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GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO. 7, PAGES 963-966, APRIL 1, 1998 Magnetic cloudfield intensitiesand solar wind velocities W. D. Gonzalez, A. L. (",Idade Go•zalez, A. Da.1La,go Institute Nacional de PesquisasEspaciais,She Jos6dos Campos, SP, Brasil B. T. Tsurutani, .1. K. Arballo, G. K. La,klfina,B. Butt, C. M. SpacePla.smaPhysics,Jet PropulsionLaboratory,C,MiforniaInstitute of Technology,Pasadena, S.-T. Wu (',enterfor SpacePlasmaand AeronomicResearch,and Departmentof Mechanicaland Aerospace Engineering,The 11niversity of Alabamain Huntsville,Huntsville Abstract. For the sets of magnetic clo•ds studied in this work [Dungcy,1961]. In the aboveexpression, v is the so- we have shown the existenceof a relationship between lar wind velocity and Bs is the southward component their peak magneticfield strengthand peak velocity of the interplanetarymagneticfield (IMF). Gonzalez values,with a clear tendencythat, cloudswhich move and Tsurutani [1987]haveestablishedempiricallythat at,higherspeedsalsopossess highercoremagneticfield the interplanetary electric field must be greater than 5 strengths.This resultsuggests a possibleintrinsicprop- mV/m for longerthan 3 hoursto createa Dst _• --100 erty of magneticcloudsand alsoimpliesa geophysical nT magnetic storm. This correspondsto a southward consequence. The relativelylow field strengthsat low field componentlarger than 12.5 nT for a solar wind velocitiesis pres•mablythe causeof the lackof intense speedof • 400 km/s. Although the positive correlation })el,weenfast (',MEs stormsduringlow speede.jecta. There is alsoan indication that, this type of behavior is peculiar for mag- and magnetic storms have been stressedand is reasonnetic clouds, whereas other types of non cloud-driver ably well understood, little attention has been paid to gaseventsdo not,seemto showa similarrelationship, the opposite question, why don't, slow CMEs lead to at least,for the data studied in this paper. We suggest rnagnetic storms? Since this question involvesthe role that, a field/speedrelationshipfor magneticclouds,as of the magnetic field of the CME, it, leads to a more that obtained in our presentstudy, could be associated generalquestion,namely how are the CME's speedand with the cloud release and acceleration mechanism a.t magnetic field related to each other? These are the the sun. questionswe wish l,o addressin this letter. Sincefor magneticcloudsthe total field tyically has a If the speed of the solar ejecta is less than the upsubstantialsouthwardcomponent,B•, our resultsin,ply stream slow solar wind plus the magnetosonicwave's that the interplanetary dawn-duskelectric field, given phasevelocity, a shockwill not forin at, the leading (anby v x Bs (wherev is the cloud'svelocity),is enhanced tisunward) edge of the e;jecta, and there will not be by both factors. Therefore, the consequentmagneto- compressed sheathfields. ttowever,one might ask "why sphericenergization(that is governedby this electric can't the ejecta t,hemselveshave fields intense enough field) becomesmore efficientfor the occurrenceof mag- to create magnetic storms with intensitiesDst < -100 netic storms. nT7". A•nong the studied relationshipof e.jecta with magneticstorms,practically all intensest,orms were asIntroduction It has been well establishedthat major (D.,t _( -100 nT) magnetic storms are associatedwith fast,Coronal Mass Ejections((•MEs) comingfrom the sun from both socia. ted with magneticclouds[ e.g. Burlagactal., 1987; Tsurutani et al., 1988] that followedshocks,although Tsurutani ctal. [1988]claimedthat the intense storm of September18 1979, wasassociatedwith a magnetic cloud without a shock (with an average speed around370 kin/s). This latter type of associationapduringsolarmaximum[Tsurulaniel al., 1988, Gosling pears to be uncommon. et al. 1991, Gonzalezet al., 1994] and the descending phaseof the solar cycle[Tsur•tani ctal., 1995]. The energytransfer mechanisnifroni the solar wind to the Method of Data Analysis. magnetosphere appearsto be magneticreconnectionbeTo address this issue first we use magnetic cloud tween the interplanetary magneticfield and the earth's magneticfield. The interplanetary dawn-duskelectric events.Magneticclouds[e.g. Burla•a, 1995]are a type field, which is given by v x B•, governs this process of ejecta that have a mesoscaledimensionof about 0.25 AU at 1 AU and crossesthe spacecrafttypically in about 24 hours. Magnetic cloudshave a large and smooth roCopyright 1998bytheAmerican Geophysical Union. tation in the field's direction, an enhancedfield strength and low proton temperature and/• values. It is thought Papernumber98GL00703. [Farragiae! al., 1997andreferences therein]that clouds 0094-8534/98/98GL-00703505.00 963 964 GONZALEZ ET AL.' MAGNETIC CLOUD are giant flux ropes formed by field aligned currents. The B•, componentof the cloud has typically an ampIf tude that represents a substantial [raction of the total IMF intensity. For example, for the intenseand super- intensestormsstudiedby Tsurutaniel, al. [1988]and Tsurutani et al. [1992],the magneticcloudsresponsible for about half of the storm events had their B., fields with intensities of 70% or more than that of the correspondingtotal IMF intensities. This association can explain why magnetic cloudswith strong magnetic fields have typically strong B, componentsand there- FIELDS AND VELOCITIES Second Set, of Clouds (1979) i ß ! ! ! ß 25 20 m 15- 10 fore cause intense storms. We have chosento examinefirst previouslypublished magneticcloudeventsbecausetheir identificationexists õ in the literatureand the readerhas the opportunityto 300 350 400 450 500 550 600 examinethe eventsin detail. We use(1) the five cloud Peak v (kin/s) eventspublishedand illustrated by Klein and Burlaga [1982];(2) the two cloudeventspublishedand illusfor the trated by Burlagaet al. [1987],•sing data.[rotl• t,he Figure 2. Scatterplot ForBr,•. vers,sv•,•,a•set of 13 •agnetic clo;•(!s,as observedby the IMP8/ISEE-3 satellites;(3) six cloudeventsreported second ISEE-3 satellite in 1979. These evc•l,s i•volved clouds by Tsurutaniet al. [1988]and Tsurutaniel al. [1992], that causedintenseand •o(!erate magneticstorn•s. usingISEE-3 data;(4) threecloudeventsdiscussed by Farrugiaet al. [1997]whoreferto previousstudiesthat useddata from the IMP8/ISEE-3 satellites;and (5) the oneevent,reportedby Burlagaet al. [1996],usingdata from the WIND served at • satellite. All these 17 events were ob- 1 AU and the criteria to select the events, that are cormnon in all these cases, are: a large rotation in the ejecta field's direction, an enhancedfield strength (typically > 10 nT), a low proton te•nperature and a duration of about, 24 hours for the ejecta to cross the spacecraft. Because most of these events were studied in asso- satellite, and identified the driver gas events using a combined criteria from those discussedby Zwickl et al. [98.q] Ts,vttani at. [988]. The combined cri- teria involved' a smooth ]nagneticfield (lack o[ Alfvdn wavesand discontinuities),high magneticfield strength, and a low proton temperature. For this study we have used5 to 15 minute averagesof the ISEE-3 data to identify the eventsand higher t,ime resolutionto checkthe smoothness of the field. These events were dividcd in two subsets,one including only clear magnetic clouds, eventsreportedby Klein and Burlaga[1982],who se- using the selection criteria mentioned in the previous lected clouds without any association with magnetic paragraph (with field rotation anglesthat for this data, storms, we have looked for another (and independent) set lie in the domain of abo]•t 200 to 320 degrees),and set of magnetic cloudsthat would not have an (a pri- the other subset was formed with the remaining driver ori) associationwith magneticstorms. We obtained gas-noncloud events. For the first s•bsot we obtained this secondset of clotadsin the followingway. We took 13 events and for the second one 24 events. As for the first, set, of clouds, we also performed the the whole year of 1979, for which a fifil set,of plaslna and magnetic field data was recordedby the ISEE-3 same study for the peak values of the solar wind speed and the magnetic field within the intervalsof thesetwo ciation with intense magnetic storms, except,the five subsets First 40 ' i ' Set, of Clouds i ß i i 35 30 to the fact that the year 1979 was near solar maxi]num, but. it, has been shown that, the solar cycle distribution of storms does not necessarilyfollow the sunspot solar 25 cycledistribution[e.g. Gonzalezctal., 1994]. On the other hand, it is also possiblethat magnetic clouds(followingshocks)are ]nest of t.!•cl,i•es asso- a0 ciated with 15 10 3OO of events. It is interesting to point, out that the secondset, of cloud events, even though they were selectedwithout looking to their relationship to magnetic storms, they were later seen to still be associatedwith magnetic storms of at. least,moderate intensities (-100 nT _<Dst <_-50 nT). One may think that this could be due , I 400 I ,500 • I 600 , I 700 , 800 Peak v (kin/s) intense storms or at least with st.orn•s of moderate intensity, becausethey tend to involvefairly largevaluesof B.• [e.g. Bnrlagact al., 1987' Tsurutani c! al., 1988]. Results. Figure 1. Scatter[)lot,k)r Bl,eat:versus•.'•,•a•: for the first set of 17 magnetic clouds,taken from severalreferences(see text), involving cloudsthat ca,•se(t intense and very intense •nagnet,icst,orn•s. The magneticcloudintensity versusspeedfor e.ach of the 17 eventsof the first set,of cloudsis shownin Figure 1, iu a scatter-plot forn•at (Prcli•]•inary results related GONZALEZ ET AL.- MAGNETIC CLOUD FIELDS AND VELOCITIES Combined 4o ß i ß i Set of Clouds ß i ß i Driver ß i Gas - ß i 965 Non Clouds ß I ß ! ß i 2o 3o m 20 lo ß 0 ß 300 I 400 • I 500 • I 600 • I 700 • i 800 300 Peak v (kin/s) • I 400 • I • 1500 I 600 • I 700 Peak v (kin/s) Figure 3. Scatter plot for the coml,ination of tlw two Figure 4. Driver gas eventsthat apparentlydid not independentsetsof cloudsshownin Figures1 and 2. involvecloudsin 1979(seetext), as measured by the ISEE-3 to thisfigurewerebrieflymentioned by Tsurutanictal. satellite. velocity is presumably the causeof the lack of intense [1998]).A linearregression fit isaddedto theplot,.The stormsd•ring low speedejecta. correlation coefficient is 0.71and the linearregression There is also an indicationthat this type of behav- line gives, ior is peculiarfor magneticclouds,whereasother types of non cloud-drivergas eventsdo not seemto show a Bp,•.(nT) = 0.047,,•,•(km/s) - 1.l similar relationship. Due to the B-v relationshipfor magneticcloudsthat Figure 2 shows a similar plot, for the secondset we presentin this paper,a fastercloudimpliesa larger of clouds,namelyfor the 1979drivergas-clearcloud amplitude of B (which typically has a substantialB• events,and Figure 3 refers to the combineddata set,of component). Thus, since B• and v are related to one thoseplottedin FiguresI and 2, involving30 clouds. another, the magnetosphericenergytransfer, which is The correlation coefficients obtained from the linear fits in Figures 2 and 3 are, respectively,0.73 and 0.7.5. From Figure 3, we note that, at cloud velocitiesof v m 400 km/s, the field magnitudescan be 1,5to 20 nT, which could have a B., componentco•npara.ble to the governedby v x Bs [e.g. Gonzalezet al., 1994],is enhanced by bothfactors and therefore becomesmore effi- cient for the occurrenceof magneticstorms.This rela: tionshipexcludesthe possibilitythat a magneticcloud with a high speed and a negligibleor absentB., field valueexpectedfromthe Gonzalezand Tsurutani[1987] could lead to the developmentof an intensestorm. To criteria, discussedin [he Introduction. It, is also inter- our knowledgeno one has observedsuch a case. This esfing[o note t,ha[ the largely documentedmagnetic lackof observationalsoreinforcesthe B-v relationship cloudevent of Jan 10, 1997, with peak valuesof about, reported in our paper. 1,5nT and 480 km/s, fits fairly well the trend shownin At.this time, the physicalcausesof the reportedrelaFigure 3. tionship betweenthe magneticfield and plasmaspeed Figure4 refersto the ISEE-3 subsetof drivergas-non of the magnetic cloudsare uncertain. Compressionof cloudevents.One canseethat this plot is largelyscat- the cloud is certainly occurring. Thus, it is possible tered, without any cleartrendfor a relationshipbetween that in some cases the field increases can be accounted the peak valuesof the magneticfield and the solarwind for by such a.neffect. speed. One can speculatethat this set of eventscould Another possibilityis that this relationshipmay be still include cloudsthat were crossedby the satellite far from their center, for which a rotation in the field was much smaller. In such a case much lower B fields are associated with the CME release and acceleration mech- anismsat the sun. For example,a recentself-consistent numericalMHD simulationstudy [Wu e! al., 1997] expectedfor a similar v value of the cloud. It is pos- showsthat the radial velocity of the center of the flux sible that other driver gas structurescould be involved rope (i.e. the centerof the magneticcloud),erupting in Figure 4, and deservea detailedinvestigation. from a helmetstreamer,is proportionalto the strength of the azimuthal componentof the magneticfield, in Discussion and Conclusions. a way consistentwith the magnetic cloud B- v re]ationshipreported in this paper. A detailedparametric For the sets of magnetic cloudsstudied in this work study of this kind of simulationalwork is currentlyunwe have shownthe existenceof a relationshipbetween der way. their peak magneticfield strength and peak velocity values,that may suggesta possibleintrinsicproperty Acknowledgments. Portions of this work were supof magneticcloudsand also imply a geophysicalcon- ported by the Fundo N acional de Desenvolvimento Cient[fico sequence.There is a clear tendency that cloudswhich e Tecno16gicoof Brazil and by the National Aeronauticsand moveat higherspeedsalsopossess highercoremagnetic SpaceAdministration/ Jet PropulsionLaboratory,Califorfieldstrengths.The relativelylowfieldstrengthat a low nia Institute of Technology. 966 References GONZALEZ ET AL.: MAGNETIC CLOUD FIELDS AND VELOCITIES T. Lee , Great magnetic storms. Geophys. Res. Lett., 19, 73 , 1992. Burlaga, L. F., Interplanetary Magnetohydrodynamics,Ox- Tsurutani, B. T., W. D. Gonzalez, A. L. C. Gonzalez, F. Tang, J. K. Arba!1o,and M. Okada,Interplanetaryorigin ford University Press, New York, 1995. of geomagneticactivity in the decliningphaseof the solar Burlaga, L. F., K. W. Behannon, and L. W. Klein, Comcycle. J. Geophys.Res., 100 (All), 21717, 1995. pound streams, magnetic clouds, and lnajor geomagnetic Tsurutani, B. T., J. K. Arballo, Y. Kamide, W. D. Gonzalez, storms, J. Geophys. Res., 92, 5725, 1987. and R. P. Lepping, Interplanetary causesof great. and Burlaga, L. F., R. P. Lepping, W. Mish, K. W. Ogilvie, A. superintensemagneticstorms,Physicsand (7hemistryof Szabo, A. J. Lazarus, and J. T. Steinberg, A magnetic the Earth, in press, 1998. cloud observed by Wind on October 18--20, 1995, NA,qA Wu, S. T., W. P. Guo, and M. Dryer, Dynamical Evopreprint, G,qFC, February, 1996. lution of a Coronal Streamer-Flux Rope System: II. A Dungey, J. W., Int.erplanetary magneticfield and the auroral Self-ConsistentNon-Planar Magnetohydrodyna,nicSimzones, Phys. Rev. Left., 6, 47, 1961. ulation, Solar Phys., 170, 265, 1997. Farrugia, C. J., L. F. Burlaga, and R. P. Lepping, Magnetic Zwickl, R. D., J. R. Asbridge,S. J. Bame,W. C,.Feldman,J. clouds and the quiet-storm effect. at earth, in Magnetic T. Gosling,and E. J. Smith, Plasmapropertiesof driver Storms, edited by B. T. Tsurutani, W. D. GonzMez, Y. gasfollowinginterplanetaryshocksobservedby ISEE-3, Kamide and J. K. Arba!1o, Amer. Geophys. lYnion Press, in Solar Wind Five, NASA Conf. Publ., CP. 2280, 711 Washington D. (',., Geophys. Mon. Set., 98, 91, ]997. pp., 1983. Gonzalez, W. D., and B.T. Tsurutani, ('•riteria of interplanetary parameters causing intense magnetic storms W. D. Gonzalez, A. L. Clfia de Gonzalez, A. dal Lago, (Dst < -100nT). Planetary ,qpaceScience, .•5, 1101, Instituto de PesquisasEspaciais,12201-970, S•.o .1os•.dos 1987. C,ampos,CP 515, SP, Brasil. (e-mail: gonzalez½$dge.inpe.br; Gonzalez, W. D., J. A..loselyn, Y. Kamide, H. W. Kroehl, br) G. Rostoker, B. T. Tsurutani, and V. M. Vasyliunas, alicia•dge.inpe.br; dallago(•dge.inpe. B. T. Tsurutani, J. K. Arballo, G. K. Lakhina, B. What is a geo•nagneticstor•n?", Review Paper. J. GeoButt, C. M. Ho, Space Plasma Physics, Jet Propulsion phys. Res.,, 99 (A4), 5771, 1994. Laboratory, California Institute of Technology,Pasadena, Gosling, .1. T., D. J. McComas, J. L. Phillips, and S. J. California, 91109. (e-mail: BTSURUTANI•jplsp2.jpl.nasa. Bame, Geomagnetic activity associatedwith earth pasgov; JARBALLO•_ jplsp2.jpl.nasa.gov; LAKt{INA½•jplsp2. sage of interplanetary shock disturbances and coronal jpl.nasa.gov;BBUTI•_jplsp2.jpl.nasa.gov; [email protected]. mass ejections. J. Geophys.Res., 96, 7831, 1991. na•a.gov) Klein, L. W., and L. F. Burlaga, Interplanetarymagnetic S.-T. Wu, Center for Space Plasma and Aeronomic clouds at 1 AU, ,1. Geophys. Res., 87, 61:3,1982. Research, and Department of Mechanical and Aerospace Tsurutani, B. T., W. D. Gonzalez, W. D., F. Tang, S. I. Engineering, The University of Alabama in Huntsville, Akasofu, and E. J. Smith, Solar wind southward B z feaHuntsville, Alabama 35899. (e-mail: [email protected]) tures responsiblefor major magnetic storms of ] 978-1979. J. Geophys.Res., 93 (AS), 8519, 1988. (ReceivedNovember10, 1997; revisedDecember26, 1997; Tsurutani, B. T, W. D. Gonzalez, W. D., F. Tang, and Y. accepted February 10, 1998.)