10 Years of SOHO10 Years of SOHO

Since its launch on 2 December 1995,SOHO has revolutionised our under-

standing of the Sun. It has provided thefirst images of structures and flows below theSun’s surface and of activity on the far side.SOHO has revealed the Sun’s extremelydynamic atmosphere, provided evidence for thetransfer of magnetic energy from the surface tothe outer solar atmosphere, the corona, througha ‘magnetic carpet’, and identified the sourceregions of the fast solar wind. It hasrevolutionised our understanding of solar-terrestrial relations and dramatically improvedour space weather-forecasting by its continuousstream of images covering the atmosphere,extended corona and far side. The findings aredocumented in an impressive number ofscientific publications: over 2500 papers inrefereed journals since launch, representing thework of over 2300 individual scientists. At thesame time, SOHO’s easily accessible,spectacular data and fundamental scientificresults have captured the imagination of thespace science community and the general publicalike. As a byproduct of the efforts to providereal-time data to the public, amateurs nowdominate SOHO’s discovery of over 1100 Sun-grazing comets.

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, USA

Joseph B. GurmanNASA Goddard Space Flight Center, Greenbelt, MD, USA

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IntroductionWe all live in the extended atmosphere of amagnetically active star. While sunlightsustains life, the Sun’s variability producesstreams of high-energy particles andradiation that can affect life. Under-standing the changing Sun and its effectson the Solar System has become one of themain goals of the SOHO mission, whichwas launched to address three fundamentalscience questions: what is the structure anddynamics of the solar interior, how is thecorona heated, and how is the solar windaccelerated?

A consortium of European spacecompanies led by prime contractor MatraMarconi Space (now EADS Astrium)built SOHO under overall management by ESA, and international consortiadeveloped its suite of 12 instruments.NASA launched SOHO on 2 December1995, inserting it into a halo orbit aroundthe L1 Lagrangian point in February1996.

The SOHO Experiment OperationsFacility (EOF), at NASA’s Goddard SpaceFlight Center, serves as the focal point formission science planning and instrumentoperations. Half of the 12 SOHO 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 of SOHO were described inBulletin 102 (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 of the Solar

System, and even pursue such esotericmatters as testing the constancy of thegravitational constant.

One of the most productive instruments,the Michelson Doppler Imager (MDI),shows oscillations of the whole Sun. It hasrevealed strong variations in the velocityof the plasma in the solar interior, andfound an ‘adjustment’ layer at the base ofthe convection zone. This layer, about220 000 km beneath the visible surface(about a third of the way down to theSun’s centre), connects the more orderlyinterior of the Sun (the radiative zone)with the more turbulent outer region(the convection zone). It is of particularinterest because this is where the solardynamo that creates the Sun’s magneticfield is believed to operate. In this region,the speed of the gas changes abruptly.Near the equator, the outer layers rotatefaster than the inner layers. At mid-latitudes and near the poles, the situationis reversed.

MDI data have revealed a fascinatingpicture of the large-scale, subsurfacedynamics of the Sun, with dramaticchanges with the solar cycle. We all know

of the crucial importance of 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 low-pressure zones, and swirling wind flowsnear active regions that vary from day today like weather patterns on Earth’ssurface.

Ever since MDI began to deliver helio-seismic information at finer spatial scalesthan previously available, the new field of‘local-area helioseismology’ has developedrapidly. New methods allowed constructionof the first true 3-D images and flow mapsof the interior of a star, and even the firstimages of the far side of our Sun. Applyingthe novel ‘acoustic tomography’ methodto MDI data, scientists could for the firsttime study the structure of sunspots belowthe Sun’s surface. They were thus able tosolve two long-standing puzzles aboutthese blemishes on the Sun, which havebeen a source of wonder ever since theywere described (and drawn in pains-taking detail) 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 of thesunspots, and sunspots are relativelyshallow.

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Solar rotation and polar flows of 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 of 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 of the convection zone has not yet beenobserved

‘Solar Subsurface Weather’ map, showing magnetic field(black/red) and average ‘wind’ flow (blue arrows) under thesolar surface

Just a little over 4 years after the launchof SOHO, scientists published anastonishing result: the first successfulholographic reconstruction of features onthe far side of the Sun. An active regionon the far side reveals itself because itsstrong magnetic fields speed up the soundwaves. The difference becomes evidentwhen sound waves shuttling back andforth fall out of step with one another. Inthe meantime, the astonishing has becomeroutine, and MDI offers daily far-sideimages online, at http://soi.stanford.edu/data/full_farside/.

Violent solar activity occasionallydisrupts satellites, radio communica-tions and power systems. Advancewarning of magnetic storms brewing onthe far side that could rotate with the

Sun and threaten Earth is therefore ofvital importance for space weatherforecasting.

The Sun’s Dynamic CoronaThe outer atmosphere of the Sun, thecorona, has a typical temperature of about1 million K and emits light mainly in theultraviolet (UV) part of the spectrum. It isthis radiation from the Sun’s hot coronathat controls the composition and

dynamics of Earth’s upper atmosphere.Water vapour and ozone are especiallysensitive to changes in the Sun’s UVoutput. Fortunately, Earth’s atmosphereprotects us from this harmful radiation.This also means that we have to leaveEarth’s atmosphere behind and observethe Sun from space.

SOHO’s Extreme ultraviolet ImagingTelescope (EIT) provides us with stunningimages of the Sun’s corona, showingdelicate coronal loops, bright flares andintriguing coronal holes. EIT and the UVspectrometers SUMER and CDS show usthat the outer solar atmosphere isextremely dynamic and changing, and thatplasma flows play an extremely importantrole.

Gone With the Solar WindThe solar wind is a stream of mainlyelectrons and protons flowing from the

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Holographically reconstructed side view of the Sun showing thetwo large active regions 10486 and 10488 as they rotate fromthe far side (left) to the hemisphere visible from Earth (right).During the previous rotation, these two active regions were thesource of the powerful ‘Halloween’ storms in 2003, whichincluded the largest ever recorded X-ray flare

Using advanced analysis techniques, SOHO/MDI can reveal the temperature and flow structure beneath sunspots

Sun faster than 3 million km/h. It isessentially the hot solar corona expandinginto interplanetary and interstellar space.It deforms Earth’s magnetosphere andionises atoms in our upper atmosphere,causing beautiful aurorae.

Interpreting data from the UltravioletCoronagraph Spectrometer (UVCS),scientists found evidence that the solarwind streams out of the Sun by ‘surfing’onwaves produced by vibrating magneticfield lines, just like an ocean wavecarrying a surfer.

Using SUMER, scientists have mappedthe outflow of plasma from coronal holes,and found a clear connection between theflow speed and the network structure ofthe chromosphere, a lower layer of theatmosphere. This is the first spectro-scopic discovery of the source of the fastsolar wind. In a later study, a Chinese-German team has used SUMER andMDI data to show that the solar wind

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Composite image of the solar corona with three wavelengths (171 Å as blue, 195 Å as yellow and 284 Å as red) combined to show features unique to each wavelength. The 171 Å filter captures emission atabout 1 million K, 195 Å at about 1.5 million K, and 284 Å at about 2.5 million K

The solar wind emerges from coronal funnels. The magenta curves are open magnetic field lines, dark grey arches show closed ones.The lower plane shows the magnetic vertical component measured by MDI (blue to red). The upper plane shows Ne VIII Doppler shiftsfrom SUMER (hatched regions have large outflows), together with the inclination of the magnetic field (blue to red), as extrapolatedfrom the MDI measurements

escapes from the Sun in funnels of openmagnetic fields.

SOHO, the Space Weather WatchdogWhile the Sun’s total radiative output isreassuringly constant, it is at the same timea dynamic and violent star. Besidesemitting a continuous stream of plasmain the solar wind, the Sun periodicallyreleases huge amounts of matter in coronalmass ejections (CMEs). These are the mostpowerful eruptions in the Solar System,with billions of tonnes of electrified gaspropelled from the Sun’s atmosphere intospace at millions of km/h. If they hit Earth,these immense clouds can cause largemagnetic storms in our magnetosphereand upper atmosphere. Researchers believethey are launched when solar magneticfields become strained and suddenly snapinto a new arrangement, like a rubber bandtwisted to breaking point.

Apart from causing beautiful aurorae,CME disturbances can damage satellites,disrupt telecommunications, endangerastronauts, lead to corrosion in oil pipelines and cause current surges in powerlines. As our society becomes increasinglydependent on space-based technologies,our vulnerability to ‘space weather’becomes more obvious, and the need tounderstand it and mitigate its effectsbecomes more urgent. In recent years,forecasting the conditions in the near-Earth environment and the ‘geo-effectiveness’ of CMEs and solar flares hasbecome one of the key research areas insolar and solar-terrestrial physics; SOHOis playing a pioneering role in this newdiscipline. While satellites that makein situ measurements in near-Earth orbitscan give only about a 2-hour alert of solarstorms, SOHO’s coronagraphs and EUV

imager observe the source of CMEs andflares and thus provide up to 3 days’warning, sufficient to save costly equip-ment. SOHO’s LASCO and EIT are theprimary source of operational informationon the location, speed and orientation ofCMEs from the Earth-facing hemisphereof the Sun. These remote-sensing observa-tions are complemented by the CELIAS,COSTEP and ERNE in situ measure-ments of the arrival of the CME andenergetic particles at the L1 Lagrangianpoint.

The LASCO team has compiled anextensive catalogue that summarises themass, speed, acceleration, angular width,position and so on of the more than 10 000CMEs observed since launch. Thiscatalogue has been used in numerousstudies, including how the number ofCMEs varies with the solar cycle: the rateincreases from 0.5 per day during solarminimum to over 6 per day during solarmaximum. Using LASCO data, scientistcan also for the first time reconstruct 3-Dimages of CME structures. Combined

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This fiery coronal mass ejection shows stunning details in the ejected material, revealed by SOHO’s LASCO coronagraph. The directsunlight is blocked (red disc), exposing the surrounding faint corona. The white circle represents the approximate size of the Sun

A coronal mass ejection heading almost directly towards Earth,observed by LASCO C2. The size of the Sun is indicated by acircle, and the x-marked circle on the Sun shows the origin ofthe CME. Panel (a) shows the total intensity (darker meansmore intense) as imaged directly by LASCO. Panel (d) is atopographic map of the material shown in panel (a). Thedistance from the plane of the Sun to the material is colour-coded; the scale in units of solar radii is shown on the side.Panels (b) and (c) show the intensity as it would appear to anobserver positioned to the side of the Sun or directly above it,respectively

with other views from the coronagraphs onthe STEREO satellite to be launched laterthis year, this technique shouldsignificantly reduce ambiguities in thereconstruction of interplanetary CMEmorphology.

During 2 weeks in October/November2003, the Sun featured three unusuallylarge sunspot groups, which gave rise to11 X-class flares (including the strongestever recorded), numerous CMEs and twolarge proton storms. Satellites, power grids,radio communications and navigationsystems were significantly affected. Theevents, among the best-observed ever, willbe analysed for years to come. The eventstriggerred unprecedented attention fromthe media and public. SOHO imagesappeared in nearly every major news

outlet. Furthermore, the great publicinterest wiped out all existing SOHO webtraffic records, with the web server servingover 31 million page requests and4.3 Terabyte of data in just one month.

Measuring the Total Solar IrradianceA crucial question is whether the Sun’stotal irradiance is changing on longer timescales. Indications of such a change wouldhave a broad social and political impactas governments would have to devisestrategies in response to global warming.SOHO’s VIRGO instrument is monitoringthe total solar irradiance (TSI), also knownas the ‘solar constant’. The VIRGO teamhas constructed composites of TSImeasurements from SOHO and otherspacecraft over the last 26 years. Fromthis composite record, it now appears thatthere is no evidence for a significant long-term trend in the TSI: the total radiationfrom the Sun does not show any systematicbrightness increase or decrease onobserved time scales. However, while thetotal solar irradiance varies by less than0.1% over the 11-year solar cycle, theirradiance in the extreme-UV (EUV) partof the spectrum changes by as much as30% within weeks and by a factor of

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Active Region 10486 unleashed a spectacular show on28 October 2003: an X 17.2 flare, a fast-moving coronal massejection and a strong solar energetic particle event. From top left:giant sunspot groups seen by MDI in white light; the flare as seenby EIT at 195 Å; the fast-moving CME in the LASCO C2coronagraph, then in the LASCO C3 coronagraph, with the particleshower becoming visible as ‘snow’ in the image. The cloud struckEarth’s magnetosphere only19 hours later, almost a record speed

The composite total solar irradiance (TSI) over two solar cycles,combining data from multiple spacecraft instruments, does notshow a long-term change. Different colours indicate differentdata sources; the blue data are from SOHO’s VIRGO

2–100 (depending on wavelength) overthe solar cycle. Detailed knowledge ofthe solar spectral irradiance is crucial forunderstanding climate variability, and todisentangle natural variations fromanthropogenic climate changes.

SOHO, the Comet FinderSOHO is providing new measurements notonly about the Sun. On 5 August 2005,Toni Scarmato, a high school teacher fromCalabria, Italy, discovered SOHO’s 999thand 1000th comets. As of February 2006,LASCO had detected over 1100 comets,most of them ‘Sun-grazers’. These cometspass very close to the Sun and growprominent tails as their icy cores areheated. Nearly half of all comets forwhich orbital elements have beendetermined (since 1761) were discoveredby SOHO, and over two-thirds of thoseby amateurs accessing LASCO data viathe Web. This is a field where amateurscan actively contribute to scientificresearch; each day, numerous peoplefrom all over the world download thenear-realtime data to search for newcomets.

As the brightest, most spectacularcomet ever observed by SOHO, cometNEAT (C/2002 V1) provided someenticing data for further study, thanks toa grazing encounter with a coronal massejection. The LASCO C3 observationsduring 16–20 February 2003 suggestinteraction between the comet’s ion tailand other magnetic fields in the outercorona at the time of the oblique impactwith the CME. This is the first time thatsuch an event has been imaged, and nocomet has ever been observed closer tothe Sun.

The analysis of high-resolution spectro-scopic observations of comet C/2002 X5(Kudo-Fujikawa) from UVCS has revealeda near-spherical cloud of neutral hydrogenand a variable tail of ionised carbon (C+

and C+2) that disconnected from the comet

and was subsequently regenerated. SOHO’s SWAN instrument monitored

the break-up of comet LINEAR C/1999

S4. The total amount of water vapourobserved by SWAN from 25 May to12 August 2000 was estimated at3.3 million tonnes. Only about 1% of thiswas left on 6 August, when observationsby the Hubble Space Telescope of thedying comet’s fragments gave an estimateof the total volume. Combining thenumbers gives a remarkably low value forthe density: about 15 kg/m3, comparedwith 917 kg/m3 for familiar non-porousice. Even allowing for an equal amount ofdust grains, 30 kg/m3 is far less than the500 kg/m3 often assumed by cometscientists.

Conclusions and SOHO’s FutureThe journey has not always been easy. Forexample, an unexpected loss of contactoccurred on 25 June 1998. Fortunately, themission was completely recovered in oneof the most dramatic rescue efforts inspace, and normal operations could beresumed in mid-November 1998 after thesuccessful recommissioning of thespacecraft and all 12 instruments. Despitethe subsequent failures of all threegyroscopes (the last in December 1998),new gyroless control software installed byFebruary 1999 allowed SOHO to return tonormal scientific operations, providing an

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Perihelion passage of comet NEAT (C/2002 V1) in February2003, as imaged by LASCO C3

even greater margin of safety. This madeSOHO the first 3-axis-stabilised spacecraftto operate without a gyroscope. A thirdcrisis occurred in June 2003, whenSOHO’s main antenna became stuck.Using the secondary antenna and softwarefor intermittent recording, however, eventhis problem was overcome, and theobservations continue.

In complex areas of research such assolar physics, progress is not made by justa few people. The scientific achievementsof the SOHO mission are the results ofa concerted, multi-disciplinary effort by alarge international community of solarscientists, involving sound investment inspace hardware coupled with a vigorousand well-coordinated scientific operationand interpretation. The interplay betweentheory and observations has deliveredmany new insights and will continue to doso for many years.

In its 10 years since launch, SOHOhas provided an unparalleled breadth anddepth of information about the Sun,from its interior, through the hot anddynamic atmosphere, to the solar windand its interaction with the interstellarmedium. Research using SOHO observa-tions has revolutionised our under-standing of the Sun and space weather.The coming years promise to be similarlyexciting and rewarding, when SOHOobservations are complemented andenhanced by those from NASA’s STEREOand Japan’s Solar-B missions, affordingnew opportunities for improved under-standing of the Sun-heliosphere system.After the launch of NASA’s SolarDynamics Observatory, SOHO’s directdescendant, the capabilities of someSOHO instruments will be eclipsed. Butnot all: the LASCO coronagraphobservations and VIRGO total solar

irradiance measurements will continueto make crucial and unique contributionsto the International Living With a Star(ILWS) programme.

AcknowledgementsThe great success of the SOHO mission isa tribute to the many people – too manyto name here – who designed and builtthis exquisite spacecraft and its excellentinstruments, to the engineers who broughtit back from the dead (twice), and to themany people who diligently work behindthe scenes to keep it up and running.

e

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Further information on SOHO and its achievements can befound at http://soho.esac.esa.int/

Composite image of comet C/2002 X5 (Kudo-Fujikawa) as it passed within 0.2 AU of theSun in late January 2003. The path of thecomet is in white, with arrows indicating thedirection of travel. The solar disc is shown asan EIT 304 Å image. The background is acombination of LASCO C2 and C3 images ofthe corona in visible light, and the blank ringrepresents the occulting disc of the C2camera. LASCO also reveals the comet in theoptical, as the fuzzy white dot to the right ofthe Sun. The zoomed-in images at right depictthe comet in the H I Lyman-alpha 1216 Å line(blue) and the C III 977 Å line (red-orange).The C III emission has never been seen beforein a comet. At upper right, the comet’s mainplasma tail has disconnected and is driftinganti-sunward even as a new, dim plasma tailis born from the nucleus. By the time of thesecond UVCS image, the new plasma tail hasbrightened and is beginning to turn as thecomet heads away from the Sun