Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 61–67

Contents lists available at SciVerse ScienceDirect

Journal of Atmospheric and Solar-Terrestrial Physics

1364-68

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jastp

Simulation of nonmigrating tide influences on the thermosphere andionosphere with a TIMED data driven TIEGCM

Q. Wu a,n, D.A. Ortland b, B. Foster a, R.G. Roble a

a National Center for Atmospheric Research, High Altitude Observatory, P.O. Box 3000, Boulder, Co 80307-3000, United Statesb Northwest Research Associates, P.O. Box 3027, Bellevue, WA 98009-3027, United States

a r t i c l e i n f o

Article history:

Received 1 August 2011

Received in revised form

6 February 2012

Accepted 8 February 2012Available online 25 February 2012

Keywords:

Nonmigrating tide

Thermosphere

Ionosphere

26/$ - see front matter & 2012 Elsevier Ltd. A

016/j.jastp.2012.02.009

esponding author. Tel.: þ1 303 4972176; fax

ail address: qwu@ucar.edu (Q. Wu).

a b s t r a c t

Using TIMED data as the lower boundary condition at 95 km altitude for both the migrating and

nonmigrating tide source, we run the NCAR Thermosphere Ionosphere Electrodynamics General

Circulation Model (TIEGCM) to simulate the mesospheric and lower thermospheric nonmigrating tidal

effects on the thermosphere and ionosphere. In this setup, the TIMED data provide a more faithful

description of the tidal forcing inter-annual and seasonal variations, while the TIEGCM provides

ionosphere electrodynamics and tidal propagation calculation. The TIEGCM also includes geomagnetic

and solar inputs. Hence, this setup will allow more detailed comparison with observations. The

simulation results show clear nonmigrating tide effects on thermospheric winds and temperature, and

ionosphere electron density. The simulation results also have consistent solar effect, which means

larger thermospheric tidal amplitudes during solar minimum. The new TIMED data driven TIEGCM

showed westward propagating wavenumber 3 diurnal tide (DE3) in the upper thermosphere compar-

able with that from earlier TIME-GCM simulation and CHAMP satellite observations.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Mesospheric and lower thermospheric (MLT) winds play animportant role in the dynamics of the thermosphere and iono-sphere. In recent years, new observations revealed a closercorrelation between the MLT nonmigrating tide and ionosphereelectron density. Such correlations are manifested as equallyspaced structures in the longitude as observed by IMAGE satellite(Immel et al., 2006). This work has generated a great interest tounderstand the MLT influences on the equatorial ionosphere.There are also many model efforts as well to simulate the MLTcoupling to the ionosphere and thermosphere (Hagan et al., 2007;2009; Wan et al., 2010; Oberheide et al., 2009; England et al.,2010; Pedatella and Forbes, 2010; Pancheva and Mukhtarov,2010; Muhktarov and Pancheva, 2011). There are two majorpotential coupling mechanisms between the MLT region and theionosphere. The first is nonmigrating tide propagation, becausemost of them are generated in the troposphere and stratospheredue to latent heat and planetary wave/migrating tide interactions.Some of these can propagate to upper thermosphere and generatethe equally spaced pattern in longitude. The second is the MLTneutral wind nonmigrating tide dynamo effect. In this case the

ll rights reserved.

: þ1 303 4972180.

nonmigrating tide does not require propagating all the way to thethermosphere. An earlier study by Wu et al. (2009) using TIMEDTIDI MLT neutral wind data and COSMIC electron density data hasshown that if the nonmigrating tide is in a symmetric mode, thereaction in the ionosphere tends to be stronger. If the nonmigrat-ing tide is antisymmetric then the ionosphere is less responsive.In the case of an antisymmetric tide, the dynamo effects from thetwo hemispheres are opposite and cancel each other out. Hence,Wu et al. (2009) showed a definitive dynamo effect between MLTand the ionosphere. However, dynamo effect is not the only linkand a more detailed understanding the link between MLT and theionosphere is needed. Wan et al. (2010) and Ren et al. (2010;2011) used TIMED TIDI diurnal eastward zonal wavenumber 3(DE3) as a lower boundary in a thermosphere ionosphere modeland simulated the DE3 effect. Oberheide et al. (2009) examinedthe inter-annual variation of the tide based on Hough ModeExtensions (HME), which includes propagation of the nonmigrat-ing tides from the tropospheric sources. No in-situ sources ordynamo effects of the nonmigrating tide were added.

While DE3 or Wavenumber Number 4 (WN4, in local time)structures were the focus of many of the past studies, it is not theonly nonmigrating tide. There are other nonmigrating tides,which are also quite significant. For example, the diurnal west-ward zonal wavenumber 2 (DW2) (Wu et al., 2009, 2008b) is alsovery strong in the MLT region. In the thermosphere, CHAMPsatellite data have shown various nonmigrating tides in the zonal

Q. Wu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 61–6762

wind component and other parameters (e.g. Luhr et al., 2007;2008; Hausler and Luhr, 2009; Hausler et al., 2010). Hence, it ishighly desirable to have a model to simulate the various non-migrating tides (not just DE3) influences on the thermosphereand ionosphere that takes into the consideration of inter-annualvariability of the nonmigrating tides, nonmigrating tide propaga-tion, and dynamo effects.

The NCAR Thermospheric and Ionospheric ElectrodynamicsGeneral Circulation Model (TIEGCM) is one of the choices for thispurpose (Roble et al., 1988; Richmond et al., 1992). The TIEGCMhas many sophisticated ionosphere and thermosphere features. Itis driven by magnetosphere and solar inputs from above and MLTinputs from below at 95 km. In this study, we will run the TIEGCMwith geophysical and solar indices. For the lower boundary at95 km, we utilize TIMED satellite observations from SABER andTIDI to provide both migrating and nonmigrating tide amplitudes,which are self-consistent and a faithful description of the lowerboundary with inter-annual variations. Hence, the new TIEGCMmodel will have inter-annual variations from both the upper andlower boundaries. Because of that, there will be smaller uncer-tainties when the new TIEGCM results are compared withthermospheric observations such as those from the CHAMPsatellite. The lower boundary conditions and related analysis ofthe TIMED data are described. Results from model run with thenonmigrating tides and without nonmigrating tides are comparedto examine the effect of the nonmigrating tide. The solar effect onthe nonmigrating tides in the thermosphere is also examined.Finally, new findings are discussed and summarized.

2. TIEGCM model description

The TIEGCM is a model of the coupled thermosphere andionosphere system. The model solves the fully coupled, nonlinear,hydrodynamic, thermodynamic, and continuity equations of theneutral gas self-consistently with the ion energy, ion momentum,and ion continuity equations. The TIEGCM outputs global winds,temperatures, major and minor species composition, electron andion densities and temperatures, and ionospheric dynamo electricfield. The horizontal resolution for this TIEGCM is 2.51�2.51 andthere are 60 pressure surfaces from 97 km to �500 km, with avertical resolution of a quarter scale height. That is double of theusual spatial resolution. We use the double resolution to properlydescribe the migrating diurnal tide which has a short verticalwavelength (�25 km). The input parameters for the TIEGCMmodel are solar EUV and UV spectral fluxes, parameterized bythe F10.7 cm index, high latitude auroral particle precipitationand convection electric field. At the model’s lower boundary, theamplitudes and phases of tides from the lower atmosphere werespecified using TIMED observations. For model runs in this paper,the high latitude ion convection pattern is specified by the Heelismodel (Heelis et al., 1982) and driven by the Kp index. TheTIEGCM usually was run with the global scale wave model(GSWM) at 95 km (Hagan et al., 1995, 1997, 2002). To understandthe nonmigrating tide effects, we run the TIEGCM with both theTIMED data based lower boundary condition and the standardGSWM lower boundary condition for comparison. In the case ofthe GSWM run, the nonmigrating tides were not included.

3. TIMED data driven lower boundary at 95 km

The lower boundary for this TIEGCM is based on TIMED SABER(Russell et al., 1999) and TIDI observations (Killeen et al., 1999; Wuet al., 2006; Wu et al., 2008a, 2008b). To provide self-consistent tidalamplitudes in geopotential, temperature, and winds, a linear

atmospheric model assimilated with TIMED observation is used. Inthis way, the linear model provides the self-consistency betweendifferent parameters and data provides spatial and temporal varia-tions of the tides. Both the diurnal and semidiurnal tides are included.The zonal wavenumber range for the diurnal (semidiurnal) tide isfrom eastward 3(2) to westward 5(6), which should cover themajority of the nonmigrating tides.

4. Model results

Since this new TIMED data driven TIEGCM just becameoperational recently, we present only the first look of the results.We examine the data by showing data from different longitudesat the same local time. For migrating tides, we should not see anylongitudinal structures in this kind of display, because for themigrating tide the longitudinal changes are solely dependent onthe local time. For the nonmigrating tides, on the other hand, wewill see longitudinal variations at the same local time. Hence,equally spaced structures in the longitudes at the same local timecan be a good indicator for nonmigrating tides. In fact, many ofthe satellite data are organized in this way to show nonmigratingtide structures (Immel et al., 2006; England et al., 2010).

4.1. Comparison with standard GSWM model run of TIEGCM

To examine the effect of nonmigrating tide on the thermo-sphere and ionosphere, a comparison with the standard GSWM runof the TIEGCM is made, which does not have nonmigrating tides.Fig. 1 shows the zonal winds at 250 km altitude and 18 LT fromboth the TIMED data driven and the GSWM driven model results.On the top, we showed a global view of the TIMED data drivenTIEGCM. Since the high latitude structures are related to thelocation of the auroral oval and not relevant with nonmigratingtide discussion, we restrict rest of discussion to regions between60S and 60N to highlight the nonmigrating tide features. It is veryapparent that the TIMED data driven model has a strong nonmi-grating tide component at 250 km, whereas in the GSWM case nononmigrating signature is present. The most prominent feature isthe wavenumber 4 at the same local time. Given that the DE3 isone of the strongest nonmigrating tides in the MLT region, thiswavenumber 4 feature is likely the results of the DE3 tide eitherthrough propagation or dynamo effect. Pancheva and Mukhtarov(2011) suggested that contributions from DW5 and SW6, SE2, andSPW4 make the ionospheric response 2 times of that from the DE3alone. Since we do not have SPW4 in our lower boundary condi-tion, we cannot assess its contribution at the moment. A futureimprovement in the lower boundary condition will add contribu-tions from planetary waves. Another less obvious feature of thenonmigrating tide is that the peaks are slightly south of theequator. We should point out that the zonal winds are affectedby the magnetic latitude in the case of the standard GSWM drivenrun. The zonal wind minima mostly follow the magnetic equator inthe GSWM case and less so in the TIMED data driven case.

Fig. 2 shows the meridional winds in the thermosphere. Theequatorial meridional winds are less affected by the nonmigratingtide and no nonmigrating tide signatures are observed. The tem-perature data also show a clear nonmigrating tide signal (Fig. 3). It isnoted that the temperature features are slightly north of the equatorwhereas the zonal wind signatures are slightly south of the equator.

Fig. 4 displays the electron density from these two differentTIEGCM runs. The electron density in the case of the TIMED datadriven TIEGCM certainly has the nonmigrating feature. Thestandard GSWM driven run does not have. The electron densitypeaks in the southern hemisphere occur at the same longitudes asthe zonal wind peaks. There are longitudinal differences between

Fig. 1. A global view of zonal wind at 250 km from TIMED data TIEGCM simulation (upper). Zonal wind at 250 km from the TIMED data from 60S to 60N (middle) and the

GSWM (lower) driven TIEGCM at 18 LT on day 243 of 2003 (August 31, 2003). The selected day coincides with the maximum of the MLT DE3 amplitude. The contour step

size is 14.6 m/s.

Q. Wu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 61–67 63

the peaks in the two hemispheres in the geographic coordinates.The northern hemisphere peaks tends to occur west of thesouthern hemisphere.

4.2. Solar effects on the nonmigrating tide

The solar effect on the migrating diurnal signal in the thermo-sphere is well known (e.g., Lei et al., 2007). In short the diurnaltide is stronger during solar minimum due to a lower neutraldensity and a smaller ion drag. In recent years similar trend is alsonoted in the nonmigrating tide (Oberheide et al., 2009). In thisnew TIEGCM simulation, the nonmigrating signals for differentsolar activity levels are examined. Specifically, the results for2003 and 2009 are compared. Since the TIEGCM includes the

ionosphere, the simulated results will reflect the influence ofchanging ion drag on the nonmigrating tides.

Fig. 5 is a comparison of zonal wind at 250 km between thesimulations for 2009 and 2003, which were under different solarconditions. The year of 2003 was close to the solar maximum while2009 to the solar minimum. The nonmigrating tide signal in 2009 islarger than that in 2003. Fig. 6 shows the thermospheric temperaturefrom 2003 and 2009 simulations. Again the nonmigrating signal isstronger in the 2009 case.

Fig. 7 is for the electron densities from these simulations. In thiscase the nonmigrating signal is smaller in 2009. That probably is notdue to a smaller nonmigrating tide. The overall electron density islower in 2009. Hence this is more likely a combined effect of a lowerelectron density in 2009 and a stronger nonmigrating tide 2009. Inthe end, the low electron density seems to be a more dominant factor.

Fig. 2. Similar to Fig. 1 for the meridional winds without the global view. The contour step size is 8.5 m/s.

Fig. 3. Same as Fig. 2 for the thermospheric temperature. The contour step size is 6 K.

Q. Wu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 61–6764

5. Discussions

This is the first look of the TIEGCM run with an observationalbased lower boundary condition for nonmigrating tide effect onthe thermosphere and ionosphere. The comparison with thestandard TIEGCM run without the nonmigrating tide inputappears to confirm the nonmigrating tide influence on thethermosphere. With an observational based lower boundarycondition, the TIEGCM can provide a more accurate descriptionof the MLT nonmigrating tide source for the thermosphere andionosphere modeling. TIEGCM also provides inputs from solar andmagnetosphere sources. Moreover the model includes thermo-sphere and ionosphere interaction. Hence, TIEGCM can provide amore comprehensive examination of the MLT influence on thethermosphere and ionosphere system. Here we discuss a few

aspects of the MLT–ionosphere coupling based on the first look ofthe simulation. Further study will be done in future studies.

5.1. Nonmigrating tide propagation

The zonal wind in the thermosphere near the equator is moreaffected by the DE3 nonmigrating tide than the meridional wind.That is understandable because the MLT DE3 is mostly in theequatorial zonal wind. To further examine the DE3 propagation, avertical profile of equatorial zonal winds is plotted in Fig. 8.Comparing with TIMEGCM model (England et al., 2010), we seemostly consistent result from this new TIEGCM. The phase changein the lower thermosphere is faster than in the upper thermo-sphere. The magnitude of the equally spaced longitudinal pattern

Fig. 4. Same as the Fig. 2 for the electron density. The contour step size is 1.1e5 cm�3.

Fig. 5. Thermospheric zonal wind nonmigrating tide comparison between solar minimum (2009) and maximum (2003) at 18 LT and 250 km. The contour step size is

13.16 m/s.

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in the thermosphere �20 m/s is also consistent with an earliersimulation from the TIMEGCM (England et al., 2010).

The comparison between 2003 and 2009 showed the solareffect on the nonmigrating tide. Overall the trend is consistentwith observation from CHAMP and larger than the HME results(Oberheide et al., 2009).

5.2. Thermospheric tidal features and geomagnetic influence

In Fig. 1, we see clearly zonal wind minimum point follow thegeomagnetic equator in the GSWM driven TIEGCM. That is a goodindication that the ion drag plays an important role in the thermo-spheric zonal wind. When the nonmigrating tide effect is introducedin the TIMED data driven TIEGCM the zonal winds beyond 201

latitude are very similar to the GSWM run. Near the equator,however, the deep valley in the GSWM case was filled with fournearly even spaced depressions, which are more close to thegeographic equator. It appears that the nonmigrating tide signatureis more symmetric in the geographic coordinate. The temperaturesignal of the nonmigrating tide is located slightly north of thegeographic equator (Fig. 3). In the case of the GSWM driven runthere is a temperature crest slightly north of the geographic equator.The nonmigrating tide signature simply is a modulation of that thecrest. Hence the temperature feature also appears to follow thegeographic coordinate.

The nonmigrating tide signature in the equatorial anomalyelectron density does not occur at the same longitudes in the twohemispheres. The northern hemisphere peaks tend to shift

Fig. 6. Same as Fig. 5 for thermosphere temperature. The contour step size is 6 K.

Fig. 7. Similar to Fig. 5 for the electron density at 03 LT. The contour step size is 2e5 cm�3.

Q. Wu et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 61–6766

westward (Fig. 4). At this point, we suspect that this is acombined effect of nonmigrating tide and the geomagneticequator offset from the geographic equator. The equatorialanomaly has longitudinal structure even without the nonmigrat-ing tide in the case of the GSWM LBC simulation. Nonmigratingtide structure should superimpose on that. The combined effectappears to shift these peaks with longitudinal offset between thehemispheres. Further investigation is needed.

5.3. Solar activity influence

Solar activity level has a great impact on thermosphericdynamics. For the migrating diurnal tide in the thermosphere, theamplitude is larger during the solar minimum due to low neutraldensity and low ion drag. CHAMP observation (Hausler et al., 2007;

Hausler and Luhr, 2009) and HME modeling (Oberheide et al., 2009)all showed the same trend for the nonmigrating tide in the thermo-sphere. However, the HME provides lower DE3 amplitude duringsolar minimum (4 m/s in 2009) compared to the TIME-GCM results7 m/s (England et al., 2010). The new TIMED data driven TIEGCMresults are closer to the TIME-GCM results. Hausler and Luhr (2009)report 10 m/s amplitude average over 2002–2005. That is largerthan HME results. Hence, new the TIEGCM results are more in linewith the CHAMP observation.

6. Summary

The new TIEGCM model run results clearly demonstrate thatthe TIEGCM with a new TIMED data based lower boundary

Fig. 8. Equatorial zonal wind profile at 18 LT for day 243 of 2009 (August 31,

2009). The profile clearly shows the nonmigrating tide propagation from the MLT

region to the thermosphere. The contour step size is 9.1 m/s.

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condition can generate nonmigrating tides effect in the thermo-sphere and ionosphere. Preliminary comparison with TIMEGCMresults showed consistent amplitude of DE3 in the upper thermo-sphere. The results also appear to agree with the CHAMP observa-tion. More analysis of the model results is planned.

Acknowledgments

This work is supported by NASA Grants NNX09AG64G andNNX11AG15G. National Center for Atmospheric Research is sup-ported by the National Science Foundation.

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