JOURNAL
The double
OF GEOPHYSICAL
oval UV auroral
RESEARCH,
VOL. 100, NO. A7, PAGES 12,075-12,092, JULY 1, 1995
distribution
1. Implications for the mapping of auroral arcs
R. D. Elphinstone,
• J.S. Murphree,
• D. J.Heam,
• L. L. Cogger,
• I. Sandahl,
2
P.T. Newell,
3 D. M. Klumpar,
4 S. Ohtani,
3 J.A. Sauvaud,
5T. A. Potemra,
3
K. Mursula,
6 A. Wright,
7 andM. Shapshak
8
Abstract. During the later stagesof the auroral substormthe luminositydistribution
frequentlyresemblesa doubleoval, one oval lying polewardof the normalor main IJV
auroraloval. We interpretthe doubleoval morphologyas being due to the plasma sheet
boundarylayer becomingactive in the later stagesof the substormprocess.ff the disturbance
engulfsthe nightsidelow-latitudeboundarylayers,then the doubleoval configurationextends
into the daysideionosphericregion. The main UV oval is associatedwith the inner portion of
the centralplasmasheetand can rapidly changeits auroralcharacterfrom being diffuse to
discrete. This transitionis associatedwith the substormprocessand is fundamentalto
understanding
the near-Earthcharacterof substormonset. On the otherhand, the poleward
arc systemin the nightsideionosphereoccursadjacentto or near the open-closedfield line
boundary. This systemactivatesat the end of the opticalexpansionphaseand is a part of the
recoveryphaseconfigurationin substormswhere it occurs. Thesetwo sourceregionsfor
nightsidediscreteauroralarcsare importantin resolvingthe controversyconcerningthe
mappingof arcsto the magnetosphere.The daysideextensionof this doubleoval
configurationis also investigatedand showsparticlesignatureswhich differ considerablyfrom
thoseon the nightsidegiving cluesto the magnetospheric
sourceregionsof the aurorain the
two local time sectors. Near-Earth substormonsetsare shownto be coupledto processes
occurringmuch further tailward and indicate the importanceof understandingthe temporal
developmentof featureswithin the doubleoval. Using "variance images," a new technique
for the investigationof thesedynamicsis outlined.
The otherview placessurgesand discreteauroraadjacentto the
open-closed
field line boundarywithin the PSBL [e.g., Burke
et
al.,
1994;
Lyons, 1991]. The central plasma sheet is then
The questionof where auroral arcs map to in the magnetoassociated
with
the more equatorwarddiffuseprecipitation.Lui
sphereis a long standingproblem. One view advocatedby
Feldstein and Galperin [1985] maps these arc systemsto the [ 1977] equatesthe centralplasmasheetwith the diffuseprecipientire central plasma sheet and associatesthe plasma sheet tationbut placesthe discreteaurorain the polewardportionof
boundarylayer (PSBL) with purelydiffuseauroralprecipitation. this region. Lui [1977, p. 2224] concludesthe discreteaurora
has a source"within but often near the polewardboundaryof
the plasmasheet."This latterpaperis an exampleof the view
1. Introduction
which existed before the the PSBL came to be associated with
• Department
of Physics
andAstronomy,
University
of Cat- the low-altitude boundaryplasma sheet (BPS). The original
gary, Calgary,Canada.
2 Swedish
Institute
for SpacePhysics,
Kimna,Sweden.
3 AppliedPhysics
Laboratory,
Johns
Hopkins
University,
Laurel, Maryland.
4 Space
Science
Laboratory,
Lockheed
Missiles
andSpace
CompanyIncorporated,Palo Alto, California.
5Toulouse
University,
Toulouse,
France.
6 Department
of Physical
Sciences,
University
of Outu,
?Mathematical
Institute,
University
of St.Andrews,
St.
distribution.
Andrews, Fife, Scotland.
s Institute
of Plasma
Physics,
Kungliska
Tekniska
Hogsko-
Copyright1995 by the AmericanGeophysicalUnion.
Papernumber95JA00326.
0148-0227/95/95JA-00326505.00
ductionand definitionof the PSBL at high altitude. The confusion betweenthe high-altitudePSBL and the low-altitudeBPS
has led many researchersto make an identification and
correspondence
betweenthe two regions. This paper is an
attempt to reconcilethese diverse views by investigatingan
auroral configuration which we term the double oval
Oulu, Finland.
tan, Stockholm, Sweden.
view of the tow-altitudeboundaryplasma signature(originally
designated
boundaryplasmalayeror BPL), whichwasput forth
by Winninghamet al. [1975], was developedbeforethe intro-
As the auroraceasesits polewardprogression,which characterizes the expansionphase, the aurora within the bulge often
fadesnoticeably. From magnetometer
data, it is known that at
this time the westwardelectrojetcan divide into two separate
electrojetsand that the fading of the auroracan correspondto a
local recovery of an H bay there [F•phinstoneet al., 1993;
Kotikov et al., 1993]. Thereforethe beginningof the recovery
phaseat least locally is associatedwith the obviousfading of
12,075
12,076
ELPHINSTONE ET AL.: THE DOUBLE OVAL AURORAL DISTRIBUTION
the aurorawithin the bulgeoncethe expansion
hasreachedits 2. Elements of the Auroral Substorm Recovery
polewardlimit. Auroralpatterns
whichoccurduringthe late
stages
of substorm
development
will be thefocusof thispaper.
Thesepatterns
will collectively
beidentified
asthedoubleoval
PhaseDevelopment
asObserved
FromViking
Figure1 illustrates
threeauroralpatternswhichexistin the
Thispaperwill address
theproblem
of mapping
thisdistribu- late stagesof substormdevelopment.Theseare all associated
tion by studyingbothhigh- and low-altitude
satellitedatain with what we call the doubleoval distribution. In a given subconjunction
with Viking auroralimagedataat timeswhenthe stormone or more of thesepatternsmay appearand the order
double oval distribution is observed. The next section of this of the patternsvaries. We shallusethe followingas a definipaperdealswith commonpatterns
observed
duringa double tion of the doubleoval configuration:An auroraldistribution
ovalauroralconfiguration.
Section3 compares
thedoubleoval which has two distinctauroralarc systemsat differentlatitudes
distribution.
as seen in the UV with other data sets recorded by Viking, which spanseveralhoursof magneticlocal time aroundmidDMSP, and CCE. In this sectiona new techniquefor investi- night. Betweenthesetwo distinctsystemstheremay be other
gatingthedynamics
of the aurorais demonstrated.
Section
4 discretefeatureswhich locally hide this larger scale pattern.
basedon the
showsthata couplingcanexistbetweenthetwo regionsof the The patternsshownin Figure 1 are schematics
double oval which can lead to a substorm onset. The last sec- viewingof approximately
45000 Viking imagesacquiredduring
tion is a summaryof the findingsin thispaper. A companion 1986. Individual events however can also be found within the
very well with the given schematics
paper[Elphinstone
et al., thisissue]investigates
in moredetail data set which correspond
here. The schematics
therefore
arecomposites
which
thetemporaldevelopment
of themostpolewardarcsystemand presented
demonstrates the association of it with the PSBL and wave represent
reasonably
well the generalcasewhilealsoaccurately
portrayingspecificindividualevents.
phenomena.
DaysideActive
Double oval
Multiplearc
westward
•:•..•_/'•
of surge
/
developes
::•••!prefetentially
inmorning)
•.,•____•'.•' •
•
N-S structures
Morning
sector
gap
Traversing
arc
Daysidespiral
18 MLT
Oouble
Oval
6 MLT
Low latitude
form
surge
12
Mostpoleward
oval
Main
UV
oval
Collapsed
"BPS"
1
o
Eastward motion
Omega-like
bands
Figure1. Threerecovery
phaseluminosity
distributions.
Thetoppanelillustrates
theformation
of thedouble
oval in the dawn sector,vortexstreets,andmultiplearcswestof the bulge. The middlepanelshowsthe dusk
sectormanifestation
of the doubleoval andthe bottompanelshowsthepatterntypicalwhenomegabandsexist
in theearlymorning.
Thesepanelsformmodules
whichcanbeputin seriesor superposed
to givean auroral
sequence
duringsubstorm
recoveryphase.
ELPHINSTONE ET AL.: THE DOUBLE OVAL AURORAL DISTRIBUTION
12,077
Figure 1 indicatesthat on occasionthe doubleoval represents
two separatelarge-scaleregionswhich can extend to within a
few hoursof local noon (see middle panel near 14-15 MLT and
bottomleft panel near 10 MLT) [Elphinstoneand Hearn, 1992;
Elphinstone
et al., 1993]. Further,while thereare many local-
formationof thesespiral forms can coincidewith the recovery
of the plasmasheetat ISEE distances[Coggerand Elphinstone,
1992]. This suddenintensificationof the polewardarc system
near midnight has been called the "traversingarc" [Murphree
and Elphinstone,1988]. Featureswithin the bulge which are
ized discrete forms within this distribution it is the extended
aligned from northwestto southeastalso develop at this time
local time nature of the two main distributions which leads to
[Nakarnuraet al., 1993]. This auroralpatternis represented
by
the characteristicdoubleoval configuration.This distribution the top panel in Figure 1.
has been seen in ISIS 2 observations[Murphree et al., 1981,
High-latitude arc systemscan form in the dawn and dusk
Figure 6]. It can also be clearly seenin other global imagesof
sectors(Figure 9 in the work by Elphinstoneet al. [1993]) and
the auroral distribution [e.g., Karnide and Akasofu, 1975; large scale stationarysurge systemscan form in the evening
Craven and Frank, 1987; Pellinen et al., 1992]. The most
sector. This resultsin the configurationshown in the middle
equatorwarddistribution or the main UV oval is the portion of
panel of Figure 1. The relation of the stationarysurge to the
the distributionwhich was found to map to the inner portion of
dayside distributionhas also been illustrated in this panel as
the centralplasmasheetby Elphinstoneet al. [1991].
well as the locationof the collapsedBPS as describedby WinIn the initial stagesof the double oval development,activity ningharnet al. [1975]. The Viking images show that omega
tends to be limited to the region near midnight. At this time
bandsoriginateon the main UV oval (the bottomleft panel of
multiple arcscan be seento the westof the surgeform [Nielsen Figure 1) and so occur during times when the inner portion of
et al., 1993]. At the poleward edge of the substormbulge the plasma sheet is active [e.g., Elphinstoneet al., 1993; Pelintensificationsoccur and multiple spiralsappearand disappear. linen et al., 1992].
These tend to develop coherentlyover several hours local time
Plate 1 shows the auroral configurationsduring six intervals
centred around the midnight meridian. In the magnetotailthe on April 9 and 10, 1986, marked in the AL diagram(Figure 2).
1040-1114UT
1417-1458
UT
EVOLUTION
OF THE AURORAL
1822-1922
UT
DISTRIBUTION
AVERAGES ON APRIL 9/10, 1986
2248-2334
UT
0759-0829
UT
1213-1236
UT
Plate 1. The development
of the auroraldistributionshowingthe correspondence
betweenthe doubleoval and
substormrecovery(see also Figure 2). Viking imageshave been averagedin CGM coordinates(basedon
IGRF 1980 model) over the northernhemisphere
portionof the orbit. Every 3 hoursMLT and 10 degrees
MLAT are shownin the panelsandthe timeperiodsfor the averaging
are displayedbeloweachpanel. White
correspondsto the most intenseemissions.
12,078
ELPHINSTONE ET AL.' THE DOUBLE OVAL AURORAL DISTRIBUTION
This Plate illustratesthe connectionbetweenrecovery phase
and the doubleoval distribution. This 2-day periodwas chosen
sincethere were many separaterecoveryphasesrecordedby the
Viking imager. Since Viking has abouta 4-hour orbital period
and viewing times are restricted to when the satellite passes
over the northernhemisphereauroraldistribution,it is not often
that a 32-hour time interval yield several separaterecovery
phase observations. This time interval was chosensimply for
APRIL9-10, 1986
-200
that reason.
It is generallydifficult to convey in a singleplate the image
data associated with hours worth of observations. Thus, in
order to illustratethe generalconfigurationsof the aurora,averages were constructed. These consistedof transformingthe
individualimagesas viewed from the satelliteperspectiveinto a
pixel map correspondingto CorrectedGeomagneticcoordinates
(CGM80). Each pixel in the original data was assigned a
specificpixel location on the CGM image grid definedby the
viewing geometry and by assumingan emissionheight of 120
km altitude. When multiple pixels from the original data
corresponded
to a given pixel locationon the CGM image, the
correspondingdata number intensitieswere averagedtogether
and the averagewas associatedwith the particularCGM pixel
element. CGM transformedimages of data number intensity
-40o
-600
D.O
D.O
D.O
O.U
ONSET
-800
AL INDEX
couldthen be constructed
from the originaldata. In orderto
-1000 , , , ....
I , , , , • , , • , , • I , • • I • , • , • , ,
get a good "averageimage" over the intervalin question,all
08.00
16.00
00.00
08.00
16.00
availableimageswere farstviewedand only thosewhich were
UNIVERSAL TIME
of goodqualitywereselectedfor the transform
process.These
"CGM 'unages"were then furtherprocessed
by averagingFigure 2. The AL index (provisional)for the 32 hour interval
showintervals
together similar CGM pixel locations at all available times. shownin Plate 1. The shorthorizontalsegments
were available. Q.U. represents
a
This produced
a CGM averaged
imagefor a giventimeinter- in whichViking observations
val. Plate1 showssix suchaverages
corresponding
to six Vik- quiet auroral distributionand D.O. means a double oval was
observed.
ing passesover the northernhemisphere
aurora.
The upper left panel showsan AL quiet time distribution
with a dawnsectorpolararc. The otheruppertwopanelsshow theseperiodsonly threeintervalsshowedthe doubleoval confitypicaldoubleovalsduringsubstorm
recovery.The bottomleft guration(i.e., 92% of thesedid not show the distribution).
panelof Plate 1 illustratesa transitiontime periodwherea new Two of these intervals occurred when the AL index was less
substormonset is beginningfrom the peak in the main UV than50 nT but werewithintwo hoursof a previoussubstorm.
oval. This representsan expansionoccurringduring the These results show that while the double oval distribution is
recoveryphaseof a previoussubstorm(consistentwith the AL generallyan AL recoveryphasephenomenon,
thereare events
index). The bottommiddle panel of Plate 1 illustratesan AL wherethis simpleclassificationschemebreaksdown.
recoveryconditionwhereboth substorm
regionsare active. In
this caseboththe mostpolewardsystemandthe main UV oval
are active regions for structuredauroral arcs. The intensifica-
3. DoubleOvalDynamics
andParticleSignatures
tionsare occurringsimultaneously
but appearto be optically In this sectiontwo auroraleventswill be studied,one in the
independent
of one another.Finallyin the bottomfightpanel morning(eventA) andthe otherin the eveninganddayside
of Plate 1, the auroraand the AL index (Figure2) have sectors(event B). The morningsectorevent coversthe sub-
returnedto a quiescentstate.
stormexpansionand recoveryphasesand illustratesthe locaThe doubleovalexamples
givenabovearespecificcasesof a tionsof auroralactivations
relativeto particleobservations
from
more generaltrendconcerning
this distribution.A studywas the low altitudeViking spacecraft
andthe highaltitudeCCE
made of Viking imagedata combinedwith the AL index cover- satellite. We shall also introduce in this section a novel teching a two week period beginningon March 15, 1986. This
niquefor the investigation
of auroraldynamics.The second
consistedof approximately60 hoursof observingtime, 63
eventillustratesthe local time dependence
of the doubleoval
satellitepassesor about3500 images. The AL indexcouldbe
particle signaturesbased on the DMSP low altitude satellites.
dividedaccordingto whetherthe AL was in recoveryor not
We shahfirst investigate
the changes
in the auroraldistribution
and similarly the image data could be used to determine
whether or not a double oval distribution existed. Of the 38
and then studythe in situ satellitedata.
separateintervalsof AL recoverywhich were found,approxi- Morning Sector Double Oval
mately 82% coincidedwith the existenceof a double oval dis-
tribution.However,sevenrecoverytimeperiodswerefoundin
Auroraldistribution.Plate2 showsan auroralconfigurawhich therewas no evidencefor this distribution.Nothing tionon July27, 1986,at a timewhentheVikingsatellite
flew
obvious was different about the AL index for these seven cases. over the 3-4 MLT sectorapproximately
coincidentwith the
During this 2 week periodtherewere 29 separateimaging timethatCCEencountered
theinneredgeof thecentral
plasma
intervalswhich were not duringsubstorm
AL recovery. Of sheet.TheAL indexshoweda sharpdecrease
at 2042LIT indi-
ELPHINSTONE
ET AL.: THE DOUBLE
OVAL
AURORAL
DISTRIBUTION
12,079
m
mm m
mm
,
ß
89
AVERAGE
AVERAGE IMAGE: 2042-2105 UT ON JULY 27, 1986
.,
,am,
q
I''e-
' 2122-2143
UT
m
m
"ram©0 0 dl ll•
IMAGE
"
.0
""
.mm
m
. •
I
me j•
m
m
I
.,
i
.if,
I
.i
,.
VARIANCE
IMAGE
ß2042-2105
UT
VARIANCE
IMAGE'2122-2143
UT
Plate 2. Viking and CCE trajectoriessuperimposed
on Viking auroraldata. The top panelsshow average
auroralconfigurations
during (left) substormexpansionphaseand (fight) recoveryphase. The bottompanels
show imagesconstructedfrom the standarddeviationsof the averageimages. White correspondsto the most
intenseemissionsor largestvariation. Points1 to 5 showlocationsalongthe Viking satellitetrajectoryat 2123,
2135, 2136:10, 2143, and 2147 UT respectively.Points6 to 10 showthe ionospheric
projectionsof the points
alongthe CCE orbit at 1910, 1930, 1940, 2120, and 2200 UT respectively.The T87 modelprojectionis shown
in white while the T89 version is given in red. Also shown are the 3 and 6 MLT meridians (CGM 1980) and
60, 70 Mlat.
cating a substormexpansionphasehad begun. This was conThe bottom panels of Plate 2 are images constructedfrom
sistentwith an explosiveonset seen in the auroral data. This normalizedstandarddeviationsof the average images. Each
index reacheda minimumnear -300 nT and beganto recover averagedpixel element has associatedwith it a standarddeviaby about2105 UT. It continuedto recoveruntil a new explo- tion. Essentiallythe maps shownare imagesof thesestandard
sive onset was seen in the auroral data at 2143 UT.
The top panelsof Plate 2 showthe averageauroraldistributions duringtheseexpansion(left panel) and recovery(fight
panel)phases. The distributionin the left panelhas beenaveraged using images acquiredbetween2042 and 2105 UT (21
images)while the fight panelrepresentsan averagefrom 2122
to 2143 UT (11 images). During expansionphase(top left
panel) the most polewardregionis relativelydim whereasduring recoveryphase(top fight panel)this regionbecomesactive
and forms the doubleoval pattern. The Viking satellitetrajectory has beenprojectedto 120 km altitudeand the CGM coordinateshave been plotted in each panel of Plate 2. This line
has points marked on it correspondingto particular signatures
seen at the satellite (points 1 to 5). Also shown in all of the
panels are the projections of the CCE satellite using two
separatemagneticfield models.
deviation maps. They have, however, been further normalized
by the expected Poisson statisticsstandard deviation for each
pixel. This latter step helpedto eliminatedayglow contamina-
tion to these images. This techniqueof producing"variance
images" is a novel means of extractingthe dynamicsof an
interval and allows it to be representedin a single image.
These imagesof "variance" illustrate where the aurora is most
spatially and temporallyactive during expansionand recovery
phases. This allows in a singleimage a methodfor illustrating
the regions of dynamic aurora over some time interval. This is
somethingwhich is difficult to showeven usinga large number
of imagesdisplayedtogether.
In the neuralnetworkdescribedby Newell et al [1991], used
to analyze particle data, the particle low-altitude boundary
plasmasheet(LABPS) is typically characterizedby temporally
and/orspatiallyvaryingelectronprecipitationin the keV energy
12,080
ELPHINSTONE
ET AL.' THE DOUBLE OVAL AURORAL DISTRIBUTION
range. Thus the dynamicIJV aurorarepresented
by thesevarianceimagesis likely to be labeledas LABPS by the neuralnetwork (the IJV camera is also most sensitiveto precipitationin
the 1 to 5 keV range). These varianceimagesthereforeallow
an importantmeans of producingtwo-dimensionalimagesof
this LABPS-like region.
The upperpanelsof Plate 2 show the changeof the auroral
distributionfrom an expansionphasemorphology(left panel) to
that of recovery(right panel). The bottompanelsillustratethat
fundamentallydifferent regions activateduring the two intervals. During expansionphase,the region near the main IJV
oval activates,while in recoverythis regionis inactiveand the
mostpolewardsystembecomesactive. The followingsubsection outlineshow these active regionscorrespondto low and
high altitudesatellitedata.
Particles, convection and field-aligned currents. Plate 3
showsthe time-energyparticlespectrogram
associated
with the
Viking trajectorymarkedin Plate 2. The electronsare shown
in the top paneland ions in the lower panel. Points1 to 5 in
Plate 2 correspondrespectivelyto the times 2123, 2135,
2136:10, 2143, and 2147 UT. The pitch angle of the particles
is shown at the bottom of the plate. The band acrossthe elec-
trons at one keV and below is an instrumenteffect. Using a
more detailed plot of the spectra,the following descriptions
apply to the electronsfor this event: (1) 2122 - 2123 UT: Low
energy (less than 1 keV) electronsexist in this region. These
occurpolewardof any IJV arcsseenby the imager. (2) 2123 2137 UT:
Structured electrons below about 1 keV.
These tend
to be more field-alignedthan the previousinterval. Several
field-aligned beams occur during the interval from 2125 to
2128 UT (one strongone is seenjust after 2127 UT). (3) 2127
- 2140 UT: Unstructuredelectronsof about 1-7 keV in energy
(i.e., electronsare isotropicexcept for an empty upgoingloss
cone). This is the definition used by SandaM and Lindqvist
[1990]. (4) 2135: An inverted-V event with acceleration to
about 7 keV. Equatorward of this point the electron fluxes
begin to decrease,disappearingfirst at the higherenergies. (5)
2140- 2143 UT: Trapped,unstructured
electrons.
Similarly, the ions show the following:(1) 2124 - 2130 lyr:
Ion conicsare seen. (2) 2125 - 2128 UT: Precipitatingions in
the 4 to 20 keV range are seen. A weak velocitydispersedion
signature(VDIS) during this interval (the highestenergiesoccur
at the highest latitude). This can be seen more clearly in a
detailedspectrogram
of this time interval. (3) 2128 - 2137 UT:
[K,eUl
Io
'H(k'•)
I .-:L.•T
'M'L"'T
!'0292
74,•i
O4,Z0
:'•83.:•3
72,4,
0:4,:O';•
':B.'354
70, 4
0:3:'54
8845
/G8,2
0::.•,,,4:.';•;
8305
':.G5,,8
0:.'3.,
.3/Z
27:35
:G,3,.•
I
0:':3,.2:'3
71:.3.6
;'G/O,.O
0::;;•,•
l,.3
Plate 3. Viking energy-timeparticlespectrogram
for the event shownin Plate 2 on July 27, 1986. Electrons
are shownin the top panel and ions in the bottompanel. The colorscorrespondto accumulatedcountsin 32
separateenergychannels(pitch angleresolutionof =5ø). An invertedV event can be seenat 2135 UT colocated with the main UV auroraloval in Plate 2. Red corresponds
to the most intensefluxes. Vertical lines
have beendrawn to correspondwith the pointsalongthe Viking trajectoryshownin Plate 2.
ELPHINSTONE
ET AL.: THE DOUBLE OVAL AURORAL
DISTRIBUTION
12,081
other (red line) is due to Tsyganenko[1989]. The points
labeled 6 through10 representthe CCE projectionsat 1910,
1930, 1940, 2120 and 2200 UT. Figure 4 showsthe lowextends between about 2137 and 2140 UT while the other
energyelectronflux changesat four separateenergylevels(1.8,
occursapproximatelyfrom 2137 to 2143 UT. These ions 4, 9 and 20 keV) during this time interval. At the time when
decreasein energy with decreasinglatitude. The boundary Viking traversedthe auroral distribution,CCE encounteredthe
betweenisotropicions and trappedions with energiesbetween inner portionof the centralplasmasheet(2120 to 2150 UT).
39 and 49 keV occurs at 2136:10 UT.
The inner edgeof the centralplasmasheetwill be considered
The peakin the energyflux into the ionosphere
for bothions to be the locationat which plasmasheetelectronfluxesdramatand electronscoincidesapproximatelywith the locationof the ically decrease. Since this occurs at different locationsfor difinverted-Vnear 2135 UT. This peak is on the orderof 10 to ferent particle energies,this edge is not a sharpone [FairfieM
15 mW/m2. This is alsothe locationof the strongest
field- and Vinas, 1984]. The outer portion of this inner edge is the
place at which the flux of particles with the highestenergy
alignedcurrentderivedfrom theparticles.
A more detailed examinationof the particle, electric field, shows a large decrease. The inner boundaryis the point at
and magneticfield observations
is summarizedin Figure 3. which no plasmasheetparticlesof any energyare found. The
This showsa schematicprofile which representsUV auroral inner edge of the plasma sheetis seenin the CCE data clearly
intensityversusuniversaltime/ latitude. A time and latitude between2120 (point 9 in Plate 2) and 2200 UT (point 10).
scale is shown on the horizontal axis so that the reader can
This regionis extendedin latitudeand lies far equatorward
of
refer back to Plates2 and 3 with regardto the auroraland par- the main UV oval. It coincidesin latitudeto the regionat Vikticle information. Figure 3 is divided into four panelseach ing altitudeswhere only the more energetictrappedions (> 20
illustratinga particularsetof Viking data. In orderfrom top to keV) are seen. Before 2120 UT this inner edgeis considerably
bottomthey showthe patternsof aurora,electrons,
ions, and more dynamicreflectingthe activeconditions.At about 1910
the field-aligned currents/convection.A few of the more UT (point6 in Plate2), an injectionof electronsfrom a distant
sourceappearsto take placewith the 20 keV electronsbecominteresting
pointsconcerning
thisfigureare listedbelow:
1. The mostpolewardion precipitation
is coincidentwith the ing enhancedprior to the 9 keV electrons. These 20 keV
decreasebeginningabout 1930 UT (point
mostpolewardarc system. This regionhas associated
with it fluxes subsequently
7) followedby the 9 keV electronsat about1940 UT (point8).
ion conicswith peakenergieslessthanabout2 keV.
2. The region 1 field-alignedcurrentsystem(downwardin This dynamic region projects to the main UV auroral oval
the morning)is colocatedwith the mostpolewardion precipita- (points6 to 8 in Figure4b andPlate2).
Interpretation and summary. Combiningthe information
tion. The senseof the field-alignedcurrentswas derivedfrom
the changein the in situmagneticfield alongthe Viking trajec- found in both the remote and in situ data two importantrelatory. The convection
reversal(westwardto the northandeast- tionscan be foundconcerningthe aurorasduring the expansion
ward to the south)lies equatorwardof theseions and the most and recoveryphasesof a substorm:
1. The active region of the aurorasduring this expansion
polewardare systemof the doubleoval. This is basedon elecphaseis locatednear the 40 keV isotropicprotonboundary.
tric field observationsfrom the Viking spacecraft.
3. The strongestregion 2 field-aligned current system This assumesthat this boundarymaintainsits relationshipto the
(upward)originatesbetweenthe two peaksin auroralintensity. main UV oval during this interval (a changeof a degreeor so
It coincideswith unstructuredion precipitationand structured is not importantin the contextof theseparticularimages).
2. The activeregionof the aurorasduringthis recoveryphase
electrons(invertedV's) lessthan about 1 keV.
4. The regionof unstructured
1 to 10 keV electronprecipita- is locatednear the polewardboundaryof the plasmasheetpartitionoverlapswith the structured
electrons
(lessthan1 keV) and cles. (It can be seen from the auroraldata that this poleward
coincides with the main UV auroral oval.
boundaryhas not movedsubstantially
over the intervalin ques5. Inverted V events occur near the peak in the main UV tion).
The ion precipitationwhich shows an energy dispersionat
oval. Equatorwardof this point the electronfluxesof plasma
sheetenergiesbeginto decrease
with the higherenergiesdisap- the highest latitudes(2125 to 2128 UT) is similar to that
pearingfirst. In thisregionthe isotropicboundaryof energetic reportedby Kovrazhkinet al. [1987] andZelenyiet al. [1990].
ions is found.
This VDIS has been associatedwith the plasmasheetboundary
6. Low-energyion dispersion
signatures
(LEIDS) canbe seen layer (PSBL) [e.g., Kovrazhkinet al., 1987; Zelenyi et al.,
in this mostequatorwardregion. The dispersionsignatureseen 1990]. In the case presentedhere, however, it is coincident
here has the same sense as the most poleward velocity with the mostpolewardarc system. If, as othershavedone,we
dispersedion signature,that is, lower energiesat lower lati- interpretthis region as the ionosphericsignatureof the PSBL,
then this region is also a sourcefor auroralarcs. This differs
tudes. Theseoccurjust equatorward
of the invertedV event.
7. A weak region 2 field-alignedcurrentand eastwardcon- from the view put forth by Feldsteinand Galperin [1985]
where the PSBL is a sourcefor diffuseprecipitationonly. This
vectiveflow extendwell equatorward
of the main UV oval.
paper[Elphinstone
The CCE satellitewas also traversingthis region at high alti- pointis furtherelaboratedin the companion
tudebetween1700 and 2300 UT. The trajectoryof the satellite eta/., 1994a].
The low-energystructuredelectronpopulationjust poleward
as it moved from L = 8.7 RE to L = 1.7 RE is shownin Plate 2
usingtwo separatefield line modelprojectionsfor Kp = 3. of this ion precipitationis below the thresholdof the UV
The trajectoryof CCE coversseveralhoursandso the coordi- imager. It is thereforeunrelatedto the UV polar arcs previnatesplottedare the magneticcoordinates
of thepoints(i.e. the ously discussedin the contextof being on closedfield lines
geographic
points would rotate significantlyrelative to the [e.g., Elphinstoneet al., 1993]. It is probablyrelatedinsteadto
aurora during this interval). The more polewardprojection the weaker red line (6300 A) polar arcs which may occur on
(white line) is due to the Tsyganenko
[1987] model while the open field lines. This electronprecipitationmay come either
Unstructured
isotropicions which have higherenergyat lower
latitudes. (4) 2136:10 - 2147 UT: Trapped ions and two
separateoverlappingdispersionsignaturesare seen. One
12,082
ELPHINSTONE
ET AL.' THE DOUBLE OVAL AURORAL DISTRIBUTION
SIGNATURES
ALONGTHE MORNINGSECTORDOUBLE
OVAL
AURORA
Transient Discrete Aurora
Main UV Oval
Diffuse
Auroral
Compo?nt
I
ELECTRONS
StructuredElectrons,1 keV(InvertedV's)
• Unstructured
Trapped
Elect• m
Unstructured
Ener
•t
Electrons
Depen
B•
ries
!
I
IONS
VDIS
•- e.•l
DD
sP_pe_e•
d;(•n_t
:-I ] Unstructured
Isøtrøpic
Iøn
Precipitatiøn
I i
or'Ions
4;20keV
I-
FIELDALIGNEDCURRENTSAND CONVECTION
Weaker
Region
UI
Strongest
Region
,,f.a.c.I
Minutes after
21 UT
MLAT
CGM
MLT
47 43
59'0'62.1'
3.6 3.8
II
35
64.2' 66.2'
67.4'
40
3.9
37
4.1
I
4.2
31 30
69.6'
4.4
I
29 28
•'
24 22
71.2'
73.9'
4.5
4.9
ELPHINSTONE
ET AL.: THE DOUBLE
a
OVAL AURORAL
DISTRIBUTION
12,083
HPCE/CCE
19:23:16 - 22:41:15 UT
July 27, 1986
079
8
6
5
4
3
2
10•
7
6
5
4
10s50
55
60
65
ProjectedC•M Latitude(Tsyganenko'87)
b
HPCE/CCE
July 27, 1986
2
19100
I • 11940UT 2120UT
2200UT
............................
.(.•.9..).
..........................
(#10Plate2)
..............................
107 .........................................................
(#6) ...[.(#•').[.'-(•½•)-.
7'
!:
.9 keV
2-
_
• 106
0keV _ --.
2
105
3
4
5
6
7
Tsyganenko '87 Projected MLT (hrs)
Figure 4. Electronfluxes at differentenergiesrecordedat the CCE satelliteversus(a) projectedT87 CGM
latitude and (b) T87 CGM local time. The universaltimes markedcorrespondto the points shownin Plate 2.
The main UV oval occursnear where the 20 keV fluxes peak.
Figure 3. (Opposite)Schematic
of the morningsectordoubleoval distribution
illustratingthe interrelationship
betweenthe variousobservationsfor the event shownin Plates2 and 3. In order from top to bottom are the
observations
of the aurora,electrons,ions and field-alignedcurrents/convection.
A schematicUV intensitycontour is plottedin eachpanel as a reference.
12,084
ELPHINSTONE ET AL.: THE DOUBLE OVAL AURORAL DISTRIBUTION
from the deeptail low latitudeboundary
layeror the tail lobes simultaneously.Plate 4 showsthe double oval distributionand
andmay be associated
with the "warm envelope
of theplasma its evolution over a 60-min time interval on July 27, 1986.
sheet" found by Zwolakowskaet al. [1992] with PROMICS-8 This event occurred about 13 hours before the event discussed
above.
high-altitudedata.
The
AL
index
at this time
varied
between
about 100
The regionof unstructured
1 to 10 keV electronprecipitation and 200 nT and was clearly recoveringby 0800 UT. An
overlapswith the structuredelectrons(less than 1 keV) and
auroral activation
coincides with the main UV auroral oval.
a doubleoval configurationby about0730 UT. Between0730
Poleward of this
occurred
at about 0715
UT which resulted in
region(2127 to 2137 UT) the ions showhigherenergiesat and 0800 UT, further auroral brighteningstook place in the 21
lower latitudes characteristic of adiabatic acceleration in the
to 0 MLT
sector.
The left panel of Plate 4 showsan averageauroraldistribuetal., 1989]. Similar characteristicsin the electrons could be tion coveringan interval of time when DMSP-F7 flew over the
interpretedas Fermi acceleration
in the centralplasmasheet. northern evening sector distributionnear 21 MLT (0738 to
This resultthen links the resultsof Sandahland Lindqvist 0742 UT and labeledN1 in the plate) and over the daysidesec[1990] andElphinstone
et al. [1991], who foundeachof these tor near 9 MLT (0729 to 0734 UT and labeledN2 in the plate).
There was also an overflight of the dusk sectorby DMSP-F6
regionsto map near the Earth.
The low-energy ion dispersionsignatures(LEIDS) can be betweenabout0731 and 0738 UT (labeledN3). The averaging
seenin thismostequatorward
region.Thesemaybe dueto sub- was chosen to illustrate the auroral configurationduring the
storminjectedionsor upflowingionsfrom the conjugate
iono- passageof thesesatellitesover the auroralregions. The change
spherewhich then separateunder the influenceof corottrion, in the polar cap intensitiesfrom the top to the bottom of the
gradientand curvaturedrifts [Sauvaudet al., 1981; Winning- imagesis due to changingsolar zenith angle altering the dayham etal., 1984]. D. M. Klumparet al. (Frejaobservations
of glow intensities(the entire polar cap is sunlightfor this event).
ion dispersion
precipitation
signatures
neartheinneredgeof the The differencesbetweenthe darkestregionsin the left and fight
plasmasheet,submittedto Geophysical
ResearchLetters,1994) imagesof Plate4 is an artifactof the scalingand averaging.
The main featuresof interestin the left panel are the followhave associated
thesewith the inner edgeof the plasmasheet
and,asin our case,alsosawinvertedV eventsjustpolewardof ing;
1. A double oval configurationis visible between21 and 03
them. This regionis associated
with the equatorward
portionof
the main UV oval andlies equatorward
of the40 key isotropic MLT. In this interval the main UV oval is the equatorward
ion boundary.This againdifferssomewhat
from the view put distributionwhile a secondsystemis evidentpolewardof it. A
forth by Feldsteinand Galperin,[1985] in that bright(in the minimum in intensity occurs between the two distributions
UV at least)aurorasare seenequatorward
of thisboundary.A althoughthe intensityis not uniformwith local time. The main
weak region 2 field-aligned current and eastwardconvective UV oval extendscontinuouslyfrom the night sectorto the dayflow extend well equatorwardof the main UV oval. Two side intensificationnear 15 MLT. The more poleward system
sourceregions(a near-Earthand tailwardsource)for region 1 doesnot extendto the daysidein the dusk sector.
2. A dawn sectorSun-alignedpolar arc (i.e., alignedparallel
field-alignedcurrentshavepreviouslybeennotedby Ohtaniet
al. [1988]. The resultspresentedhere tend to supportthat to the 0 MLT meridian in the plate) is visible between2 and 6
view.
MLT. A fainter feature is also apparentwhich representsthe
plasmasheet
[Galperinetal., 1978;Sauvaud
etal., 1981;Hardy
Inverted-Veventscan be foundcloseto the isotropicboundary of 40u keV ionsin the morningsector. The resultsof Lui
and Burrows[1978] andKirkwoodand Eliasson[1990] showed
similarresultsfor electrons.Theseinverted-Vscorrespond
with
arcson the main UV oval far equatorward
of the mostpoleward are system.The Feldsteinand Galperin[1985] view that
the discreteauroramapsto the centralplasmasheetis supported by the resultsof this paper and the view that discrete
auroramapsto the PSBL is alsosupported.Thus two impor-
weak extension
of the double oval feature into the dawn sector.
This featurebecomesmore clear in the subsequent
panels.
3. Two
unconnected
arcs are visible
between
to the above mentioned
6 and 9 MLT
which
are
features.
4. The afternoon peak intensity occurs near 15 MLT and
there is a weak separateddfuse band of precipitationequatorward of this featureextendingbetween15 and 19 MLT.
The top fight panel showsthe averageauroralconfiguration
between 0811 and 0819 UT during the time period when
tant separatemagn.etospheric
regions contributeto the discrete DMSP-F7 flew over the southerneveningsector(0815 to 0819
doubleoval auroraldistribution.The low-energyelectronsand UT and labeled S2) and DMSP-F6 flew over the southerndusk
protonswhich are presumablyof plasmasheetorigin show sectoraurora (0811 to 0815 UT and labeled S1). The plotted
trajectoriescorrespondto the southernhemisphereCGM coorditrappedfluxes which extend far equatorwardof the main UV
oval. Both the low- and high-altitudeobservations
demonstrate nates at 120 km altitude plotted as northernhemisphereCGM
that the inner edge of the plasmasheetis not a well-defined coordinates.The mappingsthereforeassumedsymmetricexterboundarybut that it is in fact a magnetospheric
regionthat nal current perturbationsof the magnetic field about the equamaps to a latitudinallyextendedionospheric
region. Therefore torial plane (nonconjugateeffects betweenhemispheresare due
it is not sufficientto simplystatethat something
mapsto the to asymmetricfield perturbationsbetween the north and the
inner edge of the plasma sheet. The main UV oval where south). The changeswhich haveoccurredrelativeto the previdiscretearcsare foundapparentlycoincides,at leastat low alti- ouspanelare given below:
1. The most polewardarc systemof the doubleoval is sometudes,with the outerboundaryof this regionwhichappearsto
what less intense but now extends further into the dusk sector.
be quite dynamic.
This arc systemhas becomea continuousarc systemfrom
Daysideand EveningSector Double Ovals
Auroral
distribution.
The double oval distribution tends to
occurpredominantlyin the morningsectorbut can be found to
occur solely in the dusk sector or in both local time sectors
about 18 MLT
to at least 3 MLT.
2. There is a continuous arc on the main UV
oval which
beginsnear 18 MLT and extendsto about23 MLT. This arc
systemintensifiesfirst near dusk and propagatesrapidly eastward into the eveningsector.
ELPHINSTONE
ET AL.: THE DOUBLE OVAL AURORAL
DISTRIBUTION
12,085
S2
VIK
AVERAGEFROM0811- 0819 U1
AVERAGE FROM 0729-0743 UT
DOUBLE
OVAL
ON JULY
27 1986
AVERAGE
FROM
0822-
0831 U
Plate 4. The double oval observedon July 27, 1986 between0729 UT and 0831 UT. The intensitiesare
represented
by false colorsrangingfrom dark blue (weakest)throughyellow,red to white (mostintense).
DMSP trajectories
S1 to S4 andN1 to N3 are shownandhavethefollowingneuralnetworkparticleboundaries
associated
with them (beginningwith the most polewardregion and moving equatorward):
N1- LABPS,
LACPS, LABPS, LACPS, LABPS, LACPS; N2- Mantle, LLBL, LABPS, LACPS; N3- LABPS, LACPS; S1LABPS, LACPS; S2- LABPS, LACPS, LABPS, LACPS; S3- LABPS, Void/Polar rain, Mantle; LABPS,
LACPS; S4- LABPS, Void/Polar rain, LABPS, Void/Polar rain, Mantle. Dots have been used to separatethe
particleregionsandhaveonlybeenplottedin thepanelscorresponding
to theUT rangein whichtheparticle
observations
occurred.
Although it may be obvious, it is perhapsuseful to comment
3. The Sun-alignedarc hasdisappeared
or mergedwith the
doubleoval configurationleaving only two regionsof peak about the apparentlack of emissionsbetween the two arc sysintensity.The doublearc systemin the late morninghas also tems near 3 MLT in the panelson the right in Plate 4. This is
not a true lack of UV emissionsbut is simply a result of the
faded.
In the bottomright panelthe auroraldistributionis shown chosen intensity scale. A given color schemeis only able to
corresponding
to the DMSP overflightsin the late morningsec- resolve a certain intensity range. Thus in order to show the
tor. The trajectory labeled S3 was a DMSP-F6 southern auroraldistributionin full sunlight(the observationsare in July
hemispherepass between0820 and 0827 UT, while S4 was a
DMSP-F7
orbit
from
about 0823
to 0831
UT.
This is also
close to the time when Viking encounteredthe polewardboundary of about5 keV ion precipitationnear 3 MLT in the plate.
The
distribution
in this case is similar
to that seen between
0811 and 0819 UT exceptthe following:
1. The most poleward system is more clearly visible near 7
MLT
and near 19 MLT.
2. The arc system on the main UV oval has developed a
spiral feature seen,in the Plate, as a green brighteningnear 21
MLT.
in the northern hemisphere) a color scheme was chosen to
enhance the global pattern. Naturally as a result, small scale
variations are lost. It should not be taken to imply that the
double oval distributionrepresentstwo systemsseparatedby no
emissions(see Figure 3).
Particles. The DMSP particle boundarieswere determined
by a neural network [see Newell et al., 1991]. In what follows,
the labels "LABPS" and "LACPS" have been usedto identify
traditionalionospheric
particleregionsknownas boundary
plasma sheet and central plasma sheet [Winningham et al.,
1975]. The prefix "low altitude" (LA) has been added to
12,086
ELPHINSTONE
ET AL.: THE DOUBLE
OVAL AURORAL
DISTRIBUTION
F7
1.2
.
El FCTRONS-
:
! (3(;JE9
4LOG
..,...._.,,•.::.•.,%
.....
.. •,.,.. ,. '-"
. ,..•'..
E AVE
I
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. d'" . .a.. n?-
•
.......
-.-.
'''1
-
.........
-.
'-..
.
ø '-' .',•.-,,- '.-g.
.
I ........
, • -
.a- -
...
' I''
•',''""'1
t
.....
ß
LOG
E FLUX
%-'•..
''''!
' '""r "'
'•
ELEC
10
''"
•.•
2
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2
"1
I"
,
•
t12
t
z3
I
2 !i
I II
! •1 '
I
07:39:{•
74.5
GLAT 68.0
GLONG
MLT
237.6
21:34
• '•
'
4 i!1II
UT
MLAT
ION
8
4
>. 2 t
•
86/208
,eø" .,•
e- .." .".' •. a'.r
-
'''''''''•
a,,.
-o .-:•.• •--'•,',._....
'""'"'
. -.
.•
,
i.
I
3
I I
tl I I!
S••
07:39:34
72.5
07:40:08
70.6
07:40:42
68.6
07:41' i0
66.6
07:41'50
64.7
07:42:24
62.7
07:42:59
60.7
235.4
21:35
233.5
21:35
231.7
21:35
230.2
21:6
228.8
21:36
227.6
21:36
226.5
21:37
66.7
•.9
63.0
61.1
59.2
57.3
S5.3
Jul 27
Plate 5. DMSP-F7 northernhemisphere
spectrogram
showingtwo sourcesof discreteauroralarcs.
Electron/ion
fluxesandenergies
aregivenin thetoptwo panelswhilethe bottomtwopanelsshowthetimeenergyspectrograms
for ionsandelectrons.This spectrogram
corresponds
to theN1 trajectoryshownm Plate
4.
theseto ensurethatthereaderdoesnotconnect
thesewith mag- of 2.7 and 8.8 keV, respectively.Equatorwardof this are four
netospheric
regions. In what followsthe particleregiondefini- alternatingregionsof LACPS, LABPS, LACPS, and LABPS all
tionsare basedon neuralnetworkdesignations.
locatedin the regionof the main UV oval (the concentration
of
In eachof thepanelsof Plate4 all of theDMSP trajectories light red dotsalongtheN1 trajectoryrepresents
thisregion). In
have been plotted as a reference to the reader. Those which the spectrogram
this corresponds
to the times from 0741:12 to
occurredwithin the intervalof time associated
with a given 0741:41 UT. Theseregionsshowhigh fluxesof bothelectrons
panel have been plotted with dots. These dots delineatethe and ionsrelativeto the morepolewardregion. The fluxesof
variousparticleregionsfound by the neuralnetwork. The tra- ions increasewith decreasinglatitude. The averageenergyof
jectoriesaretheCGM 1980coordinates
of theDMSP orbitpro- the ions (12 to 16 keV) was higherthroughthisregionthanthe
jectedto 120 km altitude. For examplethe trajectories
N1, N2, morepolewardLABPS region.The electronsshowhigheraverand N3 were northernhemisphereorbits by DMSP satellites age fluxesin the moreequatorward
LABPS regions.The most
which occurred within the time interval 0729 to 0743 UT.
equatorwardportion of the main UV oval was identified as
The time-energy
spectrogram
for theN1 DMSP-F7trajectory LACPS. In this region, both the electrons(=1 keV) and the
is shownin Plate5. The doubleoval canclearlybe madeout ions(= 5 keV) havemuchlower averageenergiesand fluxes.
as two regions of high electron flux (top panel of Plate 5) Again, similar to the morningsector,the energyof both the
separated
by a minimumnear0740 UT. Similarto themorning electronsandionsdecreases
with decreasing
latitude.
sectorcase, some low energyelectronswere seenpolewardof
A similar result is seen about 40 minutes later, one hour
the mainprecipitation
region(= 0739:15UT). The mostpole- local time to the eastnear22 MLT (profileS2 in the upper
ward LABPS region (0739:34 to 0741:12 UT) encompassesfight panel). The mostpolewardarc system,andthepoleward
both the mostpolewardarc systemandthe regionbetweenthe portion of the region betweenit and the main UV oval, were
two ovals. This regionhad averageion and electronenergies identifiedas LABPS. The arcsystemwhichhasbrightened
on
ELPHINSTONE
ET AL.: THE DOUBLE OVAL AURORAL
DISTRIBUTION
12,087
F6
12
86/208
I()NS
ELECTRONS
i. OG J E
9
4LOG E AVE
LOG
E FLUX
ELEC
10
2
ION
8
•• 3 tIill
5_
UT
MLAT
GLAT
GL()NG
MLT
-62.3
-48.6
142.9
18:06
I}8:11:42
-64.9
-5 I. 1
141.9
18:05
08:12:25
-67.4
-53.5
140.6
18:03
08:13:08
-70.0
-55.9
139.3
18:00
08:13:50
- 72.6
-58.3
137.9
17:58
08:14:33
-75.2
-60.8
136.2
17:53
08:15'16
-77.7
-63.1
134.2
17:46
08:15:59
-80.3
-65.5
132.0
i 7:36
3
Jul 27
Plate 6. DMSP-F6 southernhemispherespectrogramshowing the dusk sector extension of the double oval
configuration(same format as in Plate 5). A large flux decrease(top panel) is seenbetweenthe two arc systems. The most polewardsystemis not visible in the northernhemisphereUV auroraldistribution. This spectrogramcorrespondsto the S1 trajectoryshownin Plate 4.
the main UV oval is LABPS precipitationsurroundedby
The mostpolewardregionwasmantleprecipitation
followedby
LACPSprecipitation.
Hereagainwe seetheinverted
V events low-latitudeboundarylayerandLABPS andLACPS in orderas
on the trajectory(thedotsdelineatethe
embedded
withina regionof plasmasheetlike
diffuseprecipita- onemovesequatorward
of the regions). Similarly,near 18 MLT, the trajection. The energiesandfluxesof the particlesfollow similar boundaries
trendsto that describedin the previouscase. The mostpole- torylabeledN3 (0736-0738UT) in theupperleft panelof Plate
wardarcin thiscaseliesjust equatorward
of andadjacentto a
4 also showednothingunusualwith the polewardregionbeing
regionidentifiedas LACPS.
velocitydispersed
ion signature
(VDIS). The pointlabeled LABPS and the moreequatorward
"VIK" near3 MLT in the left panelshowsthe locationwhere Note that the LAB PS region is locatedon the samearc system
the Viking spacecraft
encounters
the polewardedgeof ions
withenergies
of about5 keV. Thispointcoincides
quitewell
withthepoleward
edgeof thedoubleovalconfiguration.
On thedaysideandalongthedawn/dusk
meridian
thesituationis considerably
different.Whatoneseesis strongly
local
timedependent.
Thetrajectory
labeled"N2" in theupperleft
which is associatedwith the equatorwardLABPS found closer
to midnightin the N1 trajectory.The LACPS regionat this
local time is associatedwith a separatedetachedportion of
auroralemission. The particle characteristics
of this trajectory
look similar to thosefound on the nightside.
The particlecharacteristics
howeverchangein the dusksec-
hemisphere
overflight(S1 in the
panelof Plate4 hasbeeninvestigated
by Ohtaniet al. [1995]. tor by thetimeof thesouthern
This showedan interesting
setof fourfield-aligned
currentsin
top rightpanel). The time-energy
spectrogram
for the S1 tra-
this sectorassociatedwith the two auroralarc systemslocated jectoryis shownin Plate6. The top panelsshowhow the ion
between6 and 9 MLT. This DMSP orbit was just east of and electronaverageenergiesand fluxeschangeand it is clear
where the double oval could be seen and so registereda rela- that there is a dramaticdepletionin the electron flux and
tivelystandard
setof dayside
ionospheric
particle.
boundaries.energybetweenthe two setsof arcs at about0813:20 and
12,088
ELPHINSTONE
ET AL.: THE DOUBLE
OVAL AURORAL
DISTRIBUTION
F6
12
86/208
IONS
ELECTRONS
-
I.OG JE
9
4L()G
E AVE
LOG
E FLUX
ELEC
ION
10
8
5
3
-2
.
il
t tl
i
UT
08:19:00
MLAT
-87.7
GLAT
-74.9
GLONG
115.9
MLT
!0:17
08:20:08
-84.6
-77.9
104.0
07:47
08:21:i6
-81.0
-80.1
85.9
07:11
08:22:25
-77.4
-81.0
61.3
06:55
08:23:33
-74.0
-80.3
36.7
06:43
08:24:42
08:25:50
08:26:59
-70.6
-67.4
-64.3
-78.2
17.4
06:34
-75.3
5.0
06:28
-71.9
356.9
06:23
Jul 27
Plate 7. DMSP-F6 southernhemispherespectrogram(same format as Plate 5) showingthe variousparticle
regionsassociatedwith the daysideextensionof the double oval configuration. This spectrogramcorresponds
to the S3 trajectoryshownin Plate 4.
0814:50 UT. There is a secondsimilar depletionat about the main precipitation
region. This constitutes
the dayside
0813:00 UT. The more polewardarc featurewas not evident extensionof the doubleoval configuration.
some40 minutespreviouslyin the northernhemisphere
(N3).
The regionsof LABPS,void/polar
rain andmantleline up
This appears
to be theextension
of themostpolewardarcsys- ratherwell for thesetwo DMSP passesindicatingthe two-
tem of the doubleoval configuration
into the daysideiono- dimensional
characterof the differentprecipitation
regions.
sphere.Note thatin thiscase,therewasno obviousUV signa- Thereis a generalcorrespondence
between
the themostpoleture of the most polewardarc systemat this local time in the wardarc systemof thedoubleoval andthe LABPSsignatures
northernhemisphere.The more equatorward
set of arcswere although
the lackof exactcorrespondence
is probably
dueto a
associated
with ions whoseenergydecreased
with decreasinglack of conjugacyof thesehigh latitudearcsbetweenthe north
latitude.
and the south. These DMSP LABPS regionshave a lower
The morningsectoralsohadinteresting
signatures
associatedelectronaverageenergythanis usuallyseenon thenightside
or
with the extension
of the doubleoval into the daysideregion. on themoreequatorward
LABPS. For theS3 passtheaverage
Furtherto the west of the N2 trajectorytwo southernhemi- energywas only 0.3 keV, while for S4 the two mostpoleward
sphereDMSP passes(S3 andS4) tookplacebetween0822 and LABPS signatureshad averageenergiesof 0.6 and 0.9 keV.
0831 UT. The dotson the S4 passseparate
the following The regionclassified
asvoid/polar
rainby theneuralnetworkis
regionsas one movesequatorward:
LABPS, void/polarrain, associatedwith a depletionbetweenthe double ovals and the
LABPS,polarrain andmantleprecipitation.Similarlyfor S3 mantleregionappearstowardsthe polewardboundary
of what
the regionsare: LABPS, void/polarrain, mantle,LABPS, and one might describe as the main UV oval at these local times.
LACPS. The time energyspectrogram
for the S3 passis Thisidentification
of mantleby theneuralnetworkis probably
shownin Plate7. The abovementioned
regionsaremarkedon not accurateand could probablybe re-labeledas LABPS since
the Plate. What is immediatelyevidentfrom Plate7 is the arc theaverage
ionenergyof 3.5 keV is ratherhighformantlepresystemnear0820 UT whichlies about7 degreespolewardof cipitation. It and the electronenergieswere, however,consid-
ELPHINSTONE ET AL.- THE DOUBLE OVAL AURORAL DISTRIBUTION
12,089
erably lower than the regionof LABPS immediatelyequator- with the morningsectorevent which showedthesefeaturesto
ward.
be coincident. It thereforeappearsthat this mostpolewardarc
with eitherthe PSBL itselfor with its
Interpretationand summary. Duringthe first part of this systemcan be associated
event(0729 to 0743 UT) the doubleoval was activein a region inner edge. What governsthis relationshipis probablyrelated
limited to 6 hours of local time centred around midnight.
DMSP satellitepassesduring this interval confirm this and
show two separateregionsof LABPS-Iikeprecipitationin the
eveningsector.The morepolewardregionwasassociated
with
the discreteauroraat the polewardedge of the doubleoval as
well as weaker featuresin the region between the two ovals
(similar to the top of Figure 3). The more equatorwardregion
lay embeddedwithin a LACPS particleregion. The existence
of the discreteaurorain this regionagain supportsthe view of
Feldsteinand Galperin [1985] and is contraryto the view that
this regionis alwayspurelydiffuseprecipitation.
The S2 DMSP passshowedthat the discretearcs near the
polewardboundarylay equatorward
of and adjacentto what
one might considerto be the ionosphericsignatureof the
PlasmaSheetBoundaryLayer (i.e., the VDIS). This contrasts
"'.',
.
.
low altitude. We have, however, seen two cases where discrete
auroralforms are embeddedwithin this innerregion. Galperin
et al. [1992] have presenteda meansby which thesearc systems might form in this inner region. In the next section we
give an example in which this inner region makes a transition
from diffuse
to discrete.
We have also seen the signaturesof the double oval in the
dawn and dusk sectorsto be considerablydifferent than those
seencloserto midnight. These arc systemsform in association
with the substormprocess[Elphinstoneet al., 1993] and form a
continuousfeature from midnight to the dayside. Although
they have the particle characteristics
of the PSBL on the night-
/
. APRI•16,brbit293
.r
to local conditionswithin the boundarylayer itself.
On the other hand, the inner portion of the central plasma
sheetis frequentlyassociatedwith only diffuse precipitationat
':.
m
i
.
q,
.t:
i
I•'I I I
.
4
!
i
i
032125 UT
0324o6 UT
ß
.
If
i
I
ß I.
4
t
/
ii
!
ß
,'
032647 UT
2
032828 UT
..,
. ,- '
'"•'
ß
,; •' -
I
._
ß
"
"
:.-•
"
I i
m
'--
,,•
r*
'1
't
I
ß
!I
I
.p
!
I
033108 UT
,
033349 UT
Plate 8. The couplingof the two portionsof the double oval. Viking imagesare shown from 0321 to 0334
UT on April 16, 1986. Pointsalong the DMSP-F7 satellitetrajectoryare shownin the lower right panel to
illustratethe the variouslow altitudeparticle regionsrelative to the auroraldistribution. Regions0, 1, and 2
correspondto no precipitation,LABPS and LACPS respectively.An arc systemcan be seento begin from the
mostpolewardoval (top right panel) and propagatetowardthe main UV oval. It couplesto the main UV oval
by about 0331 and 0334 UT an azimuthalauroralform intensifieson the main UV oval.
12,090
ELPHINSTONE ET AL.: THE DOUBLE OVAL AURORAL DISTRIBUTION
side, they resemblethe signaturesof polar arcson the dayside
(see, for example, the the polar arc observationsby Burch et al.
[1992]). We suggestthat the dayside portion of the double
oval maps to the deep tail flanks of the magnetosphere,
as was
proposedfor polar arcsby Elphinstoneet al. [1993]. The particle signatures equatorward of these arcs is dependent on
whether the Low Latitude Boundary Layer earthwardof the
sourceis on overdrapedopen field lines or not [seeElphinstone
et al., 1994]. This in turn dependson the the senseof the y
componentof the interplanetarymagneticfield.
This mapping schemeallows the double oval to be continuous from midnight to the dayside. It interpretsthe changing
particle characteristicsas being due to a change in source
region from PSBL to LLBL, and explainsthe ionosphericsignaturesusing a magnetospheric
topologywhich is simple. This
is the only method known to us which also explains as a
natural consequencethe Sun-alignednature of the polar arcs
and the featuresof the doubleoval configuration.
4. Coupling of the Two Auroral Distributions
Plate 8 shows how a coupling can exist between the most
polewardauroralarc systemand the main [IV auroraloval. At
0321 UT in the top left panel of Plate 8 a singlearc systemon
tile most poleward oval can be seen between about 21 and 0
MLT. By 0324 UT a new systemhas penetratedthe region
betweenthe two ovals and is aligned from the northwestto the
southeast. At this time it is still connectedto the most poleward system. In the next panel at 0326:47 UT this arc system
can be seen more clearly and now extendsalmostto the main
UV oval. It creates a north-southaligned connectionbetween
the two ovals. By 0328:28 UT the emissionsin the main UV
oval connectto the featureand by 0331:08 UT it hasfadedbut
the main UV oval to the west has brightenedsomewhat. A
substormonset then beginsfrom this location (lower left panel).
main UV
auroral oval from a diffuse to discrete character is
fundamental
to understanding
the substorm
onsetas havinga
near-Earth
source.
This is supported
bothby the observation
that the poleward
arcsremainpolewardof the new systemand by the observations given in Plates2 and 4 showingthe discretearcsembedded within the diffuse LACPS-Iike region. The "variance
image"resultsshownin Plate2 (eventA) furthersupportthis
view and indicate that different regions of the
ionosphere/magnetosphere
systembecomeactive during the
expansionphaseand recoveryphase. We can now begin to
understand
the dynamicsof the regionnamedby Winningham
et al. [1975] as the collapsingBPS. This is the regionbetween
the two "ovals" which can developfield-alignedpotential
dropsand form dynamicarc systemsin a previouslyinactive
ddfuseauroralarea. To understand
thisregionproperlya more
globalview mustbe adoptedand the dynamicsof the region
evaluated.It will alsobe importantin the futureto distinguish
betweenthe two typesof discreteprecipitationwhich are associatedwith eachoval in the doubleoval configuration.
Thesedynamicsallow one to reconciletwo competing
views
aboutthe originof auroralarc systems.Frequently,
the only
discretearcsoriginatein or near the PSBL. Under other condi-
tions (which are not yet clear) the inner magnetosphere
becomes
an activesourceregionfor thesearcs. This can happen at substormonset but also can continue well into substorm
recovery. One importantquestionto addressin the future is
whetherthe innermagnetosphere
is a sourcefor arcsundervery
quietconditionsand what magnetospheric
conditions
give rise
to them.
5. Summary and Conclusions
The doubleoval distribution
formsat the beginning
of substormrecovery(or end of the expansion
phase)and playsan
subsequent
auroralactivations.
time when the more poleward systemis active and a large scale integral part in understanding
The activation
at 0333 UT occurs on the main UV
oval at a
stationarysurgeexists (see, for example,the image in the lower
left panel). There was a DMSP-F7 overflightof the regionjust
to the east of the activationregionbetween0332 and 0336 UT.
The various particle regions as defined by the neural network
havebeendelineatedin the panelat 0331 UT. In this casethe
LACPS correspondsclosely to the location of the main UV
oval
where
the new
activation
occurs.
This
illustrates
an
important aspectof theseparticle definitions. A new discrete
auroralactivationhas occurredin a regionof LACPS.
Interpretation
This is an exampleof a disturbance
that was probablyinitiatedin the PSBL and eventuallyafter about10 min generated
conditionsin the near-Earthregion sufficientto begin a nearEarthsubstormonset. Bakeret al. [1993] havepreviouslyattributednear-Earthonsetsto triggeringdue to reconnection
earthward of XasM= -25 RE. Our resultsindicatethat the coupling
may be somewhatindirect, such that the propagationinward
allowsthe triggeringof an onsetin the innerregionof the magnetosphere.This type of couplingappearsto be sufficientbut
not necessaryfor a near-Earthonset.
Themoreequatorward
ovalor themain[JV ovalis a steady
feature of the auroral distribution in contrast with the more
polewardone. This main UV oval in the morningsectoris
locatednear the isotropicboundaryof 40 keV ions and substorm onset is associated with it.
The polewardoval beginsto developat theendof expansion
phaseand dependenton conditionscan developduring substormrecoveryphaseinto a distribution
covetingmostmagnetic
localtimes. Particledata supportthe linkageof the poleward
oval to boundarylayer processes
whichbecomeactiveduring
substormrecoveryphase. This most polewardsystemnear
midnightshowsperiodicactivationswhich can developinto
large scalesurgeforms. This polewardoval can be linked to
PSBLprocesses
nearmidnightbut is moreeasilyexplainedas
being a deep tail low latitudeboundarylayer phenomenon
closer to the dayside. There appears,therefore,to be an
interestingtransition region somewherebetween 18 and 21
MLT (and also03 to 06 MLT) wherethe mostpolewardare
ceases to have PSBL characteristics and takes on the character
associatedwith high latitudepolar arc events. This view then
separates
the ionospheric
projectionof the deeptail low latitude
boundarylayer and the plasmasheetboundarylayerandputs
Associated with the onset arc would be conditions such that
constraints on substorm theories.
parallel potentialdropscould be set up and the aurorabecomes
Observations
were given which show that a couplingcan
more dynamic.This occursin a region which was previously occur betweenthe near-Earthsubstormonsetsand processes
diffuse
and LACPS-like
in character.
This transition of the
originated in the PSBL. These observationsshow that the tern-
ELPHINSTONE
ET AL.: THE DOUBLE OVAL AURORAL
DISTRIBUTION
12,091
national Conference on Substorms - 1, Kimna, Sweden, Eur.
poral developmentof featuresduringthe doubleoval configuraSpaceAgencySpec.Publ., SP-335, 13-18, 1992.
tion are fundamentalto understanding
the LABPS and LACPS
signatureswhich result from local satellite observations.As Elphinstone,R. D., J. S. Murphree, D. J. Hearn, W. Heikkila,
L. L. Cogget, and I. Sandahl,The auroraldistributionand its
arcs develop in the inner magnetosphere,in associationwith
mappingaccordingto substormphase,J. Atmos.Terr. Phys.,
substormonset, regions of overlapping discrete and diffuse
55, 1741-1762, 1993.
aurorasoccur in the ionosphere. This helps one to understand
controversieswhich have arisen concerningthe mapping of Elphinstone,R. D., et al., The double oval LIV auroral distriauroral arcs. Two fundamentallydifferent sourceregions for
bution, 2, The most poleward arc systemand the dynamics
of the magnetotail,J. Geophys.Res., this issue.
nightsidediscreteauroralarcs appearto exist for magnetically
active conditions.
Elphinstone,R. D., D. J. Hearn, and J. S. Murp•ee, Dayside
To investigatethe dynamicsof the nightsidesubstormregion
aurorapolewardof the main auroraldistribution:Implications
we have developeda new techniquefor the analysisof auroral
for convectionand mapping,in PhysicalSignaturesof Magimagedata. This useswhat we havecalled "varianceimages"
netosphericBoundaryLayer Processes,editedby J. A. Holtet
and A. Egeland, pp. 189-200, Kluwer Academic, Norwell,
to produceimages of the active and dynamic regions of the
Mass., 1994.
aurora. These indicatethat fundamentallydifferentregionscan
be active during the expansionand recovery phasesof a sub- Fairfield, D. H., and A. F. Vinas, The inner edge of the plasma
storm.
sheet and the diffuse aurora,J. Geophys.Res., 89, 841-854,
1984.
Acknowledgements. The authorswould like to thank Ann
Marie Morris for helpingwith the manuscript. The Viking project was managedby the Swedish Space Corporationunder
contractto the SwedishBoard for Space Activities. The UV
imagerwas built as a projectof the NationalResearchCouncil
of Canada and this work was supportedunder grants from
Natural Sciencesand EngineeringResearchCouncil of Canada.
A. Wright is supportedby a United Kingdom PPARC
AdvancedFellowship. The Lockheedportionof this work was
supported
by NASA contractnumberNAS5-30565.The first
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structurein the high-latitudeupper atmosphere:Its dynamics
and relationshipto the large scale structureof the Earth's
magnetosphere,
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Galperin,Y. I., V. A. Gladyshey,N. V. Jorjio,R. A. Kovrazhkin, V. M.
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gradientstructuresin the tail neutralsheetas rootsof the arcs
with some effects of stochasticity,Geophys.Res. Lett., 19,
authorwould like to thank T. Lui for suggestionsand a valu2163-2166, 1992.
able evaluation of the paper and one referee for suggesting
Hardy,
D. A., M. S. Gussenhoven,and D. Brautigam,A statistchangeswhich have greatlyhelpedthe paper.
The Editor thanks T. I. Pulkkinen and another referee for
ical model of auroral ion precipitation,J. Geophys.Res., 94,
their assistancein evaluatingthis paper.
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Kamide, Y., and S-I. Akasofu, The auroralelectrojetand global
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