Solar Wind Decrease at High Heliographic Latitudes Detected from Prognoz Interplanetary Lyman Alpha Mapping Rosine Lallement, Jean-Loup Bertaux, V

Solar wind decrease at high heliographic latitudes detected from Prognoz interplanetary lyman alpha mapping Rosine Lallement, Jean-Loup Bertaux, V. G. Kurt

To cite this version:

Rosine Lallement, Jean-Loup Bertaux, V. G. Kurt. Solar wind decrease at high heliographic latitudes detected from Prognoz interplanetary lyman alpha mapping. Journal of Geophysi- cal Research Space Physics, American Geophysical Union/Wiley, 1985, 90 (A2), pp.1413-1423. 10.1029/JA090iA02p01413. hal-02889795

HAL Id: hal-02889795 https://hal.archives-ouvertes.fr/hal-02889795 Submitted on 16 Mar 2021

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNALOF GEOPHYSICALRESEARCH, VOL. 90,NO. A2, PAGES1413-1423, FEBRUARY 1, 1985

Solar Wind Decreaseat High Heliographic Latitudes Detected From Prognoz Interplanetary Lyman Alpha Mapping

R. LALLEMENT AND j. L. BERTAUX

Serviced'A•ronomie du CNRS, Verridres-le-Buisson,France

V. G. KURT

SpaceResearch Institute, Academy of Scienceof USSR, Moscow

New evidencefor a latitudinal decreaseof the solar wind massflux is presentedfrom observationsof the interplanetaryLyman alpha emissioncollected in 1976 and 1977 with satellitesPrognoz 5 and 6. The flow of interstellarhydrogen atoms in the solar systemis ionized by EUV solar radiation and charge exchangewith solar wind protons which accountsfor about 80% of the total ionization rate. The resulting gradual decreaseof the neutral H density from the upwind region down to the downwind region observedfrom Ly • intensitymeasurements allowed the determinationof the absolutevalue of the total ionization rate/• for one H atom at 1 AU against ionization. Collected in 1976 and 1977 at five placesin the solar system,the. measurements are first comparedto a model which assumesisotropy of the EUV and solar wind. Strong departuresare obvioustoward high-latituderegions, especially when the observer is in the downwind region where the solar wind ionization has had more time to act (cumulativeeffect). A model was constructedwhich included a decreaseof the ionization rate with heliographiclatitude. The adjustmentof data allowed for the measurementof the absolutevalue of the total ionizationrate and impliesa 50% latitudedecrease of the ionizationrate due to chargeexchange with the solarwind, from/•s,• - (3.9_+ 0.5) x 10-8 s-• at the equatorto/•s,• - (2.0+ 0.5) x 10-8 s-• at the pole.The correspondingabsolute value of the solarwind proton flux is (2.4- 3.6) x 108crn-: s-• at the equatorand twice lessat the pole if a constantvelocity is assumedfor the solar wind. Even if the solarwind velocityincreases from 400 to 800 km s-•, whichwould decreasethe chargeexchange cross sectionby 25%,there is stilla decreaseby about30% of thesolar wind mass ,flux from equator to pole. The Lyman alpha data from Mariner 10 (Kumar and Broadfoot,1978) had already shown a similar trendin 1974,showing a persistenceof the solarwind anisotropy for 3,-4years during a solarminimum. The large-scaleproperties of the solar wind massflux can thereforebe monitored at all latitudes by remote sensingof Ly • interplanetary emission,since the solar wind is carving the flow of interstellar H and its anisotropiesare "printed" on the interplanetaryH distribution.With uncalibratedLy • measurements,an absolutevalue of the solarwind flux canbe determinedat all latitudesaveraged over a typical 1-yearperiod.

1. INTRODUCTION The spatial evolution of the coronal holes which are associ- It has beenshown through modeling by doselynand Holzer ated with low plasmadensities and openmagnetic field lines [1975] that if there is a nonisotropicsource of the solar wind, has been deduced from ground-basedwhite light coronal the spatialdistribution of atomichydrogen in the interplah- measurements[Hundhausen et al,, 1981]. The most extended etary spacewould be greatly affected,which could be observed featuresare the polar coronal holes appearingat low solar by remotesensing with a mappingof the interplanetaryLy • activity. In the caseof near-eclipticobservations, some results emissionproduced through resonance scattering of Ly • solar about the solar protonsfluxes have been obtainedin situ by photons. Schwenn[1982] showinga decreaseof the flux in the high- More recently,Kumar and Broadfoot[1979] have observed speedstreams which are associatedwith open magneticfield and analyzed such an effect on the Mariner 10 results.In the lines. present paper we show clear evidenceof the nonisotropic Up to now there have been many local and unrelated re- character of the solar wind ionization effect from observations sults,but a generaldescription of the latitude dependenceis of the Ly 0•interplanetary emission collected with Prognoz 5 still lacking,and there is a needfor out of eclipticmeasure- and 6 Sovietspacecraft in 1976 and 1977. ments,especially of the density fluxes.The International Solar Few other recentobservations of the solar wind properties Polar Mission,scheduled to passover the solar polesin 1990, have shown latitudinal dependencesof the plasma densities will certainlygather numerousin situ measurementsat high and velocities.From analysisof both in situ measurementslatitudes. and interplanetary scintillations(IPS) observations,latitude However, the remote sensingmonitoring of the interplan- gradients in the velocities have been derived [Rhodes and etary Ly 0•emission provides a way to determineproperties of Smith,1976; Rickett and Coles,1979; Zhao and Hundhausen, the solar wind with a large spatial scale,averaged over time 1983], even at high latitude [Coles and Maa•ioe, 1972]. In scalesof the order of 1 year, by usingits ionizing effecton the general,high velocitiesare associatedwith high latitudes,but interplanetary-interstellarhydrogen gas. they also coincidewith equatorialcoronal holes or with polar 2. INTERPLANETARY LYMAN ALPHA METHOD coronal holes and their lower latitude extensions. Becauseof its relative motion in respectto the local inter- Copyright 1985by the AmericanGeophysical Union. stellar medium,characterized by the velocityvector V,• = Paper number 4A8193. -Vo, the solar systemis filled in by a permanentflow of 0148-0227/85/004A-8193505.00 neutral hydrogen and helium atoms of interstellar origin. 1413 1414 LALLEMENTET AL.' SOLARWIND AT HIGH LATITUDES

drogen absorption cell data, whereas they were guessedin DEN Mariner 10 analysis. In Figure 1 the effect of the anisotropicinteraction between UPWIND SCAN solar wind fluxes and the hydrogen flow is representedin a SCAN PLANE schematicway. The thermal velocity dispersioninside the flow PLANE I I NIZ TION RATE wasneglected, and V 0 was assumedto be in the solarequa- LOWER DENSITY torial plane for simplicity. Let us supposethat atoms follow- SOLAR EQUATORIAL PLANE ing trajectoriespassing over the polar regionsencounter a Fig. 1. Schematicillustration of the interaction between the hy- solar plasma flux with a less ionizing power compared with drogenfJow and an anisotropicsolar wind. For the sakeof simplicity the low-latitude plasma flux acting on the atoms which travel the hydrogenbulk velocityis assumedto be in the solar equatorial near the equatorial plane. Thus owing to the differentioniza- plane.Th e exact configuration.is shownin Figure 10 and has been taken into accountin the modelling.Atoms Whichtravel near the tion rates,the densitydecrease along the polar trajectoryand solar equatorialplane do sufferhigher lossprocesses than the atoms departuresfrom the axisymmetryappear gradually from the which are passingover the polar regions.Thus the densitydecrease is upstreamregion, where the flow is uniform down to the down- larger in the first case, and there are regions of enhanceddensity stream space where the distribution presentstwo regions of above and below the eclipticplane. The discrepanciesbetween the intensitiesin the directionsA and B will be much more pronounced enhanceddensity, perpendicularly above and below the eclip- in a downwind scanplane than for directionsA' and B' in an upwind tic plane. From this simple schemeit appearsthat the obser- plane of observation,since downwind the distribution has been sig- ver's position is of great importance. The maximum contrast nificantly altered by the cumulatedeffects along the wind flow. for intensitiesresulting only from solar ionizationanisotropies is for an observer on the downwind side of the sun, when he looks,for instance,in a plane normal to V 0. One of the Mar- Atoms which travel in the vicinity of the sun are submittedto iner 10 geometryof observationswas not far from this ideal the ionizing EUV radiation and corpuscularsolar fluxes.For case.This is alsothe reasonwhy we have selectedfor analysis hydrogen, the most important ionizing effect is charge ex- two periods of observationsobtained with Prognoz 6 Ly • changewith solar protons (about 80% of the whole loss pro- photometer,when the earth was very near the downwind axis. cessing), followed by EUV photoionization (•. < 91.2 nm) At variance with some other potential sources of emission (-• 20%), while charge exchangewith alpha particlesand ioni- anisotropyacting on the Ly • excitation rate of atoms (like a zation by collisions with the solar electrons have been esti- nonisotropic Ly • solar flux), the ionization anisotropy is mated of very minor importance l-Holzer, 1977-1.While pho- cumulatedalong the wind flow, as well as the resultingemis- toionization leads to the disparity of the neutrals,charge ex- sion anisotropy. One period of observation covered 15-40 changebetween a "slow" interstellar H atom (V0 is about 20 hours of data and will be referred to as an observation in the krn s-1) and a fast solar proton (_• 400 km s-•) producesa followingtext. slow new proton and a fast neutral. However, these new fast It would have been useful,as we will discussin section 3, to atoms do not contribute to the interplanetary Ly • emission have also a correspondingscan plane on the upwind side, to for the following two reasons.Because of their large radial make sure of the unique role of the solar wind by monitoring velocities,their density is much lower than the initial neutrals the contrastevolution along the flow. Unfortunately, we have (accordingto the velocity ratio, about 20 times less);but most no resultscollected when the earth was in the upwind region, important,owing to their large Doppler effectthey are only but despiteof this we could useother earth positionsand scan very weakly illuminated by the very far blue wing of the solar planesgiving upwind observations(see Figure 2) in order to Ly • line, which is lessthan 2% of the intensity at line center check that the gradual evolution described above was ef- for a velocityof 400km s-•. Thusthe resonantly scattered fectively contained in the results. interplanetaryLy • emissionrepresents practically exclusively Although the solar wind anisotropiesare revealedby photo- the slow neutral population of atoms which have not yet suf- metric measurementsand a correspondingphotometric model, feredionization. The flow of theseneutrals is progressivelythe advantageof the Prognozmeasurements is the conjunc- affected by the above mentioned loss processes,more and tion of two typesof diagnosticof the interplanetaryhydrogen, more along the "downwind" direction, (direction of the vector a spectralscanning with a hydrogenabsorption cell yielding Vo). In the theoretical case of a complete sphericalsymmetry the velocitydistributions of H atoms [Bertaux and Lallement, of the solar fluxes,the hydrogen density,the velocity, and the 1984] and a photometricmapping. The reason is that there emissivitydistributions also presenta symmetryof revolution are largeuncertainties on the determinationof the modelpa- around and axis containingthe sun and the velocityvector V0. rametersif only photometric results are used.As a matter of Thus departuresfrom this axial symmetrycan be attributed to fact, the general decreaseof the density along the flow (inde- anisotropies of solar properties. Some such departures have pendentlyof the anisotropies)is determinedat the sametime already been observedwith the Mariner 10 spectrophotometer by 4 parameters'the meanionization rate/•, the velocitymod- on Ly • results[Kumar and Broadfoot,1978, 1979;'Witt et al., ulus V0, the temperature T of the gas at "infinity," and the 1979, 1981] and have been interpretedby theseauthors as the dynamicaleffect of the Ly • radiation pressurerepresented by result of a decreasingsolar wind flux with higher ecliptic lati- the parameter#, the ratio of radiation pressureto gravitation. tude.The presentanalysis has led to similarconclusions, with A same Ly • emissionpattern may result from an infinite somewhatdifferent numerical determinations of the degree of numberof setsof parameters]•, V0,T, and# ascalculated for anisotropy. Improvements of this analysis in respectto Mar- instanceby R. Lallement and J. L. Bertaux (unpublished iner 10 resultsinclude a better statisticalaccuracy (larger data manuscript,1984). Up to now,at leasttwo of these parameters set) and a greater variety in the geometriesof observation, vderefixed a prioriwhen analyzing Ly • photometricmeasure- whichis quite importantas it will be seenbelow. Here the ments.However, it is of greatimportance when spatial vari- model takes into account the full velocity distribution, and ationsof the ionizationrate are detectedto usethe goodset of some parametersof the model were determinedfrom the hy- pFrametersin order to achievean accuratemeasurement of ß LALLEMENT ET AL.' SOLAR WIND AT HIGH LATITUDES 1415

Vw ),,,.1:.._151o

[ uP

EARTH'S

ORBIT•.( •)t•' SUN•"' 15-16/12/1977•:=85ø

•' ELIUM

ECLIPTICNORTH Z •ß • • YAXIS SPIN [DOWNWIND

•-- V s

Fig. 2. Five scan planesfrom the Prognoz 5 and 6 measurementsare shown along the earth's orbit. Each plane is perpendicular,tothe sun-earthline. The heliumcone has been represented and indicatesthe positionof the downwind direction.In a scanplane, the spin angle •b for a line of sight is countedfrom axis X oppositefrom the earth's velocity vectorVs. The date and eclipticlongitude is indicatedfor eachobservation. the anisotropy.The hydrogenabsorption cell associatedwith extrainterplanetarycontribution, (2) an initial inhomogeneous the Ly • photometerin the prognoz experimentacting like a flow of gas,(3) multiple scatteringinfluences, (4) anisotropy of high resolvingpower spectrometerallowed for the first time the illuminating solar Ly •, (5) anisotropy of the solar EUV an accuratedetermination of the dynamical parameters Vo,#, fluxes,and (6) anisotropy of the solar wind. These inconsist- and T [Lallement, 1983; J. L. Bertaux et al., unpublished encies are discussed below. manuscript, 1984] without using the measured intensities. 1. The resonancescattering of the solar Ly • line by the Thus the photometric pattern can entirely be devoted to the interplanetaryhydrogen has beenshown to constitutealmost study of ionizing effects,since there is no more ambiguity over the totality of the extraterrestrialLy • signal.An upper limit the parametersof the model. for the galactic or extragalacticcontribution has been given In the presentanalysis a seriesof positionsfor observations from a theoretical point of view [Thomas and Blamont, 1976] was used allowing both the detection of the largest contrast and has been experimentallydeduced [Bertaux et al., 1976; R. (downwindposition) and a comparisonof the contrastsfrom Lallement and J. L. Bertaux, unpublishedmanuscript, 1984]. the upwindto the downwindobserved regions. This last limit of 15 rayleigh is largely smaller than the ob- Comparisonof•Prognoz intensities with the isotropicmodel serveddiscrepancies of intensitiesbetween the best possible which dependsonly upon one parameter,the ionization rate sphericalmodel and the data.In addition,directions of en- /•, shows that there are, whatever the value of/•, systematic hancedemissivity coincide with high ecliptic(approximate he- (Figures 3a-3e and Figt•re 4) enhancementsof the intensity liographiclatitudes) and not with the galacticdisk at all. aroundthe north and southecliptic directions, which are near 2. Perturbations of the neutral part of the interstellar the directionsnormal to ¾o.Thus attributingthese departures mediumat the heliosphericboundary could introduce inho- to a nonsphericalionization effect, we constructeda noniso- mogeneitiesin the incominggas. However, sincethe distances tropic.model describedin section4. However, before dis- from the sun to the heliopauseare evaluatedto be of the order cussingthe resultsof thisnonisotropic model it is necessaryto of at least 100 AU, one would expect a spatial scale of vari- examine critically in the next section the other possibilities ations greater than the dimensionsof the volume which con- whichcould als o resultin somedepartures to the spherical tributes, in fact, to the detected emission. As a matter of fact, modeland comparetheir efficiencywith the nonisotropiCsolar owingto the r -2 radial decreaseof the solar Ly • incident wind explanation. flux, dimehsionsof the emitting volume are of the order of 5 (upwind side)to 10 (downwindside) AU. 3. CHARGE EXCHANGE WITH SOLAR PROTONS In addition, if inhomogeneities of the distribution exist AS A PRINCIPAL SOURCE OF DEPARTURES WITH beforepassing by the sun, their amplitudesshould be identical AXYSYMMETRIC HYDRoGEN EMISSIVITY DISTRIBUTION on the upwind and downwind side, and the model which is Whatis observedinitially when an isotropic(or spherical) presentedin the next sectionwhich takes into accounta grad- model of solar interaction is taken is a disagreementbetween ual apparition of anisotropiesshould not have allowed the measuredand predictedintensities. As possibleinconsistencies same good agreementfor both upwind and downwind direc- with the isotropic model hypothesiswe could propose (1) an tions. 1416 LALLEMENTET AL.' SOLARWIND AT HIGH LATITUDES o A 4-5. The most intricate problem is to disentanglethe role of solarEUV ionizationand Ly • excitationanisotropies from O• _LY-ALPHA( OFF ) thoseof the solar wind. The reasonfor treating togetherEUV and Ly • effects is that they should both induce correlated enhancementswith solar activity and spatial distributionscor- EARTH ECLIPTIC LONGITUDE 11 DEG o relatedwith the solar activeregions. In addition,in both cases o o the effect on one H atom is averagedover the full solar disk, which has a tendency to smooth out the solar irregularities. Thereare the followingthree distincteffects: 1. Independently of the hydrogen distribution, all the emissivitiesare proportional to the incident Ly • radiation o o intensityand thus departuresfrom the "isotropic"model can LF• be the "instantaneous"image of solar Ly • asphericities: longitudinal and latitudinal anisotropiesare both partially smoothedout by a full disk averaging. In fact, both longitudinaland latitudinal Ly • anisotropies o do affectthe intensitycurve corresponding to a full scanplane. o o The differentlines of sight startingfrom the point of observation contain atoms which are illuminated by different solar disks centeredon different latitudes and longitudes.Let us assumethat the intensity enhancementin north and south eclipticdirections for one observationis due to longitudinal o anisotropies:i.e., the sun's surface does present a great o LF• amount of sunspotsexactly in front of the earth and lower emissionsat the other longitudes.All the atoms of the north and south lines of sight will benefit by this enhancementin emission,since they have the same longitude. Along the two oppositeecliptic lines of sight the far atoms are facing a less o , ., I , , I , , I , , I , ,ANiG L,E ,P HI I emissive disk. 0 6 ¸ 120 180 240 300 360 There would be two consequencesin this case.First, for Fig. 3a someobservations the earth would face low brightnesssolar Fig, 3. Measurementsof the interplanetary Lyman ß emission (points) are representedas a function of spin angle ½ for the five observationsat differentecliptic longitudes and are comparedto two differentmodels of the solarwind effect.For the sakeof clarity,data pointsare plotted twice on each figure. The ordinate scale incounts _ LY-ALPHA( OFF ) per secondis valid for the lower set of data points compared to a model assuming an isotropic solar wind (A = 0). The same data points are displacedupward by 500 counts/sand are comparedto a model including anisotropyof the solar wind characterizedby the EARTH ECLIPTIC LONGITUDE 29 DEG parameterA = 0.40.The north and south ecliptic directions areindi- cated by the letters N and S. The introduction of an anisotropic ionizationrate characterizedby 40% of relativediminution from the equatorial to polar regions allows the best fit of the directions of maximumintensity for each observationand the overallshape of the curves as a function of ½. In particular, for observationsnear the downwindaxis (,;te = 58 and 85ø, Figures3c and 3d), the isotropic modelshows only one maximum and one minimum, whereas both dataand the anisotropic model show two maxima. In respectto the isotropicmodel the data points show more Lyman a emissiontoward the ecliptic pole, showingthat the solar wind was less effectiveto destroyH atoms at high eclipticlatitudes. For observationsS• to S4 (Figures 3a-3d) the calibration factor for Prognoz 6 measurements wasestimated tobe 3.7 counts s-•rayleigh-•, asdetermined froma UV star observation.According to this value the adjustmentof the modeltothe data is consistent forthe four observations andcorre- spondsto a densityof infinityn oo -• 0.04atom cm-3. The calibration factorfor the last observationsS5 is about2.1 countsrayleigh -• (Prognoz 5).

3. The presentmodel has been constructedusing the as- sumptionof an opticallythin medium.If the importanceof the multiple scatteringeffects is not negligible,as it was calculated by Keller et al. [1981], it doesnot affect the axial symmetryof the emissivity and thus it cannot be invoked as a possible explanation of the discrepancies.However, it appears neces- sary to discussthe results which will be obtained with the O • I • • I • • I , • I , I^•1 GLIE •P 60 120 180 240 300 360 optically thin approximation from that point of view (seelast section). Fig. 3b LALLEMENTET AL.' SOLARWIND AT HIGH LATITUDES 1417 disk, inducinglower intensitiesfor north and south directions. This is not at all observed. Second, the solar rotation would Y-ALPHA(OFF ) inducestrong fluctuations of the mean intensitylevel from one observation to the other. Such fluctuations are not observed.

The systematiceffect measuredof enhancementof the signal EARTH ECLIPTIC LONGITUDE 85 DEG for north and south ecliptic direction could, however, better correspondto latitudinal variations of the solar Ly • flux. However, that should mean a greater luminosity of the solar disk as viewed from an atom over the polar regionscompared to an atom traveling near the ecliptic plane. Also, that is exactly contrary to what is expected from observations of solar active regions,which appear generally at low and mid- A-O.[--.0 latitudes.Thus we can no longer invoke a latitude effect of the resonanceexcitation Ly • flux. 2. The Ly • radiation pressureacting on the atoms influ- encesthe bending of the trajectories. The present model as- sumesa permanent and stationary flow of gas, and thus a constant Ly • intensity characterized by the parameters # o (ratio of radiation pressureforce to gravitational force). Spa- tial and temporal variations of the Ly • flux lead to departures from the model trajectories. However, owing to the re- spectivetime scalesof the inflow and of the solar rotation (the mean velocity for an H atom is of 0.35 AU/month), the dynamical effect of solar longitudinal anisotropies is time-

_ averaged.As for latitude variations, significant effectswould have also introduced deviations from the model in the velocity _ distributionsat high latitude, which shouldhave been detected _ N S by the spectralstudy with the H absorptioncell, but they were not [Bertaux et al., 1984]. In fact, if we were able to find a o - ^NiG LiE •P H I good agreementwith the spectralmodeling for one constant 0 60 120 180 240 300 360 value of the parameter #, it suggeststhat the flow was not too Fig. 3d

far from being stationary and that there was no strong vari- _ LY-ALPHA( OFF ) ations of the solar Ly • flux during the one-year observation period. 3. The solar EUV flux, ionizing H atoms at 2 _<91.2 nm, EARTH ECLIPTIC LONGITUDE 58 DEG. is responsiblefor a minor contribution of the ionization pro- -- cesses.Only latitudinal variations have to be taken into account, since longitudinal effects are largely time averaged. If -- we assume a same pattern for variations of Ly • and EUV

-- solar fluxes, then we can remark that the effects(1) and (3) are

_ reversed, since a larger ionization rate in the near ecliptic regions reducesthe density, whereas a greater incident Ly • o - ^-0./-.0 o flux in the sameregion enhancesthe emissivity. L;") Even by assumingvery important differencesfor EUV photoionization rate with varying latitude, it appearsquite impossible to explain the observations with such variations. Changesof the order of 40% for the densitiesare causedby o changesof the same order of magnitude for the total ionizing o rates. Since the EUV ionizations rate representsonly 20% of the total ionization [Holzer, 1977], it would imply anisotropies of a factor of three at least of the photoionization rate, after full disk averaging,which is impossible. At this point of the discussionit is apparent that the aniso- o tropiesin the solar Ly • flux cannot have the main role in the o L;") observed effects. In addition, what is peculiar and fortunate in the interpretation of the results is that the Prognoz measurementshave been recorded during a minimum solar activity period. In 1976-1977 the Zurich sunspot number Rz did not exceed 25. I • , I , , I , , I , ,ANiGL,E,PHiI Thus there are only scarce and weak sunspots to produce 60 120 180 2/-.0 300 360 strong latitude effects of the Ly • and EUV flux. A recent Fig. 3c study of Cook et al. [1981], who have used statistical fre- 1418 LALLEMENTET AL.' SOLARWIND AT HIGH LATITUDES

tons as a result of both lower fluxes and lower cross sections

Y-ALPHA ( OFF ) [Feldmanet al., 1977; Fire et al., 1960]. Ionization by impact with a solar electronhas been calculatedby Holzer [1977], who found0.19 x 10-? s- • at 1 AU, whichis much lessthan the proton chargetransfer rate of the order of 5 x 10- ? s-x. EARTH ECLIPTIC LO•!GITUDE 151 DEG Unlike the other processes,the electron collisionalionization rate cannot be considered as a r -2 function of the radial heliocentric distance in the range of distancesused in the model and decreasesmore rapidly. We are neglectingthis fact in the modeling, assumingthis is a minor effect; however, shouldalso be noted that a variation of the mean velocityof o o the solar protons(such as the one alreadyobserved), indepen- L;') dently of the plasma flux value, implies a variation of the chargeexchange cross section (as will be discussedin the last section)and thus also a variation of the ionization rate. A=0.40 0 4. PROGNOZ RESULTS AND ANISOTROPIC 0 SOLAR WIND MODELING 0 Only the Ly • intensity measurementsobtained with the Lyman • photometerflown on Prognoz 5 and 6 in 1976 and

0 0 L;') _ LY-ALPHA( OFF )

•J S A=O EA'RTH ECLIPTIC LO•!GITUDE 11 DEG o I 0 0 6o 120 180 24o 300 360 ...... TD= 10E +6 Fig. 3e TD:2•lOE+6 TD=3•lOE+6 $ quenciesof location and contrastof solar activeregions, esti- mated at only a few percentthe decreaseof the disk averaged Ly • flux seenfrom an eclipticpoint of observationto a polar one for low solar activity periods(R: < 80). Thereforewe will neglectthis type of solar anisotropiesin the following model. o This means that we will use the stationary model with a o sphericalLy • incident flux. In addition, we will considerthat the photoionization rate is independentof the latitude. This rate has been evaluated by Holzer [1977] to be about 0.9 x 10- ? s- x at 1 AU for periodsof low activity.

6. The ionization by the solar wind is a directional effect: o at a given point of the solar systemthe solar wind ionization is determined by the local solar wind properties,related througha tube of plasmatrajectories, to a small portion of the solar surface,whereas photoionization is determinedby a full solar disk. Spatial relative variations of the solar wind fluxes are fol- A•!GL,EPHil lowed by relative variations of the solar wind ionization rate 0 60 120 180 240 300 360 of the sameorder, which is also true for the global ionization Fig. 4. Influence of the ionization rate value illustrated as an ex- rate, sincethe solar wind ionization is the most important loss ample for S•. While the relative decreaseof the ionization rate from process.From the three following ionizing effectsof the solar equatorto solar poleshas been fixed at a constantvalue (A = 0.40), wind here the varyingparameter is the ionizationrate in the solar equatorial plane /•E. The three correspondinglifetimes at 1 AU in that H + H+-•H + +H plane,TD, are indicated(T D = flE-•). The threecurves have been nor- malizedto measurementsfor the downwinddirection qb •_ 180ø, which H+He ++-•H + +He + meansthey would correspondto differentvalues of the densityat infinity.It is clearthat the measuredupwind to downwindintensity H + e---}H + + 2e- ratio precludesthe extreme values used here for the ionization life- time. However, it can be remarked that the direction of maximum chargeexchange between solar protons and atomichydrogen intensityhas been displacedfrom qb= 0ø (isotropicmodel) to the is the main effect[Holzer, 1977]. samevalue (½ _• 60ø),whatever the value of Ta. Thereforethe direc- tionof maximumintensity is onlyrelated to theparameter A andis a The chargetransfer with alpha particleshas been evaluated goodindicator for thisone. This is particularlytrue for seancesS• to to be about 200 timesweaker than chargetransfer with pro- S3, which are on the sideof the wind flow. LALLEMENT ET AL.' SOLAR WIND AT HIGH LATITUDES 1419

1977 are presentedhere. Spectralprofile studiesof the H velocity field with the absorptioncell were used to determine . L Y- ALPHA ( OFF ) someparameters of the model (V,•, T, and #). (J. L. Bertaux et - al., unpublishedobservations, 1984). A descriptionof the geometry has also beengiven in this paper.The five scanpla•nes, P• to P s, perpendicularto the sunearthline and the corre- spondingpositions of the earth are shownin Figure 2 (letters S• to S5).Two of them (P3 and P2) are very near the downwind position where the strongestanisotropic effects are expected;two others(P• and P s) are in the lateral positionsin respectto the sunwind axis, so that the scan planes contain upwindlooking directionswhere the ionizationeffects are still very weak and the last position(P2) is an intermediateone. In everyscan plan the measuredintensity is plottedin countsper secondas a function of the spin angle ½ (Figure 2) such de- ft-nedthat the direction ½ = 0ø is in the ecliptic plane, in the sensewhich is oppositeto the earth'sorbital motion; ½ = 90ø and 270ø are the north eclipticand south ecliptic directions, respectively. A=O. 30 Deficienciesof data are due either to starlight contami- A=O.dO nation in the field of view or to a too important geocoronal A =0.50 contamination. The whole set of data were corrected from the exospherichydrogen Ly ß contributionaccording to a method explainedby J. L. Bertaux et al. (unpublishedmanuscript, 1984).Here we do not usethe directionsof sight for which the relative contribution of the terrestrial Ly ß as deducedfrom _ EARTH ECLIPTIC LONGITUDE 58 DEG the method was larger than 10% of the total measuredintensity geocoronalplus interplanetary).In this casethe maximum - ANGL,E,PHI 0 60 120 180 240 300 360 relative error on the interplanetaryLy • emissionintensity resultingfrom the geocoronalcontamination is evaluated at Fig. 5. Influenceof the anisotropyA coefficient,which represents 2-3%. the relative variation of the ionization rate when the hellographic latitude is changingfrom 0 ø to 90ø. The equatorial ionization rate has Basically,the presentanisotropic model is derived from the beenfixed at /•E= 5 x 10-7 s-• (To _• 2 x 106s). The contrastbe- numericalmodel which was constructedfor line profile calcu- tween the north or south ecliptic directions(½ = 90ø and ½ = 270ø) lations (R. Lallement and J. L. Bertaux, unpublishedmanu- and the eclipticdirections (½ = 0ø or 180ø) is increasingwith increas- script,1984). The only differenceis in the calculationof the ing A. The three models correspondto the same density at infinity, extinctionfactors along eachindividual trajectory. which means that for the ecliptic directions the intensity is not very sensitiveto the value of/• at high heliographic latitudes once the We first describebriefly the isotropicmodel. At infinity the equatorialvalue is fixed. velocitydistribution of H atomsis describedby a bulk velocity Vo and a Maxwell-Boltzmanndistribution at temperature T. Atoms are submittedto the apparent solar gravity, pro- was mentioned earlier, there are enhancements of the emissiv- portional to 1 -#. The valuesof the above parametershave ity in both the north and south eclipticdirections which sug- beendetermined to be (J. L. Bertaux et al., un,publishedmanu- gestthat the solar wind axis is not far from being normal to script,1984): the ecliptic plane. As it is known, the sun spin axis is only slightly tilted from the normal to ecliptic. From a physical T--(8,000 ø _+500ø)K point of view it is likely that the solar axis of rotation will Vo= (20 _+1) km s- x have a specialrole. However, the propertiesof the solar wind are supposed to be strongly related to the solar magnetic # = (0.75 + 0.•0) features and could as a consequencepresent a symmetry and the direction of ¾o has been found identical to the corre- around an instantaneousmagnetic axis. Such an axis cotfid be spondingvalue for the flow of interstellarhelium [Dalaudier et significantlytilted from the solar rotation axis [Hundhausen, al., 1984]' ecliptic longitude(74.5 ø + 3ø) and ecliptic latitude 1978]. However, since the loss processesof the hydrogen are (-6 ø + 3ø) in eclipticcoordinates. time-averagedover severalsolar rotations,what is observedis The wind flow vector ¾o is oppositeto V•, which describes the effectof the mean spatial propertiesof the solar wind, also the motion of the sun in relation to the local interstellar averagedover at least a whole solar rotation. In the caseof a medium. tilted symmetry axis, it describesa cone which axis is the In order to computethe densityand the emissivitythe rele- rotation axis, and the final result is a symmetryof the ioniza- vant quantities obtained for the caseof a monocineticmodel tion effectsaround this rotation axis. The exact relative posi- (cold model) are integratedover the velocity spaceat infinity. tions of the equatorial plane (or of the rotation axis) and the A nonisotropic scatteringfunction is introduced for Ly 0• eclipticplane as well as the 6ø inclination of the wind velocity [Brandt and Chamberlain,1959], and the solar Ly 0• spectral vector ¾won the eclipticplane were taken into accountin the profile is assumedto be flat at its center,in the wavelength present model. The correspondinggeometry is shown on rangeof interest. Figure 7. The present anisotropicmodel assumesan axial symmetry The whole ionization rate including photoionization and of the solar wind. The choice of the axis is now discussed. As it solarwind effectsis assumedto havea r-2 radialdependence 1420 LALLEMENT ET AL.' SOLAR WIND AT HIGH LATITUDES

ß- Charge exchange scribedby their ecliptic longitude: 11ø, 29ø, 58ø, 85ø, and 151ø •> ionization rate "- for observationsSz to S5,respectively. • 1 (x 10-7 ) s- 1 4 .0 In each figure the intensity measurements(data points) are representedas a function of spin angle &. They are plotted • 5 twice, with a vertical shift. On the bottom curve they are 3 compared to a model assuming isotropy of the solar wind, whereason the top curve they are compared with our anisotropic model characterizedby A = 0.40. .--•> 5 - H+H+-'" H++H {3. • _ (H+ He++---H•'+ He+) 1 5 : > The letters N and S indicate directions toward north and

•- _ south ecliptic, respectively.The data differ from the isotropic .o_ _ model in three respects:(1) there is more intensity in the data • Euv+ H •- Hy ---H4'.e- •-, _ toward the north and south ecliptic region; (2) the maximum I--o [3e.• I H+e---H+ + 2 e •-I intensityof the model is found at & = 0ø for observationsSz 45 90 to S3, (Figures 3a-3c), whereas the data maximum is found Heliographiclatitude (o) around & = 60ø; (3) for observations S3 and S½ near the Fig. 6. Hcliographic dependenceof the global ionization rate (at downwind direction(Figures 3c and 3d) there are two maxima 1 AU) which correspondsto the adjustment presentedin Figures 3-5. instead of one in the model. This is the simpletrigonometric function /• = 5.0 [1- 0.4 sin2 4] Even in Figure 3e, where there are only subtle differences 10-? s-•. If one neglectsthe latitudinal variationsof the electron collisional ionization rate (which is here a very minor effect) and betweenthe two models,the anisotropicmodel fits still better assumes also a constant photoionization [Holzer, 1977], which is with the data. quite reasonablefor a low solar activity [Cook et al., 1981], it pro- The departure betweenthe two modelsis more pronounced vides a relative decreaseof about 50% for the chargeexchange rate. for observationsnear the downwind axis (Figures 3c and 3d), Since the charge transfer with alpha particles is about 200 times weaker than for the protons, the rate variation can be interpreted in as is expectedfrom the cumulative effect of the anisotropyas terms of a proton flux variation, provided that a heliographic vari- explainedon Figure 1. ation of the mean solar wind velocity is assumed.Results are present- In Figure 4 the data of observationSz (which probesboth ed for a constantsolar wind velocityV = 400 km s- • (right scale). the upwind and the downwind regions)are comparedto three anisotropicmodels corresponding all to the sameparameter A but with different values of/1o- The contrast for intensity be- and to be related to the heliographiclatitude/t so that tween the downwind and upwind directionsis stronglydepen- fl(r, •)= flor- 2(1- A sin2 •) dentof/1o- On the figurethe lifetimeT a = flo- 1is indicated. However, the position of the maximum is insensitiveto where/1o representsthe ionization rate at 1 AU from the sun and is only related to the value of A. in the solar equatorial plane, r is the radial distance in AU In Figure 5 the data of observationsS3 (downwind plane units, and A is a parameter (0 < A < 1) describingthe degree nearly perpendicularto Vo) are comparedto three anisotropic of anisotropy.The/1o and the A are the two parameterswhich models with the same value of/1o and different values of the we proposeto derive from a comparisonwith the data. coefficient A. Calculationsof the extinction factor at a fixed point M for A visualestimate for Figures4 and 5 togetherwith a similar every trajectory followed by an atom from "infinity" to M are exerciseon the three other observations(not shown here) presentedin the appendix. allows the determinationof the possiblerange of valuesfor The use of a trigonometricfunction for the latitude depen- the parameters/1oand A quoted above. dencewas first employedby Kumar and Broadfoot[1978] and presentsthe advantagethat it can be analyticallyintegrated 5. DISCUSSION along one trajectory(see the appendix). For our Prognoz measurementsthe determination of Without any doubt, our measurementsprovide a clear con- and A was made by a visual adjustment of the model to the firmation of the phenomenonwhich was already observedby data simultaneously for the five observations.Generally Kumar and Broadfoot[1978] from 1974 Mariner 10 data. The speaking,changes in /1o correspondto a global increaseor solar wind anisotropiesare far from being a minor effect, at decreaseof the ionization rate in all space and thus to a least at the time of the two observations. The reasons for variation of the negativegradient of H densityalong the flow which we were able to presenta better adjustmentof model to from upwind to downwind, whereas the parameter A mea- data are now briefly discussed. suresthe asphericity.One could think of two distinct adjust- From an experimentalpoint of view the Prognoz resultsare ments.In fact, owing to the great velocitydispersion, filling in characterizedby a much better statisticalaccuracy and by a of the downwind region comesfrom atoms which have trav- more detailedangular scanning (each point plotted on Figures eled at all heliographic latitudes, whatever the latitude of the 3a-3e for a bin of A& = 2ø correspondsto an averageof at observed direction. least 25 measurements,with the averagecounting rate being A good agreementcould be finally found betweenthe model -• 600 counts/measurementyielding a 1% statisticalaccuracy). and the data simultaneouslyfor the five observations,as can Concerning the geometry of the observations,it should be be seenin Figures 3a-3e. The final result is/1o = (5.0 _ 0.50) notedthat the three Mariner 10 angularscans were perfectly 10-7 s-1 and A = 0.40 _+0.10, which meansa 30-50% de- chosento provide the most important departuresfrom the creasein the ionization rate over the solar pole in comparison isotropicmodel, sincethey were performedwhen Mariner 10 with the equatorial plane. wasin the downwindregion. In fact, they correspondto our Figures3a-3e correspondto observationsS1 to S5, respec- P3 and P4 seances.As we have alreadyexplained, for a more tively, where the distributionof Ly 0•intensity was recordedin convincingstudy we decided to use also additional observa- planes perpendicularto the earth-sun line at positions de- tionsmade from as most as possibleupwind positions,where LALLEMENTET AL..'SOLAR WIND AT HIGH LATITUDES 1421 differences between data and isotropic model should be smaller,and they are indeed. Concerning the modeling, we have constructed a much more realisticmodel than thoseused by the two groupswhich analyzed independently the same Mariner 10 data !'Kumar and Broadfoot, 1978, 1979; Wittet al., 1979]. Instead of a quasi-monocineticmodel (they assumedthat all atoms were flowing at the same velocity)we took completelyinto account Fig. 8. Example of the two trajectories which lead to a given the thermal velocity dispersionat infinity, and this changes point P for a given velocity vector V = V o + V r if V o is the bulk substantiallythe density results.We also used a more appro- velocity and V r the thermic velocity. The loss factor at point P de- priate geometry(by using an axial symmetryaround the true pends on the evolution of both the distance to sun r and the helio- solar rotation axis instead of the ecliptic axis and introducing graphic latitude •l along the trajectory (as indicated for a current pointM). The latitudei canbe calculatedin termsof the threeangles the small declination of the interstellar wind). In addition, we T, r/, and y. used a realistic set of dynamical parameters (T, Vw, and #) sincethey were determinedfrom our spectralstudy (J. L. Bert- aux et al., unpublishedmanuscript, 1984), whereasthey were indexes of solar activity) then the EUV ionization rate is only estimatedin the Mariner 10 interpretation. almost constant with varying latitude (also within 3%). Thus From the Mariner 10 data the degree of anisotropy A was the latitudinal changesof the ionization are almost entirely found to be A •-0.5 by Kumar and Broadfoot [1978, 1979] due to the charge exchangewith solar wind protons. For low and A •_ 0.30 by Witt et al. [1979, 1981], from the sameset of solar activity the EUV photoionization has been estimatedto data. It appearsthat the degreeof anisotropyA is quite com- representabout 12% of the total loss process[Holzer, 1977] parable for the Mariner 10 and for the Prognoz periods of but for an estimatedglobal rate of about 7.5 10-7 s-• at 1 observations(with the restriction however that the determi- AU. In fact,we findhere a smallerglobal rate of about5 10-7 nation with a cold model is approximatein the case of Mar- s in the equatorial plane. If we assumethe same photoioniza- iner 10). Sincethe hydrogendistribution observedby Mariner tion rate, it thus represents22% of the whole rate. However, there is the small contribution to losses of the collisions with 10 in 1974 is the result of the anterior ionization back to 1973, and since the last observations by Prognoz were made in solar electrons.It is not important to neglect possibleaniso- December 1977, we concludewith the persistenceof the large tropies of this effect. Thus together with the photoionization degree of anisotropy during the whole period 1973-1977, effect we will consider that there is an isotropic part of the ionization effect with a 10-25% contribution to the whole which correspondsin fact to the 5 years of minimal solar rate. activity for the last solar cycle.Thus the anisotropicpattern is far from being an occasionalphenomenon. As a consequence,assuming as was discussedabove the What is really measuredis the decreaseof the total ioniza- independenceof the photoionization rate with latitude, we tion rate, essentially EUV photoionization and charge ex- find that a 40% decreaseof the global rate correspondsto changewith the solar wind protons. We will now briefly dis- 44-53% of latitudinaldecrease of the chargeexchange rate. cussthe implicationsof sucha latitudinal decreasein terms of The charge exchangerate is proportional to the quantity solar wind mass flux. a(V•)Fp,where a is the chargeexchange cross section, and As it was shownby the Cooket al. [1981] calculations,for a Fp= %V• is the protonflux at oneAU (% is the densityand low solar activity period the solar line center Ly 0• flux at a V• is the velocityof the protonsrelative to the neutralhy- givenpoint, averagedover the full disk, is constantwithin a drogen).Since the solarproton velocity (400 krn s-•) is much few percentwhen changingthe latitude of the point. As we largerthan the hydrogenvelocity (_• 20 km s-•), we can con- have said, all the atomic hydrogenwhich contributesto the sider that the relative velocity is the solar wind velocity. These Prognozsignal has beensubmitted to the ionizinginfluence of expressionsare related to the fact that we have assumeda a low activity sun [1975-1977]. For this period the sunspot radialdecrease as r-2 of the ionizingrates, implied by a r-2 numberwas lessthan 25, and an evaluationusing the Cook et decreaseof the density and a constant velocity of the protons al. resultsgives a relative variation for the fluxes within and {heliocentricdistances of more than 0.5 AU are only of impor- aboveecliptic plane of about 3% at line centerand -• 2% for tance here). the integrated line. If we consider that the solar EUV is The velocity dependenceof the charge exchangecross sec- stronglyrelated to the Ly 0•(they are both correlatedwith the tion has been measuredby Fite et al. [1960] for the range of proton energy corresponding to the solar wind. Indeed the velocity dependenceis such that the observedvelocity variationsof the solar protonswith latitude are of importance.As a consequence,the conclusionsabout the latitudinal decrease / / of the proton flux correspondingto a given latitudinal de-

.. creaseof charge exchangeionization strongly depend on the assumedvelocity field. From the work by Fite et al. [1960] it can be calculatedthat changesof the mean velocity from 400 km s-• at low latitudesto 800 km s-• at highlatitudes correspond to a decreaseof 25% of the charge exchange cross section. and the solar equatorial plane. By coincidence,the direction of the In this extreme case the observed decrease described by relativevelocity vector of the hydrogenflow with respectto the solar systemhas almost exactly the sameecliptic longitude as the ascending A = 0.40 correspondsto a decreaseof the mean corpuscular node of the solar equatorial plane. The used values are f• = 75ø, proton flux of 25-35% from latitude zero to above the poles. i = 7.2ø, and D = 6 ø. On the contrary, for a sphericalvelocity field the decreaseof 1422 LALLEMENT ET AL.: SOLAR WIND AT HIGH LATITUDES

the ionization rate is directly proportional to the flux decrease Vw(2w = 254¸) has almost exactlythe samedirection than the (about 50%). ascendingnode (Figure 7). The adjusted heliographicdependence of the total ioniza- For a given point P and a given initial velocity vector "at tion rate, the evaluated charge exchangerate, and the corre- infinity," V, there are two trajectoriesfor the H atoms. Along spondingprotons fluxes in the particular case of a spherical each of thesethe decreaseof the densityinside the correspond- velocity field are shown in Figure 6. ing tube of trajectoriesis integratedto yield the "lossfactor" An important point is that the Ly 0•mapping allows us to at point P. determine, even with an uncalibrated instrument, the absolute During the time interval dt, if the densityis n and the local value of the mean ionization rate, and the absolute value of ionization rate/•, then the density variation is dn = -n/•(t) dt the solar wind flux can be derived. From the measured ratio of and the lossfactor (the ratio of the densityto the initial one) is intensitiesin the upwind and downwind directionsthe equa- L = exp(-•oo v fl(t)dt). torial mean total ionization rate was found to be (5 + 0.5) By using as new variable, instead of the time t, the angular x 10-7 s- x whenour singlescattering model is used.Taking parameter 0, whoseorigin is the asymptoticdirection through into account a 10-25% contribution of the photo and electron the relation of conservationof angular momentum, it follows: collisionalionization rates we derive a chargeexchange rate of (3.3- 5)x 10-7 s-•, correspondingfor a mean velocityof 400 km s- x to a mean equatorialproton flux of (1.6- 2.4) L= exp ( fiø--r20!•p•v ) x 108 cm-2 s-:, whichis substantiallylower than the cur- wherePi is the impact Parameter,V the velocityat infinity, rentlyadmitted average values of 3.3 x 108protons cm-: s-• r0 = 1 AU,/•0 the ionizationrate at 1 AU, and 0i (i = 1, 2) the [Feldrnan et al., 1977; Hundhausenet al., 1970]. angularparameter at point M for the two typesof trajectory. However, multiple scattering will affect the upwind- If T is the angle betweenV and the radial direction SP, then downwind intensity ratio. Keller et al. [1981] computed that 01 -- •- T (direct trajectory)and 02 -- • + T (indirect trajec- for a densityat infinityof 0.05 cm-3 very near the Prognoz tory). value), multiple scatteringeffects decrease this ratio by about For the anisotropic model the ionization rate at a current 30% in respectto the singlescattering approximation. point M (Figure 8) along the trajectory is a function of the By using this result and our single scatteringmodel we can radial distance r and of the heliographic latitude 2 of M evaluate that the charge exchangerate could be at maximum •(M) = flor- 2(1-- A sin2 •). Thus underestimatedby about 50%. This leads to an equatorial protonflux of (2.4 - 3.6) x 108cm- 2 s- •, nowconsistent with L= exp _ (ro2floOi• the "classical"estimate [Schwenn, 1982]. However, from the •, PiV /l exprø•øApiV foø'sin 2 $ dO present point of view of the solar wind anisotropiesit is im- The first term is the lossfactor for the isotropicmodel L0. If portant to notice that these correctionsdo not change much • is the angle betweenthe orbital plane of the H atom and the the value of the coefficientA. In fact, since the multiple scat- equatorial solar plane (which intersect along the vector u), tering effectstend to "smooth out" the differencesbetween the then geometrical considerationsgive sin •. = sin • sin (r/+ 0), intensities in different directions corresponding to different whereM is the angle betweenV and U. densities,taking into account the multiple scatteringeffects After integration along the trajectorieswe obtain would lead to a value of A only slightly greater than the present one. L½1,2)= Lo exp (fi___o0/A sin2•, ro 2 In conclusion,it has been shown that the interplanetary Ly k,pi V 2 0• mapping provides a good diagnosticfor the out-of-ecliptic large-scale properties of the solar wind. This becomes especially true now, since the interstellar wind parameters are +(2) 20i better known and the modeling is more realistic.Of course,it The sign is changingaccording to the type (1) or (2) of trajec- would be of great interest to have such a monitoring of the tory. solar wind anisotropiesfor a full solar cycle and eventually to In order to compute the density at any given point in the search for a correlation of these low polar fluxes with the solar system the full velocity distribution due to thermal dis- presenceof polar coronal holes.Detailed studiesof suchpolar persion was integrated (classical"hot" model, modified here low density holes,as was made by Munro and Jackson[1977], for latitudinal variation of the ionization rate). are of great interest and could be consideredagain with the present new value of the solar wind flux at high latitudes as an Acknowledgments.This experiment was a cooperative effort be- upper boundary condition to their model of solar wind ex- tween the Laboratory of Ultraviolet Astronomy at the Space Re- pansion in the corona. searchInstitute (IKI) of the Academy of Scienceof USSR in Moscow Finally, it should be noted that the low-ecliptic inclination and Serviced'A6ronomie du CNRS in France. We are particularly of the interstellar wind, which is totally fortuitous, is quite grateful to H. Mironova, F. Semelin,and J. F. Brun for their long dedicationto this experiment,and to J. C. Vial for usefuldiscussions. favorable to such a study. If the wind were coming from a This work was supported by Centre National d'Etudes Spaciales high-latitude region the solar wind anisotropywould be much (France) under contractsCNES 83-201 and 84-201. less clearly "printed" on the distribution of atomic hydrogen The Editor thanks P. W. Blum and another referee for their assist- in the solar system. ancein evaluatingthis paper.

APPENDIX REFERENCES The following approached values for defining the position Bertaux,J. L., and R. Lallement, Analysisof interplanetaryLyrnan- of the solar equatorial plane have been used' f• = 75.0¸ (eclip- alpha line profile with a hydrogenabsorption cell' Theory of the tic longitude of the ascendingnode) and i = 7.2¸ (inclination Dopplerangular spectral scanning method', Astron. Astrophys., 140, on the ecliptic). The ecliptic component of the wind velocity 230, 1984. LALLEMENTET AL..'SOLAR WIND AT HIGH LATITUDES 1423

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