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VOL. 79, NO. 16
JOURNAL OF GEOPHYSICAL RESEARCH
JUNE 1, 1974
Satellite Observationsof the Equatorial Mg II
Dayglow Intensity Distribution
J.-C.
Gf•RARD • AND A. MONFILS
Institut d'Astrophysique,Universityof Likge, Ougr•e-Cointe,Belgium
Continuousmeasurements
of the Mg II (2796-2803J•) doubletintensityin the equatorialdayglow
have been obtained at dusk during the period March 19 to May 23, 1972, by using the
TD1 A UV astronomicalsatellitelookingupward.Thesedata confirmthat the glow observednearthe
equatorcan be accountedfor by resonance
scatteringof sunlightby Mg+ ionspresentat altitudesexceeding530km. Isophotemapsarepresented
showingthe morphologyanddynamicsof thephenomena.
It is clearthat the emissionzonesarecloselyrelatedto thepositionof the dip equatorand presenteithera
singleor a doublepeak. Importantlatitudinalasymmetries
are frequentlyobserved:it is suggested
that
they reflectdifferences
in the heightdistributionof the ion layer in the two hemispheres,
in agreement
with the resultsof ionosphericand satelliteobservations.
A statisticalanalysisof longitudeandplanetary
magneticactivity effectsis made. A maximumoccurrencefrequencyis reachedin the Asian sector
(60ø-150øW).
Important
day-to-day
variations
areobserved
onsome
occasions,
buttheaverage
dailyintensity is not affectedby Kp.
motions. Magnesium ions have also been detectedin the
equatorialE region by rocket-bornemassspectrometers
by
Aikin and Goldberg[1973].They were found in amountscompar•ibleto the amountof Fe+ and exhibiteda peak near 95
at
propertiesof the 2, 6300 O I radiationin the vicinityof the km reachinga densityof 10a cm-a. The densitydecreased
equator (see review by Reed [1972]). Later, measurements higheraltitudes,but no resultswere reportedabove 120 km.
were made at various wavelengths,thus completing the unRecently,Boksenberg
and Gbrard[1973]describedthe TD1
derstandingof the physicaland chemicalprocesses
involved. A satellite observation of the presenceof an UV dayglow
However, it is only the recent use of satellite-borne above 540 km, which was mainly attributed to the resonance
photometersand spectrometers
that givesa globalpictureof
scattering
oftheMg I12796-to 2803-Adoubletinthedusksubthe phenomenon.
tropicalatmosphere.
The purposeof thispaperisto describethe
morphologyof thisUV dayglowon a worldwidebasisandto ilHicksand Chubb[1970]discovered
the existence
of two UV
arcssymmetrically
centeredon the dip equator.The observed lustrateit with isophotemaps.In particular,it isshownthat imasymmetriesoftenexist.The influence
intensitywasmainlyaccounted
for by Hanson[1969]in terms portantinterhemisphere
of nighttimeradiativerecombinationof O+ ionsat 1304-1356 of Kpontheoverallintensity
isinvestigated
andturnsoutto be
,/k.The belt shapeof theseUV arcswasclearlyillustratedby negligible.The longitudeeffectsare studiedand are clearly
the photographs
takenfrom the moonduringthe Apollo 16 presentin these data.
mission [Carruthersand Page, 1972].
INSTRUMENTATION
Observationsby the Ogo 4 satelliteof the night X 6300 intensitywerereportedby Reedet al. [1973]and discussed
by
The observationshave been made by means of specChandraet al. [1973]. Thomasand Donahue[1972]measured trophotometers aboard an Esro satellite as part of
with Ogo6 the 5577-Aintensitydistributionandderivedthe astronomicalexperiment S2/S68. The satellite, labeled TD1
structureof the electrondensity.Both setsof data showedimA, was launched on March 12, 1972, from the Western test
portantasymmetries
in the integratedintensitywith respectto
range in California. The period was 95.5 min, and the orbit
the dip equator. They also gave evidenceof important was nearly circular (altitude of 530 km) and nearly polar
longitudinal
variationsin the intensitydistribution,thus (inclination of 97o55' to the earth's equator). This angle has
suggesting
someeffectsof the magneticfield morphologyon beenchosenso as to have the orbital plane precessabout the
the electron density. Both forbidden oxygen lines are rotation axis of the earth at a rate of approximately lø/d,
producedby dissociative
recombinationof O•.+, whosenight- which means that the orbit is roughly sun synchronous.
time sourcedependson chargeexchangeof O•.with the domi- Figure 1 describesthe orbital characteristicsof TD1 for the
nant ion at the altitude of emission,O +. The arcs observedin
vernal equinox configuration. The equatorial regions are
these wavelengthsare thus the optical signatureof the crossedat approximately0600and 1800LT, i.e., at dawnand
equatorial anomaly.
dusk,respectively.This situationremainsinvariant, owingto
The existenceof Fe+ ions in the equatorial F•. region was
the sun-synchronous
characterof the orbit.
detectedby HansonandSanatini [1970]with the Ogo 6 retardThe symmetryaxisof the satelliteliesalonga planeperpening potential analyzer.Those authorsexplainedthe presence dicular to the sun-satellite line. Moreover, the bottom of the
of theseions at suchaltitudeswith the 'fountain effecttheory,'
spacecraftis kept facingthe earth by meansof horizon senwhich postulatesthat metallic ions from the meteoriteablasors.
tion region are raisedto considerableheightsby E x B
The S2/S68 experiment, one of the seven experiments
aboard,is composedof a relativelylarge telescope(27.5-cm• Aspirant of the BelgianFoundation for ScientificResearch.
diameter off-axis paraboloid) followed by a specCopyright ¸ 1974 by the American GeophysicalUnion.
trophotometricdevice.
Ground-based optical observationsof the equatorial red
arcs near the magneticequator date back to more than 10
yearsago [Barbierand Glaume,1960].They haveprovidedimportant information on the morphologicaland dynamical
2544
Gi•RARD AND MONFILS: BRIEF REPORT
2545
airglow and auroras, (3) high-energyparticles,and (4)electronic
T!•
and detector
noise.
Scatteredsunlight. This wasa veryimportantbackground
noise, since it was necessaryto record faint stellar spectra
while the telescopewas illuminated at 90ø by the solar light.
No remnant influence of scattered sunlight has ever been
detected;thus any influenceof the earth or the moon except
4
for direct observations
is eliminated.
Airglow. We have mentionedthat the wavelengthslikely
to correspondto a residualairglow above 550 km have been
excluded.This was the reasonfor the lower wavelengthlimit
(1350A). A residual
UV signalof about20 countsperinterval
(apparently due to instrumentally scattered Lyman a) is,
however, observedin channelsA2 and A3. This signal is
negligiblefor A1 and A4.
High-energyparticles. The detectorshave been protected
from trapped particles by shieldingof the tube walls. The
result is roughly as was expected: in the South Atlantic
anomaly (SAA) an important backgrounddevelops,and for
channelsA1, A3, and A4 it preventsany sensitivedetectionof
light. For channelA2 the apparentfluorescence
causedin the
window is not detected by the CsI photocathode. This
channel remains almost insensitiveto the particles.
Outsidethe SAA, exceptin the auroral regions,the particle
noise in all channels is negligible. This noise is easy to
separatefrom the precedingnoisesby closingthe entrance
Fig. 1. Orbit of the TD1 A Esro satellite.The S2/S68 UV telescope
shutter; thus any light is prevented from reaching the
is looking upward from about 530 km.
photomultipliers.
Detector noise. Laboratory tests have shown that the
A very important fact is that owing to the astronomical preamplifiernoiseis negligiblein comparisonwith P.M. dark
currents.The P.M. dark currents are in turn negligiblewith
aims of the experiment,the telescope,whoseoptical axis is
parallelto the symmetryaxis of the satellite,is constantly respectto the airglow and particle noises,which along with
somescatteredsolar light in A1, appearto be the only effecpointing in a direction closeto the zenith and scansgreat
eclipticcircles.Consequently,all the data givean integration tive ones.
Table 1 givesfor the equatorialregionsoutsidethe anomaly
on an optical path rangingfrom 530 km to infinity. The
telescopehasthe obviouseffectof reducingthe observations the distribution of noise counts among the various sources
to a rather narrow beam. The wavelengthwindowsare such cited above. Figure 2 illustratesthe sensitivitythresholdas a
thattheregionextending
from1350A to a littlelessthan3000 function of wavelength,as determinedby the total noise.
A is fully coveredby means of four adjacentspectral
OBSERVATIONS
channels.The wavelengthslikely to give riseto someairglow
(1216and 1304A) havecarefullybeenexcluded.
ChannelA1, the only one to be discussed
fully below, correspondsto the passageof the beamthrougha filter limiting
the transmissionon the short-wavelengthside. The RbTe
photocathode photomultiplier thus limits the quantum
efficiencyon the long-wavelength
sideof the band,givingrise
to a roughly
Gaussian
shape
centered
on2750and300,&wide
The observationsanalyzed in the presentpaper cover 75
daysof the first summerscanningof the satellite.This period
Rayte•ghs
300
at half height. The acceptancesolid angle is 33.1 arc rain:.
ChannelsA2-A4 work on a totally differentprinciple,since
the light is dispersedby a grating.
BACKGROUND
AND NOISE
200
Our observationsbeingpart of the unexpectedsignals,it is
importantto stressthe predictedand observedbackgrounds.
Four typesof sourceswereforeseen:
(1) scatteredsunlight,(2)
100
TABLE
1.
Noise Sources in Counts/0.148
Source
A1
Sunlight
Airglow
7
Particles
Detectors
3
1
A2
25
A3
20
3
1
s
A4
A4
1500
2000
2500
3000 • (A)
Fig. 2. Sensitivitythresholdof the four channelsin rayleighsplotted
versuswavelength.
2546
200
GI•RARD AND M ONFILS: BRIEF REPORT
-
o
o
+2O
.GEOMAGNETIC
LATITUDE
Fig. 3. Exampleof the signalobservedduringa duskequatorcrossingon April 12, 1972,at 1734LT duringorbit 473.
Here the altitude is 530 km, and the longitudeis 111øE.The broad arrow indicatesthe positionof the magneticdip
equator.
has been chosenbecauseit correspondsto the correctfunctioning of the tape recorder of the satellite, which in turn
guaranteescomplete orbit recordingsand a large amount of
successiveequatorial scans.
The data, an exampleof which is reproducedin Figure 3,
appear as increasesof the continuousbackgroundin channel
Owing to the rotation of the earth under the orbital plane
the whole sphere is covered by 15 or occasionally 16
successive
orbital traces.The intensityof the airglow observed
in the region extending+30 ø from the geographicequator is
examinedexceptfor the SAA region,wherethe very high particle noisepreventsthe detectionof the dayglow signal.The
A l, i.e.•,for the bandwidthof 2750+ 150,/t duringthe dusk boundaries-ofthis region are indicated on each map.
crossingof the equatorialregion.It is very important to stress
Since each equator crossingis spacedby nearly 24ø in
the general absence of any signal on the three spec- longitude,thuscausingan apparentmotion westward,the actrophotometric channels.
curacyof the morphologyis of the sameorder of magnitude.
Althoug,h we shall not be analyzinghere the resultsof our
On each map are indicated the date, orbit numbers, and the
fellow experimentS59, it must be mentionedthat equatorial sumover 24 hoursof the 3-hour planetaryindices,• Kp.
As was mentioned above, the local time of all observations
signalshave also been recordedby S59 in all three of their
channels.Consequently,the possiblewavelengthrangeof the isnearlyconstant
•tndremains
closeto 1800LT. Owingto the
emitted light has been discussedin an earlier publication 22.5-hourdelay betweenthe first and the last satellitepasses
[Boksenbergand Gbrard, 1973] and has been shownto extend usedin each display,reproducibilityof the forms at a given
from2580to 2800A. By considering
the variouspossibilities longitudeis not to be expectedon successive
maps.Furtherof spectralidentificationof the emitting speciesthe glow has more, the equatorial passesare spaced by about 2ø in
finally been attributed to the strong resonancetransition of
longitudefrom those of the previousday.
Mg II lying at 7, 2796-2803 ,A.. Resonancescatteringof
The magneticdip equatorat 600 km is plotted after Cain's
sunlight by Mg + ions can account for the majority (or
[1969] Pogo model adjustedto 1972. All orbits usedto draw
totality) of the observedsignal.This presenceat the end of thesemaps are southbound,sincewe consideronly the dusk
the day and the absenceof any detectablesignalduring the equator crossings.Each map startswith the first passsituated
dawn crossingof the equator are in agreementwith the founwestwardof the SAA region. Three setsof two consecutive
tain effect theory developedby Hanson et al. [1972].
mapsare illustratedby Figures4-6. On two of the figures,one
If the signal observedis entirely ascribedto the Mg II
satellitepassis missingso far, thusgivingriseto a gap nearly
doublet, the detectionsensitivityis 5.6 R count-•, or 4.4 107 44ø wide in longitude. As can readily be seen,the forms are
Mg + ion cm-: count-•, as establishedin the previouspaper.
generally aligned about the magnetic dip equator, thus
suggesting
that the speciescausingthe airglowis ionized.This
MAPS
alignment is particularly remarkable in the Pacific sector
Isophote maps have been found to illustrate best the
(160øE-80øW), where the magneticdeclination is eastward.
localization, structure, and dynamicsof this UV dayglow. An exceptionto this rule can be found in Figure 6b, wherethe
The data collectedduring I day of observationare displayed form is alignedparallel to the geographicequator.
on planispheresin geodeticcoordinates.
The total latitudinal extension(as determinedby the sen-
Gl•RARD AND MONFILS: BRIEF REPORT
o
•
2547
o
Orbits
300 - 314
Mc•rch 31 - Apr 1,
rKp'
23+
1972
z•
ß
10-20 counts
•
20-40 counts
•
40-80 counts
i
> 80 counts
Fig.4. Isophote
maps
oftheUV MgII dayglow
plotted
ingeodetic
coordinates
for(a)orbits
300-314
and(b)orbits
315-329.The shaded
areais the SouthAtlanticanomaly
(SAA)region,wheretheparticlenoiseprevents
airglow
measurements.
sitivitythreshold
of the instrument)
neverexceeds
30ø but
variesstrongly
withtimeandlongitude.
Moststrikingarethe
gapsobserved
in somesectors,
principally
in theAmerican
one. The degreeof conjugacy
betweenhemispheres
is occasionally
high(Figure4b)butisgenerally
poor.Figure5billustratesthe caseof an arc in the Pacific southernhemisphere
ings [Lyon and Thomas,1963] and has been invokedby
Chandraet al. [1973]in the caseof equatorialred arcs.The
altitudeeffectis possiblycausedby transequatorial
winds
blowing from the summerhemisphere.
Figures
4(a,b) and5(a,b) showthatthemorphology
is occasionallypersistent
from day to day, at leastin the Pacific
sector.On the contrary,Figure6(a, b) illustratesan example
that has no conjugateform in the other hemisphere.A
minimumof intensitycorresponding
to the ionospheric of dramaticvariation,althoughthe magneticactivityremains
equatorial
troughis sometimes
observed
on the dip equator moderate during both days.
(Figure 5a).
Furthermore,the southernarc is generallystrongerthan
the northernoneduringApril andMay in thePacificregion.
Thislackof conjugacy
is ascribed
to differences
of altitudeof
the ion distribution north and south of the dip equator, caus-
LONGITUDINAL
VARIATIONS
Longitudinaleffectsare readilyapparentin the discontinuities observedon the maps shown in Figures 4-6.
However,thelocationof theregionwith no signalvariesfrom
all datasofar reducedhavebeen
ingthecolumn
iondensity
above
thesatellite
tobedifferent
in day to day.Consequently,
added
in
sectors
30
ø
wide
in
longitude,and the relativefrethetwohemispheres.
Sucha latitudinal
dissymmetry
hasbeen
of occurrence
havebeencalculated
in eachsectorin
observed
in the heightof theF•.peakby ionospheric
sound- quencies
2548
G!•RARDAND M ONFILS:BRIEFREPORT
A•r. 5
4•Z
o
o
-lO
-20
-30
I
I
I
I
I
-180 -160 -140 -120 -100 -80
I
-60
I
-40
I
-20
I
0
'
20
4'0 60'
80
100 , 120
140
160
180
Geographiclongitude
Orbits3?6-390
.'..':!.'.•
,•3-g.o
cou,-r_5
TKpß14
•
Apr. 6 ,
'zo•ocou•'r.•
Fig. 5. lsophotemaps of the UV Mg II dayglowplotted in geodeticcoordinatesfor (a) orbits 361-375 and (b) orbits
376-390. The shaded area is the SAA region, where the particle noise preventsairglow measurements.
order to determine whether statistical longitudinal effects
would appear.The histogramillustratingthis resultis plotted
in Figure 7. It is based on'more than 600 good-quality
equatorial passes. A deep minimum is observed in the
American zone (0ø-90øW). Since the range of latitude
covered in the 30ø-90øW
sector is less than that in the other
sectors,owing to perturbationsby particlesof the SAA, the
statisticalresultsare unreliable in this region. However, we
believethe dip to be real, sinceit is alsopresentin the adjacent
unperturbed sector. A large maximum is apparent in the
Asian sector (60ø-120øE) in 85% of the favorable observations.
Theselongitudinalvariations are most likely connectedto
the declinationangle, the mutual positionof the dip and the
geographicequator, and the strengthof the magneticfield B.
For a given value of the E field the drift velocity W =
E x B/B: is maximum in the American zone, where B is
minimum, but it has been shown above that the observed
effect works in the opposite way.
Observationsrelevantto this problem have beendescribed
by Lyon and Thomas [1963], who have shown from
ionosphericdata that the equatorialanomalydevelopsearlier
in the Asian
sector than
in the American
sector. This
has
recentlybeen confirmedby the Ariel 3 satelliteelectrondensity measurements[Hopkins, 1972],which have demonstrated
the anomalyto beginbetween500 and 600 km nearly 3 hours
earlier in the Asian sector than in the American one. Accord-
ingly, it is likely that differences
in the diurnalvariationsof
the E field existat variouslongitudes,thus causingdifferences
in the altitude reachedat a given local time by the ions. The
measurementsreportedhereintegrateupward from about 530
km; they are consequentlyvery sensitiveto such altitude
variations, which are observedas longitudinal effects. The
problem of why the anomaly startsdevelopingat local times
GI•RARD AND MONFILS: BRIEF REPORT
2549
aO
-
20
10
0
-10
-20
-30
-180 -1GO-V.0 -120 -100 -80 -GO -t,0 -20
0
20' L0
GO 80
DO 120 K0
E0 180
Cx(xjmWc Iongituck
Orl•ts
1025 -1019
I•ty 18-19, •
TKp: 7-
i
Fig. 6.
i
i
i
w,.,y
•-m,•,a
'-• ,•-m cou..m,
Z:K
p: e +
•
ao-• couwr•
Isophotemapsof the UV Mg II dayglowplottedin geodeticcoordinates
for (a) orbits1025-1039and(b) orbits
1040-1054.The shadedarea is the SAA region,where the particlenoisepreventsairglow measurements.
dependenton longitudeis outsidethe scopeof this work: it is
likely to be relatedto the dynamotheoryitself.It shouldbe
stressed that all the results described in this section concern
quiet magneticdays (Kp < 3).
Kp EFFECTS
Mg + ions in sunlight.Isophote maps were found to be convenient to illustrate the morphologyand temporal variations
of the glow. It turns out that the arcs can be either singleor
doublebut are generallyalignedparallel to the magneticdip
equator. Important interhemispheredissymmetriesare often
observed,although conjugacycan occasionallybe found in
some
sectors.
The influenceof the magneticactivity planetary index Kp
Seasonal variations are not clearly demonstrated by these
daily variationon the overallintensityhasbeeninvestigated.
measurements; however, the winter hemisphere arc is
The frequencyof observationof the UV dayglow at all
longitudeshasbeencomparedfor dayson whichKp wasless generally stronger in the Pacific sector. Gaps along the
than or greaterthan 3, the usuallimit betweenquiet and dis- equator are very frequently observed. When these
longitudinalvariationsare analyzedstatistically,they showa
turbed days.
sinuslike distribution having a maximum occurrencefreThe observedfrequenciesare 43% when Kp < 3 and 44%
quencyin the Asian sector(60ø-120øE) and a dip in the
when Kp > 3, thus demonstratingincidentally that the
Americansector(0ø-90øW). This findingis consistentwith the
presenceof metallicions at suchconsiderable
heightsis not
electron density measurements,which show the equatorial
unusual. Accordingly, it is concluded that the planetary
anomalyto developearlierin theAsianzone.The dissymmetries
magneticactivity has no noticeableeffecton the total intenandlongitudinalvariationsarepossiblydueto differences
in the
sity observed.
altitude of the ion layer. Vertical metallic ion distribution
SUMMARY
profilesas well as more quantitativeinformation regarding
windsandtheinfluenceof the magneticfieldmorphologyon the
All the characteristics
of the UV dayglowdiscussed
hereare
dynamoE field are neededto clarify this problem.
consistentwith its being ascribedto resonancescatteringof
2550
G!•RARDAND MONFILS:BRIEFREPORT
Barbier, D., and J. Glaume, Les radiations de l'oxyg•ne 6300 et 5577
A de la luminescence du ciel nocturne dans une station de basse
latitude, Ann. Geophys.,16, 319, 1960.
Boksenberg, A., and J.-C. G6rard, Ultraviolet observationsof
equatorial dayglow above the F•. peak, J. Geophys.Res., 78, 4641,
1973.
Cain, J. C., The location'of the dip equator at E layer altitude, Radio
Sci., 4, 781, 1969.
Carruthers, G. R., and T. Page, Apollo 16 far-ultraviolet
camera/spectrographearth observations,Science,177, 788, 1972.
Chandra, S., E. I. Reed,B. E. Troy, and J. E. Blamont,The equatorial
airglow and the ionosphericgeomagneticanomaly, J. Geophys.
Res., 78, 4630, 1973.
Hanson, W. B., Radiative recombinationof atomic ions in the nighttime region, J. Geophys.Res., 74, 3720, 1969.
Hanson, W. B., and S. Sanatini, Meteoritic ions abovethe F•.peak, J.
Geophys.Res., 75, 5503, 1970.
Hanson, W. B., D. L. Sterling,and R. F. Woodman, Sourceand identification of heavy ions in the equatorial F layer, J. Geophys.Res.,
77, 5530, 1972.
Fig. 7.
Longitudinal variation of the relative occurrencefrequency
of the equatorial glow for magneticallyquiet days.
Hicks, G. T., and T. A. Chubb, Equatorial aurora/airglow in the far
ultraviolet, J. Geophys.Res., 75, 6233, 1970.
Hopkins, H. D., Longitudinal variation of the equatorial anomaly,
Planet. Space Sci., 20, 2093, 1972.
Lyon, A. J., and L. Thomas,The F•.-regionequatorialanomalyin the
African, American and east Asian sectors during sunspot maximum, J. Atmos. Terr. Phys., 25, 373, 1963.
Reed,E. I., Bibliography
of observations
of OI 6300A nightairglow
at latitudes from 40øN to 60øS, GSFC X-625-72-172, Goddard
Acknowledgments. C. Thiry is gratefully acknowledgedfor her
help in data reduction.
*
,
SpaceFlight Center, Greenbelt, Md., 1972.
Reed, E. I., W. B. Fowler, and J. E. Blamont, An atlas of low-latitude
6300-AO I night airglowfrom Ogo4 observations,
J. Geophys.
Res., 78, 4630, 1973.
The Editor thanks W. B. Hanson and P. Mange for their assistance Thomas, R. J., and T. M. Donahue, Analysis of Ogo 6 observations
in evaluating this report.
of the O I 5577-Atropicalnightglow,J. Geophys.
Res.,77, 3557,
1972.
REFERENCES
Aikin, A. C., and R. A. Goldberg, Metallic ions in the equatorial
ionosphere,J. Geophys.Res., 78, 734, 1973.
(Received October 5, 1973;
acceptedFebruary 11, 1974.)