SP-402 A New Sun: The Solar Results
From Skylab
[66]
6
The Quiet Sun
- Oh, one other comment that should be of
interest.... We did get a couple of glimpses of the corona also at
the end of the orbit. It's the first time that we've seen it since
yesterday. And there are definite changes in the southwest
quadrant-a whole array of streamers-that were not there last
night.
-
- -Astronaut Owen Garriott, Sept. 7, 1973,
13:21
[67] The raging furnace
of the Sun is never quiet and its surface never still. There is
always change and, when one looks closely enough, dynamic action in
the various features that make up its many faces. The Sun described
as "quiet" still has a boiling surface of bubbling granules, a
network of heaving supergranules, and sunspots that come and go.
Spicules roughen the edge of the quiet chromosphere, shooting upward
in pointed waves that rise as high as 20 000 km into the corona.
Magnetic fields change and shift their shapes with effects felt
throughout the solar atmosphere. Long-lived, quiescent prominences
decorate the quiet Sun as does a distended and ever-changing corona.
On the Sun, as on Earth, "quiet" is a relative word.
Skylab scientists were as interested in the
quiet Sun as they were in the really dynamic action on its surface,
for the fundamental processes of the star can be seen best, and
sometimes only, on its relatively undisturbed surface. These
quiet-Sun observations lead to an understanding and important models
of the basic structure of the solar atmosphere including its
temperature, density, chemical composition, magnetic fields, and the
physical mechanisms that create and hold the chromosphere and corona
to the Sun. Truly eruptive features of the Sun, like flares, sprays,
and active prominences, can only be understood against a basic
understanding of the undisturbed background Sun.
Skylab observations of the quiet Sun gave
astronomers their first clear looks at the real extent and complexity
of the entire solar atmosphere; revealed the changing vertical
structure of previously known features such as sunspots and the
chromospheric network; and identified, for the first time, the varied
magnetic loops that make up the corona. From Skylab have come the
first clear pictures of the changes that consistently alter the form
of the outer corona as it adjusts to surface disturbances and to
changing magnetic fields. The details and changing nature of the
transition region-the shallow but important layer that separates the
cooler chromosphere and hotter corona of the Sun-were for the first
time seen. Also, the nature and importance of holes in the corona
that stand out as dark, extended open regions in X-ray and
ultraviolet pictures were established. From coronal holes the solar
wind pours outward from the Sun apparently unabated. Skylab's
continuous observations of coronal holes, coupled with measurements
at Earth of the solar wind and geomagnetic wind, clearly established
that coronal holes were the long-sought source of recurrent solar
wind disturbances that buffet the upper atmosphere of Earth, making
possible dramatic improvement in the prediction of the effects of the
Sun on Earth.
Six aspects of quiet-Sun behavior or
appearance in which Skylab observations made important advances are
discussed in this chapter: general findings from the appearance of
the Sun in the ultraviolet, the poles of the Sun, its appearance in
X-ray wavelengths, coronal holes, quiescent prominences, and the
outer corona. In each category are shown a number of selected
findings that are grouped in double-page sections. Information on
each picture may be found in the photo credits section.
[68-69]
IN THE ULTRAVIOLET: THE INVISIBLE
SUN
The great power of Skylab's solar
telescopes enabled man to see the Sun in light that from the surface
of Earth is never seen: the invisible spectrum in ultraviolet and
X-ray wavelengths that is totally absorbed by the dense atmosphere of
Earth. Solar astronomers knew that in hidden ultraviolet wavelengths
that originate in the upper chromosphere and transition region lay
the secrets of these important layers of the Sun: their chemical
composition, temperatures and densities, and physical processes-how
they changed and how they were related to the vastly different corona
above and photosphere below. There was no doubt that in the
ultraviolet lay the secrets of the changing Sun, and key pieces long
missing from the puzzle of the solar atmosphere. Rockets and orbiting
solar observatories had begun the exploration of the ultraviolet Sun,
but it was left to Skylab with its versatile, observatory-sized
telescopes of unequaled spatial resolution and staying time to
observe the invisible, ultraviolet Sun in the same intensive way that
for centuries man had seen the visible light Sun.
From analysis of the thousands of
Skylab ultraviolet pictures and spectra has come indeed a detailed
understanding of a new Sun: a full picture of spicules and the
chromospheric network; the discovery of a class of new features found
only at the solar poles; new insights into the ways that wave energy
is transmitted upward to heat the outer layer of the Sun; the first
detailed pictures of the transition region; a total picture of the
real extent of prominences, sunspots, coronal holes; and the key role
of the snarled and twisted magnetic forces that tie together so much
of what we see on the Sun. From ultraviolet spectra taken by Skylab
has already come the identifications of 65 new emission lines
originating in the transition region and corona-more than twice the
number identified in the preceding 25 years of rocket exploration,
allowing a more complete description of the physical conditions and
processes that hold the secrets of the Sun.
[70]
A SKYLAB VIEW of the ultraviolet Sun
(4) rounds out the modern picture of our nearest star. Ultraviolet
light from the Sun cannot penetrate Earth's atmosphere, yet it
carries information on the Sun's most active and interesting
layers-hotter and higher than the levels seen in solar views made
from the ground (1, 2, 3). A magnetic map of the Sun (1) locates
strong magnetic fields in the photosphere-as white areas for one
magnetic polarity and black for the other. In 
light (2) we see the low chromosphere, and m (3) a
chromospheric layer slightly above it. Dramatic new features appear
in the ultraviolet view (4). Dark regions at both poles of the Sun
and extending across the equator disclose the existence of holes in
the corona. Strong magnetic areas, or active regions, which appear
bright in (2) and calcium (3) are fluffier and more arched in the
ultravioIet, where in higher layers we begin to see loops that lace
these regions together. The remainder of the ultraviolet surface is
covered by a mottled network, also seen in (3), which traces the
all-over pattern of circulation cells in the solar atmosphere. At the
edge of the ultraviolet Sum a band of large prominences appears like
a towering line of cloudbanks tens of thousands of kilometers above
the solar surface.
[71]
ATMOSPHERIC LAYERS on the solar
horizon are peeled apart in pictures made in discrete ultraviolet
wavelengths (5-7), then reassembled in the larger view (8). Colors
were added in data reduction to distinguish different layers.
Comparisons of features in separate and combined pictures demonstrate
connections and relationships otherwise unseen. The high chromosphere
(5) at about 70 000 K, colored blue, is marked by sharp spicules at
the limb and a pronounced network of convective cells over the entire
surface. In green (6) we see the elusive transition region that
separates chromosphere and corona, sampled at a temperature of about
300 000 K. The bright ring that surrounds the edge of the Sun is an
edge-on view of this distinctive layer which then diffuses upward
into the even hotter corona. At a wavelength that samples coronal
temperatures (7), we see a vastly different view-the hot, inner
corona, stretching higher above the solar limb, thin and diffuse,
like the rosy glow of coming dawn. At this height the network of
convective cells has disappeared and in its place clumps of bright
emission outline parts of magnetic loops. Blue, green, and red
pictures were added together in (8). Where features of the separate
layers overlap, they combine to produce white, as at the network
boundaries. Features distinctive to one layer, like the tenuous
corona, show up in their respective colors.
[72]
(1)
QUILTED TOGETHER TO PORTRAY A QUIET
SUN (1) are 44 square picture segments made in ultraviolet
wavelengths 11 days before the final Skylab crew shut down the solar
observatory for the last time. The collection of square scans,
carefully recorded to insure a proper fit, kept Astronaut Gibson busy
for 3 1/2 hours, while Skylab circled Earth six times. Each square
was simultaneously scanned in seven wavelengths, sampling seven
different temperature layers of the solar atmosphere. Three (2, 3, 4)
are shown on the opposite page, color-contoured to emphasize
differences in brightness. The middle chromosphere (2), at a
temperature of about 20 000 K, shows hotter, brighter regions as
white splotches; cooler, darker ones are black. The convective cell
pattern of the chromospheric network covers the Sun, fading near the
poles. A higher, hotter layer of the chromosphere (3) presents a
different view. The white, bright ring around the Sun is spicules
seen edge-on, signaling the start of the transition region. The
ultraviolet corona (4)-a still higher, hotter layer-looks almost
unrelated to the layers below it. The network of convective cells has
disappeared; structures are now clumpy, larger, and shaped by other
magnetic fields. The anomalous polar regions, hinted at in the
chromosphere (2), are now revealed as holes in the corona. Bright
points appear like glittering flashes of light. The composite view
(1) combines the three wavelengths with new colors: blue for (2), red
for (3), and green for (4). Where bright areas coincide, their colors
add to produce white, as in the bright dots that are concentrated in
middle latitudes on the Sun, where sunspots and other activities are
born. The greenish ring around the Sun identifies the low corona,
which is absent at the polar caps where chromospheric red shows
through.
[73]
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ANOTHER SKYLAB TELESCOPE made
instantaneous photographs of the full solar disk in many
ultraviolet wavelengths, as overlapping images of the Sun.
In (5) we see two simultaneous pictures of the quiet Sun
made on the same day that (1-4) were made, (6) compares the
same pair for a more active Sun. The bright solar image in
each pair is the chromosphere at temperatures of 30 000 to
90 000 K. Below it, slightly overlapped, is a simultaneous
image of the 2.5 x 106 K corona.
The quiet corona (5) is almost blank except for the glow of
distant features on the opposite side of the Sun.
Chromosphere and corona are vastly different in (6): Bright,
active regions in the chromosphere appear without exception
as wispy arches in the hotter corona. Simultaneous pictures
of chromosphere and corona made by Skylab demonstrate that
nearly all light emitted by the hot corona comes from loops
that trace out patterns of magnetic lines of force rooted in
bright, active regions in the chromosphere.
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[74]
(1)
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(3)
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LIKE CHARCOAL PORTRAITS, these
photographs of the Sun in ultraviolet light delineate fine details
and relationships undocumented before Skylab. All are negative
prints; blacker features are in reality brighter on the Sun. The
chromosphere (1) is covered by an irregular mesh of bright, emitting
spicules. The network fades at polar regions of the Sun, where larger
spicules jut out to roughen the solar limb; it is also distorted by
centers of activity in bands of middle latitude. Ghostly prominences
hang high above the limb around the edges of the Sun, shaped and
supported by magnetic forces, made of chromospheric material too cool
to be seen in the hotter views (2, 3, 4). A simultaneous photograph
of the transition region (2) between chromosphere and corona samples
a temperature shell so thin that certain features here appear
especially sharp. The bright (black) ring around the Sun is an
edge-on view of this layer.
[75]
(2)
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(4)
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At these heights and temperatures the
network is fading and the cool, dense prominence curled around the
left lower solar limb in (1 ) appears as a white shadow that
interrupts the transition ring. Fine brushes of lines sprout from
bright chromospheric regions near the center of the disk, revealing
the lower legs of magnetic loops that stand in active regions. The
equatorial region of the Sun (3) samples coronal temperatures of
about 106 K, where the network is no longer recognizable.
Connecting active regions are higher, fuzzier parts of the same loops
whose lower legs were seen in (2). At still higher temperatures of
2.5 x 106 K, (4) the corona shows bright (black) emission only
from active regions-in complete loops that arch still higher above
the Sun, enveloping the cooler arches seen in (1-3).
[76-77]
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IMPRESSIONISTIC LANDSCAPE of
an unseen, fiery world (1) was created by combining
separate, ultraviolet pictures (2, 3, 4) made in wavelengths
that isolate the chromosphere, transition region, and
corona. Colors were added in computer reduction to label the
distinctive layers of the solar atmosphere. Green in the
composite view (1) came from the separate picture (2) that
sampled the chromosphere at a temperature of about 20 000 K.
It is marked by choppy clumps of spicules that form the
boundaries of the chromospheric network. Extended brighter
regions in (3) reveal the elusive transition region at about
150 000 K, higher and hotter than the chromosphere. The
burning bush of curving lines is formed by magnetic loops
whose roots are concentrated magnetic areas in the
photosphere. Blue (4) is light from the corona at about 1.4
x 106 K, where hot and foggy outlines enshroud the
active region loops.
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[78-79]
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THE CHROMOSPHERIC NETWORK fades and
disappears with height in a set of ultraviolet pictures (1,
2, 3) made simultaneously in wavelengths that sample
different layers of the same surface area of the Sun.
Skylab's ultraviolet pictures for the first time defined the
three-dimensional form of super-granule cells and mapped the
ever-changing circulation patterns of the solar
atmosphere.
In the chromosphere (1), where
temperature is about 20 000 K, the network is distinct:
bright segments of network cell boundaries appear in
color-coded hues of yellow brown, and white; centers of
cells, where emission is less intense, are dark. A view of
the same area at greater height (2) reveals a widening of
the same network boundaries; in the transition region, where
temperature is about 300 000 K, cell boundaries begin to
spread and blend together. The same area (3) seen in coronal
light at about 1.4 x 106 K looks
vastly different. Spicule clumps at boundaries of the cells
have spread at this height like the branches of an elm, and
we now see only the blurred tree tops.
THE BRIGHT RING at the edge of the
Sun expands in thickness over the solar pole. In this
ultraviolet view (4) we see the transition region at about
600 000 K. The balding region at the top, covering the north
pole of the Sun, is a coronal hole-an extended area where
the coronal temperature drops. In the coronal hole the
transition region, seen edge-on, widens appreciably, proving
that the solar atmosphere thins and expands over the polar
hole.
HIGH CHROMOSPHERE, seen in
ultraviolet light, looks like a paint-spattered ball in this
false-colored view (5). Bright spots show intense emission
from small areas in a thin shell enveloping the Sun where
the temperature is about 70 000 K. Specks of bright emission
come from clumps of spicules around the boundaries of
super-granule cells. Specks appear to crowd together toward
the edge of the Sun, because of its spherical shape,
producing the bright purple ring that here encircles the
Sun.
THE ABRUPT JUMP in the height of the
transition region over a coronal hole at the south pole of
the Sun is plotted (6) in measurements made from another
Skylab ultraviolet telescope, in the same spectral line as
(4), also sampling temperatures of about 600 000 K. The dots
plot individual measurements of the thickness of the
transition region above the chromosphere. The height jumps
from almost negligible thickness to about 12 000 km (the
diameter of Earth) over the coronal hole. Beneath the
plotted points is an ultraviolet picture of the
corresponding region of the Sun, seen at coronal
temperatures of about 1.4 x 106 K. The
coronal hole appears as a dark gap in the otherwise white
emission.
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(5)
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(6)
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[80-81]
AT THE POLES OF THE SUN
The north and south poles of the
Sun-the ends of the axis on which it constantly spins-were revealed
in new clarity in Skylab's ultraviolet and X-ray views. From the
ground, in visible light, they had always looked much like the rest
of the Sun, although theory had hinted that they should follow
slightly different rules. With the benefit of ultraviolet and X-ray
eyes, Skylab scientists found, as did Earth's first Arctic explorers,
that the Sun's polar areas were in truth a unique and different
world: the abode of a new class of giant spicules never seen before
and a vast area where the vertical [81] structure of tile
solar atmosphere is distended and stretched upward. Most important
was a new appreciation of the polar regions as the domain of enduring
holes in the corona-spreading bald spots in the outer atmosphere of
the Sun where the normally constraining magnetic forces relax and
open up to allow an unencumbered flow of solar particles into
interplanetary space. In Skylab's extensive pictures of the poles
were found the source of the delicate, flowing polar plumes that had
been seen only in fleeting glimpses by past generations of
astronomers at eclipse: polar plumes traced open magnetic field lines
at the solar poles, and at their roots were bright points of coronal
light.
[82-83]
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POLES OF THE SUN appear in
new perspective when seen in ultraviolet light as in this
picture of the high chromosphere made in a spectral line of
ionized helium by one of Skylab's telescopes (1). In the
photosphere and lower chromosphere the poles seem little
different from the rest of the surface of the Sun. However,
at a higher level corresponding to temperatures of 30 000 to
90 000 K, we see the balding zones at either pole where the
bright chromospheric network abruptly ends, much as trees
stop at timberline. These polar caps are long-lived coronal
holes, where coronal temperature and density suddenly
drop.
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THE POLAR ATMOSPHERE of the Sun is
dissected in three views made of the same area in different
ultraviolet wavelengths.
(2) In the upper chromosphere,
silhouettes of bright spicules jut up to give the Sun a
ragged edge. Some are unusually large and were called
"macrospicules" by Skylab solar scientists who first saw
them in ultraviolet pictures like this. Most of the spicules
and macrospicules do not rise vertically from the surface of
the Sun but are inclined to either side, following solar
magnetic fields that emanate from the polar coronal
hole.
(3) Spicules and macrospicules fade
from view at the higher temperatures of the transition
region. In the polar hole there now appears a pattern of
bright points of light.
(4) Polar features are most distinct
in coronal light. The polar hole, seen less distinctly at
lower levels, is dark and sharply bounded, at its edges the
bright shell of coronal light seen around the limb of the
Sun abruptly fades. In the hole, bright points seen in the
transition region are now revealed as the bases of polar
plumes that shoot up like searchlight beams into the void
above.
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[84-85]
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SKYLAB GETS TO THE BOTTOM of
an old question: What are the polar plumes seen in the
corona at eclipse?
Long noted in photographs of
the eclipsed Sun, coronal polar plumes spread outward from
the top and bottom of the Sun, resembling lines of force
around the poles of a bar magnet. For many years they were
taken as evidence that the Sun had a central, permanent
magnetic field, like Earth. Plumes are obvious in the
photograph of the corona (1) taken at a total eclipse of the
Sun in Australia in 1921, here printed as a negative. In a
normal print, and to the eye, the polar plumes appear white,
like the rest of the visible-light corona.
Skylab's ultraviolet views of
the poles of the Sun (also printed as negatives (2)) traced
the plumes back to the surface of the Sun to find their
source: bright points in coronal holes. The bright points
are regions of concentrated magnetic field. Plumes are
outflowing streams of coronal gas that follow magnetic lines
of force originating in these separate magnetic
regions.
The bright points and plumes
are not lasting features of the Sun. The sequence (2) tracks
the behavior of polar plumes with time. Each picture, top to
bottom, shows the same area of the solar pole, separated in
time by about 1 day. In all, 6 days are shown. Individual
plumes rotate with the Sun and may persist for several
days.
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[86-87]
GIANT SPICULES, found only in the
Sun's polar cap, were not fully recognized before Skylab's detailed
ultraviolet coverage of the Sun. Wider, higher, and longer lived than
the smaller jets that fence in the super-granule cells, macrospicules
are a new and separate feature of the Sun. Some rise to more than 40
000 km above the solar surface and are twice as wide as Earth. Unlike
smaller spicules that rise and fall in 5 or 10 min, macrospicules
last as long as threequarters of an hour. They are parts of the
churning chromosphere, at temperatures of about 50 000 K, thrown
upward at more than 150 km/s, only to fall back again.
(1)
(1) At the Sun's north pole,
macrospicules rise and fall like stormy waves dashed against a
seawall. Here ultraviolet pictures made in rapid sequence are shown
as intensity contours. Pictures are spaced about 2 min apart,
covering 14 min in the life of the polar Sun.
(2) Simultaneous views of the polar
chromosphere (green) and corona (red) were made in the ultraviolet to
test for possible connection between macrospicules and coronal polar
plumes, which here appear as wide bright red streaks. Green pictures
were produced from the first four pictures in (1), spaced about 2 min
apart. While both are features found only in the polar holes of the
Sun, plumes and macrospicules seem to lead separate, unrelated
lives.
(3) The Sun, seen with X-ray eyes,
emphasizes the dramatic nature of the polar caps, and the
relationship between bright points and the polar plumes. Bright
emission in this picture, made with an X-ray telescope on Skylab,
displays the corona at temperatures of about 2 million K. On this day
the north pole of the Sun was tilted slightly toward us, and we see
it more fully; the south pole, tilted away is hidden behind bright
loops of coronal light. The sharply bounded coronal hole at the north
pole has opened up into a more extended hole that reaches downward
across the solar equator, to regions of the Sun more than 1.5 x
106
km away.
[88]
X-RAYS FROM THE SUN
Unlike in medicine, where they are
used as a penetrating light source to make shadow pictures of solid
objects, X-rays are studied in astrophysics for what they tell of the
high-temperature gases that emit them- sources in the sky whose
temperatures exceed about 106 K. An X-ray
telescope pointed at the Sun will not see the cooler photosphere and
chromosphere, but only the hot corona high above them. This offers a
great advantage to the astronomer, for in X-rays he can observe the
corona of the Sun without the need to block out the solar disk with a
coronagraph. With an X-ray telescope he can see the corona all over
the Sun, not just at the edges as at a total solar eclipse. Thus,
X-ray observations of the corona offer an advantage like that of
viewing a tangled forest from above, as from an airplane, where
clearings and other features, hidden in ground-level, edge-on views,
can be clearly seen. X-rays offer another valuable tool in solar
observation: because they come from very-high-temperature regions,
they provide a way of sorting out the details of the most intense
features in the solar atmosphere, like the raging cores of solar
flares, as we shall see in the next chapter.
[89] As in the
ultraviolet, previous space experiments had demonstrated the power of
X-ray observation of the Sun. In Skylab's arsenal were a number of
X-ray instruments, including two high-resolution X-ray telescopes
that took pictures of the Sun on photographic film as sharp and clear
as pictures made in visible wavelengths from the ground. From these
two telescopes came almost 60 000 nearly continuous pictures,
revealing the structure of the corona as it had never been seen
before. Subsequent analysis has underlined the importance of two new
coronal phenomena, bright points and coronal holes, that after the
Skylab missions are now seen to be among the most basic features of
our Sun. Immediately obvious was the fact that the solar corona was
built entirely of magnetic structures-a loose lacing of loops and
arches the presence of which had only been hinted at before. There
was no "quiet corona"; it was all magnetic loops. Where there were no
loops, there was no corona. The view from above, in X-rays, had
dissolved a long-held, mistaken view.
Metal filters in X-ray telescopes sort
out X-rays of different strengths. allowing views of separate layers
of the corona. The most energetic or "hardest" X-rays come from hot
regions of 3 to 5 x 106 K; "softer" X-rays,
come from slightly cooler areas of 2 to 3 x 106 K.
[90-91]
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THE X-RAY PORTRAIT OF THE CORONA (1)
was taken on the same day as (2) by one of Skylab's X-ray
telescopes, when the Moon was not in the way. Features in
(1) and (2) are clearly related. The X-ray portrait shows
the less rarified regions of the corona, nearer the surface
of the Sun, but over its entire face. We see at last the
secrets of the Sun that have long been hidden from the
generations of astronomers who spent months and years
preparing and traveling to distant lands to see the Sun's
corona for but a few minutes, and then could only guess its
full three-dimensional form.
The eclipse corona in (2) was
photographed in ordinary light from the ground on June 30,
1973, during a total eclipse of the Sun that took place
while Skylab was in operation. The black disk in the center
is the Moon, which covers the bright photosphere and
chromosphere, revealing the thin, hot corona that envelops
the Sun. We see the beautiful and ethereal forms of the
corona edge on, but the Moon stands between us and the parts
of the lower corona that overlie the disk of the Sun,
precluding comparison with chromospheric and photospheric
features like sunspots, flares, and magnetic fields.
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SKYLAB'S X-RAY TELESCOPES
were not the first to see the solar corona, but instead were
the culmination of a decade of progress in attempts at
corona photography from rockets and unmanned spacecraft.
These X-ray photographs of the Sun (3-6) taken from rocket
flights between 1963 and 1969 chronicle the dramatic
improvement in X-ray imaging of the Sun over this short span
of time. The first attempts, like the first looks at the
planet with Galileo's telescope, gave only coarse glimpses
of the intricate details that later were revealed.
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[92]
(1)
(1) AROUND THE EDGE OF THE SUN the
X-ray corona floats like a hazy cloud, looking much like the lower
corona at eclipse. Over the disk of the Sun, when seen as above, its
intricate forms are at once revealed.
- (a) In a coronal hole at the north pole
X-ray emission seems utterly absent.
- (b) Extended coronal hole stretches from
the polar hole to about 45° south latitude, dominating the
half of the Sun shown here.
- (c) Tight loops lace surface active
regions, connecting areas of concentrated magnetic fields on the
solar surface.
- (d) Larger loops connect distant active
regions, thousands of kilometers apart on the solar
surface.
- (e) Large, hazy loops appear to link
separated remnants of older active regions.
- (f) Bright features in the X-ray Sun are
active regions, related to sunspots, chromospheric plages, and
concentrated magnetic fields.
- (g) Bright points appear over the entire
solar surface, including the coronal holes.
- (h) High corona above the unseen, opposite
hemisphere of the Sun glows from behind the limb.
- (i) South pole of the Sun is hidden from
view by coronal loops in the foreground and by the slight
tilt.
- (j) Feathery, dark cavity marks a line of
latitude where filaments lie in the chromosphere below.
[93]
(2)
(2) DISTANT CORONAL LOOPS blend to
form red clouds on the solar horizon in this colored X-ray panorama
of a region of the Sun near its north pole. A broad coronal hole
spreads down before us like a black valley, dotted here and there by
bright points that glitter like distant lights on a landscape. The
bright yellow glow at right betrays the seemingly tranquil scene,
telling of intense X-ray emission from an active region, bounded by
hot pink loops that outline strong magnetic fields.
[94-95]
EERIE X-RAY PORTRAIT of the Sun (1)
emphasizes the hottest, most intense parts of the corona-active
regions that glow in this colored photo like coals in the night. Each
active region is the seat of intense magnetic fields, where X-rays
escape from heated gas trapped in magnetic loops that connect points
of opposite polarity. Near the center of the solar disk (1) a curving
zone of bright X-ray emission connects with circular patches of an
even brighter active region to make a cosmic semicolon, nearly 300
000 km from dot to tail, that forms an archway through which Jupiter
could be rolled.
The same structure seen in an
ultraviolet view (2) sampling similar temperatures reveals that the
X-ray semicolon comes from a curved arcade of smaller loops.
The same Sun is seen in harder X-rays
(3) by a telescope of Skylab that samples hotter temperatures. Here
X-ray emission has been color contoured in computer reduction, and
latitude and longitude grid lines have been added for accurate
comparison with other solar data. The brightest coronal emission is
colored white, and corresponds to temperatures in excess of 5 x
106
K.
Analysis of X-ray data from the giant
curved arcade of loops at the center of (3) provides a temperature
map (4), shown here as contours at several levels between 3 x
106
and 4 x 106 K.
[96]
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SKYLAB ASTRONAUTS were the
first people in history who could watch features of the
invisible ultraviolet and X-ray Sun and see changes as they
were occurring. An extreme ultraviolet telescope on the
spacecraft delivered a live television picture to the
astronauts' solar control console to monitor the same levels
of the solar atmosphere that were seen in the X-ray
telescopes. To allow comparison with previous images, an
ordinary Polaroid camera was carried up to Skylab by the
second astronaut crew and attached to the video screen. New
film that became available in the few weeks between the
first and second manned Skylab missions made the sudden
improvement possible.
(1) Astronaut Garriott
photographs the video screen, producing an instant picture
like (2), for an album of the first informal snapshots of
the hidden layers of the Sun.
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[97]
THE EVER-CHANGING CORONA is shown in
these selected X-ray pictures that document five rotations of the
Sun- about the first half of the full Skylab mission. The Sun rotates
one-quarter of a turn in the 1-week interval between adjacent
pictures; one row corresponds to one solar rotation of about 27 days.
These glimpses are a sampling from more than 15 000 X-ray pictures
that were made during the same period by this one Skylab experiment.
[98-99]
HOLES IN THE CORONA: THE OPEN
SUN
Dark features called "holes" in the
corona are extended regions where temperature and density are
unusually low, and where solar magnetic field lines, normally arched
and closed, open out from the Sun in nearly radial directions. They
are one of the largest and, following the discoveries of Skylab, one
of the most important features of the Sun.
Coronal holes are a recent discovery
of solar physics; they are not readily apparent in the photosphere or
chromosphere, and are subtle features in coronagraph data or in
pictures of the corona made at the time of eclipse. In ultraviolet,
and particularly in X-ray wavelengths, they stand out clearly as dark
voids, nearly always present at the poles and often stretching down
into the important middle latitudes of our Sun. They were first seen
in scattered X-ray pictures of the Sun made before Skylab, and their
possible relevance to conditions on Earth was soon surmised. But, it
took thorough and continuous X-ray coverage to establish, beyond all
doubt, that coronal holes are the source of the high-speed streams of
solar wind particles that buffet the upper atmosphere of Earth,
disrupting our own magnetic field and producing other lower
atmospheric effects. This discovery, based on Skylab data, now stands
among the most firmly established solar/terrestrial connections.
Already it has been useful in the process of making daily forecasts
of geomagnetic disturbances, of practical importance to man.
[100]
HOLES IN THE CORONA were first noted
in X-ray pictures of the Sun made from rockets several years before
Skylab. These important features had been missed by generations of
eclipse watchers, for they are not easily recognized as holes in
edge-on views of the corona. Since then, and with the help of
Skylab's extensive coverage, they have been shown to be one of the
most terrestrially important of all solar phenomena.
Skylab astronauts had learned to look
for coronal holes on their live video display of the Sun in the
extreme ultraviolet region. Image (1) was taken from their onboard
video screen and transmitted to solar scientists in Houston. Computer
reduction added the color contours and gave it a needlepoint
character. Most obvious in this view of the ultraviolet corona is the
large hole that begins at the north polar cap and extends down across
the equator of the Sun, looking much like the boot of Italy on a map
of the Mediterranean.
[101]
THE BOOT OF ITALY HOLE, and others
like it, were watched continually by several Skylab telescopes. It is
traced out on a picture of the high chromosphere (2) made in the
ultraviolet one day after the preceding needlepoint view (1) was
made. Coronal holes appear at this lower, cooler level of the solar
atmosphere as facings in the chromospheric network.
In an X-ray photograph of the Sun (3),
made simultaneously with (2), the coronal hole has nearly identical
form. As in (2), boundaries of the hole have been outlined for
clarity.
[102]
(1) AS THOUGH PARTING A RED SEA, a
long coronal hole opens up a path between billowing coronal arches to
expose the cooler chromospheric surface below. Coronal holes are best
seen in soft X-ray portraits like this one, where the steep walls of
the hole can be seen in almost three-dimensional realism.
- (a) Polar coronal hole connects to another
extended hole-a winding chasm that reaches below the Sun's
equator, more than 1 1/2 x 106 km away.
- (b) Boundaries of coronal holes are
steep-sided like canyon walls.
- (c) Coronal holes are magnetically "open"
features, as opposed to the "closed" or arched-over field lines
seen else where on the Sun. Magnetic field lines diverge at
boundaries of the hole-none arch across the chasm. As a result,
electrically charged particles in the hot solar atmosphere are
free to escape from the Sun at coronal holes; elsewhere they are
trapped in closed magnetic field lines and give the structured
corona its distinctive form.
- (d) Temperature within a coronal hole is
about 106 K, two to three times lower than in active regions
and about 50 percent lower than in surrounding "quiet" areas.
Density is three times lower than in the neighboring
corona.
- (e) X-ray and ultraviolet emission is
either very weak or absent altogether in holes.
- (f) Polar plumes originate in coronal
holes at the polar crowns of the Sun and are rooted in bright
emission points.
-
[103]
CORONAL HOLES were enduring features
at both poles of the Sun throughout the 9 months of Skylab
observations. An ultraviolet panorama of the north pole (2) paints a
colored picture of a polar coronal hole. Intensity is shown as
different colors: brightest is white; darkest, black.
The same area of the Sun seen
simultaneously by another Skylab ultraviolet telescope (3) shows the
same features at a slightly lower temperature. Polar plumes spring
from bright points of emission in the polar hole. At the limb of the
Sun the hole is marked by sudden spreading and fading of the bright
ring of coronal ultraviolet emission.
[104]
THE BOOT-SHAPED CORONAL HOLE rotates
with the Sun in a sequence (1-4) of soft X-ray pictures taken about 2
days apart. Most of the apparent changes in this 6-day period result
from changing perspective. Skylab data helped demonstrate that
coronal holes are sources of high-velocity streams in the solar
wind-electrons, protons, and atomic nuclei-that spray out from the
Sun into interplanetary space. When the coronal hole is near the
center of the Sun, as in (2), the sprinkler is directed at Earth.
These high-speed streams of solar wind distort Earth's magnetic field
and disturb our upper atmosphere.
[105]
EVOLUTION OF A CORONAL HOLE in about 3
months' time is shown in soft X-ray pictures (5-8) taken at spacings
of one solar rotation-about 27 days apart. Neighboring coronal arches
encroach like crabgrass into the hole, slowly closing it, leaving
only the persistent hole at the north pole of the Sun. Curiously,
coronal holes rotate as though attached to a solid Sun, unlike the
slipping layers of the photosphere and chromosphere, which rotate
much faster at the equator than at the poles.
[106-107] SOLAR PROMINENCES:
THE RUFFLED SUN
Solar prominences are cloudlike
extensions of the chromosphere into the hot corona. Towering to
heights of 100 000 km or more, like giant solar altocumulus, they
have long been studied as one of the more intriguing features of the
Sun. Quiescent prominences are associated with slowly changing solar
activity, specifically with large-scale magnetic fields on the
surface of the Sun, whose arching lines of force give them shape and
support. They appear in varied shapes on the disk of the Sun as long,
extended ribbons and persist, in some cases, for weeks or months
before fading away. Those that erupt can cause measurable effects in
the upper atmosphere of Earth.
Skylab took these enigmatic features
under close observation in the ultraviolet, where, by recording their
appearance in different spectral lines and sampling a wide range of
temperatures, their hidden structure was dissected, layer by layer.
In this revealing process, shown on the following pages, new secrets
were ascertained. For the first time, details of their composition
were clearly seen: cooler cores, covered by ever larger shells of
hotter solar material. In simultaneous pictures, made in different
ultraviolet wavelengths, their extension into the corona, carved by
refrigerating the hotter coronal atmosphere, could be followed in
detail. In the ultraviolet, where hotter, higher shells were seen,
their real extent was appreciated for the first time; they were
larger than they had ever appeared in visible wavelengths from the
ground-immense and delicate ruffles curling above the surface of the
Sun.
[108-109]
LONG OBSERVED IN as bright clouds above the edge of the Sun, and as dark
filaments on the disk, solar prominences take on new, revealing
shapes and larger sizes when seen in Skylab's pictures made in
ultraviolet light of ionized helium. In solar portraits made 1 day
apart, two large prominences curl around the Sun's right limb. The
upper of the two can be seen both as a prominence at the limb and as
a filament on the disk. Ultraviolet prominences are significantly
larger than the same features seen in visible light because
ultraviolet emission traces out their hotter, higher reaches. As the
upper prominence rotates to the right with the Sun, we see it as a
flat ribbon that twists and turns to form a ruffle around the Sun.
Its sinuous form traces out a neutral sheet that separates large
areas of opposite magnetic polarity on the surface of the Sun.
Prominences last days weeks, and even months, gradually changing
shape, evaporating, and reappearing. Some suddenly erupt as did this
large ribbon after solar rotation carried it around to the opposite
limb of the Sun 18 days later. Here it floats serenely, suspended
nearly 100 000 km above the edge of the Sun, as tall as a stack of
seven Earths, and stretching like a giant wall of China nearly
one-quarter of the way around the Sun.
[110]
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SKYLAB DISSECTS a large
quiescent prominence that winds its way around the solar
limb. Ultraviolet pictures made simultaneously in different
wavelengths (1-3) isolate light from distinct temperature
layers. Layers are tagged with different colors in data
reduction and are recombined in (4).
At chromospheric temperatures
of about 20 000 K (1), the prominence is barely discernible
against the green background. This indicates that prominence
and chromosphere are at about the same temperature.
At temperatures of 150 000 K
(2), the lower parts of the prominence show up dark against
the hotter transition region, demonstrating that they are
cooler, denser features.
In coronal light
(temperatures of about 1.4 X 106 K), the
prominence is a complete shadow (3). Were it not dense
enough to block the background coronal light, it would be
completely invisible.
In the composite view (4),
white represents the added contributions from all three
temperature layers; where one dominates, it appears in its
distinctive color. Where the prominence curls over the solar
horizon, the blue coronal light is notched away.
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[111] NICKNAMED
"PUFF, THE MAGIC DRAGON," the same prominence, big enough to
swallow 30 Earths, is seen, crawling over the limb of the
Sun, earlier in the day, devouring the blue corona as it
goes (5). In fact, the prominence is attached to the
chromosphere and moves chiefly with the rotation of the Sun.
In the color-coded composite of three ultraviolet pictures
(5), red samples chromospheric temperatures, blue coronal,
and green the intermediate temperatures of the transition
region. Cool and dense, prominences refrigerate the hot
corona around them, carving cavities that belie their
presence. In a corresponding soft X-ray picture of the
corona (6), we see the notch that Puff has made.
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[112]
(1) CURVING PROMINENCE CAVITY AND
NEARBY CORONAL HOLE both appear as dark shadows in a bluecoded
picture made in ultraviolet light from the hot corona. The sprawling
coronal hole at lower right represents a nearly empty void in the
corona, where open magnetic field lines allow the gaseous coronal
material to escape the Sun. The curved cavity at left was excavated
by refrigeration by a quiescent prominence that still lies hidden
within it.
[113]
(2) THE HIDING, TROUTLIKE PROMINENCE
is exposed when the blue coronal view (1) is combined with a
red-coded ultraviolet picture that samples cooler chromospheric
temperatures Red also fills in the coronal hole, demonstrating that
coronal holes are not conspicuous features in the lower chromosphere.
[114]
IN LESS THAN AN HOUR an anvil-shaped
prominence at the limb of the Sun changes markedly. Composite views
(1, 2) were made from separate ultraviolet pictures that sample
temperatures typical of chromosphere (red), transition region
(green), and corona (blue). During the 49 min between (1) and (2), a
large arch fills in, revealing that the prominence is a shredded
ribbon, threaded by magnetic fields. Between pictures, Skylab
streaked halfway around Earth, keeping its solar telescopes pointed
carefully at the same small region of the Sun.
[115]
QUIESCENT PROMINENCE AND ADJACENT
LOOPS reveal varying structure when dissected into layers of
different temperature by ultraviolet pictures (3-7). Such pictures
are printed here as negatives so that black means bright. The
temperature increases with picture number in these five samples,
taken simultaneously. At chromospheric temperatures (3), the
prominence is a dense, arched cloud. At higher transition region
temperatures (4), most of the prominence fades; nearby loops, filled
with material hotter than the prominence, appear in stark outlines.
In (5-7), increasingly hot regions of the corona are shown, and the
prominence, now a (white) shadow, blocks background coronal light. In
the corona both prominence and loops are enshrouded in a foggy haze.
[116-117]
OUTER CORONA: THE ETHEREAL SUN
The Sun's corona stretches far beyond
the denser, inner corona seen in X-rays and ultraviolet light, and
beyond the limits of what we normally see in the dark sky of a total
solar eclipse. Its farthest reaches are delineated by tapered
streamers that stretch into interplanetary space, extending the
domain of our nearest star much farther than its visible disk. We see
the outer corona briefly at total eclipses of the Sun, where it
appears white and delicate against the starry background of a
temporarily darkened, daytime sky. Even then, Earth's intervening
atmosphere is bright enough to limit our view of the outer corona. At
Skylab's orbital altitude, where almost no air was left and where the
sky was starkly black, the outer corona was at last clearly seen. For
9 months it was continually observed by a coronagraph that blocked
out the solar disk.
High on the priorities of Skylab's
coronagraph observations were fundamental questions of coronal
physics that had long remained unanswered: What are the real shapes
of the coronal streamers seen flattened against the sky at times of
eclipse? How does the corona change, and in what time scales? Does it
respond to the violent changes on the Sun below? How are the inner
and outer corona associated with what we feel at the orbit of Earth
in the solar wind?
In the thousands of coronal portraits
made by Skylab in which the corona was observed more extensively than
in all the centuries of man's interest in the Sun, were the expected
answers, in clear, continuous pictures of remarkable detail. There
were changes of days, weeks, and months. The corona was constantly
altering its form, ever adjusting to the shifting forces of the
magnetic fields from the surface of the Sun that so obviously gave it
its distinctive shape. Like the rest of the Sun, and the rest of the
universe, it was never still. Skylab's coronagraph observations
coupled with X-ray pictures of the inner corona helped establish the
origin of the corona's varied forms and the important connection
between coronal holes and high-speed streams in the solar wind.
[118]
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CORONAGRAPH (1) AND X-RAY (2)
VIEWS of the outer and inner corona are compared in Skylab
pictures taken on the same day. In (3) they are combined in
proper scale Colors were added in printing.
The brightest X-ray features
at the limbs of the Sun appear m the outer corona as rays
and streamers. These outer features are electrons and
protons that have escaped from the lower corona. Their
apparent color-white-is simply reflected sunshine. Their
paths and concentrations are defined by drawn out open field
lines from magnetic areas on the surface of the Sun. More
rounded shapes of streamers close to the Sun tell of the
last restraining pulls of arched magnetic fields, before the
solar particles break away.
- (a) Occulting disk, 50 percent
larger than the image of
- (b) Shadow cast in coronagraph by
occulting disk support
- (c) Outer corona is vacant where
coronal hole appears in X-ray picture (2)
- (d) Light rings and dark bands
concentric with occulting disk, examples of noncoronal
effects of coronagraph
- (e) Outer limit of coronagraph
field of view
- (f) Coronal streamer
- (g) Coronal ray
- (h) Coronal streamers,
bulb-shaped closer to the Sun
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[119]
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ORBITAL "SNOWSTORM" obliterated the
corona (4-6) when contamination particles from the
spacecraft floated between coronagraph and the Sun. The
three pictures were taken within 5 min of each other. Soon
afterward, the view cleared. Streaks show the paths of tiny
droplets of frozen water that were vented periodically from
Skylab's living quarters Direct sunlight illuminates them
like dust motes in a sunbeam, emphasizing the coronagraph's
need for an utterly clean sky.
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[120-121]
MAN'S FIRST DETAILED LOOKS at changes
in the outer corona came from Skylab whose coronagraph saw the outer
corona flattened in projection against the sky. Long-term changes
were apparent as the Sun's rotation carried the corona around, as in
a revolving display. Front and back views of the corona are shown in
(1) and (2), taken about 14 days apart, during which time the Sun has
rotated about one half-turn. The long, pointed streamer at upper left
in (1) has swung around in the red view (2) to the opposite side of
the Sun; during this time it and other features have altered their
appearance. The shadow of the occulting disk support also shifts
because of the changing orientation of the spacecraft. Colors were
added in printing.
[122-123]
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EVOLUTION OF THE CORONA
through one season on Earth appears in views (1-6) taken on
Skylab between September 16, 1973, and January 31, 1974.
Pictures (left to right) were taken about 27 days or one
solar rotation apart, showing the same side of the corona.
Coronal streamers sharpen, then diffuse, to be replaced by
new structures, responding to changes in the global magnetic
structure on the Sun below.
CHANGES IN WEEKS (7-9) are
seen below. They are evident in the coronal streamer that
extends to the left of the Sun in (7), swings to the right
side of the Sun in (8), when the Sun has rotated one-half
turn, then appears again at left in (9), altered in shape
and direction at the end of one full rotation of the Sun.
Pictures (left to right) were taken about 14 days
apart.
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CHANGES WITHIN DAYS in the
outer corona are apparent in comparing pictures (10-12)
spaced 2 days apart. In this time the right ray vanishes,
and the streamer left of it sharpens and then widens again
as it swings around toward us. Some of the apparent changes
are the result of changing aspects of long-lived features as
the Sun rotates them through our view.
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[124]
[no photo captions in original text,
Chris Gamble, html editor]
[125]
A PSYCHEDELIC ECLIPSE OF THE SUN was
created by special photographic processing of black and white
coronagraph film from Skylab. A new Moon, here tinted yellow, moves
across a flaming pink and red corona. Truth is less exciting: In
reality, the Moon is gray and the corona white. Colors added here
distinguish subtle differences in brightness.
![LONG OBSERVED IN [image- Capital H, subscript Greek letter alpha] as bright clouds above the edge of the Sun, and as dark filaments on the disk, solar prominences take on new, revealing shapes and larger sizes when seen in Skylab's pictures made in ultraviolet light of ionized helium. In solar portraits made 1 day apart, two large prominences curl around the Sun's right limb.](p108.jpg)
![LONG OBSERVED IN [image- Capital H, subscript Greek letter alpha] as bright clouds above the edge of the Sun, and as dark filaments on the disk, solar prominences take on new, revealing shapes and larger sizes when seen in Skylab's pictures made in ultraviolet light of ionized helium. In solar portraits made 1 day apart, two large prominences curl around the Sun's right limb.](p109.jpg)
