<strong>SOHO</strong>Bernhard Fleck, Daniel Müller, Stein Haugan,Luis Sánchez Duarte & Tero SiiliSolar and Solar-Terrestrial Missions Division,ESA Research & Scientific Support Department,NASA Goddard Space Flight Center, Greenbelt,MD, USAJoseph B. GurmanNASA Goddard Space Flight Center, Greenbelt,MD, USASince its launch on 2 December 1995,<strong>SOHO</strong> has revolutionised our understanding<strong>of</strong> the Sun. It has provided thefirst images <strong>of</strong> structures and flows below theSun’s surface and <strong>of</strong> activity on the far side.<strong>SOHO</strong> has revealed the Sun’s extremelydynamic atmosphere, provided evidence for thetransfer <strong>of</strong> magnetic energy from the surface tothe outer solar atmosphere, the corona, througha ‘magnetic carpet’, and identified the sourceregions <strong>of</strong> the fast solar wind. It hasrevolutionised our understanding <strong>of</strong> solarterrestrialrelations and dramatically improvedour space weather-forecasting by its continuousstream <strong>of</strong> images covering the atmosphere,extended corona and far side. The findings aredocumented in an impressive number <strong>of</strong>scientific publications: over 2500 papers inrefereed journals since launch, representing thework <strong>of</strong> over 2300 individual scientists. At thesame time, <strong>SOHO</strong>’s easily accessible,spectacular data and fundamental scientificresults have captured the imagination <strong>of</strong> thespace science community and the general publicalike. As a byproduct <strong>of</strong> the efforts to providereal-time data to the public, amateurs nowdominate <strong>SOHO</strong>’s discovery <strong>of</strong> over 1<strong>10</strong>0 Sungrazingcomets.esa bulletin 126 - may 2006 25
ScienceIntroductionWe all live in the extended atmosphere <strong>of</strong> amagnetically active star. While sunlightsustains life, the Sun’s variability producesstreams <strong>of</strong> high-energy particles andradiation that can affect life. Understandingthe changing Sun and its effectson the Solar System has become one <strong>of</strong> themain goals <strong>of</strong> the <strong>SOHO</strong> mission, whichwas launched to address three fundamentalscience questions: what is the structure anddynamics <strong>of</strong> the solar interior, how is thecorona heated, and how is the solar windaccelerated?A consortium <strong>of</strong> European spacecompanies led by prime contractor MatraMarconi Space (now EADS Astrium)built <strong>SOHO</strong> under overall managementby ESA, and international consortiadeveloped its suite <strong>of</strong> 12 instruments.NASA launched <strong>SOHO</strong> on 2 December1995, inserting it into a halo orbit aroundthe L1 Lagrangian point in February1996.The <strong>SOHO</strong> Experiment OperationsFacility (EOF), at NASA’s Goddard SpaceFlight Center, serves as the focal point formission science planning and instrumentoperations. Half <strong>of</strong> the 12 <strong>SOHO</strong> PrincipalInvestigator Teams have residentrepresentatives at the EOF, where theyreceive telemetry and send commandsdirectly from their workstations throughthe ground system to their instruments.An overview article cannot do justice tothe 2500-plus articles published in therefereed literature and an even greaternumber in conference proceedings andother publications. Here, we touch upon afew selected results. Highlights from thefirst 4 years <strong>of</strong> <strong>SOHO</strong> were described inBulletin <strong>10</strong>2 (May 2000).Making the Sun TransparentJust as seismology reveals the Earth’sinterior by studying earthquake waves,solar physicists probe the Sun’s interior via‘helioseismology’. The oscillationsdetectable at the visible surface are due tosound waves reverberating through theSun’s inner layers. By precisely measuringthe frequencies, we can infer the Sun’stemperature, density, atomic abundances,interior structure and the age <strong>of</strong> the SolarSystem, and even pursue such esotericmatters as testing the constancy <strong>of</strong> thegravitational constant.One <strong>of</strong> the most productive instruments,the Michelson Doppler Imager (MDI),shows oscillations <strong>of</strong> the whole Sun. It hasrevealed strong variations in the velocity<strong>of</strong> the plasma in the solar interior, andfound an ‘adjustment’ layer at the base <strong>of</strong>the convection zone. This layer, about220 000 km beneath the visible surface(about a third <strong>of</strong> the way down to theSun’s centre), connects the more orderlyinterior <strong>of</strong> the Sun (the radiative zone)with the more turbulent outer region(the convection zone). It is <strong>of</strong> particularinterest because this is where the solardynamo that creates the Sun’s magneticfield is believed to operate. In this region,the speed <strong>of</strong> the gas changes abruptly.Near the equator, the outer layers rotatefaster than the inner layers. At midlatitudesand near the poles, the situationis reversed.MDI data have revealed a fascinatingpicture <strong>of</strong> the large-scale, subsurfacedynamics <strong>of</strong> the Sun, with dramaticchanges with the solar cycle. We all knowSolar rotation and polar flows <strong>of</strong> the Sun deduced from MDImeasurements. The left side shows the difference in rotationspeed between various areas. Red-yellow is faster than average,while blue is slower than average. The light orange bands arezones that move slightly faster than their surroundings. Thecutaway reveals rotation speed inside the Sun. The large dark redband beneath the solar equator is a massive fast flow <strong>of</strong> hot,electrically-charged gas: plasma. The blue lines at right show thesurface flow from the equator to the poles. The return flowindicated at the bottom <strong>of</strong> the convection zone has not yet beenobserved<strong>of</strong> the crucial importance <strong>of</strong> large-scalestreams in our atmosphere (e.g. jet stream)and in the oceans (e.g. gulf stream) forEarth’s climate. MDI has for the first timeenabled us to observe such large-scalestreams inside the Sun. Researchersdiscovered transient storms, high- and lowpressurezones, and swirling wind flowsnear active regions that vary from day today like weather patterns on Earth’ssurface.Ever since MDI began to deliver helioseismicinformation at finer spatial scalesthan previously available, the new field <strong>of</strong>‘local-area helioseismology’ has developedrapidly. New methods allowed construction<strong>of</strong> the first true 3-D images and flow maps<strong>of</strong> the interior <strong>of</strong> a star, and even the firstimages <strong>of</strong> the far side <strong>of</strong> our Sun. Applyingthe novel ‘acoustic tomography’ methodto MDI data, scientists could for the firsttime study the structure <strong>of</strong> sunspots belowthe Sun’s surface. They were thus able tosolve two long-standing puzzles aboutthese blemishes on the Sun, which havebeen a source <strong>of</strong> wonder ever since theywere described (and drawn in painstakingdetail) by Galileo Galilei: how deepdo the spots extend below the surface,and how can sunspots last for severalweeks without breaking up? The MDIteam found the answers: strong, convergingdownflows stabilise the structure <strong>of</strong> thesunspots, and sunspots are relativelyshallow.‘Solar Subsurface Weather’ map, showing magnetic field(black/red) and average ‘wind’ flow (blue arrows) under thesolar surface26esa bulletin 126 - may 2006www.esa.int
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