Flux Transfer Events at the Magnetopause and in the Ionosphere

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GEOPHYSICAL RESEARCH LETTERS, VOL. 17, NO. 12, PAGES 2241-2244, NOVEMBER 1990

FLUX TRANSFER EVENTS AT THE MAGNETOPAUSE AND IN TttE IONOSPHERE

R. C. Elphic

Los AlamosNational Laboratory

M. Lockwood

RutherfordAppleton Laboratory and Blackett Laboratory, Imperial College

S. W. H. Cowley

BlackettLaboratory, Imperial College

P. E. Sandholt

PhysicsDepartment, University of Oslo

Abstract.On December1, 1986the ISEE 1 and2 spacecraft In aneffort to capturethe effects of FTEs bothat themagne- pairpassed through the dayside magnetopause at a location topauseand in theionosphere simultaneously, a joint EIS- whichmapped approximately to ionosphericfield-line foot- CAT/ISEE campaignwas initiatedin 1986. This campaign pointsnear the fields of viewof theEISCAT radar and pho- calledfor enhancedtracking of the ISEE 1 and2 spacecraft tometersand an all-sky cameraon Svalbard. The magne- near the magnetopausefor thoseperiods when the EISCAT tosheathmagnetic field was southwardand duskward at the 'POLAR' radar experimentfield of view was nearthe cusp. time,and flux transferevents (Fl•s) were observedat the Variousobservational and operationalconstraints and prob- ISEE location. At the same time, the EISCAT radar observed lemslimited the number of joint operationintervals to roughly ionosphericflow bursts of upto 1 km s-1. Thepeak of each five. Of these,FTEs wereclearly seen at themagnetopause in burstfollowed an FTE observationat ISEE by a few minutes. one interval, 0800- !000 UT on 1 December, 1986. Neither The bursts, each lasting ten or fifteen minutes, were groundactivity nor FTEs were seenon the otheroccasions. comprisedof farsta westwardthen a polewardflow. An all; Here we examineobservations of FTEs at the magnetopause skycamera at Ny •lesundobserved dayside auroral breakup andrelated ionospheric and dayside auroral phenonomena as formsduring or shortly after the flow bursts,moving seennear the cusp footpoint by theEISCAT radar and all-sky westwardthen poleward. While these flow burstsand camerasand photometers in Svalbard. associateddayside auroral forms have been previously reportedin associationwith southwardiMF orientations,this Observations is the first observation of a direct link to FTEs at the magnetopause.On this occasion,the lower limit on the For thiscampaign the EISCAT radarused the 'Polar' beam- inferredpotential associated with the FTEs is roughly10 kV. swingingtechnique for obtainingtwo-dimensional ionospheric Their inferred east-westextent in the ionosphereranges flowvectors in theneighborhood of thedayside cusp north of between700 and 1000km, correspondingto a 3 - 5 RE local TromsO.The technique has been described in detailby Lock- timeextent at theaverage magnetopause. woodet al. [1989]. The basiccycle time is 5 minutes. ISEE 2 was inboundin the early post-noonsector in the Introduction northernhemisphere and crossed the magnetopauseat about 0925 UT at a positionof (8.43, 3.29, 5.08) RE GSM. The It iswidely believed that magnetic reconnection occurs at the left panelof Figure1 showsthe ISEE trajectoryprojected on daysidemagnetopause in both a quasi-steadyand transient theY-Z GSM planefor a 30-minuteperiod after ISEE crossed form,the latterproducing "flux transferevents" or FTEs. themagnetopause. Also shown are three traces of theTsyga- Steady-statereconnecfion leads to globalmagnetospheric and nenko[ 1987]model magnetic field lineswhich pass through ionosphericconvection, but the precise effect of transientre- thetrajectory, and the Y-Z projectionsof theobserved field connectionis lessclear. Nevertheless,impulsive or transient orientationwithin the magnetosphere on thispass. The fight reconnectionat the magnetopauseought to producesome panelshows the geographicplacement of thecenter of the manifestationin the magnetosphereand ionosphere.Bol- EISCATradar 'POLAR' range gates 1through 5,the Ny Ale- shakovaand Troitskaya[1982], GlaBmeieret al. [!984], sundall-sky camera field of view,the meridian scanned by the Lanzerottiet al. [1987]and Goertz et al. [1985],all searched photometersand the ISEE footpointsfrom 0940 to 1000UT. forground signatures of FTEs with ambiguous results. The centerof the EISCAT field of view is at roughly75.0% 13.0ø geographic,some 270 krnwest of theISEE footpoint. Figure2 showsan overviewof the !SEE 2 magneticfield Copyright 1990 by the American Geophysical Union. dataand the inferred electric potentials across the radar field of Paper number 90GL02045 view associatedwith the flows for the period0800 to 1000 0094-8276/90/90GL-02045503.00 UT on 1 December 1986. The field data are running 12 s

2241 2242 Elphicet al.: SimultaneousGround/Space FTE Observations

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Fig. 1. (Left) View from the sunof the trajectoryof ISEE 2 in GSM coordinateswith observedmagnetospheric field vectors o N •'•'•'• - mojor557'7nm events just insidethe magnetopausebetween 0930 and 1000UT on 1 • 0 • , oefineObyo.s.c.• December,1986. Also shownpassing through the trajectory arethree model magnetic field linesfrom Tsyganenko[1987]. (Right) Geographiclocations of the EISCAT radar field of 0800 •30 0840 0900 0930 0940 1000 view, showingrange gates ! through5, andthe field of view UT of theNy Jdesundall-sky-camera installation (large circle). Fig. 2. Combinedtime seriesplot of the ISEE 2 magnetic Location of the persistentcusp/cleft arc is shownby the field observationsin boundary normal coordinatesand hatchedregion. The footpointsof ISEE 2 field linesbetween inferredelectric potential across the EISCAT field of viewfor 0930 (northern-mostpoint) and 1000 UT (southern-most the interval 0800 to 1000 UT on December 1, 1986. point) are shownby the thick traceto the eastof the EISCAT Horizontal bars at the bottom of the figure denotedayside fieldof view and the Ny •lesund magnetic meridian. auroralbreakup events as seen at Ny •!esund.

averagesevery 4 s in boundarynormal coordinates. The L aratingthe two beamdirections there, but normalizedto a 200 andM componentsare in the nominal magnetopauseplane km separationfor comparisonwith (I)Ns. Thesevalues are whilethe N componentis normalto thatplane. The normalis essentiallylower limits on thepotentials since the plasma flow calculated assuming the magnetopauseis a tangential may haveextended over a muchbroader region than just the discontinuity.The resultingL andN unit vectorsare (-0.442, radarfield of view. Maxima in CI)Nsare 3 kV at 0832 UT, 10 0.019, 0.897) and (0.891, 0.131, 0.436) in GSM kV at 0850 UT, 10 kV at 0907 UT and 11 kV at 0924 UT. coordinates.The inferrednormal is about7 ø off the average Maxima in (I)Ewfollow thosein (I)Nsby roughly2 minutes, magnetopausenormal direction at the ISEE 2 location. butare smaller in magnitude.After thethird (I•s peakthe po- Minimumvariance analysis yields a similarnormal direction. tentialsnever drop back to theirearlier low levels.The begin- The databegin with ISEE 2 inboundin the magnetosheath. ningsof a fifth (I)Nsexcursion is seenbefore the dataend at After 0806 UT the magnetosheathfield was strongly 0934 UT. duskward(~ 40 nT) and southward.Using the canonical10- In Figure 3 we focuson the !SEE 2 BL and BN components nT peak-to-peakminimum BN amplitudeand 1-minutemini- togetherwith the northwardand eastwardflow components mum duration identification criteria, the first FTE occurs at from the first four rangegates from 0815 to 0945 UT. The 0845 UT (earlier BN activity around0820 UT doesnot meet highestflow speedsare seenat the poleward-mostlatitudes theacceptance criteria). Another• is observedat 0904 UT. (rangegate 4), but very similarflow behavioris also seenat Thereafter,a long seriesof BN excursionsare seen;those thelower-latitude range gates. There is a westwardswing in passingthe selection criteria occur at 0911, 0914, 0926, 0931, thelow speed(< 400 m s-1) flowsjust before0830 UT. A 0937 and 0950 UT. In the midst of this interval,at 0917 UT, strongwestward flow burstjust before 0840 UT is seenonly a BL spike is seen [Farrugia et al., 1988; Farrugia, 1989]. at gate4; thelower latitude positions reflect the enhanced flow The magnetopauseis crossedat 0925 UT, with a partial at about0844 UT. The flow persistsin a westwardsense crossinglater near 0936 UT. until0851 UT, thenswings northward. The nextwestward The bottompanel of Figure 2 showsthe north-southand flow burst begins just before 0900 UT and turns more east-westpotential drops calculated from the-VxB electric northwardby 0907 UT. This flow persistsuntil the next field integratedover the radar field of view. For (!)NSthe westward flow intensification at 0918 - 0922 UT. north-southelectric field is integratedover the 200 km spanof Horizontalbars at thebottom of Figure2 denoteauroral thefirst four rangegates. cI)]•w is derivedfrom the east-west breakupevents seen by theNy •lesund557.7 nm all-sky- electricfield at range gate four integrated over the 285 lcm sep- camerato the northand eastof the EISCAT range. These Elphicet ai.: SimultaneousGround/Space FTE Observations 2243

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ß 09:18:21 -1200 i i i 90 0 - 90 Zenith•.ngle (deg.) I ,, I 08:30 09:00 09:30 UT Fig.4. (Left)Meridian scans of the630 nm emissions over Ny ]tlesundbetween roughly 0903 and 0925 UT. Notethe Fig.3. Timeseries of theISEE BL andBN components and appearanceof a peakin emissionatlow latitudes just before EISCATVNorth and VEast flow componentsfrom range gates 0910UT. (Right)All-sky-camera images of 557.7nm emis- ! through4 for theinterval 0815 to 0945UT. Verticallines sionsabove Ny ,•lesund,represented asisophotic contours. denoteFTEs and a BL spikeat 0917 UT. Ionosphericflow Thepeak emission isshaded black. Note that the auroral form burstsare coherent over all rangegates in theinterval. beginsin the east, moves west and then north as the event pro- ceeds. This is the same motion as the westwardthen polewardflows observed by EISCAT. eventsfollow the peaks in (I)Nsand last between 4 and 8 minutes.We focushere on the secondoptical event centered at 0915UT. Figure4 showsNy ,•!esund 630 nm photometer twoclear magnetosheath FTEs observed at0845 and 0904 scansalong the local meridian and four all-sky images of UT. A thirdflow burstat 0924UT andthe beginnings of a 557.7nm intensities (represented asisophotic contours). The persistentcusp/cleft arc is seenwell to thesouth of local fourthjust before 0934 UT fall within a multiple-FTEperiod zenith.The frrst630 nm intensificationfor thisevent appears beginningat0910 UT and ending atabout 0940 UT. Thereis alsoa BLspike (often seen associated with accelerated plasma shortlybefore 0910 UT, just after the •NS peak at EISCAT. flowsin thenear-magnetopause magnetosheath [Farrugia et Theintensity peak moves gradually northward and fades by 0915UT. Meanwhilea new peak develops further south. A ai.,1988; Farrugia, 1989]) at 0917 UT, roughly five minutes beforethe westward flow peak seen at range gates 1 - 3. Ma- higherlatitude peak brightens just before 0920 UT andsubse- jordayside auroral events seen in the all-sky-camera datafrom quentlymerges with the lower latitude peak. The 557.7 nm Ny•!esund are associated withall three flow burst peaks. A all-skyimages following the flow event at 0908 UT showan fourthauroral event follows the beginnings ofthe fourth flow auroralform that brightens in theeast at 0914UT, subse- quentlymoves to the west and stretches poleward at0916 UT. burstobserved just before the end of E!SCAT data acquisition By0918 UT it is a veryelongated east-west form with its andmay be associated withthe magnetospheric FTEat 0931 western-mostedge also the most poleward. The 557.7 nm au- UT. Theionospheric flow and auroral sequences shown in roralforms reach far poleward of thepersistent cusp/cleft arc. Figures2- 4 arevery similar tothose described bySandhok et al. [1990]for otherdayside breakup events. Discussion TheFTE activity after 0910 UT isproblematic: BNsigna- turesappear tooccur with shorter durations andat shorter in- Theobservations demonstrate what appears to bea clearlink terva!sthan the 5-minute cycle time of theradar. Conse- betweenFTEs observed at themagnetopause and ionospheric quentlywecannot expect toobserve aone-to-one relationship flowbursts and dayside auroral breakup phenomena detected betweenflow bursts and FTE signatures atthis time. Never- nearthe footpoint ofthe FTE field lines. Two westward flow thelessthe increased Fi• activityafter 0910 UT maybe re- burstswith centertimes at 0851 and0907 UT closelyfollow flectedinthe higher average values ofionospheric flowd•ng 2244 Elphiceta!.- SimultaneousGround/Space FTE Observations thisperiod. Indeedthere is a generalincrease in both•NS and EISCAT/ISEEcampaign. The optical observation campaign at tI)Ew(and in VNonhand VEastat all rangegates) with time as Ny 3,!esundand work done at theUniversity of Oslowas the averagevalue of magnetosheathBL becomesincreasingly supportedby the GeophysicsLaboratory, Hanscom Air Force negativebetween 0820 and 0925 UT. Base,Massachusetts (H. C. Carlson),the NorwegianPolar The peaksof the ionosphericflow burstsfollow the two ResearchInstitute, and NAVF. ISEE dataprocessing at magnetosheathFTEs by about 6 and 3 (+ 2.5) minutes,but UCLA was doneunder NASA grantNAGS-1067. We thank theinitial rise of the ionosphericflow burstsactually precedes J. T. Gosling for useful comments. Work at Los Alamoswas theseFTEs by 4 to 5 (+ 2.5) minutes. This is expectedif the doneunder the auspices of theU.S. Departmentof Energy. Alfv•nic reconnectionsignal in themagnetosphere outraces the magnetosheathconvective FTE signal: The magnetospheric References Alfv•nic signal,traveling from the subsolarmagnetopause at ---1000km s-i, reachesthe ionosphere in roughly 100 s, while Bolshakova,O. V., and V. A. Troitskaya,Pulsed reconnec- theconvective signal, traveling at 100to 200 km s-1, reaches tionas a possiblesource of icp1 typepulsations, Geomagn. the ISEE positionin 200 to 400 s. Thus the observed240- to Acton., 22, 723, 1982. 300-sdelay times are roughly consistent with theexpected dif- Farrugia,C. J., et al., Observationsof flux transferevents, ference in Alfv•nic and convective travel times. Adv. SpaceRes., 8, 249 - 258, 1988. We can use the observed northward and westward flows to Farrugia,C. J., Observationsof solarwind-magnetosphere estimatethe respective north-south and east-west scale sizes of couplingat the Earth'smagnetopause, Phil. Trans.R. Soc. the ionosphericflow bursts.The northwardbursts have aver- Lond., A 328, 57 - 77, 1989. ageflow speeds of between150 and 300 m s-1 anddurations GlaBmeier,K.-H., et al., Pc 5 pulsationsand their possible of about 10 minutes (600 s), yielding a characteristicnorth- sourcemechanisms: A casestudy, J. Geophys.,55, 108- southscale size of 90 to 180 krn. The westwardbursts range 119, 1984. between600 and1000 m s-1 in averageflow speedwith dura- Goertz, C. K., et al., Observationsof a possibleground tionsbetween 10 and 15 minutes (600 to 900 s); the resultant signatureof flux transfer events,J. Geophys.Res., 90, scalesizes range between360 and 900 km. Assumingthe 4069, 1985. simplestmapping to the magnetopausewith lengthvarying as Gosling, J. T., et al., Plasmaflow reversalsat the dayside B-1/2, these scales become respectively 0.5 to !.0 REand 1.8 magnetopauseand the origin of asymmetricpolar cap to 4.5 RE in the north-southand longitudinal(in local time convection,J. Geophys.Res., 95, 8073, 1990. alongthe magnetopause)directions, respectively. Lanzerotti, L. J., et al., Ionosphereand ground-basedre- sponseto field-alignedcurrents near the magnetosphere Conclusions cuspregions, J. Geophys.Res., 92, 7739, 1987. Lockwood,M., et al., Interplanetarymagnetic field controlof We havepresented observations of FTEs madeby theISEE daysideauroral activity and magnetic flux transferfrom the 2 spacecraftat the daysidemagnetopause and simultaneous solarwind, Planet. SpaceSci., 37, 1347, 1989. EiSCATobservations of ionospheric flow bursts and Ny ]•le- Sandholt,P. E., et al., Midday auroralbreakup events and sundobservations of daysideauroral breakup events near the relatedenergy and momentum transfer from the magneto- ISEE field line footpoints. The flow burstswere first west- sheath,J. Geophys.Res., 95, 1039- 1060, 1990. ward,consistent with the senseof reconnectionmagnetic ten- Scholer,M., Magneticflux transferat themagnetopause based sion at the magnetopause[Gosling et al., 1990], then pole- on singleX-line burstyreconnection, Geophys. Res. Lett., ward as would be expectedif reconnectedflux tubeswere 15, 291, 1988. convectingtailward with the solarwind. Eachburst is associ- Southwood,D. J., et al., What are flux transfer events?, ated with a north-southionospheric potential whose lower Planet. SpaceSci., 36, 503, 1988. limit is about10 kV. Basedon the magnitudeand duration of Tsyganenko,N.A., Global quantitative models•" of the geo- the north-southand east-westcomponents of the flow bursts magneticfield in the cislunarmagnetosphere for different we infer the scalesizes of theseevents to rangebetween 100 disturbancelevels, Planet. Space Sci., 35, 1347, 1987. and200 km in the north-southdimension, and roughly 400 to 1000 km in the east-westdimension. Simple mapping of theselengths to the magnetopauseyields an approximate S. W. H. Cowley,Blackett Laboratory, Imperial College, inferred scale size of 0.5 to 1.0 RE in the north-south SW7 2BZ, London, U.K. dimension,and 2 to 5 RE in the east-westdimension. Thus R. C. Elphic,Los AlamosNational Laboratory, MS thelocal time extent of anFTE canbe significantlylarger than D438, Los Aimos, New Mexico, 87545 USA its north-southextent, in keepingwith themodel of FTEs by M. Lockwood,Rutherford Appleton Laboratory, Chilton, Southwoodet al. [1988] and Scholer[1988]. Didcot, Oxon OX11 OQX, U.K. P. E. Sandholt,Physics Department, University of Oslo, Acknowledgments.We thankthe director and staff of EIS- Box 1048 Blindern, 0316 Oslo 3, Norway CAT for their assistance:EISCAT is supportedby the re- searchcouncils of France (CNRS), West Germany (MPG), (Received:June 18, 1990 Norway(NAVF), Sweden(NFR), Finland(SA) andthe UK Revised: August17, 1990 (SERC). We thankthe ISEE projectfor expeditingthe joint Accepted: August28, 1990)