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Ulysses at Solar Maximum and Beyond

R.G. MarsdenSolar System Division, Space Science Department, ESA Directorate of ScientificProgrammes, ESTEC, Noordwijk, The Netherlands

IntroductionAfter a brief review of the mission and itsachievements, this article presents somerecent highlights. These include the discovery,not recognised at the time, that in 1996,Ulysses set the record for finding the longestcomet tail ever. This record is not the only oneheld by ESA’s intrepid heliospheric explorer.Ulysses attained a speed of 15.4 km/s just afterburn-out of the upper-stage motors, making itthe fastest man-made object to have left theEarth’s gravitational pull. It has also reached byfar the highest solar latitude of any spacecraft,

and travelled further from the Sun than anyother ESA-built space probe. Following a lookat some of the operational challenges ahead,the remainder of the article is devoted to thefuture, which for Ulysses continues to lookbright. The spacecraft and its payload are inexcellent condition, and there are plans toextend orbital operations until September2004. By that time Ulysses will have completedits second out-of-ecliptic orbit of the Sun.ESA’s Science Programme Committee (SPC),at its meeting on 6 June, unanimously approvedthe proposed extension, and there is goodhope that NASA will do likewise when themission comes up for review next year.

The mission to dateUlysses was launched by the Space Shuttle‘Discovery’ in October 1990. Following agravity-assist manoeuvre at Jupiter in February1992, the space probe and its suite of scientificinstruments have been pursuing goals relatedto the Sun, its heliosphere, and the region ofinterstellar space surrounding the heliosphere,all from the unique perspective of a solar polarorbit (Fig. 1). The portions of the orbit whenUlysses is above 70º solar latitude have beendesignated ‘polar passes’, and the first suchpolar passes occurred in 1994 (south) and

The year 2000 promises to be highly eventful for the joint ESA-NASAUlysses mission. Not only does it mark an important anniversary – on6 October, Ulysses will have been in orbit for 10 years – it also sees thereturn of Ulysses to the poles of the Sun. Given the spectacularsuccess of the spacecraft’s first visit to these previously unexploredregions in 1994/95, there is every reason to expect a rich scientificharvest once again. Bound inexorably by the laws of celestialmechanics, Ulysses has followed an almost identical path on its climbto high latitudes to that travelled six years earlier. The conditions it hasencountered this time have, however, been markedly different. Thefirst polar passes in 1994 and 1995 took place at a time of low solaractivity, whereas now the sunspot cycle is very close to its peak. Thishas had a clear effect on many of the phenomena recorded by thescientific instruments on board the spacecraft.

ulysses at solar maximum and beyond

Figure 1. The Ulysses orbitviewed from 15 deg above

the ecliptic plane. Dotsmark the start of each year

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1995 (north). The spacecraft takes 6.2 years tomake a complete orbit of the Sun, and so thesecond set of polar passes will take place fromSeptember 2000 to January 2001 (south), andSeptember to December 2001 (north).

Specific topics of interest to the scientistsworking with data from Ulysses include thesolar wind and its magnetic field, energeticparticles and cosmic rays, interstellar dust andgas, cosmic gamma-ray bursts, and naturalradio emission from the Sun, the planets, andthe interplanetary medium. It is a tribute to theskill of the engineers and scientists involved inthe mission that such scientific diversity couldbe achieved by nine experiments weighing only 55 kg in total. A small ESA-led teamstationed at the Jet Propulsion Laboratory inPasadena, California, conducts the missionoperations.

As reported in the many articles summarisingthe successes of the mission to date (e.g. ESABulletin 92, pp. 75-81), Ulysses has literallyadded a third dimension to our knowledge ofthe heliosphere and the space that surroundsit. While far from exhaustive, the following listhighlights some of key findings from the ‘solarminimum’ phase of the mission.– The existence of two distinct solar-wind

states, a fast high-latitude wind that onlyoccasionally extends down to low latitudes,and a slow low-latitude wind centred aboutthe heliospheric current sheet, is confirmed.These two types of solar wind are separatedby a sharp boundary extending from theSun’s corona down to the chromosphere.

– The magnitude of the radial component ofthe heliospheric magnetic field does notincrease towards the poles. The constancyof the radial field implies that the dipole-likeconfiguration of the Sun’s surface field isnot maintained, and that as a result thepolar solar wind undergoes significant non-radial expansion.

– Co-rotating solar-wind stream structureswith forward and reverse shock waves, well-studied at low latitudes and expected to beconfined to those regions, produce effectsextending to the highest latitudes exploredby Ulysses. These effects include therecurrent modulation of galactic cosmic raysand injection of accelerated lower-energyparticles into the polar regions, suggesting arevised global structure for the heliosphericmagnetic field.

– The influx of cosmic rays at high latitudes issmaller than predicted for the minimumphase of the solar activity cycle, most likelyas a result of large-amplitude waves foundby Ulysses to be present in the polarmagnetic fields.

– The density of interstellar atomic hydrogenand helium has been derived from Ulyssesdata, leading to improved knowledge of theinteraction of the local interstellar cloud withthe heliosphere.

– The ratio of interstellar 3He to

4He has been

measured for the first time. The valuesuggests that the amount of dark matterproduced in the Big Bang was greater thanpreviously thought.

– The local population of interstellar dustparticles, measured for the first time byUlysses, contains a larger number of heavygrains than predicted by observations ofstarlight. Ulysses also discovered duststreams coming from the vicinity of Jupiter.

In global terms, Ulysses has taught us that, atsolar minimum, the high- and low-latituderegions of the heliosphere are much moreintimately linked than was expected. It has alsorevealed that interstellar gas plays a surprisinglyimportant role in the dynamics of theheliosphere. Last but not least, Ulyssescontinues to be one of the cornerstones of theinterplanetary network of spacecraft recordingcosmic gamma-ray bursts. Because of itsunique orbit, Ulysses can provide significantconstraints on the location of such bursts onthe sky.

Ulysses at solar maximumThe Ulysses mission has already clearlysucceeded in extending our two-dimensional,ecliptic view of the heliosphere into a global,three-dimensional one. There is, however, afourth dimension to be considered: time. TheSun, like most other stars, undergoessignificant temporal changes in its activity.These variations in turn affect the heliosphere,the bubble in space blown out by the solarwind, in a way that is literally far-reaching.Ulysses is ideally placed to study the effects ofchanging solar activity on the large-scalestructure of the heliosphere, so this has beenone of the major goals in recent months. Beforeturning to the latest results, however, a briefreview of the solar activity cycle is given.

The solar activity cycleEven a modest telescope (with a suitable filterin place for visual observations!) will reveal thatthe Sun is not simply a featureless ball of gas.‘Blemishes’ that were already noted manycenturies ago in China, and which were firststudied in detail by Galileo at the beginning ofthe seventeenth century, mark its surface.These dark areas are what scientists now call‘sunspots’, and they are an indication of one ofthe Sun’s fundamental properties – its magneticactivity. Daily observations of the number ofsunspots on the disk provide a reasonably

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Figure 3. The solar-windspeed measured at Ulyssesplotted as a function ofsolar latitude for the currentdescent to the south pole(upper panel), and duringthe previous descent in1992-94 (lower panel)(Courtesy D.J. McComas)

Recent resultsAs noted above, when comparing the currentinterplanetary conditions with thoseencountered by Ulysses at the same locationmore than six years ago, the effects ofincreased solar activity are quite evident. Thestable solar-wind structures that swept over thespacecraft once per solar rotation in 1993 assolar activity declined have given way to amuch more complex and less repetitiveconfiguration. This is seen clearly in the plot ofthe solar-wind speed recorded by Ulyssesduring its two traversals from the equator to thesouth pole (Fig. 3). The regular appearance offast (~750 km/s) solar wind flowing from thesouthern polar coronal hole that characterisedthe earlier period has not been repeated in therecent data. Given the rapid increase insunspot number at the present time, and thecorresponding evolution of the magnetic field atthe surface of the Sun, it is unlikely that this will

reliable record of this activity going backhundreds of years. From this record, a veryclear periodicity emerges in which the numberof sunspots, and hence the magnetic activity,varies on average with an 11-year cycle. Withinthis overall pattern, however, there aresignificant variations in both the size ofindividual peaks, and the time betweenconsecutive maxima. Nevertheless, at leastduring the last 150 years for which the mostreliable observations are available, the generalpattern seems to hold. Figure 2 shows anhistorical overview of the sunspot cycle in twodifferent representations.

Sunspots typically form in pairs, and the regular11-year variation of sunspot numbers isaccompanied by similar oscillations in themagnetic polarity of these pairs. The polarity ofthe so-called ‘bipolar spot groups’ in a givenhemisphere switches from one sunspot cycleto the next, leading to a 22-year magneticcycle. A characteristic feature of the magneticcycle is the reversal of the Sun’s polar fieldsevery 11 years on average. During the mostrecently completed cycle (number 22), whichpeaked in 1989, the polar fields in the northernand southern hemisphere were predominantlyof positive and negative magnetic polarity,respectively. With the activity of the currentcycle (23) rapidly nearing its peak, the polarfields are once again reversing, this timebecoming negative in the north and positive inthe south. The change in polarity, which usuallyoccurs over a period of a year or more with onehemisphere following the other, has lagged thesunspot maximum by 1-2 years in recentcycles. Cycle 23, however, appears to beunusual in that the polarity reversal is alreadyunderway.

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Figure 2. An historicaloverview of the solar cycle,plotted in the form of a so-called ‘butterfly diagram’(upper panel), and in a moreconventional representation(lower panel)

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Figure 4. Meridional cutthrough the heliosphere

showing the surface (bluecurve), calculated from

Ulysses solar-windobservations, where the

neutral hydrogen densityshould drop to 1/e of its

interstellar value. The solarimages (not to scale) are

taken from the EIT andLASCO C2/C3 instruments

on SOHO(Courtesy D.J. McComas)

occur at all during Ulysses’ return to the polarregions this year and next. Even though ‘pure’fast wind from the polar coronal holes may notbe present, fast wind streams are clearly stillpresent at the highest latitudes encountered sofar. These streams are thought to originate inthe isolated, lower-latitude coronal holes thatdevelop near solar maximum. One of theintriguing questions to be answered is howdifferent are these fast streams from the fastwind from the poles?

The approach of solar maximum is alsoapparent in the behaviour of the energeticparticles detected by Ulysses during its climbto high latitudes. Here again, a comparison withthe earlier results reveals that the regularincreases in particle intensity once per solarrotation – a consequence of stable, long-lastingsolar-wind structures found near solarminimum – no longer exist. Instead, a morehaphazard picture is seen, due in large part tothe increase in transient events at the Sun, andthe rapidly changing pattern of solar-windstreams characteristic of solar maximum. Thedevelopment of these and other trends will bewatched with great interest as Ulysses passesover the Sun’s poles for the second time.

Ulysses data are being used to infer not onlythe local conditions in the solar wind, but alsothe global structure of the heliosphere. Chargeexchange with solar-wind protons is theprimary ionization process for interstellar

hydrogen atoms travelling through theheliosphere. Ulysses solar-wind data have beenused to examine and quantify variations incharge exchange, which has implications for,for example, the interpretation of observationsof scattered Lyman-alpha radiation. TheUlysses observations have revealed that thecharge exchange rate is higher at low than athigh latitudes, and that this rate drops off moreslowly than the inverse square of heliocentricdistance. This result is depicted in Figure 4.

Although the scientific focus of Ulysses remainsthe heliosphere, the diversity of scientific topicsaddressed by the mission has continued to beimpressive. Among the fascinating results to bereported recently was the identification of thepassage of Ulysses through the distant (3.8 AU)

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Figure 5. Artist’s impressionof Ulysses crossing thedistant tail of cometHyakutake (David A. Hardy)

right time’, Ulysses now holds the record forfinding the longest comet tail ever. An artist’simpression of this serendipitous event is shownin Figure 5.

The ten years of continuous Sun-relatedobservations acquired by Ulysses are inthemselves an impressive data set. That thesemeasurements can form the basis for studiesover a much longer time-scale is equallygratifying. Ulysses magnetic-field data havebeen used to infer global properties of theSun’s coronal magnetic field extending back intime to the mid-19th century. A critical part ofthe calculation relies on the Ulysses finding thatthe radial component of the heliospheric field isindependent of solar latitude. A particularlystriking result is the fact that the inferred

tail of comet C/1996 B2 (Hyakutake) on 1 May1996. First reported in 1998 as a ‘density hole’in the Ulysses solar-wind observations, theevent was subsequently ‘rediscovered’independently in the magnetic-field data andthe ion-composition measurements. Themagnetic field pattern at the time of the densitydrop-out was highly reminiscent of thatexpected within a comet tail, and the unusualheavy ions present in the solar-wind dataadded weight to the hypothesis. A member ofthe Imperial College magnetometer team madethe specific association with comet Hyakutake.This discovery was due in large part to Ulysses’unique, high-latitude position in theheliosphere, and the fact that both Ulysses andthe comet were immersed in the high-speedsolar wind. By being in ‘the right place at the

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Figure 7. The strength of thesolar-induced forcing

responsible for the Ulyssesnutation anomaly plotted in

arbitrary units versus time

coronal field has approximately doubled in thepast 100 years, perhaps as a result of chaoticchanges in the solar dynamo. Although not wellunderstood, a connection is believed to existbetween the Sun’s magnetic field and itsluminosity, indicating possible implications forthe global climate of the Earth.

Operational challenges in 2000/2001The Ulysses spacecraft has proven to be highlyreliable, and the members of the SpacecraftOperations Team have had to cope withremarkably few in-orbit anomalies during thepast 10 years. Nevertheless, there areoperational challenges to be faced during theupcoming phase of the mission. The thermaleffect of the Sun on Ulysses varies dramaticallyover the spacecraft’s orbit due to the largechanges in solar distance (1.34 – 5.41 AU).Thermal control is therefore an important

operational task. It is performed by dumpingpower either into various heaters inside thespacecraft body or to external radiators. Thesole power source is a RadioisotopeThermoelectric Generator (RTG) and its outputdecays exponentially with time (Fig. 6).Delivering 285 W at launch, the RTG nowprovides only 223 W, so maintaining anacceptable thermal balance while ensuring agood science data return from all of theexperiments has become more of a challenge.A number of redundant components have beenswitched off over the past few years andsimultaneous use of several power-hungryoperating modes has been avoided. If, as ishoped, the mission continues past 2001, sometime-sharing of payload elements will beinevitable. Even so, it will be possible to powera group of ‘core-science’ instruments, togetherwith a selection of ‘discretionary’ experiments,at least until the autumn of 2004.

In addition to the continuous decline inavailable power, another major operationalchallenge over the next eighteen months will bethe return of spacecraft nutation in December2000. The nutation anomaly was firstdiscovered when the axial boom was deployedshortly after launch, and appeared again in1994/95. Under certain conditions ofillumination by the Sun, the boom goes in andout of shadow as the spacecraft spins. Theresulting thermal stresses cause the boom toflex, and the body of the spacecraft then beginsto wobble or nutate. This spacecraft motionmust be kept as small as possible for thefollowing reasons:– Damage or loss of spacecraft may occur if it

is left uncontrolled.

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Figure 6. The power outputof the Radioisotope

Thermoelectric Generator(RTG) on board Ulysses

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– Flexing of the axial and wire booms mayinduce metal fatigue.

– Booms may wrap around the spacecraft,dislodge thermal blankets or even detach.

– The uncertainty of the spacecraft attitude atany given time will make data reductionmore difficult.

– Large off-pointing of the high-gain antenna will result in loss of data.

From the graph in Figure 7, we can see that theseverity of the anomaly in 2000/01 will begreater than in 1994/95. The tools andtechniques developed in 1994/95 to control thelevels of nutation are being refined, however,and will be employed again to minimise thethreat to spacecraft health. A critical element inthis regard is the presence of a continuousuplink beacon for the onboard Conscansystem, and detailed scheduling of the requiredground-station coverage is already wellunderway.

The futureAs we have seen, the heliosphere is a dynamicenvironment that undergoes large variations inboth large and- small-scale structure over theperiod of an 11-year solar cycle. It is certainthat new insights will be gained as we continueto observe the effects of increasing solaractivity, changing coronal structure, and thereversal of the solar magnetic polarity fromUlysses’ high-latitude vantage point. Based onthe success of the ‘prime mission’ (launch toSeptember 1995), ESA and NASA agreed tocontinue Ulysses operations until December2001, the end of the second north polar pass.The recent ESA SPC decision opens the wayfor a further 2.75 years of orbital operations.The spacecraft will then have reached aphelionagain, thereby completing a second out-of-

ulysses at solar maximum and beyond

Figure 8. The orbit ofUlysses, showing theplanned extension of orbitaloperations (heavy red line)

ecliptic orbit of the Sun (Fig. 8). The proposedextension would enable the effects of themagnetic polarity reversal on, for example, thecosmic-ray intensity gradients, to be studied indetail. At the same time, the additionalobservation time will be highly beneficial to themeasurement of rare pick-up ion species andcosmic-ray isotopes.

One of the ‘frequently asked questions’concerning Ulysses is: ‘Will there ever beanother Jupiter encounter?’ Although not asintimately as in 1992, the spacecraft willapproach Jupiter again in February 2004.Compared with the first fly-by, which at 6.3Jupiter radii from the planet’s centredramatically modified the Ulysses flight-path,the second ‘encounter’ has a closestapproach distance of 1682 Jupiter radii andwill not change the orbit significantly.Nevertheless, it should provide a uniqueopportunity to observe Jovian radio emissionsfrom the planet’s polar regions.

Another important argument in favour ofextending the mission is the opportunityafforded for joint observations by members ofthe ‘Solar Armada’ that includes Ulysses,SOHO, TRACE, Yohkoh, and ACE. Thisformidable fleet is soon to be joined by the fourCluster-II spacecraft. Collaborations with otherspace missions and ground-based projectsalready characterise a significant part of thescientific work carried out using Ulysses data.In all such studies, the unique high-latitudeperspective of Ulysses and its integratedinstrument payload are, and will hopefullycontinue to be, invaluable assets. r

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