GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 1, PAGES 1-4, JANUARY 1, 1998
Ulysses' return to the slow solar wind D.J. McComas, S.J. Bame, B.L. Barraclough,W.C. Feldman,H.O. Funsten,J.T. Gosling,P. Riley, and R. Skoug Los AlamosNational Laboratory,Los Alamos,New Mexico
A. BaloghandR. Forsyth ImperialCollegeof ScienceandTechnology,London,UnitedKingdom
B.E. Goldsteinand M. Neugebauer Jet PropulsionLaboratory,Pasadena,California
Abstract. After ten long years of wandering the uncharted seas,Ulyssesreturnedto his home port of Ithaca. Similarly, after its unprecedented five year odyssey through the previouslyunchartedregions over the poles of the Sun, the Ulyssesspacecrafthas returnedto the slow, variable solar wind which dominatesobservationsnear the ecliptic plane. Solar wind plasmaand magneticfield observationsfrom Ulysses are usedto examine this return from the fast polar solar wind throughthe region of solar wind variability and into a region of slow solar wind from the low latitude
streamer belt.
As i t
the high latitude regions [e.g., Phillips et al., 1995] in comparison with the generally slower and more variable speedsobservedat low latitudes. The two high latituderegions are largely unipolar and oppositely directed, indicating a substantially dipolarmagneticconfigurationof the Sunduring Ulysses'six year orbit. Brief intervals of oppositely directed fields in the fast solar wind are primarily due to large scale turbulenceand Alfvenic fluctuations[e.g., Smith et al., 1995] and occasional coronal mass ejections (CMEs), while variations betweeninward and outwardpointing IMF in the
journeyedequatorward,Ulyssesencountereda large corotating interaction region and associatedrarefaction region on each solar rotation. Due to these repeatedinteractions, Ulysses also observed numerous shocks, all of which have tilts that are
consistent with those expected for shocks generated by corotatinginteraction regions. Eventually, Ulysses emerged into a region of unusuallysteadyslow solar wind, indicating that the tilt of the streamerbelt with respect to the solar heliographicequatorwas smallerthan the width of the band of
L¾
0
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Alamo
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k
slow solar wind from the streamer belt.
Introduction
The Ulysses mission has been returning solar wind observations continuously since shortly after launch in October 1990. Ulysses' trajectory first carried it out to Jupiter,at 5.4 AU, for a gravitationalswingbywhichdeflected it into a solarpolarorbit. Ulyssesreachedits highestlatitudes of just over 80øSin September,1994 and80øNin July 1995. At the time of this writing (data through25 October 1997),
lOOO
•
1ooo
fiO0•
•'
Ulyssesis locatedat--2.5øN heliolatitudeand has nearly completedits first solarpolar orbit. Observations usedin this studyare from the Ulysses solar wind plasma experiment [Bameet al., 1992] andmagnetometer [Baloghet al., 1992]. Figure 1 summarizesthe solar wind speedas a function of heliolatitudefor Ulysses'first solar polar orbit; observations weretakenover the pastfive years,beginningnear the equator on the right side of the plot (3 o'clock position) and continuingaroundclockwise. Speedis indicatedby radial distance fromthecenterandthe superimposed color gives the magneticpolarity. A striking aspectof this plot is the high andextremelyconstantsolar windspeedobservedthroughout Copyright1998by theAmericanGeophysical Union. Papernumber97GL03444. 0094-8534/98/97GL-03444505.00
1000
Figure 1. Solar wind speedand magnetic polarity measured by Ulysses,as a function of heliolatitude, overlaid with three concentricimages taken with the NASA/GSFC E1T instrument (center),the HAO Mauna Loa coronagraph(inner ring), and the NRL LASCO C2 coronagraph (outer ring). Each 1-hour averagedspeedmeasurement has been color codedto indicate the orientationof the observedIMF: red for outwardpointing and blue for inward. Digital versions of this figure are available for scientific and educational purposes through the Ulysses/SWOOPS homepage (http://nis-www.lanl.gov/nisprojects/swoops/).
2
MCCOMAS ET AL.: ULYSSES' RETURN TO THE SLOW SOLAR WIND
region of solar wind variability generally indicate heliosphericcurrentsheet(sectorboundary)crossings. Overlaidon the Ulyssesdata in Figure 1 are three concentric imagesof the coronatakenon 17 August1996, when Ulysses wasat 29ø N heliolatitude on its way back towardthe ecliptic plane. This time was intentionally chosenbecausethe width of the high densitystreamerbelt appearedto be similar to that shown in the Ulysses observations. Due to a slight inclination of the streamer belt with respect to the solar equator,and to warpsin the streamerbelt, both high-speed, low-densityflows from the coronalholesand low-speed,highdensity flows from the coronal streamerbelt are typically presentat low latitudes. Solar rotationthen createsthe region of solar wind variability typically observednear the ecliptic plane. Becauseof Ulysses' highly elliptical orbit, the nature of the crossingsbetween fast and slow winds is quite different for observationson the left and right sides of Figure 1. The left side of the figure was obtained during Ulysses' fast latitude
scannear perihelion (-1.4 AU) when the combination of the rapid spacecraft motion and solar rotation created two completecrossingsbetweenthe fast and slow regimes. The northernof these crossingsprovideda uniqueopportunity for measuringthe thicknessesand gradientswithin the transition between fast and slow flow regions [McComas et al., 1997]. The right side of Figure 1 was obtained near aphelion (-5.4 AU) where Ulysses was moving much more slowly in heliolatitude. Here, the rotation of the Sun causesmultiple, recurrenttransitions between the high-speedflows and slowspeedwind [Gosling et al., 1993]; these transitions occur within corotating interaction regions (CIRs) where the fast wind is overtakingslow wind aheadof it [e.g., Gosling, 1996] and within rarefaction regions where the fast wind expands away from the slowerwind behindit. Observations
Plasma measurementsfrom May 1996 through our most recent observationson 25 October 1997, are shown in Figure 2. The panels display, from top to bottom, the solar wind
speed,scaledproton density, scaledproton temperature,and alpha/proton density ratio. The density was scaled by the distance to the Sun squaredand the temperaturewas scaled linearly with distancein orderto removethe distancerelated effects of radial solar wind expansion. Shading indicates regionsof outwardpointing IMF while the unshaded regions indicateinwardpointing. Heliosphericcurrentsheet (HCS) crossings occurat the boundariesbetweeninwardandoutward pointingfields. Note that all of the high-speed flowsobserved thusfar in Ulyssesreturnto the low latituderegion have come from the northernpolar coronal hole as indicatedby their outward polarities. Ulysses' returnto the band of solar wind variability was first indicated by the small, periodic reductions in speed observedin July 1996 [Gosling et al., 1997]. With each subsequent solarrotationUlyssespierceddeeperinto the band of variability until, in February1997, it no longer observed the highestspeedwind from the northern polar coronal hole. Since then Ulysses has observed progressively decreasing variationsin the flow speedand has finally becomeembedded entirely in the slow solar wind flows from the streamerbelt. While unusual,the possibility of continuousobservation of the slow solar wind over several solar rotations was predicted for Ulyssesby Gazis [1997]. Our observations indicate that underthe currentconditions,just past solar minimum, the tilt angle of the streamer belt with respect to the solar heliographic equatorand the total warp in the belt must be
Speed (km s-•) 1ooo
õoo 600 400 200
Scaled Density (em -a) 100.0 10.0
1.0 0.1
Scaled Temperature (K) lO6 lO5
[He] ratio 0.10
0.0õ 0.06
smaller than the width of the band of slow solar wind from the
0.04
streamerbelt. Consistent with this return to a pervasively slow solar wind, proton densities increasedand temperatures
0.02 0.00
Aug 96
Nov 96
Feb 97
May 97
Sep 97
decreased.
Compressions are driven by increasingspeedgradientsand are indicatedby enhancementsin the density and temperature. Conversely, rarefactions occur throughout the decreasing 35 ø 30 ø 25 ø 20 ø 15 ø 10 ø 5ø speed gradients and are accompaniedby reductionsin the Latitude density and temperature. The largest of these compressions Figure 2. One-houraveragedUlyssesplasmaobservations and rarefactions were observed at the edge of the region of from May 1996 through 25 October 1997, covering the solar wind variability (-17ø-26 ø heliolatitude for Ulysses' interval duringwhich Ulyssesreturnedfrom the uniform, fast return)wherethe largestspeeddifferenceswere present. solar wind from the northern polar coronal hole to the low Also consistentwith Ulysses' return to the region of solar latituderegion of solar wind variability. Two day running wind variability, the alpha to proton density ratio, which was averagesof the magneticpolarity are indicatedby shading quitecloseto-4.3% throughoutthe high-speedpolar coronal (outwardIMF) or clear(inwardIMF). hole flow [Barracloughet al., 1996], became lower on average Time
MCCOMAS ET AL.: ULYSSES' REFURN TO THE SLOW SOLAR WIND
and more highly variable as Ulysses penetrated to lower latitudes,as shownin Figure 2. After a year and a half of being continuously embeddedin the outwardpointing magnetic sector of the northern solar polar hole, Ulyssesfirst encounteredthe HCS on 30 September
3
[Goslinget al., 1997]. In additionto this CME, another event in mid-December1996 appearsto be associatedwith a CME eruptionfrom the Sun observedwith the LASCO coronagraph on SOHO.
Several other CMEs have been observed in the most
recent, low latitude data. The paucity of clear CMEs at mid1996 at a latitude of 25.5øN. This encounter has been latitudes, however, comparedto the number observed during explainedby a strong,localized deflectionof the HCS, created Ulysses'departurefrom the eclipticplane five years earlier, is almost certainly dueto the lower solar activity level during by an equatorialactive region on the Sun [Forsyth et al., 1997]. Similar HCS crossings and brief encounterswith the this interval. The post-solarminimum conditionsencounteredby Ulysses inward sectorIMF from the southernhemispherewere observed on many of the following rotations as indicated by the duringits returnto the near-ecliptic region are different from correlation between the speed minima and inward field both the post-maximum conditions that it left five years intervals (white banding). Many of these also correlatewith earlierandthe nearminimumconditionsobservedduringthe minima in the alpha to proton density ratio, as expectedfor fast latitude scan. In 1992-1993, when Ulysses climbed HCS crossings[Borrini et al., 1981; Barracloughet al., 1996]. southwardout of the band of solar wind variability, the The region with the largest alpha/proton density ratio inclination of the streamerbelt was -29 ø with respect to the shownin Figure 2 (15-20 October 1996) was associated with a solarequatorialplane [Bame et al., 1993]. By the time of the rapid traversal of the equatorial region in early 1995, this CME that was embedded in the interface between the low- and high-speedflows on the northern edge of the band of solar inclination had droppedto only ~10ø [Gosling et al., 1995]. wind variability. This interval had many of the classic During its return to low latitudes in 1996-1997, a localized deflectionor bulgein the HCS and streamerbelt causedUlysses signaturesof CMEs and drove a fast, forwardshock aheadof it to encounterthe HCS first in September 1996 at 25.5øN. However, Ulyssesalso continuedto periodicallysee the fastest Table 1. Ulysses shock list confirmedby both plasma and solar wind until February 1997 when it was at ~17øN. Since then, the irregularities in the streamerbelt must have become magneticfield measurements through25 October 1997. flattenedout and its overall tilt diminishedto the point where Year DOY Date Time [UT] Type Ulysses has remainedwithin slow solar wind continuously 1996
1997
219 248 274
06 Aug 04 Sep 30 Sep
0748 0809 1848
Reverse Reverse Forward
277
28 7 293
03 Oct
1855
Reverse
13 Oct 19 Oct
2150 1952
Forward (CME driven) Forward (within CME)
297
23 Oct
2211
Forward
301
27 Oct
185 3
Reverse
32 6
2! Nov
2100
Forward
340
05 Dec
1210
Reverse
360
25 Dec
0745
Reverse
020
20 Jan
1515
Reverse
03 7
06 Feb
1807
Forward
044
13 Feb
1209
Forward
063
04 Mar
04 20
Forward
071
12 Mar
1517
Reverse
08 8
29 Mar
1515
Forward
092 101 107 115 118 125 129 139 148
02 Apr 11 Apr 17 Apr 25 Apr 28 Apr 05 May 09 May 19 May 28 May
1234 07 21 1258 1025 1946 0905 1149 1923 0128
Reverse Forward Reverse Forward Reverse Forward Reverse Forward Forward
16 8
17 Jun
1900
Forward
216 234 241 254 256 271
04 Aug 22 Aug 29 Aug 11 Sep 13 Sep 28 Sep
1723 1047 0809 1626 0620 0107
Forward Forward Forward(CME driven) Forward Forward Forward
279
06 Oct
1247
Reverse
296
23 Oct
1554
Forward(CME driven)
over the last several solar rotations.
Table 1 lists all of the shocks
identified
in the combined
Ulysses plasma and magnetic field observations, from the northern polar pass through 25 October 1997. The table identifies only four CME-associatedshocks; all of the other shocksappearto be associated with the CIR. Figure3 displays the distribution of forward and reverse shocks from Table
1 in
the formatdevelopedby Gosling et al. [1993]. The density ratio plottedfor eachshockis a measureof the shockstrength. Consistent with Ulysses' observations of CIR-related shocks as it traveled slowly southwardfrom the ecliptic plane five yearsearlier,reverseshocks, which propagatepoleward,were observedto higher heliolatitudes than forwardshocks, which propagate equatorward[Gosling et al., 1993; 1997]. At
5.0 AU
4.5 AU
4.0 AU
.
-
Reverse
Shocks
-
_
-
t i I I
0
I i
5
i
i I
I ,
10
, i
,
I ! i i
15
I
I i
20
i i
i I
25
!
i i
i ! i i
30
i
, I
35
i i
i i -
40
Latitude (Deg)
Figure 3. Bar chartof forwardandreverseshockstrengths, as measuredby the density jumps across the shocks, as a functionof heliolatitude. Shocksin this figure match those listedin Table 1 by readingfrom the earliest(right side)to the most recent (left side).
4
MCCOMAS
ET AL.: ULYSSES'
•
intermediatelatitudes(11ø-26ø),many of the shocks occurred in forward-reverse pairs, again as expectedfor CIR-generated shocks. As Ulysses reached the lowest heliolatitudes, the numberof shocks dwindled,and only forward shocks were observed.It appearsfrom theseobservations that Ulysseshas dropped below the mid-latitude region where shocks are preferentially generated (Figure 2) and that (with one exception) only the forward shocks, which propagate equatorward, canreachthe spacecraft, just as only the reverse shockswere able to reachUlyssesat higherheliolatitudes. As with theIMF, CIRs andCIR-drivenshockswrapup into Archimedianspiralsowing to the rotation of the Sun. At the
relatively large heliocentricdistancesof the recentUlysses observations(>4 AU), this spiral configurationcreatesflow deflectionsacrosstheseshocksthat are orientedlargely in the north-south direction.
All of the shocks listed in Table 1 had
north-southflow deflectionsconsistentwith CIR-produced shockswhichdevelopedin the northernhemisphereaccording to the modelof Pizzo [1991] and Gosling et al. [1993; 1996]: specifically, all of the forward shocks displayedequatorward flow deflections and all of the reverse shocks displayed polewarddeflections,consistentwith a persistentnorth-south tilt predictedby this model.
TO THE SLOW SOLAR WIND
This work was carried out under the auspicesof the United States
Department of Energywith supportfromtheNASA Ulyssesprogram. References
Balogh,A., et al., The magneticfield investigation ontheUlysses mission:Instrumentation andpreliminaryscientificresults,Astron. andAstrophys., Suppl.Ser.,92, 221, 1992. Bame,S.J.,et al., The Ulyssessolarwind plasmaexperiment, Astron. Astrophys. Suppl.Ser.92, 237, 1992. Barraclough, B.L., et al., He abundance variations in thesolarwind: Observations fromUlysses,Solar Wind8, AIP Proc.382,Ed. Winterhalter, et al., 277, 1996.
Borrini,G., et al., Solarwind heliumandhydrogenstructure nearthe heliospheric currentsheet:A signalof coronalstreamers at 1 AU, J. Geophys. Res.,86, 4565, 1981. Forsyth,R.J.,et al., Ulyssesobservations of thenorthward extension of theheliospheric cutrentsheet,submitted to Geophys. Res.Lett., 1997. Gazis,P.R., The latitudinalstructureof the solarwind in the vicinity of the solarequatornearsolarminimum:1986andpredictions for 1997, Geophys. Res.Lett.,24, 627, 1997. Gosling,J.T.,Corotating andtransientsolarwindflowsin three dimensions, Annu.Rev.Astron.Astrophys., 34, 35, 1996. Gosling,J.T.,et al., Latitudevariationof solarwindcorotating stream interactionregions:Ulysses,Geophys.Res.Lett.,20, 2789, 1993. Gosling,J.T.,et al., Thebandof solarwindvariabilityat low heliographic latitudes nearsolaractivityminimum:Plasmaresults fromtheUlyssesrapidlatitudescan,Geophys. Res.Lett.,22, 3329, 1995.
Conclusion
Ulysseshas recently returnedfrom the high-speedsolar wind at high heliolatitudes to the slow solar wind near the
eclipticplane. The returnwasmarkedby repeatedcrossingsof CIR andrarefactionregionsassociatedprimarily with a large warp in the HCS and streamerbelt. With each rotation of the Sun, Ulysses passed further into the region of solar wind
variability andultimately into a region of pervasively slow, steadysolarwind. During the post-minimumsolar conditions thatprevailedduringUlysses'return,very few CME flows were observed. In addition,the streamerbelt structureappearedto be flat with little inclinationrelativeto the solarheliographic equator. Nearly all of the shocksobservedduringUlysses' return are consistent with CIR-producedcompressionsthat havesteepened into shocksby the time they wereobservedat >4 AU by Ulysses. Ulysses' upcoming second polar orbit, under solar
Gosling,J.T.,et al., Thenorthern edgeof thebandof solarwind variability:Ulyssesat -4.5 AU, Geophys. Res.Lett.,24, 309, 1997. McComas,D.J., et al., Ulyssesrapidcrossingof the polarcoronalhole boundary,in pressin J. Geophys.Res., 1997. Phillips,J.L.,et al., Ulyssessolarwindplasmaobservations at high southerlylatitudes,Science,268, 1030, 1995. Phillips,J.L., et al., Ulyssessolarwind plasmaobservations frompeak southerlylatitudethroughperihelionandbeyond,Solar Wind8, AIP Proc. 382, Ed. Winterhalter, et al., 416, 1996.
Pizzo, V.J., The evolutionof corotatingstreamfronts nearthe ecliptic planein the innersolarsystem2: Three-dimensional tilted-dipole fronts,J. Geophys.Res.,96, 5405, 1991. Smith,E.J., et al., Ulyssesobservationsof Alfven wavesin the southern andnorthernsolarhemispheres, Geophys.Res.Lett., 22, 3381, 1995.
A. BaloghandR. Forsyth,SpaceandAtmosphericPhysics, The Blackett Laboratory, Imperial College of Science and Technology, London SW7 2BZ, UK. (e-mail:
[email protected]) S.J. Bame,B.L. Barraclough,W.C. Feldman,H.O. Funsten, maximumconditions,will providea uniqueopportunityfor comparisonwith the nearly completefirst polar orbit. This J.T. Gosling, D.J. McComas, P. Riley, and R. Skoug' Los comparisonwill allow the solar cycle dependence of the 3- Alamos National Laboratory,Los Alamos, NM 87545, USA (edimensional structureof the heliosphere to be examined mail:
[email protected]) directly for the first time. B.E. Goldstein,M. Neugebauer,Jet Propulsion Laboratory, Pasadena,CA 91109-8099, USA (e-mail: bgoldstein@jplsp. Acknowledgments.We gratefullyacknowledgethe NASA/GSFC jpl.nasa.gov)
EIT instrument team,HAO MaunaLoa coronagraph team,andthe NRL LASCOC2 coronagraph teamfor theimagesusedto assemble Figure1.
(ReceivedOctober 8, 1997; acceptedNovember 14, 1997.)
