time-gcm simulations of lower-thermospheric nitric oxide seen by the halogen occultation experiment

Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 889–895www.elsevier.com/locate/jastp

TIME-GCM simulations of lower-thermospheric nitric oxideseen by the halogen occultation experiment

Daniel Marsh ∗, Ray RobleHigh Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307-3000, USA

Abstract

The Halogen Occultation Experiment (HALOE) experiment, on board the Upper Atmosphere Research Satellite, uses solaroccultation to measure vertical pro6les of nitric oxide (NO) up to an altitude of 130 km. The in;uence of dynamics on bothseasonal and diurnal time-scales is clearly seen in these observations. This study presents three-dimensional simulations ofNO in the mesosphere and lower thermosphere using the Thermosphere–Ionosphere–Mesosphere–Electrodynamic GeneralCirculation Model. The e<ects of both the residual circulation and atmospheric tides are modeled, and compare well withHALOE observations. The results con6rm that the sunrise=sunset asymmetries in HALOE data are predominantly the resultof tidal vertical motions. c© 2002 Elsevier Science Ltd. All rights reserved.

Keywords: D-region; Nitric oxide; Thermosphere; Mesosphere; Tides

1. Introduction

Modeling the distribution of nitric oxide (NO) in theterrestrial atmosphere is a particularly complex task wellsuited to a general circulation model. To accurately de-scribe NO production in the high latitude thermosphere,the model must include an ionosphere and the e<ects ofparticle precipitation in auroral regions. In addition, Jouleheating must also be accurately represented, since the rateof thermospheric NO production is sensitive to changes intemperature. At low latitudes it becomes important to accu-rately calculate solar ultraviolet and soft X-ray absorption.In fact, as was pointed out by Barth (1992), almost any en-ergy input into the thermosphere results in NO production.In the mesosphere longer NO lifetimes mean that NO canact like a tracer, and so an accurate representation of dynam-ical motions is required. All these processes are representedin a self-consistent manner in the the NCAR Thermo-sphere–Ionosphere–Mesosphere–Electrodynamic General

∗ Corresponding author. Tel.: +1-303-497-1160; fax: +1-303-497-1400.

E-mail address: [email protected] (D. Marsh).

Circulation Model (TIME-GCM) (Roble and Ridley, 1994).Comparisons between TIME-GCM predicted NO and obser-vations are therefore useful in not only helping to understandfactors that in;uence NO distributions, but also in testingcurrent models of the highly coupled mesosphere and lowerthermosphere (MLT).

Prior to the launch of the halogen occultation experiment(HALOE) on board the Upper Atmosphere Research Satel-lite (UARS), there were no long-term observations of meso-spheric NO. Previous NO measurements made by the solarmesosphere explorer (SME) were restricted to the thermo-sphere (Siskind et al., 1998). The HALOE instrument andthe solar occultation measurement technique used are dis-cussed in Russell III et al. (1993). Speci6cs on the NO re-trieval algorithms and validation are presented by Gordley etal. (1996). This paper presents an analysis of pro6les fromversion 19 of the dataset.

In the next section we describe the TIME-GCM modelchemistry. This is followed in Section 3 by model=data com-parisons of both general NO morphology and the diurnalvariability. In Section 4 we present arguments supportingthe hypothesis that the majority of the observed diurnal vari-ation is due to tidal vertical advection. Finally, conclusionsfrom the study are presented in Section 5.

1364-6826/02/$ - see front matter c© 2002 Elsevier Science Ltd. All rights reserved.PII: S1364 -6826(02)00044 -5

890 D. Marsh, R. Roble / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 889–895

2. Model description

The TIME-GCM is described in Roble (1995), which de-tails the neutral and ion chemistry included in the model.The model’s vertical extent is from 30 to 500 km. A previ-ous version of the model with a lower boundary of 100 kmwas used to model thermospheric NO observations madeby SME (Siskind et al., 1989). We describe here the modelcomponents relevant to NO in the MLT.

Above 80 km, odd-nitrogen (NOx) is predominantlyground state atomic nitrogen N(4S) and NO. The majorityof NOx is produced through the reaction of excited nitrogenand molecular oxygen:

N(2D) + O2 → NO + O: (1)

Production through the reaction

N(4S) + O2 → NO + O (2)

does occur, but this rate is highly temperature dependent,and at MLT temperatures proceeds at a much slower ratethan the rate of Eq. (1). Major sources of N(2D) includedin the model are recombination and charge transfer:

NO+ + e → O + N(2D); (3)

N+2 + e → N + N(2D); (4)

N+2 + O → NO+ + N(2D): (5)

Twenty-6ve further ion reactions are included that encom-pass the chemical scheme of Torr et al. (1990). At mid andlow latitudes, photoionization by solar ultraviolet and softX-rays are the primary source of ions. Photoionization ratesare calculated over 37 wavelength intervals using the EU-VAC solar ;ux model and photoionization cross sections ofRichards et al. (1994). The daily and 81-day average solar10:7 cm ;uxes used by the EUVAC model to scale the solarspectrum were 6xed at 150 (10−22 W=m2=Hz), correspond-ing to moderate solar activity. In auroral regions particleprecipitation is the predominant mechanism for ion produc-tion. Auroral ionization rates are evaluated using the schemedescribed by Roble and Ridley (1987). The total energyinput due to particle precipitation in auroral regions was6xed at 16 GW. Ion convection is speci6ed by the Heeliset al. (1982) model, with a cross-polar cap potential dropof 45 kV. Tidal forcing was speci6ed at the lower boundarybased on annual mean values from the Global Scale WaveModel (Hagan et al., 1999), and was not varied with sea-son. Any seasonal tidal variability in the mesopause regionis the result of variations in wave propagation caused by thebackground atmosphere and to changes in in situ forcing.

3. Model=data comparisons

Figs. 1a and c show monthly mean HALOE NO vol-ume mixing ratio for northern hemisphere winter and spring.

(a) (b)

(c) (d)

Fig. 1. Comparison of HALOE and TIME-GCM nitric oxide (log10 volume mixing ratio). HALOE ratios represent the mean of all sunriseobservations during (a) January and (c) April of 1993 and 1994. (b) and (d) are the zonal mean of TIME-GCM simulations for January10 and April 10, respectively. Shaded regions indicate regions of mixing ratio in excess of 100 ppmv.

D. Marsh, R. Roble / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 889–895 891

HALOE data are comprised of all sunrise observations thatfall within January and April of 1993 and 1994. In the alti-tude range of 50–140 km observed NO volume mixing ra-tios range from¡1 part per billion (ppbv) to¿100 parts permillion (ppmv). TIME-GCM simulations shown in Figs. 1band d capture the 6ve orders of magnitude change in mixingratio with altitude, and place the maximum and minimummixing ratios at the same altitude as the observations. NOis at a minimum near 70 km due to NO photodissociationand the resulting recombination with N(4S):

NO + h� → N(4S) + O; (6)

N(4S) + NO → N2 + O: (7)

In the MLT, the reaction rate for Eq. (7) is much fasterthan the rate for Eq. (2), and so invariably when NO isphotolized, the result is the loss of 2 NOx. The largest mixingratios are seen at high latitudes near 130 km. This is theregion of increased NO production resulting from particleprecipitation in auroral regions.

TIME-GCM simulations show that during northern hemi-sphere winter, high-latitude NO concentrations are greatlyincreased below 100 km compared with spring-time values.This has been modeled previously (see e.g. Garcia et al.,1987; Siskind et al., 1997) and is the result of transport by thedownward arm of the residual circulation during perpetualpolar night. Fig. 2 depicts the residual circulation ( Kv∗; Kw∗)(Dunkerton et al., 1981) calculated from the model duringJanuary. Downward velocities can exceed 2 cm=s near 50◦Nfor much of the mesosphere. Since during night-time NO ispractically inert, the chemical lifetime is greatly increasedallowing a build up of NO in the lower mesosphere. Theincreases in observed NO near 80 km between 40◦N and50◦N are likely due to horizontal transport of NO from thisenhanced polar region by planetary waves (Garcia et al.,1987).

Even out of polar night, the chemical lifetime in the meso-sphere can be on the order of days. The fact that HALOE

Fig. 2. The residual circulation ( Kv∗; Kw∗) calculated from theTIME-GCM for January.

sunrise and sunset observations are often markedly di<er-ent (seen 6rst by Siskind et al., 1998) indicates that dy-namics must play a role in causing NO diurnal variability.Marsh and Russell III (2000) showed that the diurnal pertur-bations appear anti-correlated with tidal vertical velocities,and proposed that tidal advection produced the observedsunrise=sunset asymmetry. Fig. 3 shows both the amplitudeand phase of the diurnal component of variations in the ver-tical wind and NO mixing ratios. Wind perturbations tendto maximize at the equator, reaching in excess of 30 cm=s.Secondary maxima in the wind perturbation 6eld are mod-eled near ±30◦ latitude. Perturbations at these latitudesare approximately 12 h out of phase with variations at theequator. This pattern indicates a diurnal wind componentdominated by the (1,1) mode of the diurnal tide. The NOperturbation amplitudes coincide with wind perturbationsand can be ¿ 60% of the zonal mean. Vertical winds a<ectNO in this region because a steep gradient in NO mixingratios means that downwelling will tend to increase mixingratios by advecting air rich in NO from the thermosphere.The converse is also true—upwelling will decrease NO be-tween 80 and 105 km.

HALOE’s solar occultation technique limits observationsto sunrise and sunset, and so the full diurnal cycle is not ob-served. However, by comparing di<erences in sunrise andsunset observations, some estimate of the diurnal variabil-ity can be made. If the di<erences in sunrise and sunsetobserved NO are in fact due to vertical advection by thediurnal tide, then the asymmetry should be greatest at theequator, where perturbations maximize. Also, asymmetriesare expected to be small at ±20◦ latitude, where the (1,1)mode has a node, and at ±30◦ any asymmetries should bein the opposite sense to those at the equator. In addition,since the vertical wavelength of the diurnal tide is ∼ 25 km,this structure should also been seen in the ratio of sunrise tosunset observations.

Fig. 4 shows the ratio of modeled and observed sunriseto sunset observations for April, 1993 for three di<erentlatitudes. At the equator below 94 km the ratio is very muchless than 1.0, i.e. sunset NO mixing ratios exceed thosesunrise. Above this altitude, the reverse occurs, with largersunrise values. At 30◦N, the opposite pattern exists, andthe model correctly predicts the heights at which the ratioreaches its maximum and minimum values. At 20◦N far lessdiurnal variations are observed or modeled, with ratios closeto one. All this appears consistent with a tidal advectioninterpretation, but does not rule out a tidal temperature e<ectwhich would have the same latitudinal and vertical structureas the vertical wind. This possibility is investigated in thenext section.

In Fig. 5 the temporal relationships between modeled NOand the dynamical 6elds w and T are presented at two al-titudes. As expected, local time variations in NO near theequator are approximately sinusoidal, with a 24 h period.Modeled vertical winds and temperatures also show a si-nusoidal variation, characteristic of the diurnal tide. The


(a) (b)

(c) (d)

Fig. 3. Magnitude expressed as a percentage of the zonal mean (a) and phase (b) of the diurnal component of variations in NO mixingratio for April 10. Phase is the local time (hrs) of maximum positive variation. Diurnal components of variations in the vertical wind 6eld(cm=s) are shown in (c) and (d).

Fig. 4. Ratio of sunrise to sunset NO mixing ratios for TIME-GCM (solid line) and HALOE (dotted line).

vertical wind reaches a maximum of 28 cm=s at 99 km and22 cm=s at 89 km. Temperature perturbations are ∼11% ofthe mean temperature at each altitude. Model NO reaches adiurnal maximum within three hours of the vertical wind re-versing direction from downward to upward. NO minimummixing ratios occur almost simultaneously with the reversalof the vertical wind from an upward to downward direction.

NO perturbations appear to be in phase with temperatureperturbations at 89 km, and lag temperature perturbations atmost 1–2 h at 99 km.

For comparison, the average of HALOE observations thatfall within 5◦ latitude of 2:5◦N are also shown. Both sunsetaverages comprise of 15 observations taken between the2nd and 3rd of April, 1993. Sunrise averages are from 14


(a)

(b)

Fig. 5. Local time variation in TIME-GCM NO mixing ratio (left axis), vertical wind and temperature (right axes) for (a) 86 and (b)99 km at 2:5◦N. Model data shown for April 10th. Positive wind values denote upward directed winds. HALOE observations are shownfor comparison (squares), with the average variance from the sample mean shown as a vertical bar.

pro6les taken on the 11th and 12th of the same month.During this period the 10:7 cm wavelength solar ;ux was∼123 (10−22 W=m2=Hz). 1� error bars shown are calculatedfrom the average variance from the sample mean pro6le,and include not only instrument errors but any geophysicalvariance. Uncertainties in an individual pro6le are typically¡ 0:8 ppmv at 99 km, and¡ 0:1 ppmv at 89 km. The modelappears to predict the observed sunrise=sunset asymmetry,and predictions fall within 1� of the observations.

4. Discussion

At 6rst, it may appear that the good correlation betweenchanges in NO and temperature seen in Fig. 5 indicatesthat the diurnal variation in NO is the result of changes intemperature a<ecting chemical reaction rates and not dueto tidal advection. The relative e<ects of tidal motions andtemperature variations can be examined by linearizing thethermodynamic and continuity equations in a similar mannerto Lindzen and Goody (1965):

T ′t + uT ′

x + v′ KTy + w′( KT z + g=cp) = 0; (8)

ft + uf′x + v′ Kf y + w′ Kf z =−AT ′ − Bf′; (9)

where subscripts denote di<erentiation with respect to in-dependent variables time, latitude, longitude, and height(t; x; y; z). The total NO mixing ratio (f) and temperature(T ) are the sum of zonal mean and perturbation terms (in-dicated by an overbar and prime, respectively). The zonal,meridional, and vertical velocities (u; v; w) have similarlybeen separated into mean and perturbation terms. Here wehave assumed adiabatic ;ow (right-hand side of Eq. (8)is zero), and no mean meridional circulation ( Kv = Kw = 0)because we are considering tidal motions that operate ontime-scales of hours. The terms A and B are the di<erentialchanges in NO production and loss rates for changes in tem-perature and mixing ratio, and g=cp is the adiabatic lapse ratefor dry air. Unlike Lindzen and Goody (1965), we do notconsider perturbations in temperature induced by changes inf, since NO is radiatively unimportant in the MLT. Whilethis simple system fails to include the complex, non-linearprocesses included in the general circulation model, the useof linear theory can provide insight into 6rst order e<ectson NO mixing ratios. From Eq. (8) it can be shown thatif the motion is sinusoidal, that temperature perturbationslag vertical velocity perturbations by �=2. For the simpli6edcase where the meridional wind perturbation terms can beignored and w′ is of the form w0 exp(i(kx + � − �t)), the


solution for Eq. (8) is

T ′ =i( KT z + g=cp)(uk − �)

w′ ≡ i�w′: (10)

Here, k is the zonal wavenumber, � is the wave frequency,and � is de6ned as �t0 − kx0. If the right-hand side of Eq.(9) is zero, both equations have the same form and so tem-perature and mixing ratio variations should be in phase orof opposite phase, depending on the sign of the mixing ra-tio gradient. Above 80 km; Kf z is positive, and so T

′ and f′

are in phase. This appears to be the case near 89 km (seeFig. 4b). The presence of terms A and B will alter this phaserelationship, and may explain why f′ and T ′ are not entirelyin phase at 99 km. For the case of non-zero A and B, elimi-nating T ′ from Eq. (9) yields the following solution for f′:

f′ =(i Kf z − �A)

(uk − �)− iBw′: (11)

This allows the possibility of mixing ratio perturbations be-ing in phase with the vertical wind perturbations.

It is now possible to estimate the magnitude of the e<ecton NO mixing ratios due to changes in reaction rates result-ing from temperature perturbations. The relative importanceof this e<ect compared to advective changes can be deter-mined by comparing the size of �A to Kf z . Out of all thereactions important in determining NO mixing ratios in theMLT, only the reaction rate constants for reactions (2) and(7) have temperature dependencies (Brasseur and Solomon,1986). The term A is then

A= [N(4S)]([O2]n

@k2@T

− Kf@k7@T

); (12)

where n, [O2] and [N(4S)] are total, molecular oxygen, andatomic nitrogen number densities, respectively. k2 and k7 arethe reaction rate constants for Eqs. (2) and (7), and oncecalculated, the partial derivatives are evaluated at the meanzonal mean temperature ( KT ). Using density and temperaturesfrom the TIME-GCM, the amplitude of �A was found to bearound 1% of the amplitude of the vertical gradient in Kf. Ittherefore seems plausible that the temperature e<ect can beruled out as a cause for the phase shift between T ′ and f′,i.e. the failure of f′ to lag w′ by �=2.

It remains to determine the importance of the chemicaldamping term B by comparing the amplitude of B to �. B isequivalent to the inverse of the chemical lifetime (�) de6nedas [NO]=L([NO]), where [NO] is the nitric oxide density,and L([NO]) is the total loss rate for nitric oxide. In the MLT

B = k7[N(4S)] + JNO; (13)

where JNO is the photolysis rate for Eq. (6). Values for Bwere estimated using TIME-GCM loss rates, and found tovary signi6cantly during the diurnal cycle. For the case of thediurnal tide, �(=2�=24 h−1) is much larger than Kuk, and sothe phase di<erence ( ) between w′ and f′ is approximately

= tan−1(�B

)− �: (14)

Fig. 6. Calculated phase lag in hours of peak NO mixing ratiobehind the maximum in the vertical wind.

In Fig. 6 the phase lag is shown for the two altitudes usedin Fig. 5. During nighttime, when loss rates are small, NObehaves like a tracer and lags the vertical wind perturbationby �=2 (6 h). However, during the daytime near 99 km, NOlifetimes are shortened due to photolysis, and B is compa-rable to � resulting in the phase lag increasing. This shiftin lag is apparent in Fig. 5b, where at midnight NO is ata minimum when the vertical wind reverses directions, andbecomes downward, indicating a lag of 6 h. However, dur-ing the day, peak NO occurs closer in time to the verticalwind minimum and so indicates a lag nearer to 8 h.

5. Conclusions

A self-consistent three-dimensional global circulationmodel has been used to simulate the distribution of NO inthe mesosphere and lower thermosphere. Good agreementis found between model predictions and observations madeby the UARS HALOE instrument on both seasonal anddiurnal time-scales. Modeling of lower thermospheric NOshows that the di<erences in sunrise and sunset low-latitudeNO observed by HALOE are consistent with a diurnalvariations induced by atmospheric tides. TIME-GCM NOsimulations show that variations are primarily the resultof vertical motions in a background atmosphere that has asteep vertical gradient in NO.

This 6nding has an immediate consequence for the inter-pretation of electron density data in the lower ionosphere.The relation between NO and the electron density of theD-region has been studied by Friedrich et al. (1998). Theynoted that diurnal asymmetries in electron densities wereconsistent with the asymmetries in NO seen by HALOE(the consequence of Lyman-� photolysis of NO being a sig-ni6cant source of D-region electrons). The modeled tidalvariation in NO could be used to predict D-region elec-tron densities throughout the daytime that could be directly


compared with radiowave absorption measurements, provid-ing a further test of the tidal hypothesis for NO variations.

Acknowledgements

The National Center for Atmospheric Research isoperated by the University Corporation for AtmosphericResearch under sponsorship of the National ScienceFoundation.

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time-gcm simulations of lower-thermospheric nitric oxide seen by the halogen occultation experiment

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