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Advances in Space Research 47 (2011) 729–735

Evidence for the lowering of the centroid of daytime thermosphericO(1D) 630.0 nm emission over the magnetic equator: First results

T.K. Pant ⇑, C. Vineeth, S.G. Sumod, Lijo Jose

Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695022, Kerala, India

Received 31 October 2004; received in revised form 3 August 2010; accepted 4 August 2010Available online 11 August 2010

Abstract

It is shown in this paper for the first time that the intensity of the daytime thermospheric O(1D) 630.0 nm airglow as measured by theground-based dayglow photometer over Trivandrum (8.5�N; 77�E; dip lat. 0.5�N), a geomagnetic dip equatorial station, exhibit a directcorrelation with the electron density at 180 km. This altitude is about �40 km lower than the believed centroid of the O(1D) 630.0 nmdayglow emission i.e. 220 km. This observation is contrary to the understanding of the behavior of O(1D) 630.0 nm dayglow over equa-torial/low latitudes. Over these latitudes, the variations of the measured intensity of O(1D) 630.0 nm dayglow are known to be associatedwith the changes in the electron density at altitudes around 220 km, the centroid of this emission. In this context, the present results indi-cating the lowering of the peak altitude of O(1D) 630.0 nm emission from �220 to �180 km over the dip equator is new. Recent resultson solar XUV flux indicate that this could be an important parameter that controls the O(1D) 630.0 nm dayglow excitation rates throughmodulations in the neutral and ionic composition in lower thermosphere-ionosphere region. However, the lowering of the centroid ofO(1D) 630.0 nm emission, as shown in this study, has been ascribed primarily to the fountain effect associated with the equatorial ion-ization anomaly.� 2010 Published by Elsevier Ltd. on behalf of COSPAR.

Keywords: Equatorial ionosphere; Dayglow

1. Introduction

The energetics of the daytime upper atmosphere is pri-marily driven by the solar radiation absorbed at differentaltitudes in this region. A complex web of photochemicaland dynamical processes further modulates the upperatmospheric energetics and dynamics. Quite a few atmo-spheric constituents undergo specific spectral transitionsas a result of these processes. The atmospheric emissionsresulting from these transitions constitute the ‘airglow’.The broad classification of the airglow phenomenon is day-glow, nightglow or twilight glow depending on the time it isobserved. These airglow emissions serve as tracers for thealtitude regions, and processes therein, from which they

0273-1177/$36.00 � 2010 Published by Elsevier Ltd. on behalf of COSPAR.

doi:10.1016/j.asr.2010.08.011

⇑ Corresponding author. Tel.: +91 471 2562597; fax: +91 471 2706535.E-mail addresses: tarun_kumar@vssc.gov.in, tkpant@yahoo.ca (T.K.

Pant).

emanate. Some very extensive and excellent textbooks sum-marize the early work on airglow (e.g. Chamberlain, 1991).

The most challenging part of the airglow studies hadbeen, and still continues to be, the investigation of the air-glow during daytime. Though the daytime observation ofthe O(1D) 630.0 nm was made first by Noxon and Goodyas early as 1962, not many studies focused on this aspectin the later years (Noxon and Goody, 1962). However, asignificant amount of work in this area of research has beendone in India and over other latitudes during last two dec-ade (Desai et al., 1979; Sivjee et al., 1980; Hernandez, 1982;Rees and Greenway, 1983; Narayanan et al., 1989; Biondiet al., 1995; Meriwether et al., 1986; Sridharan et al., 1991,1994, 1998; Sekar et al., 1993; Conde and Smith, 1997;Slanger et al., 2003; Zhang and Shepherd, 2004).

During daytime, the O(1D) 630.0 nm thermospheric air-glow is produced primarily through three different chemicalchannels. The photoelectrons of energy >1.96 eV and their

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impact on the ground state Oxygen i.e. O3P is the firstimportant source. The photodissociation of molecular oxy-gen in the 135–175 nm range (Schumann Runge contin-uum) of the incoming solar radiation is the secondimportant process for the generation of O1D, dissociativerecombination of the molecular ion Oþ2 being the third interms of their relative contribution to the overall O(1D)630.0 nm dayglow emission intensities. In addition to these,there are some more processes leading to the generation ofdaytime O1D but their contribution is very small in com-parison to the dominant processes discussed above (Solo-mon and Abreu, 1989; Zhang and Shepherd, 2004 andreferences therein).

Space based studies over mid-latitude regions haveshown that the peak of thermospheric O(1D) 630.0 nm day-glow lies at an altitude of �220 km (Hays et al., 1978; Sol-omon and Abreu, 1989). Using systematic measurementsof the daytime O(1D) 630.0 nm emissions over a low lati-tude location, it has already been shown that the variabil-ities of O(1D) 630.0 nm emissions strongly corroboratewith the F-region electron density at 220 km (Sridharanet al., 1991). It was shown that these variabilities in thedayglow intensity were caused primarily by the dissocia-tive-recombination process. At the same time, it was foundthat this process contributes relatively less towards theoverall dayglow intensity than the other processes (Sridha-ran et al., 1991). The process of dissociative recombinationat F-region altitudes, especially in context of O(1D)630.0 nm dayglow, is known to have a direct dependenceon the insitu electron density. Moreover, the F-region elec-tron density over low and equatorial latitudes during day-time undergoes significant changes owing to the presenceand evolution of the electrodynamical feature namely theEquatorial Ionization Anomaly (EIA) over these latitudes.In fact, quiet time measurements from the magnetic equa-tor in India show that the EIA through equatorial electro-dynamics has definite imprints on the O(1D) 630.0 nmdayglow, at times with a time-delay (Sridharan et al.,1994; Pallamraju et al., 1996). At the same time, no system-atic studies concerning the dayglow intensity and its depen-dence on the electron density at 220 km i.e. the centroid ofOI 630.0 nm dayglow over dip equator have beenperformed.

This study presents the initial results from magneticequator showing that the O(1D) 630.0 nm dayglow inten-sity grossly corroborates with the electron density ataround 180 km, not 220 km. This is a new result indicatingthat, perhaps over magnetic equator, the centroid of O(1D)630.0 nm dayglow emissions is lowered in altitude andreaching as low as �180 km, unlike the latitudes away fromequator where it lies around 220 km. These results are pre-sented and discussed here.

2. Experimental details

The daytime airglow intensity measurements were madeusing a unique Multi Wavelength Dayglow PhotoMeter

(MWDPM) on O(1D) 630.0 nm at Trivandrum (8.5€�N,77�E, 0.5�dip lat.) during March 2004 and 2009. The mea-surements presented here are limited for 6 days for brevity.These daytime measurements were made typically between0800 and 1800 Indian standard time (IST) for zenith sky atevery 20 s interval. The details of this photometer havebeen described earlier (Vineeth et al., 2005 and referencestherein). The ionospheric densities were obtained using acollocated Ionosonde. For completeness, simultaneousmeasurements on the equatorial electrojet (EEJ) inducedmagnetic field for the relevant days are also presented.These magnetic field measurements were made using aground-based proton precision magnetometer (PPM).

3. Observations

To check with the centroid of emission of daytime O(1D)630.0 nm over Trivandrum, 6 days of daytime measure-ments during the year 2004 and 2009 are considered. Theperiod of March 2004 was geomagnetically moderately dis-turbed whereas March 2009 was geomagnetically quiet.The days are considered from the year 2004 and 2009, inorder to find out the effect of solar activity, if any. Thesolar flux as seen from the F10.7 cm radio flux was almostdouble during the year 2004 in comparison with that of theyear 2009. The days when the dayglow intensity measure-ments presented here experienced different levels of geo-magnetic activity. Top panels of the Figs. 1–6 show thetime variation of the O(1D) 630.0 nm dayglow intensityon March 3, 2004; March 8, 2004; March 9, 2004; March2, 2009; March 4, 2009 and March 9, 2004, respectivelyover Trivandrum. The Ap indices are also shown at thetop of each panels to indicate the level of geomagneticactivity respectively. All the days, the dayglow exhibitedsignificant variability and shows short period (<1 h) fluctu-ations. At the same time, the trend in the dayglow variabil-ity also showed marked differences in terms of the time ofmaximum dayglow intensity. For instance, the dayglowintensity on March 9, 2004 peaked about 2–3 h lateraround 1400 IST than that of other days and March 23,2009 exhibited a sharp increase at around 1230 IST.

Over the magnetic equator, it is known that the largescale EIA process inflicts significant variabilities in the ther-mospheric O(1D) 630.0 nm dayglow. The EIA in turn isgoverned by another equatorial process, the equatorial elec-trojet (EEJ) current, whose time evolution can be seen interms of its induced magnetic field on the surface. The vari-ations of this magnetic field for all the days of dayglow mea-surements are presented in the top panels of each Figure.The magnetic field, as seen on March 8, 2004 represents atypical quiet time evolution of the EEJ current on a givenday over the dip equator. The magnetic field does not exhi-bit any significant short-time variations that usually areindicative of either changes of the gravity wave activitiesat EEJ altitudes or impending geomagnetic disturbances.The magnetic field on March 9, 2004 shows significant vari-abilities which, in this case, are an outcome of the ongoing

Fig. 1. Temporal variation of (a) O(1D) 630.0 nm dayglow intensity andEEJ induced magnetic field at surface over Trivandrum on March 3, 2004;(b) electron density at 180 km (c) electron density at 200, 220 and 240 kmaltitudes.

Fig. 2. Temporal variation of (a) O(1D) 630.0 nm dayglow intensity andEEJ induced magnetic field at surface over Trivandrum on March 8, 2004;(b) electron density at 180 km; (c) electron density at 200, 220 and 240 kmaltitudes.

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moderate geomagnetic disturbance. These variabilities infield could be associated with the perturbations associatedwith the setting up of a global disturbance dynamo. Thesimultaneous presence of variabilities in O(1D) 630.0 nmthermospheric dayglow intensity and the magnetic fieldindicate the disturbance dynamo effect. On March 3,2004the field shows the same nighttime mean as on March 8,2004 but the daytime maximum attained is significantlylower (�50 nT) than that of the same on March 8. Similarly,the magnetic field and airglow intensities during the year2009 also showed significant day-to-day variability. Thevarying nature of the magnetic field on all these days indi-cates that the extent of electrodynamical control on the dis-tribution of the ionization density over and off equatoriallatitudes is different from one day to another.

Since the time resolution of the ionosonde measure-ments used in this investigation is 15 min, the mean airglowmeasurements for every 15 min are also depicted in eachpanel. The electron density at heights 200, 220 and

240 kms for each day are plotted in bottom panels ofFigs. 1–6 (panel c). To facilitate a better comparison, elec-tron density at 180 km is plotted in a separate panel(Figs. 1–6, middle panels). Variations in electron densitybelow 200 km exhibited opposite trends to those above itespecially in the morning and evening times. The noontimedecrease in the electron density above 200 km as measuredhere on all three days, is indicative of the presence of EIAthat drift the plasma from these altitudes to low latitudesalong the magnetic field lines thereby causing the bite outin the ionization density.

Comparing the mean variabilities of the dayglow, elec-tron densities and magnetic field, it is well evident that (a)the mean O(1D) 630.0 nm dayglow variations are oppositein phase with changes in electron density at 220 km irrespec-tive of the EEJ strength, geomagnetic condition and solarflux (b) the dayglow emission variations are always in phasewith the electron density at �180 km. These observationsare discussed in detail in the following sections.

Fig. 3. Temporal variation of (a) O(1D) 630.0 nm dayglow intensity andEEJ induced magnetic field at surface over Trivandrum on March 9, 2004;(b) electron density at 180 km; (c) electron density at 200, 220 and 240 kmaltitudes.

Fig. 4. Temporal variation of (a) O(1D) 630.0 nm dayglow intensity andEEJ induced magnetic field at surface over Trivandrum on March 2, 2009;(b) electron density at 180 km; (c) electron density at 200, 220 and 240 kmaltitudes.

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4. Discussions

As mentioned earlier, there are three important pro-cesses, namely the photoelectron impact, photodissociationand the dissociative recombination (DR), that result in theproduction of O(1D) i.e. the excited state of oxygen whichsubsequently gives daytime O(1D) 630.0 nm emission.There are other sources too that give rise to the excitedO(1D) state, but their overall contribution is rather smallas compared to these three dominant processes (Solomonand Abreu, 1989; Zhang and Shepherd, 2004). Volumeemission rate calculations based on data obtained usingvisible airglow experiment (VAE) onboard the atmosphericexplorer (AE) satellite showed that the photoelectron con-tribution to the dayglow is largest followed by photodisso-ciation and dissociative recombination, respectively (Hayset al., 1978). Further, in a study Zhang and Shepherd,2004 have shown that, for a given day, the daytime red lineemission rate is linearly correlated with the solar zenith

angle. They also have shown that, the behavior of theheight of the maximum emission rate is complicatedbecause the effects of the solar zenith angle and solar fluxon the emission height are different. It was found that atthe peak emission height of 220 km the contribution dueto DR is larger by a factor of 1.5–2 than the other twomechanisms, over low latitudes while at high latitudes theyare comparable (Solomon and Abreu, 1989). Moreover,the various photochemical and electrodynamical processesthat control the dayglow emission maximize at differenttimes of the day. Owing to their relative influence on theemission, significant temporal changes are induced in thedayglow emission intensity on a given day. However,changes in the relative contribution of these processes asa function of altitude would also manifest as variationsin the measured dayglow intensity.

In the present study, the observation that the variabili-ties in dayglow intensity corroborate with variations inthe magnetic field highlight the fact that E-region zonal

Fig. 5. Temporal variation of (a) O(1D) 630.0 nm dayglow intensity andEEJ induced magnetic field at surface over Trivandrum on March 4, 2009;(b) electron density at 180 km; (c) electron density at 200, 220 and 240 kmaltitudes.

Fig. 6. Temporal variation of (a) O(1D) 630.0 nm dayglow intensity andEEJ induced magnetic field at surface over Trivandrum on March 23,2009; (b) electron density at 180 km; (c) electron density at 200, 220 and240 km altitudes.

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electric field is indirectly controlling the plasma drift in thealtitude region of O(1D) 630.0 nm dayglow emission. Theredoes not seem to be any time-delay between the two. Theprevalent electric field at �110 km in the presence of terres-trial magnetic field over magnetic equator causes a drift onthe plasma which is ambipolar and upwards during day-time. This upwards motion of plasma causes a modulationin the plasma density at altitudes higher above, which inturn manifests as changes in the O(1D) 630.0 nm dayglowemission intensity (Sridharan et al., 1992). On occasions,there could be a time-delay before the plasma density mod-ulations appear as dayglow intensity changes (PallamRajuet al., 1996). Since the O(1D) 630.0 nm dayglow intensityand magnetic field both are height integrated ground-basedmeasurements, the observed correlation between the day-glow intensity and magnetic field mentioned above isexpected and explainable.

However, in view of the above discussion, one wouldalso expect the temporal changes in O(1D) 630.0 nm day-

glow intensity to be correlating more with electron densitychanges around altitudes of it’s peak emission/centroid i.e.�220 km. The processes involving photodissociation andphotoelectron being dependent on the solar flux, their con-tribution to overall dayglow would exhibit sharp changesin it’s intensity during morning and evening hours as thesolar v angle changes (Zhang and Shepherd, 2004). None-theless, if the centroid of emission itself undergoes a changein the altitude owing to various photochemical and electro-dynamical processes, expected correlation between ioniza-tion density at centroid and the dayglow intensity wouldnot be forthcoming. In fact, the O(1D) 630.0 nm emissionintensity would then correspond to electron densitychanges at the altered centroid height. The observationsin the present study seem to indicate towards such a changein the altitude of centroid of emission.

The anti-correlations/correlations between the electrondensity changes at various representative altitudes withdayglow intensity on individual days clearly indicate that

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the centroid of dayglow emission is lowered in altitude. Theelectron densities at 180 km show a strong correlation(0.65 ± 0.12) with the mean dayglow variation indicatingthe emission peak to be lying around this altitude. It isexpected that small changes of �5–10 km in the centroidwould not affect the integrated airglow emission as the typ-ical width of the O(1D) 630.0 nm emission is �40 km.

As is understood, the emission profile of the O(1D)630.0 nm on the topside is governed by the decreasing den-sity of O2

+ ions and the lower limit is controlled by thequenching of O(1D) by molecular species namely N2, O2.While collisions of O2 with photoelectrons leading toO(1D) states are known to be active at most altitudes, theSchumann–Runge photodissociation of O2 predominatesonly at the lower thermospheric heights. Over dip equator,the interaction of the EIA and it’s associated effects e.g. theneutral anomaly are known to alter the relative ion compo-sition in and around the F-region. The solar XUV flux has asignificant role in ionizing this region and also for the pro-duction of O(1D). Measurements on the photoelectronshave indicated that the solar XUV flux is at least 3 timeshigher than that assumed in the existing models (Hintereg-ger et al. 1981; Richards and Torr, 1984; Winningham et al.,1989; Feng et al., 1989; Ogawa et al., 1990; Bailey et al.,1999). This has very significant implications in the daytimeairglow studies because XUV photons produce additionalionization through the photoelectron impact on thermo-spheric neutral species. As a consequence, the relative con-tribution of the photoelectrons towards the O(1D) 630.0 nmdayglow would enhance significantly shifting the peak emis-sion altitude of the dayglow profile to lower altitudes i.e.<200 km. These results indicate that the photochemicalequilibrium of the neutral species like O2 and O, whichare the dayglow emitting species in present case, gets signif-icantly altered by the changes in solar XUV flux. Along withthis, a shift in altitude where the diffusive transport beginsmay occur on account of the low solar flux during low solaractivity epoch. However if the observed lowering of cen-troid of emission is dependent on the solar flux, it shouldnot have restricted to the dip equator alone.

In view of the above discussion, the observed loweringof the centroid of O(1D) 630.0 nm dayglow emission ismore likely associated to the possible changes in the com-position due to the EIA associated vertical plasma driftand the electron density distribution. However, quantifica-tion of these changes cannot be done without simultaneousmeasurements on the composition changes and a solarEUV flux. Whether such a decrease in altitude of the lowerboundary of diffusively controlled thermosphere-iono-sphere is due to photoelectrons or other photochemical/electrodynamical process remains to be ascertained usingextended database.

5. Conclusions

The present study from the magnetic equator shows thatthe centroid of thermospheric O(1D) 630.0 nm dayglow is

lowered in altitude to be around 180 km during March2004, while it is known to be around �220 km over lowand high latitudes. The inferred lowering is ascribed tothe possible changes in the plasma and neutral distributionat the altitude profile of O(1D) 630.0 nm dayglow due to theupward drift associated with the EIA. Varying solar XUVflux can significantly modulate the photoelectron abun-dance in lower thermosphere. However, this study is limitedin discussing the Solar XUV effects for want of simulta-neous flux measurements. Nevertheless, the observed lower-ing of the altitude of peak airglow emission necessitates abetter understanding of the control, photochemistry alongwith other large and small-scale neutral dynamical pro-cesses, has on the O(1D) 630.0 nm dayglow variabilities overthe equator under different geophysical conditions.

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