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A Powerful Solar Observatory in Space

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A Powerful Solar Observatory in Space Source of Acquisition NASA Washington, D. C. I L Orb . .. so/;. -..: .. -i .b' Q' - A Powerful Solar Observatory in Space he Sun, the nearest star, is c~ucialfor life on Earth ancl has always heen an olject of intense stucly. Indeed, Inally of the fundamental principles of astrophysics, nrhich seeks to unclerstancl the physical nature of the universe, have I~eenestablished through stucly of Magnetic activity within the Sun elrives powerfill events on its surface anel in its atmosphere. Fro111 the Earth, these events can he seen as active regions with their sunspots ancl solar tlares. But to obse~vesuch phenomena in detail requires more finely-tuneel and powerfi~linstruments, operating in space, across a I,roacler range of the electromagnetic spectrum. than have ever I~eenavailal~le in the past. Earth's atmosphese has always I~lurredthe iniages received 11y visil~le-lighttele- scopes on the grouncl. The atn~ospherealso acts as a harrier to [he ultraviolet, extreme ultraviolet, anel x rays that are siinilarly emitted fro111 the Sun. All these wavelengths neecl to Ile captureel, at the same moment in time, before the complex interrelationships of solar activity can be truly understood. The Orbiting Solar Lahoratosy (OSL), the prime NASA solar mission for the 1990's, is i~niquelydesigned to "see" across the wavelengths anel so ohse~lfethe fine evolving cletails of a nride range of phenomena on the surface of the Sun. By the turn of the centu~y,OSL nrill enable scientists to s~lve mysteries that humankincl has sought to i~nravelfor thousantls .,- - - . 8 . ..-. 8.- 8 ,, ::I,- * :'--:: ,. --:I A mognetogmm of the full disk of the Sun, in ivhich white oreos represent outwordpointing niognehi. field, with closeup view of two sunspots. Lis groundbosed image hos o resolution of 2 arc , ,seconds, which OSL willimprove by o fucb~ofIS. '. c4 --+A- - The Sun 4 The Orbiting Solor loboratory: Abw fro in Solar 0b;ervatians from Soace 8 *W%lar Cycle and Solar Magnetism I 10 1I ' The Sulface and Interior of the Sun 12 BeSun's Chromosphere and Corona 14 abgnetic Structure of the Corona 16 Solar Flares 18 The Sun and the Earth's Magnetosphere and Atmosphere 20 The Sun and Astrophysics 22 The Orbiting Solar laboratory: The Mission 28 - . ~h;orbitin~ Solar Laboratory: A Solar Research Facility - - Summary The Sun Why Study the Sun? ce prehistoric times, humankind has observed the of radiation. The solar engine is driven by countless nuclear ( Sun. Its rhythms intrigued our distant ancestors in fusion reactions in its core, where the temperature reaches locations as far apart as Stonehenge and the Big fifteen million degrees kelvin (K). Its energy encompasses the I9 Horn Medicine Wheel of Wyoming. The Sun is the source Earth, interacting in complex ways with the Earth's magnetic of the heat and light that make possible life on Earth and it field and reaching out beyond the solar system into interstellar creates a spectrum of other significant effects on the Easth and in space. the interplanetary environment. It is known that very slowly, over millennia, the Sun is In addition, because the Sun is very much closer to Earth changing. About four and a half billion years ago, it is believed, than any other star, it can prwide detailed inforn~ationabout the the Sun was born out of a cloud of dust and gas. Approxi- nature of the stars and the universe that can be learned nowhere mately five billion years from now it will be transformed into a else. Since Galileo first studied sunspots in the seventeenth red giant star. It will consume the inner planets of the solar century, the Sun has fascinated scientists. system and spew half of its mass into space in a series of The Sun is also a unique laboratory for physics. Because gigantic expansions and contractions before subsiding into old of its huge scale, processes can be studied on the Sun that age as a white dwarf. cannot be duplicated on Earth in any physics or engineering Fortunately for life on Earth, the Sun in its present phase laboratory. Study of the Sun expands our knowledge of the way provides relatively constant radiation of heat and light. in which the physical world works, both on Earth and througp- Although the Sun's brightness has increased by some 40 out the universe. percent since its creation, the amount of solar energy retained The Sun has been, and will continue to be, a compelling by the Earth and its atmosphere has remained roughly the and important object of study. What is the Sun? What has all this study taught scientiseand what do they now need still to learn? It is known that the Sun is a sphere of hot, seething gases, of sudden outbursts and surges fie main feohes of he Sun. I NASA Headquarters Library 3!J0 E St. SW Rm. 1120 Washington, DC 20546 same during its lifetime. That continuity has made possible life visible to the naked eye only when the Sun's bright disk is as it is known on Earth today. A small reduction in the average eclipsed. Beyond, flowing out into interplanetary space, is a surface temperature of the Sun would initiate a major advance stream of positively and negatively charged particles-a of the Earth's glaciers; a small rise would melt the icecaps, plasma-known as the solar wind. flood the continental coasts, and eventually turn the land The solar wind catches the Earth in its flow and sweeps masses to Saharan deserts. the lines of the terrestrial magnetic field-which forms a cavity in However, on much shorter timescale~very11 years- the solar wind known as the magnetosphere--out into the shape magnetic activity deep within the Sun reaches a climax, causing of a long tail. outbreaks of dark sunspots, solar flares, and other forms of As mighty forces contend within the Sun's million-mile increased activity. The energy reaching the Earth during this sphere, their effects are manifested on the photosphere and period of maximum solar activity, known as the solar maxi- spread outward in waves throughout the layers of the solar mum, then rises some tenths of a percent compared with solar atmosphere. The effect of this process can be seen in visible minimum. We do not yet understand the mechanisms that drive light in the photosphere. Effects in the chromosphere can be this cycle. detected by ultraviolet light, and those in the corona by The Sun's bright surface is known as the photosphere extreme ultraviolet and x rays. The solar wind itself changes as (from Greek words meaning "sphere of light"). Above the these events unfold, carrying the direct effects into Earth's own photosphere lies the chromosphere, or "sphere of color," so space. called because of its bright red emission from hot hydrogen. I The Orbiting Solar Laboratory (OSL) will be designed Still higher above the solar surface lies the corona, or "crown," specifically to read the messages in the three outer layers of the Sun-the photosphere, the chromosphere, and the corona-thereby giving us solutions to some of the deepest solar mysteries: the )olar cycle, sunspots, flares, and many other phenomena. A representation of the Earth's magnetosphere: he solar wind from he Sun on the light envelopes the Fa& nowing oound the I terrestrial mognek field. - lain OSL Tele The Orbiting Solar Laboratory: A New Era in Solar Observations from Spate he key feature of the Orbiting Solar Laboratory nlission In addition, OSL has been designed as a major international is that it will place in space, in an almost continually solar facility for use by large numbers of scientists. Guest sunlit orbit around the Earth's poles, the most powerful Obsenrers and Guest Investigators will be selected. A theory Tsolar telescope ever flown. This telescope will be able to program and OSL Graduate Student and Postdoctoral Programs focus on a small area on the solar surface and observe the Sun will be set up. in unprecedented detail almost continuously for most of the year. Coordinated Observations of the Sun Strangely, the dense gases of the photosphere and Why Is This So Critical? chromosphere are relatively cool compared to the much less Study of the Sun from the ground is hampered by the dense, but very hot, gases of the corona. Sonlehow, the low- Earth's overlying atmosphere in two fundamental ways. First, energy, cooler gases at the solar surface are heated to become the atmosphere causes blurring of images from even the best high-energy, hot gases in the corona. As a result, the Sun ground-based telescopes, and this seriously degrades obsel-va- displays an enormous range of interrelated phenomena over an tions of key solar phenomena. Second, some of the most , equally enormous temperature range. Observations in visible meaningful and informative emissions from the Sun occur at light show the interaction of turbulent motions and magnetic ultraviolet and x-ray wavelengths, which are not able to fields at the solar surface, and observations in ultraviolet and penetrate the Earth's atmosphere to ground level. Additionally, x-ray wavelengths show how the overlying hot chromosphere the day-night cycle experienced on Earth limits the time during and corona respond to these interactions. But a complete which important events can be continuously observed. The picture of these processes requires simultaneous and coorcli- solution to these problems is to make solar obset-vations from nated observation. at visible, ultraviolet, and x-ray wavelengths. space, where instrunlents can sharply capture the entire range Using such &bservations, scientists can correlate activity in the of electro~nagneticenergy from visible light through x rays, and photosphere with that in the upper layers of the Sun's atmo- where the observing platfo~mcan orbit independently of the sphere, the chromosphere and the corona.
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