Dec 1, 1995 - Reiff and Burch [1985] discussed the convection in the tail; when IMF By is comparable to or larger than Bz in magni- tude, the open field lines ...
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL.
100, NO. A12, PAGES 23,663-23,675, DECEMBER
1, 1995
GEOTAIL observation of magnetosphericconvection in the distant tail at 200 in quiet times A. Nishida, T. Mukai, T. Yamamoto, and Y. Saito Instituteof SpaceandAstronomicalScience,Sagamihara,Japan S. Kokubun
SolarTerrestrialEnvironmentLaboratory,NagoyaUniversity,Toyokawa,Japan K. Maezawa
Departmentof Physics,NagoyaUniversity,Nagoya,Japan
Abstract. Magneticfield andplasmaobservations by GEOTAILin the distanttail at x = -200 REare studiedfor a geomagnetically quietintervalwhenIMF Bz is predominantly northward and IBylis largerthanBz on average.In the distanttail duringthisinterval,Bz is northwardand Ey is directeddusk-to-dawn on average.This combinationof Bz andEy doesnot seemto representthetailwardconvectionof the closedfield lines,becauseEy is muchweakerin the lobe than in the plasmasheetso that the field lineswould be piled up at the plasmasheetboundaryif they were closed.The observations suggestinsteadthatthe plasmaandfield linesare convectedparallel to the neutralsheetacrossthe tail at the sametime as they flow tailward. The directionof thiscross-tailconvectiondependson thepolarityof IMF By andis antisymmetric with respect to the neutralsheet,whichcanbe twistedby tensof degreesunder the influenceof IMF By. A consistent pictureis obtained from observations both insidethe tail andat the tail magnetopause. This convectionprofile agreesin topologywith the cuspreconnectionmodel, but it occursmainlyin theplasmasheetwhile only the lobefield linesareexpectedto be involvedaccording to thismodel. Observations showthatnot only the cold-denseionswhichcanbe linkeddirectlywith the entrantsolarwind plasmabut alsothe hot-tenuous ionsin theplasmasheettake partin the convection. Introduction
While there exists a standardpicture for the convection in the magnetotailundersouthwardIMF, widely different views have been held on the convection
under northward
IMF.
Heikkila [1987], who foundthat magneticfield in the neutral sheetat 220 R• is northwardwith an averageBz of 0.7 nT suggestedthat magnetotail is dominatedby boundary layer plasma flowing on closed field lines. Noting that Bz is northwardon average when the tailward flow speed is less than 400 km/s, while it is southwardwhen this speed is larger, Heikkila [1988] concludedthat the slow flow representsthe boundarylayer. Slavin et al. [1987] also suggested that the centralplasmasheetbeyond80-100 REis threadedby closedfield lines that are being carriedslowly tailward, since the magneticfield in the plasmasheetis northwardout to 240 R•.duringgeomagnetically quiet times(IAEI < 100 nT) despite the presenceof the tailwardflow. Fairfield [ 1993] studiedthe observationsat 225 R•. during several a few-hour intervals of northwardIMF and suggested that at thesetimes of noahward IMF the magnetotailconsistsmostly of field lines that close earthwardof the above distance and only a narrow tail remains. Tailward convectionof northwardmagneticfield lines has also been observedat 230 REby the GEOTA•L satellite in
quiettimeswhenKp = 0 or 1, but Nishida et al. [1994] have pointed out that these field lines are not likely to be the closedones since suchinterpretation leads to an unrealistically large transportof closedfield lines away from the Earth. They suggestedthat thesefield lines traversethe neutral sheet northwardand are connectedto IMF at both endsin high latitudes. Richardsonet al. [1989l, on the other hand, suggested that the tailward flow is producedby reconnection inside the tail by attributingthe accompanyingnorthwardIMF to twisting and tilting of the tail. Useful
information
Papernumber95JA02519. 0148-0227/95/95JA-02519505.00
in the distant
tail is ob-
where the flow is directed sunward between the two cells.
This
convection, which has come to be called reverse convection
as its direction is opposite to the antisunwardflow which is more commonlyseenin the polar cap, is consistentwith satellite observations of the electric field and field-aligned current [e.g., Burke et al., 1979; Zanetti et al., 1984]. There have been attemptsto interpret these observationsby distorting the standardtwo cell patterninsteadof introducingthe reversecells [Heppner and Maynard, 1987], but Reiff and Heelis [1994] have shownthat this alternativeinterpretationcannot be made consistent
Copyright1995 by the AmericanGeophysicalUnion.
on convection
tainable from the ionospheric convection in the polar cap. Maezawa [1976] found that in geomagnetically quiet times the convectionconsistsof two cells in the daysidepolar cap
with
the
overall
observations
without
postulatinga severeflow kink and extra field-alignedcurrents which are typically not observedin times of northwardIMF. When Bz is northwardbut IByl is larger than Bz, one of the 23,663
23,664
NISHIDA ET AL.: CONVECTION IN DISTANT TAIL IN QUIET TIMES
two cells of the reverse convection
becomes dominant.
The
rotation senseof the dominantcell is opposite in opposite hemispheres; in the northern(southern) polarcapfor IMF By
> 0, thedawnside (duskside) c•11is muchlargerandis clockwise (counterclockwise) as viewed from above.
Hence the
flow aroundmidnightis directedduskwardin the northernpolar cap and dawnwardin the southernpolar cap when IMF By > 0 [e.g., Knipp et al., 1993; Taguchiet al., 1994]. Projection of this pattern to the distant tail suggeststhat the convection in the distanttail would have a substantialcomponent across the tail whose direction differs between northern and southern sectors.
A generationmechanismof convectionunderthe influence of northwardIMF was first consideredby Dungey [ 1963]. Reconnectionoccurspoleward of the daysidecuspwheresouthwarddirectedmagnetic field of the lobe makes contact with northwardIMF. Russell 119721developedthis model by considering that the cuspreconnectioninvolves only the open field lines. Maezawa[1976] consideredthat the open field lines are transportedinto the tail from the dawn/duskflanks and form convection
cells that are confined
to the lobe re-
gions. Crooker [1979] built a unifying model of convection undergeneralIMF directionsby assumingthat reconnection is constrainedto occuronly wherethe fields are antiparallel acrossthe interface. When IMF B z is northward and By is positive (negative), this model producesa circular convection cell in the polar cap which is clockwise(counterclockwise) in the northern polar cap, in agreementwith the observedconvection. By adopting the antiparallel reconnection model, Reiff and Burch [1985] discussedthe convection in the tail; when IMF By is comparableto or larger than Bz in magnitude,the open field lines in each(i.e., northernand southern) sector of the tail convect in a single cell whose sense is clockwise for IMF By > 0 and counterclockwisefor By < 0 in both sectorswhen viewed toward the Earth. Lyons and Pridmore-Brown[1994] obtained the radial range of such open field lines in the tail by using a simple quantitative model in which a 2-nT portion of the IMF penetratesthe magnetosphere. Akasofu and Meng [19861 showedthat the neutral sheet is twisted
in the direction
of IMF
when
IMF
B z is
northwardby usinga vacuumsuperposition model whereIMF is simply superposedon the field inside the magnetosphere. Reiff and Burch [1985] suggestedfurther that the lobe cells of the northern and southern sectors of the tail make direct con-
IMF condition. (1) Because of the orbital motion of the Earth, a radial solar wind appears to come from several degreeswest of the sunand this aberrationis reflectedin the di-
rectionof the tail axis[e.g.,Fairfield, 1993]. An averageradiusof the distanttail is 24 RE [Fairfield, 19921. (2) The tail seems to be twisted and flattened under the influence of IMF.
During strong By
of IMF, the northern lobe is observed
southof the ecliptic plane andthe boundarynormalsdeviate markedlyfrom the radially outwarddirection from the usual tail axis [Sibecket al., 1985a]. (3) A substantial fraction of IMF By entersthe tail. In the boundarylayer of the distant tail, about 31% of By of the adjacentmagnetosheath is observed[Sibecket al., 1985b]. Preliminarystudyof GEOTAIL data duringquiet intervalshas confirmedthat By in the neutral sheet has the same sign as IMF By [Nishida et al., 1994]. Penetrationof IMF By has beendiscussed in severaltheory papers [Cowley, 1981; Moses et al., 1985; Pudovkin et al., 1993]. (4) A broadboundarylayer plasmaoftenexistswithin the distant tail lobes directly adjacent to the tail magnetopause.At ISEE 3 the electrondensityandtemperature immediatelyinsidethe magnetopause differ only slightly from the plasmaimmediatelyoutside[Goslinget al., 1984]. Lobe plasmadensitieson opposite sidesof the neutral sheet show differencesthat dependon the sign of IMF By and the dawn/duskposition of the observing site [Gosling et al., 1985]. It has been noted that these observations can be in-
terpretedin termsof the open characterof the field lines. (5) A statisticalsummaryof GEOTAIL magneticfield datasuggests thatthe flaring angleof the magneticfield in the xy plane is
largerthan the expansionangle of the magnetopause in the distanttail (particularlybeyond 150 RE) [Yamamotoet al., 1994]. This suggeststhat magneticfield lines at the distant magnetopauseare linked at least partially with those in the magnetosheath. In this paper, we studymagnetic structureandconvection
profileof the distanttail undernorthwardIMF using the data obtainedat the distanceof about200 R• by the GEOTAIL satellite duringa geomagnetically quiet interval.By virtueof large geometricfactorof the low-energyplasmaanalyzer,a detailed studycan be madeof the dynamicsof the tenuousplasmain the distant plasma sheet and lobe down to the density of
0.01/cm 3. Theresults showthatnorthward magnetic field lines are convected in the dawn/dusk direction across the tail
tact at the neutralsheet,insteadof being separatedby a layer of closedfield lines, and arguedthat reconnection occursbetween the open field lines of the northern and southernlobe
at the sametime astheymovetailward. This convectionprofile is essentiallyconsistentwith whatis expectedfrom reconnectionunderthe northwardIMF at the cuspregion of the magnetopause,but the convection occurs mainly in the
cells.
plasma sheet rather than in the lobe.
The convection cell of closed field lines that results is
called reclosurecell. They suggestedthat the ionospheric footprintsof the closedfield lines of the reclosurecell move sunwardacrossthe polar cap. In Russell's [1972] model some of the open field lines producedby reconnectionare drapedover the daysidemagnetopause. Topological development of open field lines after the cuspreconnectionwas discussedby Crooker [1992] including the overdrapedfield lines as one of the key elements. When overdrapedfield lines reconnectwith the open field linesof the nightsidewhich have a closed-likeconfiguration but have beenreconnectedwith IMF at the cuspof the opposite hemisphere,a new type of field lines is producedon the nightside;they appearto be closedfield lines but are detached from the ionosphereat both ends. Past observations
in the distant
other basic features which
tail have revealed several
are not limited
to the northward
Data
The GEOTAIL satellite waslaunchedon July 24, 1992. In the first phaseof the orbit that lasted till November 1994 its apogeewaskepton the nightsideof the Earth andbeyondthe orbit of the moon by the doublelunar swingby maneuvers. The datausedin this paperwereobtainedduringthis orbital phase. The magneticfield is measured by Magnetic Field (MGF) experimentusing two setsof fluxgatemagnetometers which
aredeployedat distances of 7.15 and5.12 m from the spacecraft spinaxis [Kokubunet al., 1994]. The spinperiodis 3 s. Sampledsignals from the outboardmagnetometerare averagedto 16 vectors/s.In the presentanalysistheir 12-s averagesare used.
NISHIDA ET AL.: CONVECTION IN DISTANT TAIL IN QUIET 73MES 1•
•'
2o
.
O
_
.
1o _
23,665
i
.
:
:
.
---,•---•-•--? .................. i........................................................................................................ i.................. •................
-
$ _=............. :................. •... ............................................................................... i..................................... i................ i................ c• :24. _
o ß ....•................... i.................... !......................................... i.................... •........................................ :.................... i................... =-............ '*6...... *:;'g•' i ! i , , -
0
andaremovingoutof it if Bx < 0, while the normalcomponent of B is directedinto the tail for both signs of Bx (see Figure 15B). In the presentdata set there are 12 casesof Bx >
in theyz planedependon the signof Bx in sucha way thatis consistentwith the identificationof the enteringand the exit-
0 and 4 cases of Bx < O.
(a)
I ' Bx>O "Bx
