” rays that overlie twin loop arcades and separate coronal holes of the same polarity. [Courtesy of Miloslav Druckmüller, Peter Aniol and Serge Koutchmy, and adapted from Yi-Ming Wang, et al. (2007).]


Coronal Streamers and Comet

Coronal Streamers and Comet

. A coronagraph on the Solar and Heliospheric Observatory (SOHO) shows bright helmet streamers near the Sunís equatorial regions (center right and center left) near the minimum in the Sunís magnetic activity cycle. The edge of the visible solar disk, or photosphere, is indicated by the white circle, and the larger circle marks the outer edge of the occulting disk of the coronagraph. A comet is also shown (bottom left) during its fiery plunge into the Sun; more than 400 Sun-grazing comets have been discovered with this instrument. (Courtesy of the SOHO LASCO consortium. SOHO is a project of international collaboration between ESA and NASA.)


Transition Region

Transition Region

. The temperature of the solar atmosphere decreases from values near 6,000 degrees Kelvin at the visible photosphere to a minimum value of roughly 4,400 degrees Kelvin about 500 kilometers higher up. The temperature increases with height, slowly at first, then extremely rapidly in the narrow transition region, less than 100 kilometers thick, between the chromosphere and corona, from about 10,000 degrees Kelvin to about one million degrees Kelvin. (Courtesy of Eugene Avrett, Smithsonian Astrophysical Observatory.)


Chromosphere Bright Spots

Chromosphere Bright Spots

. Numerous ultraviolet bright spots are seen in this image from the SOlar and Heliospheric Observatory (SOHO). It shows the chromosphere at temperatures of 60,000 to 80,000 degrees Kelvin. Two intense active regions with numerous magnetic loops are also seen, as well as a huge eruptive prominence at the solar limb. This image was taken on 14 September 1997 in the extreme ultraviolet resonance line of singly ionized helium (He II) at 30.4 nanometers. (Courtesy of the SOHO EIT consortium. SOHO is a project of international cooperation between ESA and NASA.)


The Sun in X-rays

The Sun in X-rays

. The bright glow seen in this X-ray image of the Sun is produced by ionized gases at a temperature of a few million degrees Kelvin. It shows magnetic coronal loops which 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 (SXT) aboard the Japanese Yohkoh satellite on 1 February 1992, near the 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 Ė see Fig. 5.29. (Courtesy of Gregory L. Slater, Gary A, Linford, and Lawrence Shing, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Magnetic Shapes in X-rays

Magnetic Shapes in X-rays

. A coronal X-ray image (top) from the Yohkoh Soft X-ray Telescope on 26 December 1991, scaled to show the brightest parts, is compared with a magnetogram (bottom) from the National Solar Observatory, Kitt Peak, taken on the same day. In the magnetogram, black and white denote different magnetic directions or polarity, and the intensity is a measure of the magnetic field strength. Strong regions of opposite magnetic polarity are joined by magnetic loops that constrain the hot, dense gas that shines brightly in X-rays. This comparison shows that the structure and brightness of the X-ray corona are dictated by their magnetic roots in the photosphere. (Courtesy of David A. Falconer and Ronald L. Moore.)


Coronal loops

Coronal loops

. The corona is stitched together with the ubiquitous coronal loops that are created when upwelling magnetic fields generated inside the Sun push through the photosphere into the overlying 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 underling photosphere and below, the coronal loops twist, rise, shear, writhe 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.


Long Thin Loops Heated from their Base

Long Thin Loops Heated from their Base

. The low solar corona and transition region are filled with bright, thin magnetized loops that extend for hundreds of millions of meters above the visible solar disk, or photosphere, spanning 30 or more times the diameter of Earth. The coronal loops are filled with gas that is hundreds of times hotter than the photosphere. In this TRACE image, taken on 06 November 1999, the hot gas is detected in the ultraviolet light emitted by eight-and nine-times-ionized iron (Fe IX/X at 17.1nm) formed at a temperature of about 1.0 million degrees kelvin. Such detailed TRACE images indicate that most of the heating occurs low in the corona, near the bases of the loops as they emerge from and return to the solar disk, and that the heating does not occur uniformly along the entire loop length. (Courtesy of Markus J. Aschwanden, the TRACE consortium, LMSAL, and NASA.)


X-ray View of the Solar Cycle

X-ray View of the Solar Cycle

. Dramatic changes in the solar corona are revealed in this four-year montage of images from the Soft X-ray Telescope (SXT) aboard Yokhoh. The 12 images are spaced at 120-day intervals from the time of the satelliteís launch in August 1991, at the maximum phase of the 11-year sunspot cycle (left), to late 1995 near the minimum phase (right). The bright glow of X-rays near activity maximum comes from very hot, million-degree coronal gases that are confined within powerful magnetic fields anchored in sunspots. Near the cycle minimum, the active regions associated with sunspots have almost disappeared, and there is an overall decrease in X-ray brightness by 100 times. (Courtesy of Gregory L. Slater and Gary A, Linford, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


The Ultraviolet Sun

The Ultraviolet Sun

. This composite image, taken by two SOHO instruments and joined at the black circle, reveals the ultraviolet light of the Sunís atmosphere from the base of the corona to millions of kilometers above the visible solar disk. The region outside the black circle, obtained by UVCS, shines in the ultraviolet light emitted by oxygen ions flowing away from the Sun to form the solar wind. The inner image, obtained by EIT, shows the ultraviolet light emitted by iron ions at a temperature near two million degrees Kelvin. Dark areas, called coronal holes, are found at both poles of the Sun (top and bottom) and across the disk of the Sun; they are the places where the highest-speed solar wind originates. The structure of the corona is controlled by the Sunís magnetic field which forms the bright active regions on the solar disk and the ray-like structures extending from the coronal holes. [Courtesy of the SOHO UVCS consortium (outer region) and the SOHO EIT consortium (inner region). SOHO is a project of international cooperation between ESA and NASA.]


Magnetic Carpet

Magnetic Carpet

. Magnetic loops of all sizes rise up into the solar corona from regions of opposite magnetic polarity (black and white) in the photosphere, forming a veritable carpet of magnetism in the low corona. Energy released when oppositely directed magnetic fields meet in the corona, to reconnect and form new magnetic configurations, is one likely cause for making the solar corona so hot. (Courtesy of the SOHO EIT and MDI consortia. SOHO is a project of international cooperation between ESA and NASA.)


Magnetic Connection

Magnetic Connection

. Material moves in both directions away from an explosive center (left), observed as Doppler shifts when the SOHO SUMER instruments scanned across it. This is explained by a magnetic reconnection model (right), involving magnetic field lines (dashed lines with arrows for magnetic direction) and plasma flow (solid arrows). Material flowing inwards from each side, at speeds much less than the Alfvťn velocity, VA, carry magnetic fields together. At the center X, magnetic fields that point in opposite directions meet and join together, hurling material in both directions away from the point of magnetic contact at about the Alfvťn velocity.


Spiral Path Of Interplanetary Electrons

Spiral Path Of Interplanetary Electrons

. The trajectory of flare electrons in interplanetary space as viewed from above the Polar Regions using the Ulysses spacecraft. As the high-speed electrons move out from the Sun, they excite radiation at successively lower plasma frequencies; the numbers denote the observed frequency in kiloHertz, or kHz. Since the flaring electrons are forced to follow the interplanetary magnetic field, they do not move in a straight line from the Sun to the Earth, but instead move along the spiral pattern of the interplanetary magnetic field, shown by the solid curved lines. The squares and crosses show Ulysses radio measurements of type III radio bursts on 25 October 1994 and 30 October 1994. The approximate 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.)


Coronal Magnetism at Minimum Activity

Coronal Magnetism at Minimum Activity

. Near the minimum in the 11-year cycle of solar magnetic activity, the coronal magnetic fields lines have a dipole-type geometry and equatorial current sheet. The high-speed wind escapes along the open magnetic field lines in the Polar Regions. At the equator, where the slow wind originates, the magnetic field lines have been pulled outward by the solar wind into oppositely directed, parallel magnetic fields separated by a neutral current sheet.


Spiral, Warped Current Sheet

Spiral, Warped Current Sheet

. Near the minimum in the Sunís 11-year activity cycle, the Sunís magnetic field is primarily directed outward from one pole and inward in the other. The oppositely directed magnetic field lines meet near the solar equator, forming a thin, wave-like neutral current sheet that divides magnetic fields directed away from the Sun and those directed toward it. The neutral current sheet winds out above the Sunís equator, with a spiral shape and a warped structure that resembles the skirt of a ballerina or a whirling dervish. As it rotates with the Sun, the current sheet sweeps regions of opposite magnetic polarity past the Earth. (Courtesy of J. Randy Jokipii, and the Advanced Composition Explorer mission).


Fast and Slow Winds

Fast and Slow Winds

. A composite picture of an ultraviolet image of the solar disk and white-light image of the solar corona form a backdrop for a radial plot of solar-wind speed versus latitude. These velocity data, obtained by the Ulysses spacecraft between 1991 and 1996, show that a fast wind escapes form the polar regions where coronal holes are found, while a slow wind is associated the Sunís equatorial regions that contain coronal streamers (Courtesy of the Ulysses mission. It is a project of international collaboration between ESA and NASA.)


Coronal Holes, Magnetic Network, and the High-Speed Solar Wind

Coronal Holes, Magnetic Network, and the High-Speed Solar Wind

. Dark coronal holes at the Sunís polar regions (top and bottom) are the source of much of the high-speed solar wind. The inset provides a close-up, Doppler velocity map of the million-degree gas at the base of the corona where the fast solar wind originates, taken in the extreme ultraviolet light of ionized neon (the Ne VIII line at 77.0 nanometers). Dark blue represents an outflow, or blueshift, at a velocity of 10,000 meters per second; it marks the beginning of the high-speed solar wind. Dark red indicates a downflow at the same speed. Superposed are the edges of the ďhoney-combĒ shaped pattern of the magnetic network, where the strongest outward flows (dark blue) are found. The relationship between the outflow velocities and the network suggests that the high-speed wind emanates from the boundaries and boundary intersections of the magnetic network. These observations were taken on 22 September 1996. [Courtesy of the SOHO EIT consortium (full disk) and the SOHO SUMER consortium (velocity inset). SOHO is a project of international collaboration between ESA and NASA.]


Polar Plumes

Polar Plumes

. Close-up, extreme ultraviolet views of the south polar coronal hole (large dark region) obtained with the SOHO EIT instrument on 8 May 1996. The fast component of the solar wind emanates from these coronal holes. Plumes can be seen emerging from tiny bright spots, but they are not the main source of the high-speed winds. These images were taken in the extreme ultraviolet light of highly ionized iron - the Fe XII emission line at 19.5 nanometers (top), formed at a temperature of about 1.5 million degrees Kelvin, and the Fe IX/X emission lines near 17.1 nanometers (bottom), formed at a temperature of about 1.0 million degrees Kelvin. (Courtesy of the SOHO EIT consortium. SOHO is a project of international collaboration between ESA and NASA.)


Expanding Magnetic Loops in an Active Region

Expanding Magnetic Loops in an Active Region

. Difference images reveal the expansion of active-region loops in a time sequence, running from top to bottom, lasting about one half an hour. The fainter loops near the top are expanding with an apparent velocity of about 40 thousand meters per second. These images were taken on 22 April 1992 with the Soft X-ray Telescope (SXT) on Yohkoh. (Courtesy Alan McAllister, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Rising Temperatures Near The Photosphere

Rising Temperatures Near The Photosphere

. The increases in temperature with height in a coronal streamer (top) and a coronal hole (bottom) have been inferred from data taken with the Soft X-ray Telescope on Yohkoh. Here the temperature is given in millions of degrees Kelvin, or MK, and the distance, r, from Sun center is specified in units of the Sunís radius. At a given height in the low corona, the temperature of coronal streamers is hotter than the temperature of coronal holes. Both regions seem to be fully heated by between 1.3 and 1.5 solar radii from Sun center. [Yohkoh is a project of international cooperation between ISAS and NASA.]


Fast Winds Accelerate Rapidly

Fast Winds Accelerate Rapidly

. The fast solar wind accelerates to its high velocity very close to the Sun, within at least 10 solar radii. Interferometric observations of interplanetary radio scintillations are marked with circles and squares. The vertical bar on each data point is the 90 percent confidence limit, and the horizontal bar indicates the distance range over which the scintillation estimate is averaged. The upper and lower bounds of the Ulysses measurements are plotted as horizontal dotted lines, and the mean Ulysses fast-wind speed is marked with an arrow at 100 solar radii. The flow speed from a wave-assisted acceleration model is plotted as a dashed line, and the apparent scintillation velocity calculated from this model is plotted as a heavy solid line. The point nearest the Sun is estimated from Spartan-201 coronagraph measurements.


Blobs Accelerate Slowly

Blobs Accelerate Slowly

. Time-lapse coronagraph images show prominent features, sometimes called blobs, that move radially out in equatorial regions from helmet streamers. Their speed typically doubles between 5 and 25 solar radii from the Sun, with a slow acceleration of about 4 meters per square second through most of this distance. This data was taken on 30 April 1996 with the SOHO LASCO instrument, at a time near a minimum in the 11-year solar activity cycle. The solid line denotes the best fit to the data using an exponential function starting at a distance of 4.5 solar radii from Sun center and with an asymptotic terminal speed of 418.7 thousand meters per second at a distance of about 15 solar radii from Sun center. [Courtesy of the SOHO LASCO consortium. SOHO is a project of international collaboration between ESA and NASA.]


Heavier Ions Move Faster In Coronal Holes

Heavier Ions Move Faster In Coronal Holes

. Outflow speeds at different distances over the solar poles for hydrogen atoms (dark gray, H0) and ionized oxygen ( light gray, O5+). Here the distances are given in units of the solar radius. These data, taken in late 1996 and early 1997 with the SOHO UVCS instrument, show that the heavier oxygen ions move out of coronal holes at faster speeds than the lighter protons, and that the oxygen ions attain supersonic velocities within 2.5 solar radii from the Sun center. The solid lines denote the proton outflow speed derived from mass flux conservation; for a time-steady flow, the product of the density, speed and flow-tube area should be constant. (Courtesy of the SOHO UVCS consortium. SOHO is a project of international collaboration between ESA and NASA.)


Magnetic Waves Block Cosmic Rays

Magnetic Waves Block Cosmic Rays

. Within the ecliptic, or the plane of the solar equator, the magnetic fields of the solar wind are wound up in a spiral pattern due to the rotation of the Sun. Since the solar rotation velocity is lower at higher latitudes, the magnetic fields are oriented more or less radially above the solar poles. Scientists therefore predicted that the abundance of cosmic-ray ions would increase over the Sunís polar regions. However, Ulysses did not find the expected increase of cosmic rays, apparently because strong magnetic waves above the poles act to repel the high-energy ions back into space.


Distributions of solar wind speeds at solar minimum and maximum

Distributions of
    solar wind speeds at solar minimum and maximum

. Plots of the solar wind speed as a function of solar latitude, obtained from two orbits of the Ulysses spacecraft (top panel). The north and south poles of the Sun are at the top and bottom of each plot, the solar equator is located along the middle, and the velocities are in units of kilometers per second, abbreviated km s-1. The sunspot numbers (bottom panel) indicated that the first orbit (top left) occurred through the declining phase and minimum of the 11-year solar activity cycle, and that the second orbit (top right) spanned a maximum in activity. Ultraviolet images of the solar disk and white-light images of the inner solar corona form a central backdrop for the wind speed data, and indicate the probable sources of the winds. Near solar minimum (top left), polar coronal holes, with open magnetic fields, give rise to the fast, low-density wind streams, whereas equatorial streamer regions of closed magnetic field yielded the slow, gusty, dense winds. At solar maximum (top right), small, low-latitude coronal holes gave rise to fast winds, and a variety of slow-wind and fast-wind sources resulted in little average latitudinal variation. (Courtesy of David J. McComas and Richard G. Marsden. The Ulysses mission is a project of international collaboration between ESA and NASA. The central images are from the Extreme ultraviolet Imaging Telescope and the Large Angle Spectrometric Coronagraph aboard the SOlar and Heliospheric Observatory, abbreviated SOHO, as well as the Mauna Loa K-coronameter.)


Fast and slow winds, open and closed magnetism

Fast and slow winds, open and
    closed magnetism

. The solar atmosphere, or corona, is threaded with magnetic fields (yellow lines). Regions with open magnetic fields, known as coronal holes, give rise to fast, low density, solar wind streams (long, solid red arrows). In addition to permanent coronal holes at the Sun’s poles (top and bottom), coronal holes can sometimes occur closer to the Sun’s equator (center). Areas with closed magnetic fields yield the slow, dense wind (short, dashed red arrows). Comparisons of TRACE images with solar wind ACE data indicate that the speed and composition of the solar wind emerging from a given area have deep roots in the chromosphere. There is a shallow dense chromosphere below the strong, closed magnetic regions with a slow, dense, solar-wind outflow; deep, less dense chromosphere is found below the open magnetic regions with fast, tenuous, solar-wind outflow. This image was taken on 11 September 2003 with the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, aboard the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the SOHO EIT consortium. SOHO is a project of international collaboration between ESA and NASA.)


The heliosphere

The heliosphere

. With its solar wind going out in all directions, the Sun blows a huge bubble in space called the heliosphere. The heliopause is the name for the boundary between the heliosphere and the interstellar gas outside the Solar System. Interstellar winds mold the heliosphere into a non-spherical shape, creating a bow shock where they first encounter it. The orbits of the planets are shown near the center of the drawing.


Edge of the Solar System

Edge of the Solar System

. Voyager 1 and 2 spacecraft, located at a distance of about 90 AU and 70 AU, approach the place where the Solar System ends and interstellar space begins. One AU is the mean distance between the Earth and the Sun, and the edge of the Solar System is located at roughly 100 times this distance. At the termination shock, the supersonic solar wind abruptly slows from an average speed of 400 kilometers per second to less than one quarter that speed. Beyond the termination shock is the heliosheath, a vast region where the turbulent and hot solar wind is compressed as it presses outward against the interstellar wind. The edge of the Solar System is found at the heliopause, where the pressure of the solar wind balances that of the interstellar medium. A bow shock likely forms as the interstellar wind approaches and is deflected around the heliosphere, forcing it into a teardrop-shaped structure with a long, comet-like tail. (Courtesy of JPL and NASA.)


Stellar bow shock

Stellar bow shock

. A crescent-shaped bow shock is formed when the material in the fast wind from the bright, very young star, LL Ori (center) collides with the slow-moving gas in its vicinity, coming from the lower right. The stellar wind is a stream of charged particles moving rapidly outward from the star. It is a less energetic version of the solar wind that flows from the Sun. A second, fainter bow shock can be seen around a star near the upper right-hand corner of this image, taken from the Hubble Space Telescope. Both stars are located in the Orion Nebula; an intense star-forming region located about 1,500 light-years from the Earth. (Courtesy of NASA, the Hubble Heritage Team, STScI, and AURA.)