Solar Physics in the Space Age. INSTITUTION National Aeronautics and Space Administration, Washington, D.C

DOCUMENT RESUME

ED 312 125 SE 050 890

AUTHOR Dittmer, Phil D.; And Others TITLE Solar Physics in the Space Age. INSTITUTION National Aeronautics and Space Administration, Washington, D.C. REPORT NO NASA-NP-105 PUB DATE 83 NOTE 100p.; Colored photographs may not reproduce well. PUB TYPE Reports - Descriptive (141)

EDRS PRICE MF01/PC04 Plus Postage. DESCRIPTORS Aerospace Technology; Astronomy; *Physics; Satellites (Aerospace); Science History; *Science Programs; *Scientific Research; *Solar Energy; *Space Exploration; Space Sciences IDENTIFIERS *Sun

ABSTRACT This amply illustrated booklet provides a physical description of the sun as well as present and future tasks for solar physics study. The first chapter, an introduction, describes the history of solar study, solar study in space, and the relevance of solar study. The second chapter describes the five heliographic domains including the interior, surface atmosphere, inner corona, outer corona and solar wind, and the .un-earth interface. The third chapter discusses current problems in solar physics such as: solar neutrinos; helioseismology; magnetic fields; temperature of the corona; solar flares; holes in the corona; solar wind; coronal mass ejections; solar constant changes; and effects of the changing sun. The last chapter describes types of space missions and future solar research. (YP)

* Reproductions supplied by EDRS are the best that can be made * * from the original document. * ******* Solar Physics In The Space Age

U.E. DEPARTMENT OF EDUCATION Office of Educational Research and Improvement EDUCATIONAL RESOURCES INFORMATION CENTER (EMI XThis document has been rep, oduced as received from the Person or Cegintzation or cenatong it r Monet changes have been made to improve reproduction Quality

Points of view or opinions stated on INS dOCu ment do not necessarily represent official 2 OERI position or policy A Few Solar Facts and Figures

Diameter 1.4 million kilometers (109 Earth diameters)

Sun-Earth Distance 150 million kilometers (107 Sun diameters)

Volume 1.4 billion billion cubic kilometers (1.3 million Earths)

Density At center 160 grams per cubic centimeter (160 times density of water) At surface one gram per thousand cubic meters In corona one gram per ten cubic kilometers

Temperature At center 15,000,000 K (degrees Kelvin) At surface 6,000 K In sunspots 4,200 K In chromosphere 4,300 K to 50,000 K In corona 800,000 K to 3,000,000 K

Emission Total 383 billion trillion kilowatts At top of Earth's atmosphere 1.36 kilowatts per square meter

Magnetic Field Strength In sunspots Up to 3,000 gauss Elsewhere on Sun 1 to 100 gauss Earth, at pole 0.7 gauss

Age 4.5 b'llion years

Life Expectancy about t: billion more years

14 So'ar Physics In TheSpace Age Figure Credits

Page

2, 16. Carol Cranell, NASA Goddard Space Flight Center

9. bottom Gary Emerson, NASA Marshall Space Flight Center

19. left J. Zirker, National Solar Observatory

21. left High Altitude Observatory

23. left Palomar Observatory Photograph right Solar wind data from M. Neugebauer and C.W. Snyder, Journal of Geophysical Research, Vol. 70, No. 7, pp. 1587-1591 (1965), copyright by the American Geophysical Union

31. J.W. Harvey, National Solar Observatory

33. bottom D.M. Rust, The Johns Hopkins University Applied Physics Laboratcry

37. top left American Science and Engineering and Harvard College Observatory bottom Naval Research Laboratory

39. American Science and Eng::-.ering and Harvard College Observatory Digest 4(4), 41. T.A. Potemra, "Magnetospheric Currents," Johns Hopkins APL Technical 276-284 (1983).

43 Naval Research Laboratory and High Altitude Observatory

47. J.W. King, "Sun-Weather Relationships," from the April 1975 Astronauticsand Aeronautics, copyright American Institute of Aeronautics and Astronautics.

Text Credits

Text Dr. Phil H. Dittmer with support from Dr. Adrienne F. Pedersen andthe NASA Solar Physics Management Operations Working Group (Dr. J. David Bohlin,Chief of Solar Physics Branch, Chairman), with major contributions from Drs David M. Rust, Carol Crannell, and Harjit S. Ahluwalia. Introduction Solar Study through History

From the Stone Age to the Space Age, we triggers the violent "flare" explosions have been keenly interested in the Sun. associated with these spots? How do these Even in ancient times, we built observato- phenomena affect the Earth? ries like Stonehenge to record the Sun's changing path across the sky. We learned Today, we pursue the answers to these how the Sun governs our days and sea- questions from the unique vantage point of sons, but knew little about the Sun itself. space. Powerful space solar observatories Most early observers were content with have begun to probe all the Sun's complex the opinion of the ancient astronomer and dynamic structure unencumbered by Ptolemy that the Sun was a brilliant per- the limits our atmosphere imposes on fect sphere circling the Earth. dynamic range and spectral coverage. The solar space program will continue to yield When Galileo turned the telescope hea- breakthroughs, helping us master our own venward in 1610, he opened a whole new environment and contributing to other dis- era of solar inquiry. Subsequent observers ciplines from the minute world of elemen- have developed increasingly sophisticated tary particles to the cosmic questions of ground-based instruments, like the great astrophysics. solar observatories on Mount Wilson. These instruments have shown us a Sun which deviates from Ptolemaic perfection by virtue of intricate phenomena and dynamic processes. Each new discovery has spawned new questions. What causes the mysterious dark spots on the Sun? Why does the number of spots increase and decrease in a regular way? What

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12 1 Why Study the Sun from Space?

The setting Sun reminds us vividly that we these radiations and the information they live at the bottom of an ocean of air. The air bring from the Sun. For example, the Sun's scatters and absorbs the Sun's light, leav- ultraviolet light displays an intricate net- ing the Sun distorted and discolored. These work near the top of the Sun's middle effects delight the eye, but they also frus- atmosphere, the chromosphere. Solar X- trate the solar scientist. This frustration rays brilliantly highlight the magnetic persists even with the Sun overhead, be- arches and streamers which shape its cause air currents blur the smallest solar outer atmosphere, the corona. Still higher structures, which otherwise could be seen energy gamma rays come from the heart of through telescopes. From space we can solarire explosions, and may yet reveal surmount these difficulties; a large tele- how and why thesexplosions begin. Skylab scope in space will permit us to study solar features many times smaller than those With scientific instruments in space, we visible from the surface of the Earth. can go beyond observing the Sun to sam- pling the Sun's materials directly. Particles Space offers even greater advantages for and fields from the Sun flow continually studying the Sun's invisible radiations. outward past the earth. The Earth's mag- These radiations come from the Sun's netic field protects us by turning aside this outer atmospheres, where temperatures flow of particles and fields, but not before exceed one million degrees under "quiet" the flow compresses or inflates the Earth's conditions and are driven to tens of mil- magnetic domain. Spacecraft traveling out- lions of degrees in the violent explosions side this protective magnetic cocoon can called flares. The Earth's atmosphere com- measure these particles and fields, helping pletely blocks 3se radiations, protecting us determine their specific solar origins life on Earth , om their harmful effects. and terrestrial effects. Only instruments in space can receive

8 4 The Sun in X-Rays Satellites No Limit (Above 120 KM)

Rockets up To 250 KM

The Sun in Balloons Ultraviolet Light 40 KM (Above 80 KM)

Aircraft 15 KM

Ground -Be The Sun in Visible Light (From the Ground)

The Sun from Space

15 9 The Reievance of Solar Study 1-- Why do we study the Sun? location, 275,000 times closer than any other star, makes it unique. The Sun's ( "r E: ii the reasons is that our life depends proximity serves us like a powerful tele- or the Sun its light and heat make life on scope, permitting the discovery and study 7.rth possible. We are so certain of the of sunspots, flares, convection, rotation, Sun s constancy in performing this func- solar wind, and atmospheric structure, all tion that the Sun's rise each morning has of which have later been discovered on became a metaphor for certainty. Recent other stars. Furthermore, the Sun's mass, spacecraft measurements have shown that temperature, and ace have been bench- the Sun's output is not constant but is marks in developing the tneory of stellar marked by subtle variations. Similar varia- structure and evolution. Even today, at- tions may have triggered the ice ages of tempts to measure tt.3 products of the The Whirlpool Galaxy (M51) the cast. The Earth's environment is also Sun's thermonuclear reactions reveal affe.:ted by the Sun's fluctuating emis- flaws in existing astrophysical theories. sions 44 particles, magnetic fields, and Hence, the Sun continues to act as a invisiole electromagnetic radiation. These Rosetta Stone, helping us to decipher the emissions disturb our magnetic field and secrets of astrophysics. create the aurorae and the ionosphere. For measuring these phenomena and identify- Finally, the Sun provides scientists a uni- ing their effects on the Earth's environ- que laboratory for testing the laws of mod- ment, instruments in space have been ern plasma physics. These laws, which invaluable. govern the interaction of hot gases with magnetic fields, cannot for reasons of We also studythe Sun as a star. Astrophys- scale be tested adequately within the con- icists classify the Sun as a star of average fines of an Earth-bound laboratory. size, temperature, and brightness a typical dwarf star just past middle age. But its

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Why We Study the Sun

I 9

The Five Heliographic Domains

From the crushing pressure and density of The Sun-Earth Interfacethe Sun's its core to the near vacuum conditions in electromagnetic, particle, and magnetic its magnetic field laced outer atmosphere, field emissions and the processes by the Sun and its atmosphere span a vast which they affect the Earth's space range of physical conditions. To compre- environment. hend the discoveries and unanswered questions of solar physics, it is helpful to The age of space exploration has seen introduce the five heliographic domains. dramatic increases in knowledge of each domain, in many cases resulting from the The Interiorsite of the nuclear burn- expanded sensitivity and spectral coverage ing which heats the Sun, the Earth, and available from space. mankind. The Surface Atmospheresthe Sun's golden, visible surface, the photosphere, and its hotter overlying skin, the chromosphere. The Inner (Visible) Coronathe glowing halo of the Sun visible from Earth at times of solar eclipse. The Outer Corona and Solar Wind the region where the Sun's super- heated outer atmosphere overpowers its constraining gravity and magnetic fields and streams outward into space.

ri I3 14

The Solar Interior

The Sun's surface is not solid like the do not yet know the depth of the zone Earth's, but its temperature and density where convection omrs, a fact which make it just as difficult to see through. may help us predict the behavior of surface Hence, all we know of conditions within magnetic fields. The unknown abundance the Sun must be inferred by measuring the of elements in the interior affects the rate things we can seeits size, mass, and sur- energy leaks from the core and the way the face brightness and compositionand by Sun and other stars evolve. If the Sun's applying the fundamental laws of physics. core rotates rapidly or contains strong magnetic fields, Einstein's General Theory The Sun's nuclear furnace is in its center, of Relativity may need to be revised. where intense heat (15 million degrees) and pressure (200 billion atmospheres) Answers to these questions will be sought fuse hydrogen nuclei into helium. The by two recently developed techniques. One photons of energy produced by fusion technique uses the detection of neutrinos, travel at the speed of light. Uninterrupted, the tiny fast remnants of the nuclear burn- these photons would reach the surface ing process which escape unimpeded from within three seconds, but in the Sun's the core. The other technique, which is crowded interior, these photons are called helioseismology, involves the mea- absorbed and re-radiated so many times surement and interpretation of surface dis- that their journey to the surface takes ten turbances caused by sound waves travel- million years. In the Sun's outer layers, its ing through the Sun's interior. Thus far, outward flowing energy is lifted to the sur- these techniques have yielded results face primarily by rising bubbles of hotter, which differ from those predicted by theory. lighter material pushed upward by buoy- For this reason, current plans for more ant forces in the process known as con- sensitive solar neutrino experiments and vection. for improved ground- and space-based measurements of solar oscillations are of There are important uncertainties remain- the utmost importance. ing in this picture of the Sun's interior. We

16 27., Creation of Energy

Hydrogen Helium Nuclei

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Transport of Energy Radiation Scattered Radiation

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Emergence of Energy

Visible Light Convection 6000 K \

Cooler Hot Cooler Gas

Energy Processes within the Sun

3 17 The Surface Atmospheres

We call the golden surface of the Sun the tions known collectively as "solar activity." photosphere. Even when the Sun is in its These fields cool and darken the photo- quiet state, this surface seethes with mosphere, producing the well-known sun- tion. Bubbles of hotter material well up spots. These same fields arch through high- from within the Sun, dividing the surface er atmospheric layers, heating them and into bright granules that expand and fade creating glowing, bright "active regions." in several minutes, only to be replaced by At times active regions explode with an the next upwelling. Simultaneously, the intense release of magnetic energy called surface undulates with wave motions a solar flare, causing sudden large in- which repeat at roughly five-minute inter- creases of radiation and expelling huge vals. quantities of energetic particles into space.

Above the photosphere is the chromo- Solar activity displays an enormous range Spacelab 2 sphere, so named because it may be seen of time scales. Flares begin in seconds and briefly during eclipses as a reddish rim end in minutes or hours. Active regions around the Sun. Like the photosphere, the last many weeks, and may flare many chromosphere is mottled by a cellular con- times before fading away. The number of vection pattern, but the cells are thirty sunspots and active regions rises and falls times larger than granules and last several in a mysterious eleven-year cycle. Behind hours. Solar material flows horizontally all of these phenomena and time scales outward from the center of these "super- are the Sun's magnetic fields, deriving granules," sweeping magnetic fields to the their energyfrom the interplay of the Sun's cell boundaries. Field concentrations along rotation and convection motions. To ob- cell boundaries are marked by vertical jets serve and interpret the intricate interac- of material called spicules. tions of fields and matter, we need to operate a large, high resolution solar telescope Sometimes huge magnetic field bundles outside the Earth's own turbulent at- break through the surface, disturbing this mosphere. quietly simmering Sun with a set of condi-

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The Quiet SunSupergranules with BoundariesMarked The Active SunActive Region Loops at Limb by Spicules

The Sun's Surface Atmospheres

3 ek The Inner Corona

The Sun's wispy halo, the corona, reaches Our ability to launch instruments into more than a million miles into space, mak- space has also made it possible to observe ing the corona larger than the Sun itself. the corona in X-rays, which do not pene- But the Sun's brilliant disk blinds our view trate the Earth's atmosphere. X-rays are of the corona except when the moon cov- produced abundantly in the ultra-hot cor- ers the disk during an eclipse. Onlyfifty ona, and reveal corona! temperatures and years ago, scientists learned that the coro- densities more directly than the corona's na's puzzling spectral signature is not visible light, which is mostly second-hand caused by a new chemical element ( "coro- light from the Sun's surface scattered by Skylab S-054 X-Ray Telescope nium"), but by a temperature much hotter corona! material. Furthermore, because than the underlying surface. This discov- the corona is much brighter at X-ray wave- ery defies intuition and theory, and the lengths than the Sun's underlying cooler corona's two million degree temperature surface, it can be observed against the face remains an intriguing mystery despite of the disk. From this vantage point, it is many attempts at explanation. clear that magnetic arches and streamers dominate the structure of the corona. To increase opportunities for corona) obser- Large and small magnetic active regions vation, the French astronomer Lyot invent- glow brightly at X-ray wavelengths, while ed the coronagraph in 1930. This instru- open magnetic field structures appear as ment artificially eclipses the Sun's surface gaping holes in the corona. To understand so the corona can be observed at any time. the physical processes by which magnetic Coronagraphs are especially effective in fields shape corona! structure and contrib- space, where the corona can be seen with ute to its extremely high temperature, we exceptional clarity against the black back- need to orbit X-ray telescopes of greatly ground of space. improved resolution and sensitivity.

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Corona Observed from Space with X-Ray Telescope

Corona Observed from Space with Coronagraph

The Inner Corona

21 38 The Outer Corona and Solar Wind

One of the great discoveries of space phy- wind fields, allowing some solar whid sics is that the Earth is immersed in the energy to leak into the terrestrial environ- Sun's expanding outer atmosphere. Eclipse ment by a variety of processes. observations of the Sun's corona gave an early clue of the outflow of solar material. What causes the solar wind to flow? Its Dual comet tails were another early clue; energy comes from the heat of the corona, the straight, clumpy tail is caused by the but we have been unable to observe its Sun's radiation pressure, and a more dif- acceleration because the action takes place fuse tail by outf lowing corona! material. In in the inner corona, which has not yet been 1958, theorist Eugene Parker developed a studied by telescope or by spacecraft. Ob- theory predicting this outflow, which he servation of solar wind acceleration re- called the solar wind. This prediction was quires new observing techniques, or decisively confirmed in 1962 by the Marin- spacecraft that can withstand the tremend- er 2 spacecraft on its way to Venus. ous heat close to the Sun, where the accelerating wind can be measured di- Continued spacecraft measurements re- rectly. veal that the solar wind is much faster (a million miles per hour), thinner (a few par- How far does the solar wind go before it ticles per cubic centimeter), and hotter succumbs to the magnetic influence of the (several hundred thousand degrees) than galaxy? instruments on the Voyager space- any wind on Earth. The solar wind is hot craft continue to measure the wind beyond enough to be a "plasma," meaning that its Saturn's orbit, and we hope these instru- atoms are divided into electrically charged ments will record the wind's termination particles electrons, protons, and ions. As in interstellar space. a plasma, the wind carries with it magnetic fields from the corona, exposing the Earth alternately to the influence of the Sun's north and south magnetic poles. The Earth's own magnetic field deflects solar wind particles, but it interacts with solar

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Soir tr Hhr Cornet M Mariner 2 Spacecraft Solar Effects on the Earth

The Sun continually bombards the Earth layers of the atmosphere which reflect with energy in three forms: particles, radio waves and make tong- distance short- magnetic fields, and electromagnetic radi- wave radio transmission possible. ation (radio, infrared, visible, ultraviolet, X- rays, and gamma rays). Since each form of The particles and fields of the solar wind energy affects our environment, we need are responsible for the aurorae anc; for per- to understand the solar patterns and pro- turbations in the Earth's magnetic field. cesses which produce this energy. Some of the largest auroral and geomag- netic disturbances occur when solar flares Most of the Sun's energy output is in the expel large quantities of hot gas into form of visible light. This light provides the spacehot gas which the solar wind chan- energy for photosynthesis and for the heat, nels toward the Earth. Spaceborne instru- wind, and even precipitation of our weath- ments have detected other solar wind er. Not yet known are the effects of the patterns and features which disturb the subtle changes in the Sun's visible light Earth but whose solar source is invisible emission which have recently been de- from the ground. These instruments have International Sun-Earth tected by sr rcecraft. Space-based instru- also shown that the strenctf, of solar wind Evplorer (ISEE) ments have also measured much larger disturbances of our envirc nment depends variations in the Sun's ultraviolet and X- not only on wind velocity and density, but rays, which are absorbed by the Earth's also on its magnetic fieln orientation. It is upper atmosphere. The absorption process this orientation which &term Ines whether heats the atmosphere and causes it to the solar wind leaks through or flows expand. The atmospheric density change around the Earth's magnetospheric shield. which accompanies this expansion exerts a changing drag on low-altitude Earth- Din. ;t observations of the solar wind strik- orbiting spacecraft and thus determines ing the magnetosphere are no longer regu- how long such spacecraft will stay in orbit. larly available. A new and more advanced Ultraviolet and X-rays also break atmos- solar wind measuring spacecraft is urgent- pheric atoms and molecules into electrons ;y needed. and ions ar.d produce the ionospherethe

24 43 4 4 Solar Effects on the Earth (Drawing Notto Scalo) Current ProblemsIn Solar Physics

vrF Why Are There So Few Solar Neutrinos?

Scientists would love to have a glimpse the Sun has undergone large scale mixing deep into the heart of the Sun. The inferno for some unknown reason. Alternately, the there is the source of the Sun's (and the Sun may vary slowly, with a cool core Earth's) energy, and knowledge of condi- temperature now implying a lower surface tions there may help us to harness the temperature in a few million years. This energy of nuclear fusion on Earth. The may explain past (and future?) ice ages. A Sun's light cannot bring us this knowl- cool central temperature may result from edge; light can travel only short distances intense, trapped magnetic fields cr from a Brookhaven Neutrino below the Sun's blazing surface. But a rapidly rotating core. (The latter explana- Detector glimpse of the Sun's very center has tion would require adjustments in Ein- recently become possible using neutrinos stein's General Theory of Relativity.) The tiny subatomic part'cles which are pro- most exotic explanation is that the Sun's duced by nuclear reactions within the Sun. energy is not generated by fusion at all, but Neutrinos travel with the speed of light, but by matter falling into a gravitational singu- unlike light, they rarely interact with mat- larity (popularly known as a black hole) in ter, and can stream from the Sun's center the Sun's center. An explanation which undisturbed. To detect these elusive neu- leaves current solar theories intact is that trinos on Earth, a 100,000 gallon tank of neutrinos change to an undetectable form cleaning fluid has been placed deep within between Sun and Earth. This would have a mine (out of reach of cosmic rays). Cur- significance ranging from elementary par- rent solar theories predict that this tank ticle physics to cosmology. will capture six n^utrinos per day, but it has detected only one third this many. To help choose the correct solution, efforts This unexpected result seriously chal- are underway to build a more sensitive lenges current solar theories. neutrino detector using gallium, a rare metallic element used in semiconductors. The magnitude of the challenge can be These measurements will be complement- appreciated by a survey of proposed expla- ed by the emerging techniques of helio- nations. Solar theories can match the seismology, which use the Sun's own observed neutrino flux if material within acoustic waves to probe its interior.

49 51.1 28 The Problem

Predicted Observed Neutrino Neutrino Flux Flux

Proposed Explanations: I

Neutrino Change of ; State Large-Scale Mixing Rapi Core Rotation

,.,I i Trapped Magnetic Black Hole Fields Energy Source /ki ,

The Solar Neutrino Problem

r1 J r r) 0 4. 29 What Will Helioseismology Uncover?

Much of what we know about the Earth's Sun's shallow outer layers. The other rea- interior comes from seismologythe study son is that an observatory in space can of earthquake waves traveling below the operate without interruptions for night or surface. What might we learn if we could weather. Long, uninterrupted observations put seismographs on the Sun? In a sense, are essential to detect and measure the solar scientists have learned to do just hour-long oscillations which penetrate the that. By measuring minute shifts in the Sun's deepest layers. Interruptions may be color of sunlight, we can measure solar avoided by putting the spacecraft in a halo motions of less than one meter per second, orbit around the zero-gravity point between which is the pace of a leisurely walk. These Sun and Earth. This orbit also keeps space- motions are caused by acoustic waves trav- craft motions relative to the Sun small, eling below the Sun's surface, waves minimizing a potential source of error. which can be used to probe the Sun's interior. This procedure, which is *Ailed helio- Instruments suitable for helio,3eismology Ray Path of One Acoustic seismology, has already been used for pre- observations from space are already Wave Mode Traveling through the Sun's Interior liminary measurements of the depth of the planned, as are ground-based observing convection zone and the rate of solar rota- networks. These plans not only promise a tion below the surface. It may yet help to powerful tool for addressing the solar neu- solve the mystery of the missing solar trino problem, but also the prospect of dis- neutrinos. covering and exploring new surprises which the Sun now hides below its sur- Helioseismology was developed using face. ground-based observations, but there are two tantalizing reasons for pursuing this technique with an observatory in space. One reason is to surmount the Earth's shimmering atmosphere. From space, we can observe clearly the small -scale solar motions which are ideal for probing the r 30 53 Solar Waves: Blue and Red Represent Expandingand Contracting Regions

55 How Do We Explain the Behavior of the Sun's Magnetic Fields?

From the dark sunspots of the photosphere promises to improve dynamo theories by to the glowing arches of the corona, mag- letting us measure the rotation and con- netic fields are responsible for many inter- vection below the surface where magnetic esting things that happen on the Sun. Any dynamo action takes place. serious effort to explain these phenomena inevitably leads to the problem of explain- The behavior of magnetic fields on small ing the behavior of the Sun's magnetic scales may be an even greater mystery. fields. There is no explanation for the stability of the large, intense magnetic field concen- On a large scale, magrItic field behavior is trations which we observe in sunspr s. marked by the eleven year rise and fall in The problem is compounded by the recent the number of sunspots. During succes- discovery that most magnetic fields out- sive eleven year periods, magnetic fields side of spots are concentrated in small in spots and at the Sun's poles reverse bundles of high field strength. This discov- Orbiting Solar Laboratory (OSL) polarities. Pence, the fundamental cycle is ery challenges some assumptions of the Sun Sync Free Flyer a twenty-two year cycle of the Sun's mag- dynamo theories since these intense bun- netic fields. This cycle results from the dles cannot be moved about easily by the solar -magnetic dynamo," the stretching Sun's convection and rotation motions. It and twisting of magnetic fields by the may also be the key to understanding the Sun's upwelling convective motions and relation between magnetic fields and cor- its more rapid rotation at the equator than onal heati.ig. But the discovery itself is at at the poles. Successful dynamo theories the limits of ground-based observations, must explain not only the eleven year so further confirmation and exploitation cycle, but also variations in the strength of mus await high resolution velocity and cycles, including the nearly complete dis- magnetic field measurements available appearance of cycles which occurred be- only from a large telescope in space. tween 1645 and 1710. Helioseismology

56 32 , 1 The Macro Problem The Sunspot Cycle

Sunspot Number

1610 1650 1700 1750 1800 1850 1900 1950 1970 Year The Micro Problem Small-Scale Solar MagneticFields

An Active Region Photographed in White Light A Magnetic Field Map of t;ie Same Region

The Sun's Magnetic Field Why is the Corona So Hot?

One of the Sun's most puzzling and per- Since that time, solar scientists have de- sistent mysteries is the temperature pro- vised several alternate new theories to file of its atmosphere. Instead of becoming explain coronal heating. One theory is that cooler further from the surface, the atmos- magnetic fields convert sound waves into phere becomes hotter. The temperature magnetohydrodynamic waves, whose ma- rises steadily in the chromosphere, then terial motions are hard to detect. Another jumps abruptly in the corona to a level theory is that jets of material thrust upward thirty times hotter than the surface. While along magnetic field lines give the corona the corona's energy must come from the its energy. The high coronal temperature Sun, this flow of energy seems to contra- may come from the direct dissipationof Orbiting Solar Observatory coronal magnetic fields resulting from the (OSO -8) dict thermodynamic principles requiring heat energy to flow from a hotter object to twisting and tangling of their photospheric a cooler one. roots.

For decades, the preferred explanation has Choosing the correct theory requires sen- been that energy flows from the Sun's sur- sitive measurements of gas motions and face to the corona in the form of sound magnetic fields throughout the Sun's at- waves generated by convective upwelling mosphere. Only observations from space motion:, Ground-based instrumen'3 ob- can resolve the smallest solar magnetic served sound waves in the Sun's lower field structures and extend to the ultra- atmosphere, but these instruments could violet and X-ray wavelengths characteris- not trace them through hotter, higher lev- tic of the Sun's upper atmosphere. Hence, els of the atmosphere. Space-based ultra- a sophisticated solar space observatory violet observations in the late 1970s proved with a battery of high - resolution, instru- that sound waves carry their energy high ments offers our best hope of unravelling enough to heat the lower chromosphere, the mystery of solar and , teller coronal but not high enough to heat the corona, so heating. the mystery remains.

34 60 7

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The Photosphere The Upper Chromosphere The Corona in Visible Lig'it Sound Waves inJltraviolet Light in Soft X-Rays

6000 K 50,000 K 1-2 Million K

5,000 KM Above the 10,000-100,000 KM Photosphere Above the Photosphere

Corona, Heating

2 v335 Why Do Solar Flares Occur? .M11111

In less than an hour, flares release more Most flare energy is carried from the co- energy than a billion nuclear explosions. rona by thermal conduction electrons ex- Terrestrial by-products include strong au- panding outward along coronal magnetic rorae, magnetic field perturbations, and loops. This energy heats the chromosphere ionospheric disturbances. In spite of these to 10 million degrees and produces an effects, flares were late to be discovered impulsive chromospheric brightening vis- (1859) because much of their energy is ible in the red light of hydrogen atoms. This released in forms not directly visible from light provided the primary meansstudy- Pinhole/Occulter Facility the Earth's surface. All forms of flare ing flares before space observations. The energy can now be studied from space- flare-heated chromosphere also radiates craft, greatly expanding our understanding much energy at ultraviolet and X-ray wave- of flare phenomena and processes. lengths (X-ray emission during a large flare may reach 10,000 times the normal Flares:: begin when complex coronal mag- level). Heated chromospherir material ex- netic fields become explosively unstable. pands upward into the corona and may The resulting energy release raises temper- escape into the solar wind. atures to 100 million degrees and acceler- ates electrons and protons to near the While flare phenomena are now well speed of light. Particles accelerated out- known, one fundamental question remains: ward produce distinctive patterns of radio Why and how is magnetic energy released interference at the Earth. Particles travel- suddenly and explosively in flares? Flare ing inward strike the solar atmosphere. onset is known to occur very suddenly and Protons colliding with ions in the Sun's to be highly localized in coronal magnetic atmosphere produce nuclear reactions, loops. Hence, imaging space based high- whose gamma rays and neutrons have energy detectors are needed to solve this been detected from spacecraft. These fundamental question. r)diations can be used to measure the abundance of different chemical elements on the Sun, a key parameter in theories of stellar evolution.

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Solar Flures

f6 37 Why Are There Holes in the Corona?

Closed Fields When X-ray telescopes which could form What is the cause of the holes? Calcula- Low-Speed Wind images of the Sun's corona were devel- tions of coronal magnetic fields show that oped and orbited, observers were surpr sed most coronal magnetic field lines are an- Open Fields to discover that the corona sometimes chored to the Sun in two places, forming Sun High-Speed Wind appears to open into gaping holes. magnetic loops which confine the corona. But in a few places, the magnetic fields What is the significance of these holes? In open outward into space, allowing the cor- addition to being a surprising and dramatic ona to escape freely to form fast, low den- discovery, they solve a half century old sity streams in the solar wind. The rapid mystery of solar-terrestrial relations. The escape of energy leaves behind a cool, low mystery dates to the 1930s, when Bartels density coronal region that emits few X- Coronal Hole Magnetic recognized that there are sometimes dis- rays aod hence appears to be a hole in Field Pattern turbances in the Earth's magnetic field X-ray photographs. One question remains which repeat every 27 days. Since this is unanswered: Why does the Sun's mag- the Sun's rotation period, Bartels conclud- netic dynamo sometimes produce open ed that the disturbances must be caused magnetic field regions? The question must by some solar feature, but he could find no be addressed ' nrough a combination of feature which correlated with the distur- long term coronal studies and high resolu- bances. Bartels therefore labelled the re- tion magnetic field measurements. The sponsible solar features M-regions, where solution will help us to anticipate holes and M stands for mystery. When spacecraft the effects they produce on the Earth. discovered that Bartels' disturb periods correspond to high speed streams in the solar wind, the mystery was half solved However, it was only with the discovery of coronal holes in th" early 1970s and their subsequent correlation with high speed streams that the mysterious M-regions were identified.

38 S C 1

4t,

10-

Corona! Holes - Observed from Skylab ,I=ms

7O Now Different is the Polar Solar Wind?

How limited our understanding of the random, but is organized into large sectors Earth's weather would be if our measure- like the pieces of a r, a. Within a sector, the ments were limited to a few degrees from magnetic field is predominantly in one the equator! Yet our knowledge of the direction, either away from or toward ti Sun's "weather" suffers from just such a Sun. It is now believed that these sectors limitation. The Earth's orbit keeps it within are simply the extensions of the "north" or a few degrees of the Sun's equator, and WI "south" polarity fields at the Sun's poles, spacecraft heretofore sent to measure the which reverse signs every eleven years. solar wind have operated near the plane of The boundary between these regions is the Earth's orbit (the "ecliptic"). warped like the skirt of a twirling ballerina. The rotating Sun sweeps c uccessive bound- What reason is there to believe the solar aries past the Earth, but tie boundary is so wind is different over the Sun's poles? One thin that terrestr:al effects are small. How- reason is that X-ray photographs have ever, for cosmic rays carrying an electric shown large, long lived corona' holes over charge, this magnetic Iield boundary pre- the poles, and we know that near the equa- sents a major obstacle. Ccrnparisons over tor such holes correcnond tt solar wind time have proven that solar wind magnetic

Ulysses (International flows of increased si_ .and reduced den- fields are the solar system's first line of Solar Polar Mission) sity. Large high speed streams from the defense against cosmic rays. Hence a poles may mean that the Sun is losir satellite over the Sun's poles can not only mass to the solar wind much faster than explore this changi ig magnetic field geom- has been inferred from our measurements etry, but may also have a unique opportun- near the equator. ity to measure these high energy particles streaming in from the galaxy and beyond. There are also indications that the solar wind's magnetic field character is com- pletely different over the poles. From our limited perspective near the Sun's equator, the solar wind magnetic field is not

40 7 2 Three-Dimensional Structure of the Boundary between Magnetic Field Sectors in the Solar Wind (Artist's Concept).

"4 41 What Triggers Coronal Mass Ejections?

Aided by t!'e improved quantity and quality tion; it evidently results from the gradual of corona! observations from spaceoraf, evolution of corona! magnetic fields to an we have learned that the Sun's loss if unstable configuration. material to the solar wind is not always a slow, steady process. The corona is some- If we could measure the particles and times disrupted by large transient mass fields expelled by such an ejection, we ejections traveling outward from the Sun. could learn more about the Sun's composi- These ejections have the appearance of tion and magnetic field structure, espe- huge bubbles or clouds expanding out- cially in the region where the ejection orig- ward through the coro la. Notwithstanding inated. Unfortunately, such measurements their delicate appearance, + ansient ejec- have not y t been possible because the tions "onsist of billions of Lons of solar ejections we see with coronagraphs are on Solar and Heliospheric material thrown outward so forcefully that the edge of the Sun, and pass the Earth's Observatory (SOHO) they ravel millions of kilometers within a orbit 90 degrees (three months travel time) few 'lours. These ejections are the most ahead of, or behind, the Earth. Therefore, enerptic events in the solar system. coordinated measurements require either a spacecraft 90 degrees before or behind Why do these events occur? Some are the Earth in its orbit, or a high resolution associated with solar flares, although it is X-ray telescope which can identify against unclear why some flares produce visible the Sun's disk ejections directed toward ejections while many do not. An even the Earth. Transient ejections from the more frequent identifiable cause is an Sun's corona are of interest not only eruptive prominence in which material because of their terrestrial effects, but also suspended over the Sun's surface ay mag- as an example of impulsive stellar mass netic fields (a "prominencd") is abruptly loss. Mass loss processes affect many expelled outward. In some cases, no sur- stars, and may have an important influ- face event can be found to cause the ejec- ence on whether they evolve to a quiet whimper or a final bang.

PiG 42 I tt 1

Corona! Transient Ejection - Observed from Skylab

78 43 Why Does the Solar Constant Change?

While s'ilar wind particles and flares have must determine how, when, and where given us new knowledge of the Sun, the this missing energy appears. Sun's radiant energy remains the Earth's primary source of heat and light. The total Measurements have also detected a small of this energy (the "total solar irradiance") but unmistakable decline in irradiance on is about two calories per square centime- a time scale of years. The decline is only ter per minute, meaning the Earth receives one tenth of one percent in four years. But more than 100 trillion kilowatts of solar its importance may be appreciated by not- power. This total quantity has long been ing that if toe Sun continued to dim at this called the "solar constant" because r o rate, it would be left with only one half its variations in total solar irradiance could be current brightness in 2800 years. This detected from beneath the Earth's chang- decline has reversed with the beginning of ing atmosphere. Now, a very accurate the new solar cycle, starting in 1987, Solar Maximum Mission -- radiometer on the Solar Maximum Mis- when the number of sunspots began to (SMA4) sion (SMM) spacecraft has proved that the increase We are searching for the source "solar constant" changes. of this variation and studying how it affects our environment. From SMM investigations, cne type of change quickly recognized was a decrease To further characterize and confirm these in irradiance lasting about a week. It was observations, the SiV1M radiometer will found to correspond to, and to be explained have to be supplemented by similar instru- by, the rotation of a large group of sunspots ments on other spacecraft operating over across the face of the Sun. This discovery many years. This investigation requires was a surprise since it had been thought great patience, but it is justified by its great that the light blocked by sunspots would scientific z.nd practical importance. appear elsewhere on the Sun Now we

30 44 Short-Term Long-Term

Effect: Dip Lasting a Few Days T 1 I Effect: Long-term Decrease A Ni6 0 11., Ii T ip, 04 ) A 0.05 / Mean: qhvilL Li*/ "Vii\ L Li ,I t i s RI 0 L Total Solar Flux A -0.05 R Solar Maximum Mission 0.05% 1 I R I R I A -0.15 D March 1980 April 1980 I A N C E -0.25 1980 1981 1982 1983 Cause: `,locking by Sunspots

Cause: Unknown

8 April 1980

Variations in Total Solar Irradiance ("Solar Constant') VII

62 E3 45 Does the Changing Sun Change Our Weather or Climate?

The Sun is a variable star. It varies slightly Recent studies have found stronger evi- its total energy output, and varies greatly dence that very long term solar variations its output of some forms of energy, includ- affect our climate. For example, during the ing ultraviolet and X-rays, particles, and reign of the Si King Louis XIV (1643- magnetic fields. This varying Sun is the 15), there were very few spots on the energy source fr our "weather engine." It Sun, and northern hemisphere tempera- provides heat, moves around air masses, tures were abnormally low. This trend is and evaporates the water vapor which consistent with SMM data showing declin- becomes precipitation. But our weather is ing irradiance in years of declining sunspot so complex that we have until now been number. There iF thin but tantalizing evi- unable to prove that the changing solar dence extending the connection between output causes measurable changes in our spots and climate backward for thousands weather. of years.

Our ignorance persists in spite of years of While these correlations are impressive, investigation. Since the sunspot cycle was this subject remains controversial because Upper Atmosphere Research discovered in 1842, many investigators no physical mechanism has been found by Satellite (WRSj have looked for corresponding cycles in which small changes in solar energy input the Earth's we .,her. Some impressive can govern the large energy stored in the correlations have been reported. For ex- Earth's weather system. It is a situation ample, years with more sunspots corres akin to a flea biting an elephant; we are pond to increased rainfall at 70 to 80 looking for a small input which produces a degrees north latitude, and decreased rain- very large response. The search requires fall at 60 to 70 degrees north latitude. But accurate measut ements over an extended an apparent correlation at 50 to 60 degrees time period of all solar inputs to the Earth's north latitude switched abruptly to anti- environment. It is a labor of great patience correlation, making a physh-al connection which can be completed only from space. between the two phenomena unlikely, and casting doubt on all such correlations

46 S 5 84 LL-80°N- Latitude of Barrow, AlaskaI i 1

I I /r.', NA 1 / i \\/ `, i CO7791-- Latitude of Reykjavik .1 N 1 , / , ; \ ,."'N / / 1 / t ' \ 1 / I .S. 1 1 / 1, ..... % \ / j/ 1. 1 50°-60"N I Latitude of London 1\ / \

Sunspot Number Rainfall (Annual Excess) .___I 1880 1890 1900 1910 1920 1930 1940 1950 1960 Year

Solar Effects on Weather?

F 6 The Future ofSolar Physics

11 r:T

E' 7 S 49 Missions to Solve the Sun's Mysteries

Solar scientists are justifiably excited about A helioseismology observatory ;Mould the unsolved problems of modern solar be placed in a solar orbit between Earth physics. These problems ar: diverse, ex- and Sun where there are no day-night tending over a wide range of conditions interruptions or large line-of-sight and phenomena. They are of fundamental motions. importance for astrophysics, geophysics, A spacecraft using a swingby of Jupiter and plasma physics. And most exciting is to escape the Earth's orbital plane (the the availability of technology which brings ecliptic) is needed to measure the solar the solution of these problems within our wind over the Sun's poles. (This space- reach. craft, Ulysses, is ready for launch.)

Space-based missions, with their expand- Flights on balloons and rockets are ed spectral coverage and improved spatial ideal for new instrument development. resolution, will continue to play a vital role Balloon-borne instrument packages can in the investigation of these problems. It is be reedy for flight before the next solar revealing to describe likely key compo- maximum (1991). nents of the solar space prcgram and to Many instruments developed earlier assess their contribution to solving the will be included in the Advanced Solar Sun's mysteries. Observatory. It will be mounted on the A one-meter diameter space tele- Space Station, offering great advan- L'pace Shuttle scope operating at visible, ultraviolet, tages for instrument maintenance or and infrared wavelengths is needed to incremental upgrades as new observ- resolve fine structure magnetic fields ing techniques 1.acome available or and gas motions. Interactions between new solar problems are discovered. these fields and gases drive the magnetic dynamo, solar activity, and corona' heating. To study flare processes, we need a new generation of improved resolution detectors of X-rays, gamma rays, and energetic particles. 9U U 50 89 Problem High-ResolutionHigh-Energy Sun-Orbiting Out-of- Rockets- Integrated Addressed Optical Observatory Dbservatory Ecliptic Balloons Observatory

Solar Neutrinos

. . Magnetic Fields f e

Coronal Heating

Flares

Coronal Holes

Polar Solar Wind

Coronal Ejections

Solar "Constant"

Solar Weather Changes . .., The Unknown

In-porta;a of Contribution Primary Secondary

Current and Manned Space Solar Physics Missions

(11 51 The Sun: Pathfinder for Astrophysics

NI. What part does the Sun play in modern astrophysics will apply with the Great astrophysics? Not only is it unique among Observatories Program. The successful astronomical objects because of its practi- repair in orbit of the Solar Maximum Mis- cal importance, but it is an object of oreat sion spacecraft has demonstrated the fea- scientific importance as well. The ,,,..n's sibility of instrument and spacecraft main- proximity has permitted the discovery of tenance in space. This maintenance capa- such phenomena and processes as rota- bility has significantly influenced the tion, flares, spots, chromospheres, and design of the next generation of space coronae, paving the way for similar dis- astronomical missions coveries throughout astrophysics. Similar- ly, tools and techniques developed for use Non-solar astrophysics also anticipates on the Sun have aften found subsequent broad application of the answers to such application in nor solar astrophysics. Solar current questions in solar physics as precedence is not universal (radio waves neutrino problem and magnetic field be- SMM Repair Mission from the galaxy were detected before those havior. The observational connection is from the Sun), but it is typical because the .summarized by Robert Rosner, theoretical abundant fluxes of various forms of energy astrophysicist at the Harvard College Ob- we receive from the Sun are much easier servatory: "Solar observations can often to detect and analyze than the fluxes from be used both 1, constrain the physics and distant stars or galaxies. to suggest the appropriate observational tools: thus the Sun and its environs pro- Current plans presage a continuation of vide us with a directly observable labora- this fruitful symbiotic relationship. The tory for studying magnetohydronamics and development of imaging X-ray optics for plasma physics on astrophysical scales." Skylab provided experience and confi- dence essential for the Einstein Observa- tor ,, and for the future Advanced X-Ray Astrophysics Facility. Skylab also demonstrated the utility c simulaneous multi- spectral observation, a concept non-solar

f A ' `i 52 93 Tool or Technique Application to Astrophysics

Solar Non-Solar

Visible Spectroscopy 1802 1 w,"0 s

Radio Wave Observations 1'2 ;: 1932

Ultraviolet Obsei ;ations 1949 i 4,_-,(--;

X-Ray Spectra 1949

X-Ray Imaging 1963

Simultaneous MultispectrcAl 1973 Observations

Manned Orbiting Observatory 19 73

Instrument Repair in Orbit 1973

Solar Physics: Develop:1g the Tools of Astrophysics

.C J r Solar Research: A New Era Ahead

Solar physics crossed an important thresh- the ground. For example, other space mis- old when instruments capable of observ- sions will be needed for instrumentation ing the Sun were first lifted above our development and to exploit the advantages atmosphere. An age of accelerated discov- of unique orbits (for example, orbits out of ery ensued, aided by continued improve- the ecliptic or between Earth and Sun). ments in the size and quality of spaceborne instruments and the duration of Itis appropriate that solar physics be their operation. This progress points to an included as a major participant on the important future milestonethe assembly Space Station. Not only do the Sun's light of the Advanced Solar Observatory on the and other emissions affect us in diverse Space Station. It will include a full spec- ways, but our study of the Sun continues to trum of advanced instruments operating shed light throughout astrophysics and from space but with opportunities for peri- other disciplines. In the words of Eugene odic servicing or evolutionary upgrading Parker of the University of Chicago: "The v. t..ch are now available only for ground- observed behavior of the Sun defies theo- based instruments. retical understanding, and therein lies the future of solar physics. Indeed, it is the To exploit coming opportunities like the future of all stellar physics, for a dilemma Space Station, solar physics must con- posed by the Sun is a dilemma for all stars tinue its advances in instrument develop- otherwise too distant to be properly stu- ment, observational techniques, and basic died. The Sun, after several decades of theory. Even when the Advanced Solar scrutiny, has become as enigmatic as the Observatory becomes a reality, it will not quasar, but with the advantage that the eliminate the need for other space-based mysteries can be intimately probed by new observations any more than they have techniques and instrumentation.- eliminated the utility of observations from

54 97 la

A New Era of Sola' Research Advanced Solar Observatory Mountedon Space Station (Extreme Left End of the Beam Structure)

55 NASA National Aeronautics and Space Administration NP-106 100