the scientific possibilities for coronagraphy from the solar orbiter

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Advances in Space Research 36 (2005) 1367–1374

The scientific possibilities for coronagraphy from the Solar Orbiter

Ester Antonucci

Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Torino, Italy

Received 18 January 2005; received in revised form 21 June 2005; accepted 7 July 2005

Abstract

This paper briefly discusses the major open issues relative to the understanding of the solar corona and the origin of the solarwind in the context of the opportunities offered by the Solar Orbiter, future mission of the European Space Agency. The Solar Orbi-ter mission profile offers unique opportunities for the study of the initiation and propagation of coronal mass ejection, of the evo-lution of the global corona and its restructuring in response to the abrupt changes induced by coronal activity, and for theassessment of the role of the coronal magnetic topology in controlling the physical parameters of the fast and slow wind.� 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Solar corona; Solar instrumentation; Space missions

1. Introduction

During the last solar cycle two large coronagraphs,LASCO and UVCS, have been observing the outer cor-ona of the Sun from the Solar and Heliospheric Obser-vatory, SOHO, in orbit at the lagrangian point L1. Theirdata have enormously contributed to a deeper under-standing of the eruptive activity of the Sun and the ori-gin and acceleration of the solar wind. A wealth ofdetailed information on the frequency, evolution andpropagation of coronal mass ejections, CMEs, has beengathered with LASCO (Brueckner et al., 1995) by al-most continuously imaging, at very high sensitivity,the visible corona in the range from 2Rx to 30Rx. Eventhe fainter coronal transients, such as halo CMEs andstreamer blowouts, moving outward at the ambient slowsolar wind velocity (Sheeley et al., 1997), have been de-tected. The uninterrupted monitoring of halo mass ejec-tions over a cycle has allowed a far better perception ofthe influence of these events on the terrestrialenvironment.

0273-1177/$30 � 2005 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/j.asr.2005.07.014

E-mail address: [email protected].

Spectroscopy of the ultraviolet emission of the outercorona, performed for the first time with UVCS, UltraViolet Coronagraph Spectrometer (Kohl et al., 1995),has allowed the direct observation of the coronal windand revealed, among many other important effects, theexistence of a strong anisotropy of the velocity distribu-tion of oxygen ions and differential heating between ionsand protons in coronal holes (e.g., Kohl et al., 1997,1998; Cranmer et al., 1999). The anisotropy of the ionvelocity distribution, evidence for preferential heatingof ions across the coronal magnetic field, is interpretedas a signature of energy deposition in the regions wherethe fast wind is accelerated. Ultraviolet spectroscopy hasalso significantly contributed to the comprehension ofstreamers, where strong ion abundance anomalies aredetected (e.g., Raymond et al., 1997), and coronal massejections, by measuring the temperature and velocitystructure of the ejected material (e.g., Antonucci et al.,1997; Ciaravella et al., 2000).

Notwithstanding the progress in observational capa-bilities achieved with the two powerful SOHO corona-graphs, the knowledge of a few fundamental physicalquantities, crucial for a full understanding of the physicsof the outer corona, is still missing. Namely we still do

ved.

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1368 E. Antonucci / Advances in Space Research 36 (2005) 1367–1374

not measure in corona the magnetic field and the abun-dance, temperature and outflow speed of the heliumcomponent, second major constituent of the coronalplasma and the solar wind. The importance of the roleplayed by the magnetic field in terms of structuring thesolar corona and providing the energy for its heatingand activity is undeniable, yet direct quantitative knowl-edge of coronal magnetic fields is still absent. Variationin the abundance of helium can produce significantdynamical modifications of the proton outflows arisingfrom proton–helium frictional forces, thus influencingthe solar wind proton flux and asymptotic proton flowspeed. In the recent years, however, significant effortshave been made to develop new diagnostic methodsand improved technologies enabling to obtain, on theone hand, monochromatic imaging of the inner and out-er corona in the intense Hell 304 A line, in order to mea-sure helium abundance and outflow speed, and, on theother hand, the detection of coronal magnetic fieldsvia the Hanle effect that exploits the fact that resonancepolarization in coronal spectral lines, falling in the ultra-violet domain, is deeply modified by the presence of amagnetic field.

2. Major issues in the physics of the outer corona

Energy deposition in coronal holes, necessary toaccelerate the fast solar wind to the heliospheric asymp-totic speed of about 800 km s�1, remains an open issue.The question is whether indeed dissipation via ion cyclo-tron resonance by the high-frequency part of the spec-trum of outward propagating Alfven waves, caninduce preferential heating of the oxygen ions acrossthe coronal magnetic field, as observed with UVCS.Further new data, relative to the behavior of other ionswith different charge-to-mass ratio and neutral hydro-gen/protons. are needed however to confirm this inter-pretation and thus definitively solve the issue of theidentification of the mechanism of energy deposition re-quired to explain the fast wind.

Heliospheric and coronal observations are suggestingthat the coronal magnetic topology might play animportant role in controlling the solar wind outflowvelocity (e.g., Wang and Sheeley, 1990), the heliosphericion abundance (Aellig et al., 2001) and possibly even theenergy deposition itself at coronal level (Antonucciet al., 2005). These hypotheses however are still to befully verified. We need to establish whether the physicalparameters of the coronal wind, such as outflow speed,ion velocity distribution across the magnetic field andcoronal abundances are indeed modulated by the degreeof expansion of the magnetic flux tubes channelling thewind in corona.

The anomalies of ion abundance observed in stream-ers may be revealing the existence of low-speed coronal

outflows, that might have a significant role in formingthe slow solar wind. Yet whether the depletion of oxy-gen, observed in the core of the solar minimum quies-cent streamers with UVCS, is due to gravitationalsettling in a magnetically confined plasma (e.g., Ray-mond et al., 1997) or to slow flows along open field lines,expected in magnetically complex streamers (e.g., Nociet al., 1997), is still object of debate.

The pre-eruption phase of coronal mass ejections is acrucial element for the identification of the mechanismthat is initiating these phenomena. Yet we have stillquite scarce information on the evolution of the large-scale coronal magnetic fields prior to their destabiliza-tion leading to the violent expulsion of matter we ob-serve. Little is known also on the longitudinal extentof the fields involved in the disruption giving origin toa mass ejection. Furthermore the modification of thestreamer belt and of the global corona, readjusting in re-sponse to coronal mass ejections, cannot be fully ob-served due to the fact that the outer corona is onlydetectable at the limb of a rotating Sun.

2.1. Energy deposition in the regions of acceleration of the

solar wind

The hot corona cannot be in static equilibrium andmust continuously expand outward. However in orderto obtain the speed of the fast solar wind observed inthe heliosphere, additional energy is required and hasto be supplied in coronal holes above the base of the so-lar corona, beyond the sonic point. During solar mini-mum in polar coronal holes, UVCS has observed,beyond 1.8Rx, extremely high kinetic temperatures ofthe oxygen ions five time ionized, reaching approxi-mately 1 · 108 K, across the magnetic field. These tem-peratures are indicating extremely broad ion velocitydistributions due to preferential heating of minor ionsacross the field. These data suggest that collisionlesswave dissipation is acting. Damping of the high fre-quency part of the Alfven spectrum via ion cyclotronresonance is indeed the most probable mechanism to ex-plain the observed anomalous heating of oxygen ions(e.g., Cranmer, 2002). In this context, it is crucial to ob-serve the effect of energy dissipation also on the hydro-gen and helium components, the major constituents ofthe solar wind.

The importance of ion cyclotron dissipation of Alf-ven waves in accelerating the solar wind can be definitelyproved by measuring the velocity distribution across themagnetic field of ions with different charge-to-mass ratioZi/Ai, since this parameter characterizes the ion cyclo-tron resonance effect. A wave of a given frequencywould first become resonant with ions with lowcharge-to-mass ratios, before becoming resonant withions with higher ratios. Essential in this respect then be-comes the observation of the effect of wave dissipation

E. Antonucci / Advances in Space Research 36 (2005) 1367–1374 1369

on protons (Zi/Ai) = 1 and singly ionized He+ ions (Zi/Ai = 0.25) that have a different charge-to-mass ratiofrom O5+ ions (Zi/Ai = 0.31). Protons and helium ionswould undergo preferential heating at different coronallevels. From the observational point of view however,whether and where in corona protons are preferentiallyaccelerated across the field is not yet well established.The data obtained with UVCS from the neutral hydro-gen HI Lyman a emission, at 1216 A, are not sufficientto elucidate this point. On the other hand, no informa-tion on the helium ion behavior is available yet. It isthen crucial to acquire the capability of determiningthe atom/ion velocity distribution of the hydrogen andhelium components together with the study of their out-flow velocity. This would clarify the nature of the mech-anism of energy deposition and quantify the degree ofenergy transferred to the major constituents of the solarwind.

A long-lasting question not yet fully solved concernsthe role of polar plumes in the generation of the fastwind. The inner corona observations obtained with SU-MER (Wilhelm et al., 1995) indicate that the flow rate inplumes is higher than in the interplume regions (Gabrielet al., 2003). On the contrary, in the outer corona UVCSobserves higher flow rate in the dimmest coronal laneswhere the kinetic temperatures of minor ions is higher.This latter effect indicates higher energy deposition inthese regions (Giordano et al., 2000). The very natureof the almost radial bright coronal features observedat solar minimum in the outer corona is indeed still elu-sive. We have not yet established whether the bright re-gions observed in ultraviolet and visible light are thesame and if they indeed correspond to the extension ofthe plumes observed close to the disk. Their brightnessin visible light might be due either to the enhanced den-sity of these features relative to the ambient corona, orto a line of sight effect deriving from a casual alignmentof denser coronal threads. These issues need to be clar-ified in order to fully understand the fast wind.

2.2. Coronal mass ejections and global corona evolution

It is commonly accepted that coronal mass ejectionsare associated with regions of strong non-potential mag-netic fields in proximity of neutral lines and that they de-rive their energy from that stored in the field. Massejections are due to an abrupt disruption of balance be-tween an upper pressure, exerted by the strongly shearedmagnetic fields that are presumed to exist in filamentchannels and a downward pressure, that can be eitherof magnetic nature or it can be ascribed to the weightof an overlying mass. The shear is slowly increasingdue to the action of the photospheric motions associatedwith turbulent convection and differential rotation, ofthe order of 0.5–1 and 2 km s�1, respectively. The dis-ruption occurs when the magnetic tension of overlying

fields is abruptly removed either via magnetic breakout,when the overlying fields reconnect with neighboringfield systems (e.g., Antiochos, 1998), or via reconnectionof the overlying fields, in the flux rope models (e.g., For-bes and Isenberg, 1991). Magnetic buoyancy leads to thecollapse of the overlying mass and the expulsion of aflux rope, in the mass-loading models (e.g., Low, 1996a).

The difficulty in studying the eruption mechanism de-rives from the limitations inherent in coronal observa-tions. The pre-eruption phase can be monitored in theinner corona by observing either bright active filamentson disk or active/eruptive prominences at the limb;whilst in the outer corona, the overlying large-scalecoronal structure can be examined just at the limb.Long-lived, evolving filaments can be followed on diskduring their rotation from the East to the West limb.The overlying coronal structures are instead visible atthe limb for a much shorter time interval, no more than2–3 days, due to solar rotation (about 13�/day, synodicperiod of equatorial coronal rotation). Hence, the evolu-tion of the global coronal structure involved in, andleading to, a mass ejection (filament and overlying fields)can be examined for a very short period. The eruption isfollowed by the outward propagation of a system oftenconsisting of a bright leading shell, surrounding a darkcavity which contains the ejected cool prominence mate-rial. Most of the information gathered on this phasewith the SOHO instrumentation comes from images ofthe corona above 2Rx. Scarce information exists insteadon the earlier propagation in the intermediate layer be-tween, 1.3Rx and 2Rx, and is limited to the early phaseof the SOHO mission (when the LASCO C1 corona-graph was operating). The longitudinal extent of a coro-nal mass ejection is also of difficult assessment in eventsobserved at the limb. Another crucial issue, related tothese phenomena, concerns the total flux injected intothe heliosphere. The bulk of it probably comes fromthe large-scale coronal field overlying the erupting mate-rial, rather than the relatively small flux of the eruptingfilament/prominence itself. Again in this case the under-standing is limited by the temporal restrictions imposedby the nature of coronal observations.

The fact that at the limb we can observe, just for ashort time, a narrow longitudinal portion of the coronaat the plane of the sky limits also our understanding ofthe global corona and its readjustment in response to dif-ferent phenomena, varying from moderate episodic heat-ing to disruptive CMEs occurring with frequencyincreasing from about 0.5 CMEs per day during mini-mum to about 5 CMEs per day duringmaximumof activ-ity (e.g., Gopalswamy, 2004). In response to moderateheating the temperature in closed field structures raisesup thus increasing thermal radiation and downward ther-mal conduction. In the slowly evolving magnetic fields, asthose of a quiescent streamer belt, thermal balance can beachieved through a quasi-static adjustment of forces such


that the structure remains in approximate equilibrium.The evolution can imply also heated material rising upalong the closed magnetic fields (e.g., Low, 1996b). Thestudy of the streamer belt evolution however requireslong-term observations and also the capability of deter-mining the streamer longitudinal extent. The belt longitu-dinal structure is at present derived from the short-termlimb observations, used to reconstruct synoptic coronalmaps that mainly preserve the non-evolving patterns withsuperimposed short-term variations.

2.3. Coronal composition and solar wind

The core of solar minimum quiescent streamersshows a strong depletion of oxygen ions, five times ion-ized. This effect can be interpreted in terms of gravita-tional settling of heavier ions in a coronal plasmaconfined by closed field lines (Raymond et al., 1997).According to the suggestion of Raymond et al., the re-gions bright in oxygen emission, surrounding the de-pleted core, are sources of slow wind. Theconfiguration of the solar minimum streamers howeveralso suggests a complex magnetic structure consistingof sub-streamers separated by open field lines. Thereforethe oxygen depletion might also be ascribed to a processof dynamic stratification due to reduced Coulomb drag-ging of the ions in the slow wind. This is channelledalong open field lines where the proton flux is reducedbecause of the peculiar local magnetic topology (Nociet al., 1997; Marocchi et al., 2001). On the other hand,Ofman (2000) predicts higher oxygen abundance outsidethe streamer core, ascribed to slow proton outflows thatincrease oxygen density by Coulomb friction. The issueof oxygen abundance in streamers is then strictly relatedto the issue of the origin of the slow wind. In order todistinguish between these different interpretations, thecomparative study of the streamer helium abundancewith the well-known oxygen abundance structure, is cru-cial, since helium is light and thus less affected by grav-itational settling than heavier ions such as oxygen. Onthe other hand, its drag factor in the slow wind is theleast favorable (Geiss et al., 1970).

A decade of Ulysses observations of the heliosphericsolar wind and of SOHO observation of the outer coro-na have clearly shown that the evolution of the large-scale magnetic structure of the corona through the activ-ity cycle influences the solar wind speed. The Wang andSheeley (1990) hypothesis of a direct relation betweenexpansion factor of the flux tubes channelling coronalexpansion and solar wind velocity finds support in re-cent observations of the slow coronal wind. This flowsat the edges of coronal holes where the coronal flux tubedivergence is the largest one (Antonucci et al., 2005). Inthis interpretation slow and fast, wind are indeed origi-nating from the same coronal source and they differenti-ate further out in corona because they are channelled in

flux-tubes with different expansion factors. Further-more, the wind velocity is correlated to the heliospherichelium abundance (Aellig et al., 2001). This quantity at1 AU varies markedly from minimum to maximum, asdoes the wind speed, and increasing speed is correlatedto increasing helium abundance. The reason for this so-lar cycle dependence of helium abundance is likely to re-side in the evolution of the large-scale magneticstructure of the equatorial corona through the cycle.The strong over-expansion of the coronal magnetic fieldnear the streamer boundaries characteristic of the solarminimum conditions is not present during solar maxi-mum, when coronal flux tubes expand more gradually.If the helium abundance variability is controlled bythe expansion factor of the coronal flux tube, consider-ing that at any given time the open field lines in coronaexpand with different expansion factor at different lati-tudes, we should expect a latitudinal correlation of he-lium abundance and wind velocity in corona. This issuggested by the fact that, if the expansion factor ofthe coronal flux tube controls the proton flux, it alsocontrols the Coulomb drag of helium ions into the cor-ona and thus ultimately the helium abundance. Theabundance variations then can denote temporal or spacedifferences in the expansion factor of the open magneticfields, and be considered a signature of magnetic topol-ogy in corona. Helium abundance thus provides a deci-sive test of the role of the flux tube divergence incontrolling the coronal wind properties.

3. Solar Orbiter mission profile uniqueness

Solar Orbiter will perform the first out-of-ecliptic andthe first heliosynchronous observations of the Sun andthe solar corona, thus allowing unique science investiga-tions. The Orbiter mission profile has been designed inorder to fulfill the following requirements: (i) low perihe-lion allowing pseudosynchronous viewing of Sun, last-ing 10 days, and high-resolution observations, (ii) highinclination with respect to the solar equator, (iii) aph-elion not higher than the Venus heliocentric distance.The trajectory is composed of three phases: the cruisephase (1.86 years long), the nominal mission (2.9 yearslong), during which scientific operations are performed,the extended mission (2.3 years long), to better meet therequirements of high inclination (ESA Pre-AssessmentStudy Report, 1999; ESA Assessment Study Report,2000). Investigations of the outer corona with the Orbi-ter instrumentation have been discussed by Antonucci(2001).

3.1. Corotation

The early phase of the Solar Orbiter nominal mission,which lasts 2.9 years corresponding to seven orbits of


the spacecraft around the Sun, is characterized by a fewperiods of about 10 days, when the spacecraft quasi-cor-otates with the Sun. The orbital rate at the passage atperihelion will be 12.7�/day in the first three orbits and8.8�/day in the subsequent three orbits. In this intervalperihelion will vary from 0.22 to 0.26 AU. From thepoint of view of the investigation of the outer corona,the Solar Orbiter corotation provides a unique opportu-nity to study the evolution of the global corona overtime-scales a few times longer than the passage at thelimb of coronal structures, with an equatorial siderealrotation rate of about 14�/day (e.g., Antonucci and Do-dero, 1979). In these periods, the effect of solar rotationis almost completely cancelled and the outer corona ap-pears as frozen at the limb for several days, thus allow-ing to trace for the first time the evolution of magneticfields, elemental composition and plasma outflows in acoronal region, without the temporal limitations im-posed by solar rotation.

The full sequence of pre-eruption, eruption and prop-agation of a coronal mass ejection, as well as reconfigu-ration of the corona following the CME, can beobserved during a corotation interval. This would beinstrumental to: (i) identify the mechanism driving theeruption, (ii) ascertain whether the main source of fluxinjection into the heliosphere is indeed residing in theouter corona, (iii) study the restructuring of the globalcorona following mass ejections. These observationsneed to be performed at very high space resolution in or-der to resolve the fine structure of the magnetic fieldswhen evolving toward the disruption leading to a coro-nal mass ejection. During most of the nominal missioncorotation occurs at perihelion (0.22–0.26 AU), there-fore an improvement of almost one order of magnitudein space resolution relative to the present SOHO corona-graphs can be easily achieved. The extended observationat the limb of the same portion of corona is also essen-tial to address the study of streamer evolution. This willfocus, depending on the mission launch date, either onthe quiescent, slowly evolving streamer belt or on therapidly varying active streamers. These scientific objec-tives can be addressed by studying the density structureof the full corona via polarized visible light measure-ments. The detection of monochromatic images of thefull corona in intense UV lines with a strong radiativecomponent, such as the HI 1216 A and the He II304 A, adds the capability to measure coronal flowsnot detectable, as in the case of the solar wind outflow,through the motion tracers in the plane of the sky.

The investigation of the interrelation of solar windacceleration and magnetic topology of the flux tubeguiding the expansion, and the study of gravitationalsettling in the confined regions of the streamer belt,necessitate as well long-term observations. The opportu-nity to freeze a streamer section at the limb offers, inaddition, the possibility of increasing dramatically the

statistics, being, in this case, only limited by the intrinsicevolution of the structure and not by solar rotation. Asignificant increase in statistics is then coupled withthe possibility of observing at high spatial resolution;this allows us to resolve the fine structure in the slowwind coronal regions, thus enabling the test of the differ-ent hypotheses concerning the wind origin. The regionsto be better resolved include: (i) streamer borders, wherethe wind is guided by over-expanded magnetic fluxtubes, (ii) open field lines separating sub-streamers,and (iii) streamer legs surrounding the core, character-ized by higher oxygen abundance. Of course this kindof studies cannot be only based on the knowledge ofelectron density and morphology of the corona observedwith visible light instrumentation. The UVCS resultshave indeed shown that the spatial distribution of theelectron/proton density in streamers is quite differentfrom that of minor ions. Hence, the above scientific is-sues have to be addressed by observing simultaneouslythe corona in visible light, ultraviolet HI emission andin the UV lines emitted by minor ions (e.g., HII 304 Aand OVI 1032, 1037 A). Monochromatic imaging inUV lines would allow to reach a higher statistics thanthat obtained with a scanning spectrometer. Full imagingof the corona, moreover, would increase the probabilityto observe the full spectrum of the coronal large-scalefeatures during the corotation phase. An even betterapproach to such problematics might by provided bycoupling imaging and spectroscopic capabilities.

A coronagraph, designed to observe the solar atmo-sphere closer to the disk than the SOHO coronagraphs,allows to address during the corotation phase also theplume issue. At present, there is controversy on the nat-ure of the elongated radial features observed in the outercorona in visible light and UV emission and defined as�plumes�. Are they the extension of the features observedlower down close to the disk, or more simply tinythreads with larger density that are by chance alignedalong the line of sight? These alternatives can be verifiedby observing plumes both in a rotating and in an almostnon-rotating Sun. In the latter case the changes in mor-phology are only due to the �plume� intrinsic evolution.The nature of plumes can be identified by studying thedensity morphology. How the plumes are related tothe source of the fast wind has to be investigated bymeans of monochromatic imaging of the radiative UVlines, in order to be capable to measure wind streamswith different outflow velocity in the polar regions.

3.2. Out of the ecliptic

The Solar Orbiter is the first observatory carrying re-mote-sensing instrumentation out of the ecliptic, thusallowing for the first time a view of the longitudinal/azi-muthal structure of the global corona. This occurs to-ward the end of the mission when the Orbiter has a


30� inclination angle with the ecliptic, and in the ex-tended mission, lasting 2.3 years, when the inclinationangle further increases from 30� to 38� (while perihelionincreases from 0.27 to 0.36 AU). During solar minimum,from high latitudes the Orbiter views the streamer beltrunning at low-latitude, or close to the equator, as anapproximately continuous annular structure aroundthe solar disk. The out-of-ecliptic observations can pro-vide information on main unknowns, such as the longi-tudinal and global structure of the streamer belt and thelongitudinal extent of the magnetic system involved incoronal mass ejections. In conjunction with corona-graphic observations performed in the ecliptic, this al-lows the reconstruction of the three-dimensionalgeometry of these events. Furthermore, the out-of-eclip-tic vantage point is indeed providing another means toobserve the corona not affected by solar rotation. Whenthe morphology is relatively simple and the relevantcoronal features are at low-latitude, or close to the eq-uator, their intrinsic evolution can indeed be easilyseparated from rotational effects, if viewed from high-latitudes.

Another interesting aspect concerns the possibility tomeasure the dynamics of coronal expansion all aroundthe streamer belt. During solar minimum, slow solarwind studies would be privileged, since their low-latitudeand equatorial sources are predominant on the plane ofthe sky. It would then be possible to (i) study the globaldynamics of the corona and streamer belt from a high-latitude perspective, (ii) assess the contribution to theslow wind of sporadic reconnection events, such as thecoronal blowouts, and the total magnetic flux injectioninto the heliosphere, from blowouts and coronal massejections, all along the streamer belt. In case high incli-nation with respect to the ecliptic is reached when theSun is active, the most interesting scientific objectivesare those of investigating: the modification of the longi-tudinal extent and 3D geometry of the streamer belt in-duced by solar activity, the mass loss and magnetic fluxinjection into the heliosphere by the active Sun and the3D geometry of mid-, high-latitude coronal mass ejec-tions. The latter have been identified during the lastmaximum with the LASCO coronagraph and occurwhen the solar magnetic fields reverse at the poles. Mostof the new aspects that can be revealed by the out-of-ecliptic observations concern the morphology of the glo-bal corona and therefore they can be addressed bymeans of visible light measurements. However, theinformation on the dynamics and the abundance in openand closed field line regions has to come from the mono-chromatic imaging of the corona in UV lines.

3.3. Close to the Sun

The proximity of the Solar Orbiter to the Sun, withperihelion varying from 0.22 at the beginning, to

0.30 AU at the end of the nominal mission, and to0.36 AU at the end of extended mission, is facilitatinghigh-resolution coronal observations. An improvementof almost an order of magnitude in spatial resolutionrelative to the SOHO coronagraphs can be easilyachieved in imaging the outer corona with instrumenta-tion of relatively limited dimensions. This allows thedetection of the fine structures of plumes, streamersand coronal mass ejections. At the closest approach, res-olution below 200/pxl can be achieved. This will degradealmost by a factor of 2 at the end of the extendedmission.

The proximity to the Sun gives also the uniqueopportunity to infer the three-dimensional velocity dis-tribution of the coronal neutral hydrogen atoms andprotons. At present ultraviolet spectroscopy allows usto derive the hydrogen/proton velocity distribution,from the line broadening of the HI Lyman a line, mainlyacross the coronal magnetic fields. Information on thefull 3-D distribution of hydrogen at the coronal layerswhere neutral atoms decouple from the charged protons(around 3–4 solar radii) is carried by the neutral hydro-gen atoms in their outward propagation and thus it canbe detected �in situ� at 0.2 AU (D�Amicis et al., in prep-aration). We think that these measurements at perihe-lion are crucial to reveal the existence of an anisotropyin the velocity distribution of the hydrogen atoms andprotons in the coronal region where the solar wind isaccelerated. A direct measurement of anisotropy relatedto preferential heating of protons across the field woulddefinitely prove that wave dissipation via ion cyclotronresonance (Cranmer et al., 1999) is indeed the mainmechanism acting to accelerate the dominant wind com-ponent in corona. At last, the capability of inferring the3-D distribution of coronal hydrogen coupled with thatof imaging monochromatically the outer corona in HI1216 A, allows to reconstruct the line intensity and pro-file. This is equivalent to acquire spectrometric capabil-ities by simply adding a neutral atom detector to anultraviolet coronagraphic imager onboard of thespacecraft.

4. Coronagraphic techniques

The traditional visible light imaging of the coronaprovides information on the electron density structuresand their temporal and spatial evolution. The introduc-tion of monochromatic imaging of the outer corona inthe most intense ultraviolet lines, the Lyman a HI,1216 A and He II, 304 A, allows us, on the one hand,to use the powerful diagnostics of Doppler dimming toderive velocity maps of the full corona, which describethe outflow of the hydrogen and helium componentsof the coronal wind, and, on the other hand, to derivefull corona maps of the abundance of helium relative


to hydrogen. Spectroscopy of ultraviolet coronal linesemitted by hydrogen and minor ions, as proved byUVCS can add a wealth of information on the velocitydistribution of the emitting atoms and ions. The�in situ� detection of neutral hydrogen in proximity ofthe Sun can add information on the third dimensionof the neutral hydrogen and proton velocity distributionin corona. That is, in simple magnetic configurationssuch as those observed during solar minimum, the�in situ� detection of neutral hydrogen can indirectlyprobe the atom/proton velocity distribution also alongthe coronal magnetic field, whilst the ultraviolet spec-troscopy of the HI line probes the distribution acrossthe field.

Visible and ultraviolet coronagraphs can easily ex-tend the imaging of the corona down to the level reachedeither by ground-based coronagraphs or by ultravioletdisk imagers, such as EIT (Delaboudiniere et al.,1995). This is important since the present observationalgap between 1.2 and 2 solar radii prevents the monitor-ing of the evolution of the magnetic fields prior to andduring the eruption and the early propagation of coro-nal mass ejections. Imaging of this layer has been ob-tained in the early SOHO mission when the LASCOC1 visible light coronagraph was operating. When per-formed in the two most intense coronal lines, HI 1216,and Hell 304, UV imaging allows a continuous studyof the helium abundance and dynamics of the solarwind, via Doppler dimming, from the limb to the outercorona. Moreover, Solar Orbiter is ideal for ultravioletobservations of the solar corona, since the detection ofultraviolet emission from spacecraft in Earth�s orbit issignificantly affected by the local geocorona.

Ultraviolet polarimetry can provide for the first timethe diagnostics to measure coronal fields via the Hanleeffect. There are strong reasons to believe that this effectcan operate in the ultraviolet coronal lines of the Lymanseries and in one line of oxygen VI at 1032 A. These linesshould present in the solar corona the phenomenon ofresonance polarization that should be modified by thepresence of magnetic fields in a predictable way. Ultra-violet polarimetry however might be not sufficiently ma-ture for the Solar Orbiter mission.

5. Conclusions

Energy deposition in coronal holes and in general inthe regions of acceleration of the solar wind affects thevelocity distribution of coronal protons as well as neu-tral atoms within 3–4 solar radii, where charge-exchangeis effective. Therefore, this problem can be effectively ad-dressed by inferring both the three-dimensional velocitydistribution of the coronal neutral hydrogen (via �in situ�techniques) and the outflow velocity of neutral atoms(via Doppler dimming) by exploiting ultraviolet mono-

chromatic imaging and spectroscopy. This coupling ofdifferent diagnostic methods can be only performed inproximity of the Sun when the Orbiter is approaching0.2 AU, since further away neutral atoms are not ex-pected to retain information on the coronal conditions.

Solar Orbiter observations, performed during corota-tion and out-of ecliptic, will give a major contribution tothe study of the evolution of the streamer belt and globalcorona, of the role of the coronal magnetic field topol-ogy in controlling the solar wind dynamics and abun-dance, and of abundance anomalies in streamers,important for understanding whether the slow windhas a component originating in streamers. Most of therelevant observations have to be performed for periodssignificantly longer than the passage of coronal struc-tures at the limb and from a high latitude perspectiveto determine the longitudinal extent of the large scale-structures. The observation of a corona almost not af-fected by rotation is also required in the study of themagnetic field evolution prior to a coronal eruptionand the readjustment of the corona in response to massejections. These topics can be fully addressed by per-forming visible light imaging and monochromatic imag-ing in the intense hydrogen and helium H I 1216 A andHe II 304 A lines, in order to derive the dynamics andabundance of the two major solar wind componentsvia Doppler dimming. If possible, coupling of imagingand spectroscopic capabilities in UV lines emitted byhydrogen, helium and minor ions would provide richerdiagnostic capabilities.

It is clear that a simplified solar minimum configura-tion offers excellent observational conditions for mostof the coronal studies discussed above. This is true inparticular for observations performed out of the eclip-tic, since during minimum the corona shows a com-pletely different configuration when viewed eitherfrom the ecliptic or from high latitudes. It would betherefore important that the late phase of the nominalSolar Orbiter mission and the start of the extendedmission could coincide with the minimum of activity.However, out-of-ecliptic observations during solarmaximum would also be of special interest in orderto investigate, for instance, the reversal of the solarmagnetic fields and the formation of the high latitudecoronal mass ejections.

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