SP-402 A New Sun: The Solar Results From Skylab

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picture of the Sun

7

The Active Sun

 

Johnson Space Center (June 15, 1973, 14:09): Skylab, Houston. Be advised we understand in active region 31 there is a subnormal flare that is now in progress, and two of our instruments are recording information from it.
 
Astronaut Joe Kerwin, M.D. (12 minutes later): Houston, Skylab. I'd like you to be the first to know that the pilot is the proud father of a genuine flare.

 

[127] The surface of the Sun, once thought to be smooth and unblemished, has been shown with modern telescopes to be a seething inferno that is never still. Today, with the best modern telescopes, at the best observing sites, and on the best days, we can watch the roaring chromosphere in almost frightening detail, as in the dramatic picture at right from the Sacramento Peak Solar Observatory. But even in these superb, ground-based pictures, we see but a limited part of the real action on our Sun. In the chromosphere, and in the even more unstable layers above it, which are not visible from the ground, a variety of catastrophic events occur: explosions and eruptions on scales unknown on Earth. Skylab observing programs were planned to make the best use of the chance to record these same violent events in X-ray and ultraviolet wavelengths to seek their causes; to trace, for the first time, their full extension through all the layers of the solar atmosphere; and, with the coronagraph, to observe their impact in the outer corona of the Sun. Sunspots were easy targets, but detailed observations of more explosive and shorter lived events required constant vigilance, both in the spacecraft and by the many Skylab support scientists on the ground. When the Sun became more active, an even closer watch was kept. The effort paid off: Skylab observations of action on the Sun were the most fruitful and comprehensive ever made.

Skylab observations revealed the magnetic nature of active regions on the Sun, how they are woven from looped lines of magnetic force, and how new active regions are born, interconnect, and eventually die. From carefully taken Skylab data in ultraviolet and X-ray wavelengths have come important new insights into the elusive nature of solar flares, their exceedingly high temperatures and confinement-in crucial early stages only seen by Skylab-to tiny and intense loops of magnetic force near the surface of the Sun. Flare precursors, found in real-time X-ray pictures taken by Skylab astronauts, may help in making practical predictions of these terrestrially important solar events. Skylab observations of the active Sun established the fundamental role of another new solar feature of the X-ray and ultraviolet Sun called "bright points." Associated with intense, compact mag-....

 

close-up photo of the Sun

 

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Solar features called <<bright points>> are small, pointlike sources of X-ray and ultraviolet emission, like tiny lights glowing in the upper atmosphere of the Sun.

BRIGHT POINTS: THE GLITTERING SUN

Solar features called "bright points" are small, pointlike sources of X-ray and ultraviolet emission, like tiny lights glowing in the upper atmosphere of the Sun. They were first observed in earlier rocket images made in X-ray wavelengths, and confirmed and better studied in Skylab's extensive coverage of the Sun. Smaller than sunspots, but also associated with intense, localized magnetic fields of opposite magnetic polarities, they have now been shown to be an unappreciated and basic element of solar activity, perhaps as fundamental as sunspots, which for so long have served as the standard index of change on the Sun. Unlike sunspots and larger active regions, bright points were found in Skylab coverage to extend all over the Sun, even appearing at the poles and in coronal holes. A significant fraction occurred predominantly in active regions in lower solar latitudes, following the usual pattern of activity on the Sun. About 10 percent show explosive brightening, like miniature flares, increasing in brightness by a factor of 10 in about 10 min. Unlike other known classes of solar activity, bright points are of short duration, lasting less than half an hour. Several surprising features of bright points have come from the analysis of the Skylab films. They seem to increase in overall numbers on the Sun as other known signs of activity fall, as though to balance the overall budget of activity on the Sun. Of equal significance is the Skylab finding that the total magnetic flux contained in bright points is at least as great, and possibly greater, than that customarily assessed in counts of sunspots and active regions. Because magnetic activity rules the Sun, bright points may indeed prove to be the most important feature of the star. If that is so, in counting sunspots and plotting sunspot numbers, solar astronomers may long have been distracted.

 

...-netic regions, bright points account for a major fraction of the magnetic energy on the surface of the Sun. Like sunspots, though of shorter duration, they come and go, but by a surprising set of rules: all over the Sun they increase in numbers and magnetic flux as sunspots and active regions wane, perhaps to balance the overall magnetic budget of the Sun. As such, they add a new dimension to standard pictures of varying solar activity and alter long-held concepts of the nature of cyclic, solar change. Caught in the net of Skylab's coverage of the outer corona were a truly dramatic find-scores of the immense clouds of coronal material called "transients" that are propelled outward from the Sun by the initial force of flares and prominence eruptions. Some expand to exceed the size of the Sun itself and travel outward toward the planets, with recognized terrestrial effects.

 

 

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FIRST SEEN IN X-RAY PICTURES, scores of bright points of light dot the solar disk, like scattered jewels. The glittering points (1) are found all over the Sun and are not limited to lower latitudes where the large-scale active regions lie. Nor are they absent in coronal holes. Detailed comparison with magnetic maps of the Sun show that X-ray bright points overlie compact magnetic regions that have both positive and negative polarity. Bright points come and go, lasting on the average about 8 hr. Some last for several days; others flare up and fade in a few minutes. At any time, 100 or more can be seen on the visible hemisphere of the Sun. Because there are so many bright points, they account for much of the Sun's magnetic energy. SKYLAB found that bright points dot the ultraviolet Sun as well, when seen in wavelengths that sample the transition region and corona. Here at coronal temperatures (2), bright points are found to coincide with bright fragments of the chromospheric network, where magnetic field is concentrated.

FIRST SEEN IN X-RAY PICTURES, scores of bright points of light dot the solar disk, like scattered jewels. The glittering points (1) are found all over the Sun and are not limited to lower latitudes where the large-scale active regions lie. Nor are they absent in coronal holes. Detailed comparison with magnetic maps of the Sun show that X-ray bright points overlie compact magnetic regions that have both positive and negative polarity. Bright points come and go, lasting on the average about 8 hr. Some last for several days; others flare up and fade in a few minutes. At any time, 100 or more can be seen on the visible hemisphere of the Sun. Because there are so many bright points, they account for much of the Sun's magnetic energy.

SKYLAB found that bright points dot the ultraviolet Sun as well, when seen in wavelengths that sample the transition region and corona. Here at coronal temperatures (2), bright points are found to coincide with bright fragments of the chromospheric network, where magnetic field is concentrated.

When plotted on a map of the Sun (3), X-ray bright points show a slight preference for middle latitudes, and seem equally at home in active regions, coronal holes, or the undisturbed Sun. individual bright points observed in X-ray images of the Sun during the first Skylab mission are shown in this map as single dots. Outlines of coronal holes seen during the same 28 days are also drawn in, and active regions are shown as squares.

When plotted on a map of the Sun (3), X-ray bright points show a slight preference for middle latitudes, and seem equally at home in active regions, coronal holes, or the undisturbed Sun. individual bright points observed in X-ray images of the Sun during the first Skylab mission are shown in this map as single dots. Outlines of coronal holes seen during the same 28 days are also drawn in, and active regions are shown as squares.

 

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BRIGHT POINTS in the ultraviolet (4) are related to patterns of magnetic field (5) concentrated at boundaries of supergranule cells in the chromospheric network.

BRIGHT POINTS in the ultraviolet (4) are related to patterns of magnetic field (5) concentrated at boundaries of supergranule cells in the chromospheric network. Color-coded ultraviolet pictures (4) of a small area of the solar surface distinguish chromosphere (blue), transition region (red), and corona (green) contributions. Green corona is absent in a sprawling, red and blue coronal hole. Within the hole, and elsewhere in the area, bright, circular patterns outline the boundaries of super-granule cells. Bright (white) points fall on the network super-granule cell boundaries.

 

Magnetic map of the same area (5) delineates the same patterns, cell-for-cell, and ties bright points to magnetic concentrations at super-granule cell boundaries.

Magnetic map of the same area (5) delineates the same patterns, cell-for-cell, and ties bright points to magnetic concentrations at super-granule cell boundaries. Concentrated magnetic fields are shown in the magnetogram in red for one polarity and in light blue for the opposite polarity. In this area of the Sun, super-granule cell boundaries are predominantly red.

 

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(1-6) A PAIR OF X-RAY BRIGHT POINTS flash alternately off and on, as in a lighted warning sign.

(1-6) A PAIR OF X-RAY BRIGHT POINTS flash alternately off and on, as in a lighted warning sign.

(1-6) A PAIR OF X-RAY BRIGHT POINTS flash alternately off and on, as in a lighted warning sign. Then a whole arcade of loops brightens, revealing that short-lived bright points also appear in the legs of individual X-ray arches. The sequence (1-6), made in hard X-rays that sample temperatures of 3 to 5 x 106 K, covers an interval of only 17 min. Hazy X-ray emission from the bright regions at top and bottom of each picture, more typical of what is seen in the quiet corona at these high temperatures, contrasts the minute size of X-ray bright points, where energy is highly concentrated. Throughout the display, bright points, which in this case resemble small solar flares, are more intense than any other feature.

(7) NEAR THE SUN'S SOUTH POLE a point of light the size of Earth suddenly brightens, calling attention to itself in a large coronal hole. Flaring bright points like this, unseen before Skylab, constitute a class of small flares that, unlike more common flares seen in active regions, appear all over the disk of the Sun.

 

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ANOTHER FLARING BRIGHT POINT (8-10) flashes out in X-ray pictures taken about 12 min apart. This example also occurs in a polar coronal hole, near the limb of the Sun, where we see it in slanting view.

ANOTHER FLARING BRIGHT POINT (8-10) flashes out in X-ray pictures taken about 12 min apart. This example also occurs in a polar coronal hole, near the limb of the Sun, where we see it in slanting view.

AN ULTRAVIOLET BRIGHT POINT, portrayed simultaneously in three wavelengths (11), rises and falls in brightness in a few minutes. Here brightness is shown as a contour map of a small area of the solar surface; brighter features appear as spikes. Bottom row (red) portrays chromospheric temperatures, the middle row (green) portrays the transition region, and the top row (blue) portrays the 1.4 x 106 K temperatures of the corona. Red, green, and blue pictures in each column were made at the same instant, in three samples taken 5 min apart. The bright spike that comes and goes in the center of each contour map is from a small area about the size of Earth. Barely distinguishable in the chaotic chromosphere, it is most spectacular in the transition region, and a distinct feature of the corona, where it would also be seen in X-rays. Skylab found that most bright points do not flare up like this sample, but live more constant lives.

AN ULTRAVIOLET BRIGHT POINT, portrayed simultaneously in three wavelengths (11), rises and falls in brightness in a few minutes.

 

[134-135] ACTIVE REGIONS: THE VIOLENT SUN

For years astronomers have recognized that activity on the surface of the Sun, in the photosphere and chromosphere, evolves in local areas called <<active regions>> that are associated with areas of strong, localized magnetic fields.

For years astronomers have recognized that activity on the surface of the Sun, in the photosphere and chromosphere, evolves in local areas called "active regions" that are associated with areas of strong, localized magnetic fields. These fields come and go in recognized ways, lasting weeks or months and following known patterns on the Sun: appearing at higher latitudes early in the sunspot cycle and gradually shifting toward the equator as the cycle first waxes and then wanes. Within the bounds of active regions are found all the classic signs of solar activity: sunspots, prominences, flares, and coronal condensations. With Skylab's ultraviolet and X-ray coverage, it was possible for the first time to study in detail the extensions of active regions in the upper chromosphere, transition region, and lower corona, where most of their energy lies. High-resolution pictures, made simultaneously in selected wavelengths in the ultraviolet and in X-ray wavelengths, allowed their important upper reaches to be traced in detail before unknown. A fundamental finding, apparent in all the Skylab pictures, was the fact that active regions are defined by soaring magnetic fields; they consist of hot loops of high-temperature gas, trapped and contained by arching lines of force whose footpoints stand in opposite regions of high magnetic field on the surface of the Sun. In the ultraviolet and in X-rays, the important connections between these magnetic regions were clearly revealed, including the lacing together of magnetic regions often far apart on the surface of the Sun. The birth and death of solar active regions were well observed in Skylab's net of continuous pictures. Magnetic loops in new, old, and middle-aged active regions account for most of the emission seen in Skylab's ultraviolet and X-ray images of the Sun , confirming the feeling that it is solar magnetism that calls the tune of changing solar behavior.

 

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In hard (1, 4) and soft (2) X-rays, individual active regions stand out in sharp contrast, illustrating that they are areas of locally high temperature.

REGIONS OF MOST VIOLENT ACTIVITY on the Sun are restricted to two parallel belts on either side of the solar equator; at the beginning of each 11-year solar cycle they appear at about 45° solar latitude and gradually converge toward the Sun's equator. At the time of Skylab's flight, late in the declining solar cycle, active regions were found in the two low-latitude bands shown here (1-5). In hard (1, 4) and soft (2) X-rays, individual active regions stand out in sharp contrast, illustrating that they are areas of locally high temperature. In (1) brightness is color-contoured from the hard X-ray photograph (4), with brightest shown as white. White regions correspond to local temperatures in excess of 5 x 106 K, a thousand times hotter than in the photosphere below, where the same active regions appear as cool sunspots. In a simultaneous magnetic map of the Sun (3), we see the same active regions in the photosphere as strong magnetic fields of opposite polarity, here coded red and blue. At coronal temperatures sampled in X-ray photographs (2, 4), active regions are revealed as fuzzy loops that connect these areas of opposite magnetic polarity. The tops of the loops are also the source of varying radio noise from the Sun, as seen in (5), where a map showing locations of bursts of radio static from the Sun is overlaid on the X-ray photo (4). The Sun is a constant emitter of radio waves- the most intense and variable of which can be traced with radio telescopes to active regions, where trapped electrons bounce back and forth along the arched loops, emitting static as they go.

 

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In hard (1, 4) and soft (2) X-rays, individual active regions stand out in sharp contrast, illustrating that they are areas of locally high temperature. In (1) brightness is color-contoured from the hard X-ray photograph (4), with brightest shown as white. White regions correspond to local temperatures in excess of 5 x 106 K, a thousand times hotter than in the photosphere below, where the same active regions appear as cool sunspots. In a simultaneous magnetic map of the Sun (3), we see the same active regions in the photosphere as strong magnetic fields of opposite polarity, here coded red and blue. At coronal temperatures sampled in X-ray photographs (2, 4), active regions are revealed as fuzzy loops that connect these areas of opposite magnetic polarity. The tops of the loops are also the source of varying radio noise from the Sun, as seen in (5), where a map showing locations of bursts of radio static from the Sun is overlaid on the X-ray photo (4).

 

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SKYLAB ULTRAVIOLET PICTURES disclose the patterns of magnetic loops that holds hot, ionized gases above all solar active regions.

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SKYLAB ULTRAVIOLET PICTURES disclose the patterns of magnetic loops that holds hot, ionized gases above all solar active regions.

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SKYLAB ULTRAVIOLET PICTURES disclose the patterns of magnetic loops that holds hot, ionized gases above all solar active regions.

SKYLAB ULTRAVIOLET PICTURES disclose the patterns of magnetic loops that holds hot, ionized gases above all solar active regions.

 

 

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CLOSE-UP X-RAY VIEWS from Skylab (1-4) reveal systems of loops that lace together areas of intense opposite magnetic polarity on the surface of the Sun; these areas appear in an accompanying magnetogram (5) as light and dark patches. Magnetic fields on the Sun are far stronger than that of Earth, and are created and maintained by large-scale motions in hot ionized gas. In active regions the fields and loops bottle up prodigious stores of energy, which from time to time are suddenly released in violent eruptions and solar flares. Solar eruptions in turn affect our Earth, when bursts of high-energy radiation and clouds of charged electrons, protons, and other atomic particles are received from the Sun. PROGRESSIVELY LONGER X-RAY EXPOSURES (6-8) bung out the magnetic loops that stitch segments of a solar active region together. The shortest exposure (6) shows only the intense emission from the innermost loops. In this small area of the solar surface, about the size of Jupiter, a tangle of magnetic loops is seen near the limb of the Sun. Some errant lines in the longest exposure (8) arch over to outlying bright points and magnetic clumps in the surrounding network.

CLOSE-UP X-RAY VIEWS from Skylab (1-4) reveal systems of loops that lace together areas of intense opposite magnetic polarity on the surface of the Sun; these areas appear in an accompanying magnetogram (5) as light and dark patches. Magnetic fields on the Sun are far stronger than that of Earth, and are created and maintained by large-scale motions in hot ionized gas. In active regions the fields and loops bottle up prodigious stores of energy, which from time to time are suddenly released in violent eruptions and solar flares. Solar eruptions in turn affect our Earth, when bursts of high-energy radiation and clouds of charged electrons, protons, and other atomic particles are received from the Sun.

 

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PROGRESSIVELY LONGER X-RAY EXPOSURES (6-8) bring out the magnetic loops that stitch segments of a solar active region together. The shortest exposure (6) shows only the intense emission from the innermost loops. In this small area of the solar surface, about the size of Jupiter, a tangle of magnetic loops is seen near the limb of the Sun. Some errant lines in the longest exposure (8) arch over to outlying bright points and magnetic clumps in the surrounding network.

 

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ULTRAVIOLET DISSECTION OF ACTIVE REGIONS near the limb paints a colored picture of the hot loops that arch high into the solar atmosphere. Here light from the low chromosphere (9) is colored green; from the transition region (10), red; and from the corona (11), blue. In the larger photo (12) the three pictures and colors are added together. The set of active region loops is too hot to be visible at (green) chromospheric temperatures and too cool to show up sharply in the hotter blue corona, where we see only the diffuse envelope of their outer sheaths. Temperatures in the transition region (red) are just right, and by sampling temperature. of about 450 OOO K (10). we see loops most distinctly. Sequences like this from Skylab enabled solar scientists to pinpoint temperatures and other conditions in the roaring solar atmosphere 150 million km away.

ULTRAVIOLET DISSECTION OF ACTIVE REGIONS near the limb paints a colored picture of the hot loops that arch high into the solar atmosphere. Here light from the low chromosphere (9) is colored green; from the transition region (10), red; and from the corona (11), blue. In the larger photo (12) the three pictures and colors are added together. The set of active region loops is too hot to be visible at (green) chromospheric temperatures and too cool to show up sharply in the hotter blue corona, where we see only the diffuse envelope of their outer sheaths. Temperatures in the transition region (red) are just right, and by sampling temperature. of about 450 000 K (10). we see loops most distinctly. Sequences like this from Skylab enabled solar scientists to pinpoint temperatures and other conditions in the roaring solar atmosphere 150 million km away.

 

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THE BIRTH OF TWIN ACTIVE REGIONS was witnessed by the Skylab X-ray telescope. Sequence (1-3) covers a day and a half in the life of the Sun. Here X-ray pictures sampling coronal temperatures of 3 to 5 x 106 K are printed as negatives, with black indicating bright emission, and white as cooler areas. In one day, between (1) and (2), two black patches of X-ray emission appear, each about the size of Earth, to the left and slightly above and below the center of the Sun. As the lower one is born, looping arms reach out from an older, larger active region on its left, as though to clasp it to the Sun. By (3), a half day later the ties are complete. Magnetograms (4, 5) taken about midway between the three Skylab X-ray pictures show that both newborn active regions consist of adjacent positive (white) and negative (black) areas of solar magnetism that have risen from within the Sun to break through the surface of the photosphere. TWO DAYS LATER, between (6) and (7), another new active region emerges, as a smaller dot of X-ray emission just below the center of the Sun; it is also connected by magnetic loops to older, active regions. Between the two exposures, made about 12 hr apart, X-ray loops in other regions (left of center) arch across the solar equator to connect active regions in northern and southern latitude bands. The connected regions in the photosphere lie over the solar horizon from each other, as far apart as the Moon and Earth. Had this connection occurred at the limb of the Sun, we would have seen it as a gigantic coronal arch that straddled the solar equator. A magnetogram (8), made shortly after (7), identifies the magnetic areas that are laced together by hot X-ray arches in (6,7).

THE BIRTH OF TWIN ACTIVE REGIONS was witnessed by the Skylab X-ray telescope. Sequence (1-3) covers a day and a half in the life of the Sun. Here X-ray pictures sampling coronal temperatures of 3 to 5 x 106 K are printed as negatives, with black indicating bright emission, and white as cooler areas. In one day, between (1) and (2), two black patches of X-ray emission appear, each about the size of Earth, to the left and slightly above and below the center of the Sun. As the lower one is born, looping arms reach out from an older, larger active region on its left, as though to clasp it to the Sun. By (3), a half day later the ties are complete. Magnetograms (4, 5) taken about midway between the three Skylab X-ray pictures show that both newborn active regions consist of adjacent positive (white) and negative (black) areas of solar magnetism that have risen from within the Sun to break through the surface of the photosphere.

TWO DAYS LATER, between (6) and (7), another new active region emerges, as a smaller dot of X-ray emission just below the center of the Sun; it is also connected by magnetic loops to older, active regions. Between the two exposures, made about 12 hr apart, X-ray loops in other regions (left of center) arch across the solar equator to connect active regions in northern and southern latitude bands. The connected regions in the photosphere lie over the solar horizon from each other, as far apart as the Moon and Earth. Had this connection occurred at the limb of the Sun, we would have seen it as a gigantic coronal arch that straddled the solar equator. A magnetogram (8), made shortly after (7), identifies the magnetic areas that are laced together by hot X-ray arches in (6,7).

 

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AS SKYLAB WATCHES, a solar active region dies, much more slowly than it was born. X-ray loops gradually spread and fade, like smoke from a dying fire, in pictures (10-12) spaced about 27 days, or one solar rotation, apart. A magnetogram (9), made on the same day as (10), shows in light and dark patches the areas of opposite magnetic polarity in the photosphere that are connected by the X-ray loops. An picture (13), made on the same day as (9) and (10), portrays the same region in the chromosphere. sequence (13-15), made in step with the X-ray pictures above it, documents the parallel fading and disappearance of the active region in the lower chromosphere. X-ray loops preserve the longest memory of a solar active region, lingering as faded, ghostly loops long after the end of its life in chromospheric layers below.

AS SKYLAB WATCHES, a solar active region dies, much more slowly than it was born. X-ray loops gradually spread and fade, like smoke from a dying fire, in pictures (10-12) spaced about 27 days, or one solar rotation, apart. A magnetogram (9), made on the same day as (10), shows in light and dark patches the areas of opposite magnetic polarity in the photosphere that are connected by the X-ray loops. An Capital H, subscript Greek letter alpha picture (13), made on the same day as (9) and (10), portrays the same region in the chromosphere. Capital H, subscript Greek letter alpha sequence (13-15), made in step with the X-ray pictures above it, documents the parallel fading and disappearance of the active region in the lower chromosphere. X-ray loops preserve the longest memory of a solar active region, lingering as faded, ghostly loops long after the end of its life in chromospheric layers below.

 

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Many solar flares were observed in detail by Skylab, including the crucial early phases that give new clues to the sources of their energy and its explosive, sudden release. Ultraviolet spectra taken of different flare phases revealed the existence of localized temperatures within flares that exceeded 20 x 106 K, as hot as conditions in the searing interior of the Sun.

 

FLARES: THE EXPLOSIVE SUN

Solar flares are the most powerful and explosive of all forms of solar activity and, as might be expected, the most important in terrestrial effect. For many years astronomers have watched for them in the chromosphere, where they appear as sudden and generally unexpected brightenings, lasting from a few minutes to half an hour, usually in strong or complex magnetic regions. In the chromosphere they appear much like a pool of gasoline that has suddenly been ignited. Flares are more than flashes in the pan. Atomic particles released from the Sun during larger flares sweep against our upper atmosphere, disturbing Earth's magnetic field, disrupting the ionosphere and radio communications, and causing auroras and other effects.

For some time it has been known that ultraviolet and X-ray radiation from the Sun increases dramatically during flares, causing other important effects in our upper atmosphere. These increase suggested that flares were high-temperature phenomena whose secrets would be found in the higher, hotter regions of the solar atmosphere Thus a prime objective of Skylab's solar observing program was the observation of the complete behavior of flares in X-ray and ultraviolet wave lengths[145], taking advantage of Skylab's position above the atmosphere of Earth and its unprecedented ability to see fine details on the Sun in these hidden wavelengths.

Many solar flares were observed in detail by Skylab, including the crucial early phases that give new clues to the sources of their energy and its explosive, sudden release. Ultraviolet spectra taken of different flare phases revealed the existence of localized temperatures within flares that exceeded 20 x 106 K, as hot as conditions in the searing interior of the Sun. High-resolution pictures made in hard X-rays revealed an unappreciated confinement of the initial flare disturbance to intensely heated regions so small that they taxed the resolving power of Skylab's telescopes. In several important cases, X-ray and ultraviolet observations demonstrated that the important seat of flares lay in the tops of small, intense magnetic loops and that certain flares consisted of the sequential brightening of loops in a series, or arcade, like the popping off of a string of solar firecrackers. Energy released in flares in the ultraviolet and X-ray wavelengths was far greater than the emission seen customarily from the ground in visible wavelengths, confirming that what was seen of them in the past was but a fraction of their most important features.

 

[146-147] WHEN OR WHERE flares will occur is not known in advance. This made Skylab's chances of observing a flare less certain. Moreover, because Skylab was launched late in the solar cycle, when the general state of solar activity was low no one was sure that it would see any large flares at all. It was hoped that several would occur, and that through rapid communications and quick reactions by the astronaut crews, at least one could be caught in the sights of Skylab's battery of solar telescopes.

The Sun obliged by producing many flares, of all sizes; more than 100 smaller flares were caught in Skylab's net of routine observations of the entire disk of the Sun in X-rays. When rapid responses and precise pointing were required for taking spectra and other more detailed observations of flares, Skylab's alert crews time after time brought home their quarry, as revealed in the record of Skylab Capital H, subscript Greek letter alpha pictures (2-7), that document where Skylab's telescopes were pointed during six of the larger flares. At the telescope control console (8) is one of the flare hunters, Astronaut Bill Pogue.

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When rapid responses and precise pointing were required for taking spectra and other more detailed observations of flares, Skylab's alert crews time after time brought home their quarry, as revealed in the record of Skylab pictures (2-7), that document where Skylab's telescopes were pointed during six of the larger flares. At the telescope control console (8) is one of the flare hunters, Astronaut Bill Pogue.

The Sun obliged by producing many flares, of all sizes; more than 100 smaller flares were caught in Skylab's net of routine observations of the entire disk of the Sun in X-rays.

 

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Skylab's telescopes found that in X-rays (1) flares are triggered by the brightening of small, low-lying loops in active regions.

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ULTRAVIOLET PICTURES from Skylab (2,6) spread a flare out in wavelength, like a deck of cards, to find what it looks like at different layers and temperatures.

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A PRIME OBJECT in Skylab's hunt for flares was the answer to a simple question: What does the start of a flare look like in the high-temperature regions of the solar atmosphere-above the level where we see them as spreading fires in the chromosphere? What strikes the match that sets off the inflammable pool? Sought was the secret of what causes solar flares and, from this, better predictions of when and where they will occur.

Skylab's telescopes found that in X-rays (1) flares are triggered by the brightening of small, low-lying loops in active regions. X-ray flares began in tiny areas of individual loops, as smaller features than many would have guessed. But like the quick, small flash in the sparkplug of an engine these intense brightenings set off other, larger effects. The spreading flares seen by Skylab in Capital H, subscript Greek letter alpha were aftereffects of initial disruptions in hot coronal loops, where momentary temperatures exceeded 20 x 106 K-hotter than in the core of the Sun itself where, in denser material, nuclear reactions occur.

ULTRAVIOLET PICTURES from Skylab (2,6) spread a flare out in wavelength, like a deck of cards, to find what it looks like at different layers and temperatures. At chromospheric temperatures (full disk in (2)), the flare is a double ribbon, much as it appeared in slightly cooler Capital H, subscript Greek letter alpha . To the left in (2), spread along the bright horizontal streak, we see simultaneous images of the same flare in several other spectral lines, sampling hotter temperatures more typical of the corona. Enlargements of the last two images of the flare in (2) are shown in which, at temperatures of 3 x 106 K (3) and 2 x 106 K (4), we see that it is an arcade of brightened loops whose opposite footpoints rest on the chromospheric ribbons. These views (2-4) sample the flare slightly after its hottest stage.

A FLARE at the limb of the Sun (5,6) affords an edge-on view of the loop where it all began. These negative prints show bright regions as black. In (5), three overlapping ultraviolet images of the flare are strung out in wavelength. They sample temperatures, left to right, of about 3, 14, and 2 x 106 K, from iron vapor which in the hot inferno of the Sun has been stripped of most of its 26 electrons. A bright ribbon runs along the footpoints of an arcade of brighttopped loops. Nestled under the loops, and seen best in the higher temperature lines, left, is a small, low-lying loop where the flare began. Other spectral pictures of the same flare (6) show the same low-lying loop at even higher temperatures, in the light of iron vapor at 17 million K, which has but three electrons left, and that of calcium ionized 16 times at about 5 million K.

 

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A SOLAR FLARE in an active region near the limb of the Sun flashes out and then fades in color-contoured pictures (1-6) made by Skylab in the ultraviolet.

A SOLAR FLARE in an active region near the limb of the Sun flashes out and then fades in color-contoured pictures (1-6) made by Skylab in the ultraviolet. In a sequence (1-5) covering 23 min, the brightest features are colored yellow; each samples the transition region, at about 300 000 K, where flare brightening is accentuated. In (6) the same flare, at the same time as (4), is shown in a composite view: chromosphere (blue), transition region (green), and corona (red), where loops lace the flaring region. This flare burst out near midnight in the central United States, while American solar observatories were shut down for the night. Skylab, in continuous contact with solar observatories all over the world, operated in another realm, where days were roughly 60 min, and nights were half as long.

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A SOLAR FLARE in an active region near the limb of the Sun flashes out and then fades in color-contoured pictures (1-6) made by Skylab in the ultraviolet.

(6)

 

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DURING A SKYLAB FLARE (7-9), magnetic loops in an arcade go off like a string of firecrackers as a wave of excitation sweeps along them, end to end.

DURING A SKYLAB FLARE (7-9), magnetic loops in an arcade go off like a string of firecrackers as a wave of excitation sweeps along them, end to end. A color-contoured X-ray picture (7) shows the brightest areas yellow, fainter in green, red, and blue. We look down on the tops of about 10 green loops, strung out in a slightly curved arcade. As Skylab watched, the loops brightened one by one, flashing off and on several times-the most intense emission at their tops. An ultraviolet view of the same region (8), here printed as a negative, catches the brightening (black) of the right-hand end of the arcade about 6 min later. The flare occurred along the border of a sunspot, in the active region identified in the Capital H, subscript Greek letter alpha picture (9).

 

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SPECTRAL IMAGES OF A FLARE are again spread out like a deck of cards in this ultraviolet spectrogram (1).

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SPECTRAL IMAGES OF A FLARE are again spread out like a deck of cards in this ultraviolet spectrogram (1). The flare appears in the full-disk image of the chromosphere as a y [gamma]-shaped spot, so bright that it was reversed, or "solarized," on the original Skylab film. Images of the same flare appear at many other wavelengths in the nearly continuous streak of light that runs horizontally across this solar spectrum. At the extreme left we see the flare in light of iron vapor ionized 23 times, at temperatures of about 17 x 106 K, where it appears as a small cloud of emission. Images of other, nonflaring active regions on the Sun are also spread out, but are wholly absent in spectral lines that sample hottest temperatures. The unexpected but well-observed flare occurred late in the first Skylab mission, bringing cheer to a Skylab crew that was eager to observe one.

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RICH PRIZES FROM SKYLAB'S SPECTROGRAPHS included detailed ultraviolet spectra of solar flares, such as (2), showing about 5000 spectral lines, each the unique signature of a specific element in the flaring region of the Sun.

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RICH PRIZES FROM SKYLAB'S SPECTROGRAPHS included detailed ultraviolet spectra of solar flares, such as (2), showing about 5000 spectral lines, each the unique signature of a specific element in the flaring region of the Sun. The scale at left gives the wavelength in angstroms. The identities and shapes of spectral lines-which appear as dark, vertical streaks in this negative print-contain encoded information on the physical conditions within the flare.

The broad, lens-shaped line near the top is the strongest spectral line of hydrogen, most abundant of all elements in the Sun. To secure this spectrum, taken in the late stages of the same flare as shown in (1), Skylab astronauts aimed the massive telescope mount at the point of brightest flare emission with a precision that could split the thickness of a dime the length of a football field away.

 

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A FLARE ERUPTS

WHERE A FLARE ERUPTS (3), the ultraviolet spectrum changes abruptly. Lines in the same, limited region of the solar ultraviolet spectrum are compared in (4) for the quiet Sun and in (5) during a Skylab flare. Both are negative prints: black, vertical spectral lines are really bright features of the solar spectrum. Four strong lines (due to ionized silicon and carbon) brighten and broaden dramatically during the flare; and many new bright spectral lines appear between them, as ultrahigh flare temperatures excite and ionize solar atoms in the vicinity of the flare.

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Lines in the same, limited region of the solar ultraviolet spectrum are compared in (4) for the quiet Sun and in (5) during a Skylab flare.

 

[154]

Solar prominences classified as <<active>> are those that suddenly erupt, often rising as though propelled outward through the corona by loaded springs.

ACTIVE PROMINENCES

Solar prominences classified as "active" are those that suddenly erupt, often rising as though propelled outward through the corona by loaded springs. Others have been noted to mysteriously and quickly disappear, their giant forms suddenly erased from chromospheric pictures made in wavelengths visible from the ground. Combined ultraviolet and X-ray observations of active prominences by Skylab captured many of these dynamic features of the Sun, and recorded the details of their eruption and disappearance, and the apparent magnetic origin of their expulsion. Clearly visible in the ultraviolet dissections of giant, eruptive [155] prominences were the twisted sinews of magnetic force that tore them from the Sun. Prominences that vanished in ultraviolet wavelengths that sampled cooler temperatures in the solar atmosphere were followed as they appeared in other wavelengths, exposing the secrets of their apparent vanishing act. The persistent coverage of Skylab's ultraviolet telescopes caught and followed the details of some of the largest and most dynamic prominence eruptions that have ever been seen on the Sun, and documented their association as triggers for other, even larger eruptions in the outer corona. Concurrent observations from Skylab's unique battery of solar telescopes provided an almost immediate understanding of the largest explosive prominence yet seen on the Sun.

 

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SOME OF THE PROMINENCES that float like lazy clouds above the solar surface suddenly erupt and break away from the Sun in cataclysmic action.

SOME OF THE PROMINENCES that float like lazy clouds above the solar surface suddenly erupt and break away from the Sun in cataclysmic action. Much larger than flares and not always associated with them, eruptive prominences constitute some of the Sun's most spectacular sights.

The "elbow prominence" was accidentally photographed by Astronaut Garriott while observing a small flare near the limb of the Sun beneath the mighty arch. We see it here in ultraviolet light of ionized helium, which tells us that its temperature was in the range of 30 000 to 90 000 K-far cooler and denser than the million-Kelvin corona through which we see it pass. Its intricate form traces out twisting magnetic field lines as they are torn from their roots in the chromosphere. The eruption began as a low-lying prominence that soared upward; here, after about 20 min, it has reached a dizzying altitude of more than 600 000 km above the Sun-almost twice the distance that separates Earth and the Moon. Our planet, for scale, is smaller than the black dot near the limb of the Sun beneath the arch.

 

[158]

SKYLAB TAKES THE TEMPERATURES of two active prominences by observing them in different ultraviolet spectral lines, here distinguished in color code. In each case they prove to be extensions of the cooler chromosphere into the hot corona.

SKYLAB TAKES THE TEMPERATURES of two active prominences by observing them in different ultraviolet spectral lines, here distinguished in color code. In each case they prove to be extensions of the cooler chromosphere into the hot corona.

(1,2,3) are color-coded dissections of the active prominence shown in composite form in (4), which is a feature seen edge-on above the solar limb. The active prominence is made of material mostly at chromospheric (red) temperatures, with some hot strands (blue) at about 150 000 K. At coronal temperatures (green) it is too cool to be seen.

A more active prominence (5-8) seems made of chromospheric (red) material in twisted strands, with an outer (green) sheath at 150 000 K. In the corona (blue), it almost disappears; in (8) the three color-coded views are combined.

 

 

[159]

THE SUN ENJOYS A MIDNIGHT FLING, casting off 6 billion metric tons of filmy outer dress at speeds of more than 100 km/s. This immense eruption (9), arching here more than 300 000 km above the Sun, was caught in Skylab's ultraviolet eye just before midnight, Houston time, about 10 min after it had left the surface of the Sun. (10-13) THE SUN ERASES A PROMINENCE in the chromosphere, only to see it reappear in the X-ray corona.

THE SUN ENJOYS A MIDNIGHT FLING, casting off 6 billion metric tons of filmy outer dress at speeds of more than 100 km/s. This immense eruption (9), arching here more than 300 000 km above the Sun, was caught in Skylab's ultraviolet eye just before midnight, Houston time, about 10 min after it had left the surface of the Sun. Pushing coronal material ahead of it, the eruption produced an even larger transient disturbance in the outer corona, and eventually dissipated in interplanetary space. Here, in light of ionized helium, at temperatures of 30 000 to 90 000 K, we see its clearly twisted form-revealing the coiled, magnetic springs that tell the secret of its expulsion from the Sun.

 

(10-13) THE SUN ERASES A PROMINENCE in the chromosphere, only to see it reappear in the X-ray corona. Between Capital H, subscript Greek letter alpha chromospheric pictures (10) and (11), 14 hr apart, the prominence (which appears on the disk of the Sun as a dark black filament) disappears. A Skylab X-ray view of the corona (12), made about the same time as (10),reveals a dark cavity above the filament, like a small coronal hole cut to fit the prominence below it. After the filament disappears from the chromosphere, the X-ray cavity fills in with hot, bright material (13), suggesting that the prominence has ascended, heating up drastically as it goes. Disappearing filaments have long been noted in Capital H, subscript Greek letter alpha in the chromosphere; X-ray pictures of the upper solar atmosphere illustrate that what disappears at one level may reappear at another.

 

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BREAKING THE GRIP of the closed magnetic loops that constrain other gases around it, a spray of chromospheric material surges upward, free of the Sun. An ultraviolet sequence (1-5) depicts action in pictures made about 5 min apart, each a composite of separate images made in chromospheric (red), transition region (green), and coronal (blue) temperatures. Eruption begins (2) as material in or near a small, compact loop develops enough energy to overcome the Sun's magnetic bonds. In (6) another, similar spray breaks loose, as seen in the light of ionized helium, color-contoured with brightest emission white.

BREAKING THE GRIP of the closed magnetic loops that constrain other gases around it, a spray of chromospheric material surges upward, free of the Sun. An ultraviolet sequence (1-5) depicts action in pictures made about 5 min apart, each a composite of separate images made in chromospheric (red), transition region (green), and coronal (blue) temperatures. Eruption begins (2) as material in or near a small, compact loop develops enough energy to overcome the Sun's magnetic bonds. In (6) another, similar spray breaks loose, as seen in the light of ionized helium, color-contoured with brightest emission white.

 

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AN ACTIVE PROMINENCE (7,8) printed in colors to represent different temperatures, alters its appearance in 3 hr, while Skylab circled twice around Earth. (9-13) COLOR COMPUTER PROCESSING of Skylab ultraviolet images differentiates subtle changes in a wispy prominence arch.

AN ACTIVE PROMINENCE (7,8) printed in colors to represent different temperatures, alters its appearance in 3 hr, while Skylab circled twice around Earth. Later the same prominence erupted, vanished, and subsequently reformed filling the arched magnetic lines with new material from the chromosphere.

(9-13) COLOR COMPUTER PROCESSING of Skylab ultraviolet images differentiates subtle changes in a wispy prominence arch. In wavelength-composite pictures (9,10), little change is apparent over a 34-min period. The same scene when shown as time differences in each of three separate wavelengths (11-13), reveals that during this time the cooler regions (11) are changing, while hotter ones (12,13) are not. In each of these three views, made at chromospheric transition region, and coronal temperatures, the earlier picture was colored red and the latter, green. Yellow, their sum, signals regions of no change. In this case the corona (13) seems to care little about what goes on at lower temperatures within it, and at transition region temperatures (12) the effects are also slight. With the layer of change identified, another primary color, blue, is added to the chromospheric view in (14), to introduce a third, intermediate time: Blue is 17 min later than green, which is 17 min later than red. Their sum, where no change occurs, is white.

 

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Sequence (1-4) recorded the imminent eruption for over an hour, but little change occurred.

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A GIANT PROMINENCE

A GIANT PROMINENCE (5), one of the largest seen in a decade, lifts off from the Sun. Sequence (1-4) recorded the imminent eruption for over an hour, but little change occurred. Each picture is a color composite of separate ultraviolet views that isolate different temperature layers. The three contained in (1) are stacked below (2); the hottest is blue and the coldest, red. Invisible at coronal temperatures (8), the prominence is mostly 20 000 K material (6), with hotter strands at about 70 000 K in green (7), which hint of twisted sinews of magnetic force. In (5), taken half an hour after (4), the prominence erupts and is caught in Skylab's net. In (9), taken later, a rain of prominence material falls back on the area of the solar limb where the prominence once stood.

the prominence is mostly 20 000 K material, with hotter strands at about 70 000 K in green

In (9), taken later, a rain of prominence material falls back on the area of the solar limb where the prominence once stood

 

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WITH STEADY HAND, a Skylab ultraviolet telescope dissects a prominence eruption as it leaves an active limb of the Sun. All three pictures (1-3) show the same region at the same instant.

WITH STEADY HAND, a Skylab ultraviolet telescope dissects a prominence eruption as it leaves an active limb of the Sun. All three pictures (1-3) show the same region at the same instant. At chromospheric temperatures (1), the prominence stands out like a sharp-nosed rocket poised to leave the solar surface. Magnetic field lines that channel its flight are far better seen in (2), where material at transition region temperatures of about 600 000 K outline fine loops that soar from place to place on the solar limb. At coronal temperatures of about 2 1/2 x 106 K (3), the prominence is lost in searing clouds of hot iron vapor, where it is far too cool to be seen.

 

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SOLAR PROMINENCES IN ACTION: These are some of Skylab's many-splendored views.

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SOLAR PROMINENCES IN ACTION: These are some of Skylab's many-splendored views.

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SOLAR PROMINENCES IN ACTION: These are some of Skylab's many-splendored views. Interpretation of the rich story of Skylab ultraviolet solar data was facilitated by computerized color enhancement of the original black and white images, highlighting subtle but important brightness differences. Color enhancement of solar data was among the new techniques of data reduction brought into use by Skylab scientists.

 

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CORONAL TRANSIENTS: THE EXTRAVAGANT SUN

As though held back through all the years, the Sun released for Skylab's view a shower of surprises in the outer corona in scale and numbers wholly unexpected.

As though held back through all the years, the Sun released for Skylab's view a shower of surprises in the outer corona in scale and numbers wholly unexpected. Indications of large, transient disturbances traveling through the Sun's outer corona had been rioted in solar radio records and found in coronagraph observations from earlier, unmanned spacecraft, but never had man seen these gargantuan loops rushing outward at fantastic speeds with the sharp detail of Skylab's coronagraph coverage. When the first was seen, early in the first manned mission, astronomers were elated to have captured an event of such surprising proportions: detailed pictures of the expulsion from the Sun of an eruption bigger than the disk of the Sun. Then, more and more were seen until, at the end of Skylab's life, more than 100 had been recorded. They now loom as our Sun's biggest and most dynamic intrusions into the world of interplanetary space, and a major reason for reassessing our picture of activity on the Sun. Because of Skylab's extensive, concurrent coverage in ultraviolet and X-ray wavelengths, discovery was followed, in subsequent analysis, by a full description of the features and causes of coronal transient disturbances. Their numbers clearly followed the state of activity on the Sun, increasing in frequency as sunspot numbers rose. It was clear that some of the coronal transients were initiated lower in the solar atmosphere by flares; another, larger class was associated with prominence eruptions. More than threefourths of the observed coronal transients originated in coronal active regions and more than 70 percent were initiated by eruptive prominences that acted as triggers to set off and propel the giant coronal loops. Flares that were accompanied by mass ejections were always followed by coronal transients. They contained too much material to be simply expanding prominences, but were recognized as a coronal phenomenon: large pieces of the outer corona, pushed outward at speeds of 100 km/s to more than 1200 km/s, drastically altering the corona's outer form. Though truly immense and much larger than prominences, coronal transients contain very little solar matter, and carry away but a negligible part of the Sun's total mass. Perhaps most important, they were found to be associated with a class of disturbances in the solar wind, demonstrating that they were propelled at least as far as the orbit of Earth to make their imprint on the upper atmosphere of our planet, 150 x 106 km away. Larger coronal transients have now been identified as one of the previously missing links by which part of the explosive energy of solar flares is carried to Earth, where it disrupts the ionosphere and geomagnetic field, with significant impact on our everyday life.

 

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SKYLAB CAPTURES a gargantuan disruption in the corona as it develops and moves outward from the Sun (1-4). Rapid picture rates were needed to catch its movement. Sequence (2-4) covers a period of but 23 min, during which time the outer shell of the disturbance traveled nearly a million kilometers.

SKYLAB CAPTURES a gargantuan disruption in the corona as it develops and moves outward from the Sun (1-4). Rapid picture rates were needed to catch its movement. Sequence (2-4) covers a period of but 23 min, during which time the outer shell of the disturbance traveled nearly a million kilometers.

 

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SKYLAB CAPTURES a gargantuan disruption in the corona as it develops and moves outward from the Sun (1-4). Rapid picture rates were needed to catch its movement. Sequence (2-4) covers a period of but 23 min, during which time the outer shell of the disturbance traveled nearly a million kilometers. An hour before, in (1), the corona gave little hint of the action that was to come.

The trigger of this coronal transient, like many others seen by Skylab's coronagraph, was an eruptive prominence that surged outward from the limb of the Sun, ejecting matter that disturbed the outer corona. We see the surge in action in (5), in ultraviolet light of ionized helium. Simultaneous observations like this made possible an almost immediate understanding of the new-found cosmic phenomenon.

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The trigger of this coronal transient, like many others seen by Skylab's coronagraph, was an eruptive prominence that surged outward from the limb of the Sun, ejecting matter that disturbed the outer corona. We see the surge in action in (5), in ultraviolet light of ionized helium.

 

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ANOTHER CORONAL BUBBLE is blown outward from behind the oversize occulting disk that hides the Sun. In but half an hour it exceeds in size its parent star-and still it grows, expanding outward in the void of space. Sequence (1-4) covers 89 min, about the time for Skylab to circle once around the Earth-a world insignificant on the scale of these largest known solar features. In (5), outlines of the moving transient are shown for the times of (1-4) with a corresponding X-ray image of the Sun for scale.

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ANOTHER CORONAL BUBBLE is blown outward from behind the oversize occulting disk that hides the Sun. In but half an hour it exceeds in size its parent star-and still it grows, expanding outward in the void of space. Sequence (1-4) covers 89 min, about the time for Skylab to circle once around the Earth-a world insignificant on the scale of these largest known solar features. In (5), outlines of the moving transient are shown for the times of (1-4) with a corresponding X-ray image of the Sun for scale.

(3 - left) and (4 - right)

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ANOTHER CORONAL BUBBLE is blown outward from behind the oversize occulting disk that hides the Sun. In but half an hour it exceeds in size its parent star-and still it grows, expanding outward in the void of space. Sequence (1-4) covers 89 min, about the time for Skylab to circle once around the Earth-a world insignificant on the scale of these largest known solar features. In (5), outlines of the moving transient are shown for the times of (1-4) with a corresponding X-ray image of the Sun for scale. Although coronal transients are immense, they are mostly empty space-diffuse clouds of electrons and other atomic particles that represent but a negligible part of the Sun. A trillion coronal bubbles would weigh no more than our solid Earth. Had the Sun expended one of these transient features each day since it was born, the material lost would still be an insignificant part of the total mass of the Sun.

A CORONAL TRANSIENT of another kind blows apart nearly half the corona in these views (6,7) taken less than 2 hr apart. Skylab found that transient disturbances passing through the outer solar corona often rearranged the entire configuration of magnetic lines and loops that define coronal forms.

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A CORONAL TRANSIENT of another kind blows apart nearly half the corona in these views (6,7) taken less than 2 hr apart.

 

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A COLOSSAL CORONAL TRANSIENT balloons outward from the Sun, dwarfing the disk of the Sun and the surge in the chromosphere that started it. Sequence (1-3) covers only 29 min. In (3) a simultaneous image of the ultraviolet Sun is superposed, revealing the eruptive prominence that lashes out like a tongue of fire propelling the great disturbance. The eruptive prominence was well observed from the ground in [image- Capital H, subscript Greek letter alpha] . In (4), colored outlines trace the initial stages of the [image- Capital H, subscript Greek letter alpha], eruption at times of 1 hr (green), 47 min (red), and 36 min (blue) before (1). A contour map of hard X-ray emission from the low corona over the disturbance on the limb is shown in (5), for time midway between (1) and (2). Brightest X-ray emission comes from regions of the corona where the local temperature exceeds 3 x 106 K.

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A COLOSSAL CORONAL TRANSIENT balloons outward from the Sun, dwarfing the disk of the Sun and the surge in the chromosphere that started it. Sequence (1-3) covers only 29 min. In (3) a simultaneous image of the ultraviolet Sun is superposed, revealing the eruptive prominence that lashes out like a tongue of fire propelling the great disturbance. The eruptive prominence was well observed from the ground in [image- Capital H, subscript Greek letter alpha] . In (4), colored outlines trace the initial stages of the H`, eruption at times of 1 hr (green), 47 min (red), and 36 min (blue) before (1). A contour map of hard X-ray emission from the low corona over the disturbance on the limb is shown in (5), for time midway between (1) and (2). Brightest X-ray emission comes from regions of the corona where the local temperature exceeds 3 x 106 K.

(3)

A COLOSSAL CORONAL TRANSIENT balloons outward from the Sun, dwarfing the disk of the Sun and the surge in the chromosphere that started it. Sequence (1-3) covers only 29 min. In (3) a simultaneous image of the ultraviolet Sun is superposed, revealing the eruptive prominence that lashes out like a tongue of fire propelling the great disturbance. The eruptive prominence was well observed from the ground in Capital H, subscript Greek letter alpha . In (4), colored outlines trace the initial stages of the Capital H, subscript Greek letter alpha , eruption at times of 1 hr (green), 47 min (red), and 36 min (blue) before (1). A contour map of hard X-ray emission from the low corona over the disturbance on the limb is shown in (5), for time midway between (1) and (2). Brightest X-ray emission comes from regions of the corona where the local temperature exceeds 3 x 106 K.

 

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A LASHING PROMINENCE (1-8) whips a delicate disturbance through the gossamer form of the outer corona (9-12). Sequence (1-8), in [image- Capital H, subscript Greek letter alpha] and ultraviolet light of helium, spans half an hour. The first coronal picture (9) was made at the same time as (6); in (12), an hour later, the coronal transient-a feature much brighter and more substantial than the diffuse, radial threads through which it travels-has sailed beyond the reach of the coronagraph, more than 3 x 106 km from the edge of the Sun. In (13,14), simultaneous images of the rupturing prominence, made in many wavelengths by Skylab's ultraviolet spectroheliograph, classify the fragments as the prominence flies apart. A LASHING PROMINENCE (1-8) whips a delicate disturbance through the gossamer form of the outer corona (9-12). Sequence (1-8), in [image- Capital H, subscript Greek letter alpha] and ultraviolet light of helium, spans half an hour. The first coronal picture (9) was made at the same time as (6); in (12), an hour later, the coronal transient-a feature much brighter and more substantial than the diffuse, radial threads through which it travels-has sailed beyond the reach of the coronagraph, more than 3 x 106 km from the edge of the Sun. In (13,14), simultaneous images of the rupturing prominence, made in many wavelengths by Skylab's ultraviolet spectroheliograph, classify the fragments as the prominence flies apart.

A LASHING PROMINENCE (1-8) whips a delicate disturbance through the gossamer form of the outer corona (9-12). Sequence (1-8), in [image- Capital H, subscript Greek letter alpha] and ultraviolet light of helium, spans half an hour. The first coronal picture (9) was made at the same time as (6); in (12), an hour later, the coronal transient-a feature much brighter and more substantial than the diffuse, radial threads through which it travels-has sailed beyond the reach of the coronagraph, more than 3 x 106 km from the edge of the Sun. In (13,14), simultaneous images of the rupturing prominence, made in many wavelengths by Skylab's ultraviolet spectroheliograph, classify the fragments as the prominence flies apart.

A LASHING PROMINENCE (1-8) whips a delicate disturbance through the gossamer form of the outer corona (9-12). Sequence (1-8), in Capital H, subscript Greek letter alpha and ultraviolet light of helium, spans half an hour. The first coronal picture (9) was made at the same time as (6); in (12), an hour later, the coronal transient-a feature much brighter and more substantial than the diffuse, radial threads through which it travels-has sailed beyond the reach of the coronagraph, more than 3 x 106 km from the edge of the Sun. In (13,14), simultaneous images of the rupturing prominence, made in many wavelengths by Skylab's ultraviolet spectroheliograph, classify the fragments as the prominence flies apart.

 

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