the solar orbiter mission and its prospects for helioseismology

Astron. Nachr. / AN 328, No. 3/4, 362 – 367 (2007) / DOI 10.1002/asna.200610743

The Solar Orbiter mission and its prospects for helioseismology

J. Woch� and L. Gizon

Max-Planck-Institut fur Sonnensystemforschung, Max-Planck-Str. 2, D-37191 Katlenburg-Lindau, Germany

Received 2006 Dec 19, accepted 2007 Jan 2Published online 2007 Feb 28

Key words Sun: atmosphere – Sun: helioseismology – Sun: oscillations

Solar Orbiter is intended to become ESA’s next solar mission in heritage of the successful SOHO project. The scientificobjectives of the mission, its design, and its scientific payload are reviewed. Specific emphasis is given to the prospects ofSolar Orbiter with respect to helioseismology.

c© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

The successful ESA/NASA solar observatory SOHO whichwas launched in 1995 has provided and still is providingan unprecedented breadth and depth of information aboutthe Sun, from the Sun’s interior, through the hot and dy-namic atmosphere, to the solar wind and its interaction withthe interstellar medium. The SOHO mission still constitutesthe prime focus for the research of the European solar sci-ence community. Specifically, data returned from the MDIinstrument onboard SOHO have given a tremendous pushto the field of helioseismology. The Solar Orbiter missionhas been configured as the logical advancing step in the ex-ploration of the Sun after SOHO. By approaching the Sunas close as 48 solar radii (0.22 AU), Solar Orbiter will viewthe solar atmosphere with a spatial resolution of less than150 km (1 arcsec) and will perform the closest ever in-situmeasurements. In the course of the mission the inclinationof the spacecraft orbit to the ecliptic will incrementally in-crease to reach heliographic latitudes larger than 30◦ whichwill provide the first ever views of the Sun’s polar regions.A further unique aspect of the mission is the combinationof comprehensive remote sensing and in-situ instrumenta-tions allowing to trace solar features from the photosphereto interplanetary space.

Of high interest for helioseismology is the large rangeof spacecraft-Sun-Earth angles covered by Solar Orbiter. Incombination with observation from Earth (ground-based ornear-Earth orbit), Solar Orbiter will offer the opportunityfor stereoscopic helioseismology.

2 Scientific objectives

Missions such as Helios, Ulysses, Yohkoh, SOHO, TRACEand RHESSI have advanced significantly our understandingof the various atmospheric layers of the Sun, the solar wind

� Corresponding author: [email protected]

and the three-dimensional heliosphere. Further progress isto be expected from missions like STEREO, Solar-B, andthe Solar Dynamics Observatory (SDO), the first of NASAsLiving With a Star (LWS) missions.

Each of these missions has a specific focus, each ful-filling its part within a coordinated solar and heliosphericresearch effort. Solar Orbiter with its unique orbit and ad-vanced suite of in-situ and remote sensing instruments con-stitute an important augmentation to this effort. Solar Or-biter will for the first time

– explore the uncharted innermost regions of our solarsystem,

– study the Sun from close-up,– fly by the Sun tuned to its rotation and examine the solar

surface and the space above from a corotating vantagepoint,

– provide images and spectral observations of the Sun’spolar regions from out of the ecliptic.

According to the ESA Solar Orbiter Science Require-ment Document the top-level scientific goals of the SolarOrbiter mission are to

– determine the properties, dynamics and interactions ofplasma, fields and particles in the near-Sun heliosphere,

– investigate the links between the solar surface, coronaand inner heliosphere,

– explore, at all latitudes, the energetics, dynamics andfine-scale structure of the Sun’s magnetized atmosphere,

– probe the solar dynamo by observing the Sun’s high-latitude field, flows and seismic waves.

3 Mission design

Solar Orbiter is a three-axis stabilized spacecraft. On thesun-facing side a heat shield protects the spacecraft and itspayload from the compared to Earth 25 times higher radi-ation. The heat shield has to provide the apertures for theremote sensing instruments.

c© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Astron. Nachr. / AN (2007) 363

Fig. 1 The projection of the orbit of Solar Orbiter into the eclip-tic plane. Earth and Venus orbit as well as the position of variousspacecraft manoeuvres are indicated. Figure adapted from a pre-sentation by R. Marsden.

Fig. 2 The solar latitude of Solar Orbiter as a function of missionduration. Figure adapted from a presentation by R. Marsden.

According to the current baseline (as at December 2006)Solar Orbiter is scheduled to be launched in May 2015 ona Soyuz-Fregat 2-1b. The spacecraft will use a chemicalpropulsion system and multiple gravity assist manoeuvresat Venus and Earth to reach its science orbit. The initialcruise to the science orbit will last 3.4 years. In the sci-ence phase, Solar Orbiter will be in a 3:2 resonant orbitwith Venus (period 149.8 days). The Venus fly-bys are usedto slowly increase the inclination of the orbit. The total mis-sion duration will be approximately 10 years. The minimumperihelion distance during the science phase will be 48 solarradii (0.22 AU) with a maximum solar latitude of 34◦. Theprojection of the baselined orbit into the ecliptic x-y planeand the solar latitude as a function of mission duration areshown in Figs. 1 and 2, respectively.

Figure 3 shows the time profile of the maximum solarlatitude reached in each orbit together with a predicted pro-file of the solar activity based on the sunspot number. In2015, Solar Orbiter will be launched into the solar activityminimum, with the cruise phase extending through the min-imum phase. Solar activity is expected to increase again atthe start of the science phase. While the activity further de-

velops, Solar Orbiter will advance to higher latitudes. Dur-ing the declining phase of the activity cycle the spacecraftwill reach highest latitudes. This offers the possibility tostudy the transition from an active to a quiet Sun from ahigh-latitude vantage point. The current mission baseline al-lows observations to be carried out for about 10 days aroundthe perihelion passes and/or the high-latitude segments ofeach of the science orbits.

4 Scientific payload

The current Solar Orbiter scientific payload ia a compre-hensive set of remote sensing and in-situ instrumentation.It offers the unprecedented opportunity for complementaryobservation of solar processes as they evolve from the pho-tosphere into near-Sun interplanetary space. The near-Suninterplanetary measurements together with simultaneous re-mote sensing observations of the Sun will permit to disen-tangle spatial and temporal variations during the corotationphases. They will allow to understand the characteristics ofthe solar wind and energetic particles and link them to theplasma, neutral atmosphere and magnetic conditions in theirsource regions on the Sun. The Solar Orbiter remote sens-ing instruments will deliver images of the solar atmospherewith a spatial resolution of 0.5 arcsec pixels, equivalent to80 km on the Sun at 0.222 AU, which means resolving so-lar features as small as 160 km (ESA Solar Orbiter ScienceRequirements Document). The reference core payload, theirscientific goals and allocated resources (according to ESASolar Orbiter Payload Definition Document) are shown inFig. 4.

5 Current status

Solar Orbiter is currently targeted for a launch in 2015. InJuly 2006 ESA has issued a letter of intent (LOI) to proposeinstruments for the Solar Orbiter mission with deadline forresponses in September 2006. ESA has received 23 LOIs,documenting the interest of the solar community in SolarOrbiter. However, due to the financial constrains of ESA, themission is currently reviewed again and undergoes a con-solidation phase with the aim of cost reduction. Beside de-scoping, one potential way of limit costs to ESA is to mergeSolar Orbiter with the Sentinel mission of NASA. The con-solidation phase will be used to advance the readiness ofthe mission, specifically in respect to the critical heat shield– instrument interfaces by further refinement of the instru-ments based on interactions with potential principal investi-gator teams. Final decision on Solar Orbiter is expected inNovember 2007.

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364 J. Woch & L. Gizon: The Solar Orbiter mission

Fig. 3 The maximum solar latitude reached for each of the science orbits as a function of mission duration for various launch datestogether with the predicted sunspot number. Figure adapted from a presentation by R. Marsden.

Fig. 4 The scientific core reference payload of Solar Orbiter, adapted from the ESA Solar Orbiter Payload Definition Document.

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6 The VIM instrument and its perspectivefor helioseismology

The Visible Light Imager and Magnetograph (VIM) is oneof the remote sensing instruments in the core reference pay-load of Solar Orbiter. VIM will measure the structure, dy-namics and evolution of the full magnetic vector and of theflow field in the solar photosphere as well as waves and os-cillations that penetrate into the solar interior. VIM will givethe first view of the magnetic and velocity field in the polarregions. VIM consists of two telescopes, a High ResolutionTelescope (HRT) and a Full Disk Telescope (FDT), both ofwhich feed a filtergraph. They provide high resolution mea-surements within a limited field-of-view and low resolutionmeasurements covering the full solar disk. Maps of the thefull photospheric magnetic field vector, the line-of-sight ve-locity, and the continuum intensity will be obtained with acadence of less than a minute. This requires recording lin-early and circularly polarized intensities (Stokes vectors) atdifferent wavelengths in a Zeeman-sensitive photosphericspectral line and the nearby continuum.

Among the Solar Orbiter instrumentation, VIM will de-liver the most relevant data for helioseismology.

The method of global-mode helioseismology (Christen-sen-Dalsgaard 2002) has allowed to map the solar differ-ential rotation as a function of latitude and radial distancethroughout most of the convection zone with a very goodlevel of precision, while the sound speed profile averagedover spheres has been measured down to the solar core. Athigh heliographic latitudes, however, the picture of the so-lar interior is far from complete and is fuzzier with greaterdepth. In particular, global mode frequency inversions forrotation cease to be accurate above 70◦ latitude and are un-certain in the radiative interior.

The developing science of local helioseismology (Gizon& Birch 2005), which aims at producing three-dimensionalimages of the solar interior, has provided information in theupper third of the convective envelope and only at latitudesless than 50◦. High latitudes are difficult to study becausethe sensitivity of the line-of-sight Doppler measurements tothe nearly radial pulsations drops as a function of center-to-limb distance. In addition, the reduction of the effectivespatial sampling on the Sun toward the solar limb impliesthat moderate to high degree modes are more difficult to de-tect away from disk center in the center-to-limb direction.That measurements are difficult to make close to the solarlimb has implications not only for studies at high latitudes,but also for probing deep into the interior. Indeed, acous-tic ray bundles that penetrate deep inside the Sun connectsurface locations that are separated by large horizontal dis-tances: less signal at high latitudes means a reduced abilityto probe deep at moderate latitudes. In short, much remainsto be learned above 50◦ heliographic latitude and for radialdistances less than, say, 0.9 solar radius.

The VIM instrument will return the necessary Dopplerimages of the polar regions of the Sun. In addition, VIM

Doppler images combined with Doppler images taken fromEarth will allow to demonstrate the concept of stereoscopichelioseismology. Thus, VIM data as input for helioseimicmethods offer the unique opportunity to address some out-standing key questions of solar physics:

1. Internal rotation at high latitudes: The large-scale dif-ferential rotation is an essential dynamical property ofthe Sun. Inversions of global-mode frequency splittingssupport the idea that rotation is special at heliographiclatitudes above 70◦. In particular, some inversions sug-gest the existence of a buried jet-like zonal flow (Schou1999) and a surprisingly slow rotation rate above 75◦

compared to a smooth extrapolation from lower lati-tudes (Schou et al. 1998; Birch & Kosovichev 1998). Inaddition, the near-surface radial gradient of the angularvelocity, may switch sign near latitude 50◦ (Corbard &Thompson 2002). These results have been derived us-ing global-mode techniques, which do not necessarilyrequire local observations at high latitudes. Using ob-servations out of the ecliptic plane together with tech-niques of local helioseismology, VIM may offer the op-portunity to check the validity of the global-mode fre-quency inversions at high latitudes. Of course, directDoppler measurements will also give important infor-mation about the surface rotation rate.

2. Meridional circulation: Surface Doppler measurementshave shown the existence of a meridional flow from theequator to the poles with a maximum amplitude of about15 m/s near 30◦ latitude. Meridional circulation plays arole in the evolution of the large-scale surface magneticfield. In flux-transport dynamo models based on theBabcock-Leighton mechanism, it carries the poloidalfield generated at the surface down to the bottom of theconvection zone, transports the toroidal magnetic fieldequatorward, and sets the period of the solar cycle (Dik-pati & Charbonneau 1999). Due to mass conservation,the meridional flow amplitude at the base of the convec-tion zone is expected to be extremely small. Techniquesof local helioseismology are sensitive to the first ordereffect of meridional circulation. However, the merid-ional flow at latitudes above 50◦ is poorly known andshould be an important target for VIM. One question iswhether the meridional flow has a single-cell or multi-cell structure. The determination of the latitudinal com-ponent of the meridional flow in the near polar regionsis likely to be extremely difficult (a very small signal isexpected).

3. Polar region variability: The polar regions appear tobe very dynamical. There is evidence that the differen-tial rotation near the poles may vary on a short timescale (Ye & Livingston 1998). The magnetic proper-ties of the polar regions at solar minimum determinethe strength of the forthcoming solar maximum (Schat-ten 1998); a new sunspot cycle starts in the polar re-gions. Thus, studying the dynamics and variability ofthe polar regions will help us understand how the so-


366 J. Woch & L. Gizon: The Solar Orbiter mission

lar dynamo works. Using global-mode helioseismology,it was found that there may be a significant variationof the subsurface high-latitude rotation as a function ofthe phase of the solar cycle (Birch & Kosovichev 1998;Schou 2003). Does the meridional circulation, which isknown to vary with time near active latitudes, also varyat high latitudes? In particular, does it exhibit multi-cellstructures that come and go with the phase of the solarcycle? We may also ask about flow patterns across thepoles. Answering these questions will require long-termmonitoring of the polar regions.

4. Convection at high latitudes: Near-surface convectiveflows act as an important driver of solar activity. Bycontrolling the evolution of the magnetic network, su-pergranulation affects chromospheric and coronal activ-ity. Near-surface convective flows could support localdynamo action too. Is the dynamics of supergranula-tion different in the polar regions where the effect ofthe Coriolis force is expected to be much stronger thanthat at lower latitudes? What is the dynamical couplingbetween near-surface flows in the near polar regions andphotospheric/coronal magnetism? Flows at supergranu-lar scales can be measured with seismic holography ortime-distance helioseismology with sufficient temporalresolution. However, a good spatial resolution does re-quire that f-modes be observed (Duvall & Gizon 2000).Possibly, flows at larger spatial scales than supergranu-lation can be measured with the technique of ring dia-gram analysis.

5. Structure and variability of the Tachocline, dynamoflows: The solar tachocline is believed to be the seatof the generation of the global magnetic field (Gilman2005) and is thus an important target for helioseismo-logical studies. A proper determination of the figure ofthe solar tachocline obviously requires high latitude in-formation. It has recently been suggested that progradefluid jets may be generated in the solar tachocline athigh latitudes, which may be measurable by methods ofhelioseismology (Rempel & Dikpati 2003; Christensen-Daalsgard et al. 2004). The tachocline also appears tosupport quasi-periodic oscillations with a period of ap-proximately 1.3 year (Howe et al. 2000), which maybe a deep extension of the torsional oscillations. Thereare indications that surface magnetic fields often clus-ter at a few specific longitudes for several rotationperiods (Henney & Harvey 2002). This may indicatethe presence of an underlying non-axisymmetric mag-netic structure and could be a manifestation of non-axisymmetric modes of the dynamo (Ruzmaikin 1998;Berdyugina et al. 2006). Local helioseismology is theappropriate tool to search for longitudinal variationsdeep in the convection zone. Stereoscopic local helio-seismology may be an important advance to detect thesignature of deep dynamo flows and their longitudinalvariations inside the Sun.

6. Stereoscopic helioseismology: Stereoscopic helioseis-mology would benefit both global and local methods ofhelioseismology. Stereoscopy requires several observa-tories. Both SDO-HMI and GONG (or another ground-based facility) are expected to be operational at the timeof Solar Orbiter operations. Combining Solar Orbiterwith any one of them will open new windows into theSun. Looking at the Sun from two distinct viewing an-gles results in an increase of the observed fraction of theSun’s surface. Global helioseismology would naturallybenefit from this situation, since the modes of oscilla-tion would be easier to disentangle (reduction of spatialleaks). The precision on the determination of the modefrequencies would improve at high frequencies for allspherical harmonic degrees. Spatial masks for extractingacoustic modes will be closer to optimal. With stereo-scopic local helioseismology, new acoustic ray pathscan be considered to probe deeper layers in the interior.Observations from two widely different viewing anglesallow the probing of the solar interior at any depth, andin particular the solar tachocline (see above). It even be-comes possible to target regions in the solar core. Thisaspect of seismic stereoscopy is revolutionary. The otheraspect of stereoscopy is that it is possible to observe acommon area from two different viewing angles. Thiscase is useful to understand systematic errors and line-of-sight projection effects in local helioseismology. It isalso potentially important to minimize the contributionof convection noise in spectra of solar oscillations. Fi-nally, there is the possibility of observing the same areawith two different spectral lines formed at two heights inthe solar atmosphere to study vertical wave propagation.In essence, VIM observations in combination with othersolar Doppler imaging have the capability to demon-strate the promising concept of stereoscopic helioseis-mology.

7 Requirements for helioseismology

Many of the above scientific objectives are difficult to ac-complish. Which objective can be fulfilled and to which ex-tent depends on the actual performance of the VIM instru-ment. High quality image stabilization, accurate knowledgeof pointing and precise timing are a prerequisite for usefulmeasurements.

The observation duration is one of the most critical pa-rameter: the larger the observation interval T , the smallerthe contribution of random noise to helioseismic measure-ments. For nearly all quantities of interest, the level of ran-dom noise due to the stochastic nature of solar oscillationsbehaves like T−1/2. The current mission baseline of So-lar Orbiter allows observations to be carried out for aboutT = 10 days during each orbit, near perihelion. Studies ofthe solar nearsurface layers require days to weeks of data.Investigations of the deep interior typically require dura-tions from weeks to months. In practice, the longer the data

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coverage, the better. It would be very meaningful to at leastconsider the possibility of making observations during thecruise phase and outside perihelion passages, even at a verylow telemetry rate. Line-of-sight vectors other than Earth-Sun are prefered for stereoscopic helioseismology.

The temporal sampling needs to be at least as short as1 minute to make possible observations of all solar waveswith significant power; faster sampling with, say, 40 s maybe desirable to detect the highest frequency waves. The useof high-frequency waves is potentially important to studymagnetic effects and to provide more localized information.

The field of view must be large to resolve the low-degreemodes that penetrate deep into the interior, as well as to en-sure the study of large scale solar inhomogeneities. A largefield of view also gives the ability to track local areas on theSun over long periods of time to compensate for the space-craft motion in frames that co-rotate with the Sun. A full-disk view is ideal, especially to make the most of stereo-scopic analyses.

The spatial resolution measured on the Sun’s surfaceis a function of detector pixel size and center-to-limb dis-tance. Solar oscillations have significant power at all wave-lengths above, say, 5 Mm (f-mode frequency at 3 mHz).Typically, a spatial sampling of 1 Mm (measured on theSun) is sufficient for nearly all applications of helioseismol-ogy. A higher spatial sampling is not needed, while a lowerone is still acceptable for a large range of applications thatinvolve long-wavelength deep-penetrating modes of oscilla-tion.

The telemetry rate will likewise have critical impact.The limited telemetry rates require powerful lossless and

lossy data compression techniques. Intelligent methods ofspatial binning, Fourier filtering and low-pass filtering haveto be applied to match the data volume to the telemetryrates.

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the solar orbiter mission and its prospects for helioseismology

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