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Incoherent scatter radar observations of AGW/TIDevents generated by the moving solar terminatorV. G. Galushko, V. V. Paznukhov, Y. M. Yampolski, J. C. Foster

To cite this version:V. G. Galushko, V. V. Paznukhov, Y. M. Yampolski, J. C. Foster. Incoherent scatter radar observa-tions of AGW/TID events generated by the moving solar terminator. Annales Geophysicae, EuropeanGeosciences Union, 1998, 16 (7), pp.821-827. �hal-00316410�

Incoherent scatter radar observations of AGW/TID eventsgenerated by the moving solar terminator

V. G. Galushko1, V. V. Paznukhov1, Y. M. Yampolski1, and J. C. Foster2

1 Institute of Radio Astronomy, National Academy of Sciences of Ukraine, Kharkov, Ukraine2MIT Haystack Observatory, Atmospheric Sciences Group, Westford, MA 01886, USA

Received: 13 October 1997 / Revised: 6 February 1998 /Accepted: 10 February 1998.

Abstract. Observations of traveling ionospheric distur-bances (TIDs) associated with atmospheric gravitywaves (AGWs) generated by the moving solar termina-tor have been made with the Millstone Hill incoherentscatter radar. Three experiments near 1995 fall equinoxmeasured the AGW/TID velocity and direction ofmotion. Spectral and cross-correlation analysis of theionospheric density observations indicates that ST-generated AGWs/TIDs were observed during eachexperiment, with the more-pronounced e�ect occurringat sunrise. The strongest oscillations in the ionosphericparameters have periods of 1.5 to 2 hours. The groupand phase velocities have been determined and showthat the disturbances propagate in the horizontal planeperpendicular to the terminator with the group velocityof 300±400 m s)1 that corresponds to the ST speed ationospheric heights. The high horizontal group velocityseems to contradict the accepted theory of AGW/TIDpropagation and indicates a need for additional inv-estigation.

Key words. Ionosphere (wave propagation) áMeteorology and atmospheric dynamics (waves andtides)

1 Introduction

Investigations of the atmospheric gravity waves (AGW)manifesting themselves at ionospheric heights as travel-ing ionospheric disturbances (TID) are of great impor-tance due to their role in the energy and momentumexchange between di�erent regions of the upper atmo-

sphere. A distinctive characteristic of this kind ofwavelike disturbances is their persistence (the mainAGW parameters such as spatial periodicity, velocityand direction of motion are retained over great distancesfrom their origin). Hines (1960) was the ®rst to give afundamental explanation of AGWs and their relation-ship to TIDs. Since then, a number of review papers(Yeh and Liu, 1972; Hines, 1974; Francis, 1975; Hines,1980; Hunsucker, 1982; Mayr et al., 1990; Crowley,1991; Leitinger, 1992; Hocke and Schlegel, 1996) havedescribed the essential results of AGW/TID research.Gravity waves in the upper atmosphere can be observedeither directly as neutral gas ¯uctuations (for example,with the use of interferometers or satellite-borne massspectrometers) or indirectly as ionospheric plasmavariations using various remote radio techniques (inco-herent scatter, IS, HF Doppler, TEC, etc.). But despitethe extensive research that has been performed, there arestill many open questions in the problem of thegeneration and propagation of AGWs/TIDs.

AGWs can be generated by di�erent sources either ofnatural or anthropogenic origin (particle precipitationat high latitudes, moving solar terminator, jet streams,wind shears, earthquakes, hurricanes, tsunami or in-dustrial accidents, ionosphere modi®cation experiments,powerful blasts, chemical releases, etc.). It is worthnoting that among all the sources of gravity waves, themoving solar terminator (ST) has a special status sinceit is a predictable phenomenon whose characteristics arewell known. The ST is characterized by sharp changes inatmospheric parameters such as energy, temperature,pressure, and electron density at a given altitude. Themain energy source, the solar heat input and itsabsorption, depends on the photochemistry of theatmosphere together with convection and di�usione�ects. In the vicinity of the ST, the atmospheric gasis not in an equilibrium state. This situation and themotion of the ST cause both waves and turbulence.Such processes have been reviewed by Somsikov andGanguly (1995). Considering the ST as a stable andrepetitive source of AGWs, one can derive informationCorrespondence to: J. C. Foster

Ann. Geophysicae 16, 821±827 (1998) Ó EGS ± Springer-Verlag 1998

about atmospheric conditions from the response of themedium to this input. Such a source is suitable forinvestigating gravity wave parameters and could be usedas an atmospheric diagnostic, using the ST-generatedwaves as probe signals. However, experimental mea-surements of the e�ects of the solar terminator as ageneration region for AGWs are quite rare (Bezrodny etal., 1976; Popov and Yampolsky, 1981; Galushko andYampolski, 1983; Beley et al., 1995). The work reportedhere was directed towards determining the main pa-rameters of ST-generated AGWs/TIDs through a set ofspeci®cally designed and analyzed incoherent scatterradar observations.

2 Experiments

Experimental observations were obtained with the high-powered UHF (440 MHz) incoherent scatter radar atthe Millstone Hill Observatory (42.6°N, 288.5°E).Ionospheric data investigated include electron density,electron temperature, ion temperature, and ion driftvelocity observed along the line of sight of the fullysteerable 46-meter or the zenith-directed 68-m antenna.An initial investigation sought direct evidence in theradar data of an ionospheric response to the passage ofthe solar terminator used previously-acquired data froma March 1992 zenith-directed, ®xed-beam, radar exper-iment. Spectral analysis identi®ed low-frequency distur-bances with the period of more than 1 h which weregenerated by the ST. Subsequently, special experimentswere carried out during the autumn of 1995 toinvestigate further these wavelike disturbances, and todetermine their main parameters such as spatial peri-odicity, velocity, and direction of motion. These dedi-cated experiments were performed in near-equinoxconditions when the terminator is best aligned alongthe meridian and were designed to look for the altitude/latitude signatures, predicted by the ST/AGW-genera-tion theory. To investigate altitude and space/timee�ects, a three-beam operating mode was chosen, whosescheme is shown in Fig. 1. Switching between antennapositions in the sequence ``zenith-west-zenith-north-zenith'' was done with a cycle time of 5 min and datawere integrated in each position for 68 s. Hence, thesampling time was equal to 5 min for positions ``west''and ``north'' and 2.5 min for the zenith position.Ionospheric parameters were determined over the heightrange 154 km±427 km with a height resolution of�20 km.

In order to determine TID characteristics unambig-uously, we need to have Dt < T=2, where Dt is the datasampling time and T is the period of the TID. Spatialsampling is done with DLh

ZÿW ; DLhZÿN < K, where

DLhZÿW ; DLh

ZÿN are the horizontal spatial distancesbetween positions ``zenith-west'' and ``zenith-north'' atheight, h, and K is the TID wavelength. As follows fromtheoretical works (e.g. Somsikov, 1983; Somsikov andGanguly, 1995) the temporal period and spatial scale ofST-generated TIDs depend on the Mach numberM � VT =VS , where VT is the velocity of the ST and VS

is the sound velocity. At mid-latitudes, three regions ofthe atmosphere can be di�erentiated based on themagnitude of M . The ®rst corresponds to inequalityM2 > 1 and is located below the altitude of 120 km.Both acoustic and gravity waves can be generated in thisregion. The second region lies between 120 km and180 km where 1 > M2 > 0:8. The waves generated bythe ST in this region, as a rule, are decaying. Finally, athird region where M2 < 0:8 begins from �180 km andstretches up to 600 km. In this region the ST generatesgravity waves with a spatial scale of about 1000 km andwith period more than 30 min. Taking into account thelocation of the Millstone Hill Observatory and theheight range where the measurements of the ionosphericparameters were performed, one concludes that theseexperiments are most likely to observe TIDs related withgravity waves with spatial scales of a few hundreds ofkilometers.

It is known (Hargreaves, 1982) that gravity waveshave frequencies less than Brunt-VaÈ isaÈ laÈ frequency, xB.For the height range where the measurements werecarried out, the period corresponding toxB is TB � 1=xB � 10min. Therefore, the shortest ex-pected TID period is equal to 10 min and an experi-mental sampling time of 5 min was chosen. For a roughestimate of TID wavelength, we assume that thedisturbances move at the velocity of the ST, whichvaries slightly over the height range of interest. For240 km (near the height of the daytime electron densitymaximum), the TID period was assumed to be equal tothe shortest period expected, 10 min. Thus, the shortestexpected wavelength is K � VT � T � 360 km sÿ1�600 s � 216 km.

So, it is necessary that DL240ZÿW and DL240ZÿN be less than216 km. In our measurements they both were chosen toequal 200 km by setting the elevation angle of antennabeams to be 40° from vertical. As mentioned already,the most likely TID periods are expected to be greaterthan 30 min, and therefore the experiment plan was wellmatched to its objectives.

Fig. 1. Schematic representation of the 3-beam radar experiment usedto determine TID/AGW propagation characteristics. Radar beams at40° elevation angle to the north and the west were interleaved withvertical observations with the Millstone Hill zenith antenna

822 V. G. Galushko et al.: AGW/TIDs generated by solar terminator

3 Data processing

Of the four ionospheric parameters investigated, the ST-generated disturbances were most consistently observedin the electron density data. Segments of electron densityrecords obtained on September 21, 1995 are shown inFig. 2. Curves for di�erent heights are arti®cially shiftedto avoid overlapping. Times of the sunset and sunriseterminator passage at ionospheric heights over theobservatory are indicated. The ST-generated distur-bances manifest themselves as a ``peak'' in electrondensity at sunset as well as ``fast'' oscillations aftersunrise. However, the pronounced diurnal trend of theparameters makes it di�cult to observe the e�ect ofAGWs/TIDs generated by the ST and complicates theirspectral analysis. Accordingly, the data were detrended

before further processing by subtracting data smoothedby a moving rectangular time window. Disturbanceswith periods >2.5 h±3 h can be associated with tidalmodes (Hocke, 1996), so a smoothing window width of2.5 h was chosen to remove variations which were notassociated with the ST. This slowly varying trend, hNi,was subtracted from the original density data to yieldthe time-varying part of the electron density record,dN � N ÿ hNi, which was further analyzed with astepped Fourier transform

S�x; Ts � n � Dt� �ZTs�n�Dt�Tj

Ts�n�Dt

dtdNeÿjxt �1�

which determines a time-sequence of spectra where Dt isthe step between adjacent spectra, n � 1; 2; 3; . . . ; Tj isthe integration time, x � 2pf is the angular frequency,Ts corresponds to the start of the records. The results ofapplying this procedure to the data collected in thezenith position on September 21, 1995 are presented inFig. 3 where Tj was 2.5 h, and Dt was 10 min (2samples). Some strengthening of the spectral intensityafter sunrise and sunset over a wide height range isapparent. It is worth noting that the strengthening of the¯uctuations does not occur across the entire frequencyrange, but only at selected frequencies (this is quiteevident for the spectra at 301 km). Such spectral ®nestructure indicates the presence of wavelike processes inthe ionosphere. From Fig. 3 one can see that the mostpowerful spectral components are concentrated withinthe frequency band which corresponds to time periodsof 1.5±2.5 h. The most powerful spectral componentsthen were selected by a numerical rectangular ®lter. Theband-pass ®lter had cuto� frequencies corresponding tothe time periods Tmin � 90 min and Tmax � 130 min.Figure 4 demonstrates ®ltered data for the three anten-na positions (``zenith'', ``west'', and ``north''). Thespatial/temporal structure of the disturbance is repre-sented by the isolines of electron density and theintensity of the ¯uctuations is given in gray scale. Themaximum intensity of the disturbances is observed nearthe solar terminator. It is clearly seen that the e�ect isnot over the whole height range, but occurs only atheights lower than �300 km.

We have used a triangular method to determine TIDcharacteristics, including velocity and direction of mo-tion. To estimate these parameters, measurements areneeded in four di�erent points, not in the same plane(Fig. 1, points 1±4). To determine time delays betweenvariations for di�erent measuring points, cross-correla-tion analysis was applied to the ®ltered records near thetime of ST passage for positions ``zenith'', ``west'', and``north'' and results are given in Table 1. The cross-correlation values are su�ciently high over the heightrange 217±301 km to conclude that the disturbances aremoving as ``frozen-in irregularities'', i.e., all spectralcomponents move with the same velocity. The timedelays between variations for the di�erent points werederived from the maxima of the correlation functions.TID group velocity, Vg, and direction of motion were

Fig. 2. Un®ltered electron density observations (log 10) observed atseveral altitudes during the dedicated TID experiment onSeptember 21, 1995 are shown for the three antenna positions ofFig. 1. Curves for di�erent heights are arti®cially shifted to avoidoverlapping. Times of the sunset and sunrise terminator passage ationospheric heights over the observatory are indicated

V. G. Galushko et al.: AGW/TIDs generated by solar terminator 823

determined for the height range where values of cross-correlation functions were high. The results of theseestimates of Vg are given in Table 2. In the frame ofreference where Vg was derived, c is elevation angle and/ is azimuth measured counter-clockwise from thenorth. TID phase velocity was determined as thevelocity of a monochromatic wave. To estimate thephase velocity of the TID motion, the strongest spectralcomponent (2h-wave) was selected from the spectrum ofthe ®ltered records. Phase di�erences between variationsfor four measuring points were derived from the cross-spectra. Propagation velocity of the 2h-wave was esti-mated as the speed of motion of a plane wave. The

phase velocity and direction of propagation of thestrongest component of the wave packet are given inTable 3.

We are able to determine the spatial wavelength ofthe disturbances from the temporal period and velocityof the TIDs motion. Figure 5 illustrates Vph versus Kdependencies which follow the dispersion law forionospheric wavelike processes. The data were derivedfor the most powerful spectral harmonics of the electrondensity variations for di�erent heights. It is evident thatfor longer wavelengths, the wave moves faster. A similarbehavior of AGWs has been mentioned by otherauthors (e.g. Hocke and Schlegel, 1996).

Fig. 3. Spectral power of the detrendeddensity data at four F-region altitudesreveal strong intensi®cation of wavelike(banded) disturbances in the two hoursfollowing sunrise (at �09 UT). The mostpowerful spectral components corres-pond to temporal periods of 1.5 h±2.5 h

824 V. G. Galushko et al.: AGW/TIDs generated by solar terminator

4 Discussion

The results of our investigations demonstrate that TIDsare generated by the moving solar terminator in theionosphere. As seen in Fig. 3, the e�ect is observed bestat sunrise, while at sunset it is rare. The measuredvelocity and direction of the propagation strongly

Fig. 4. Intensity of the disturbance in the®ltered spectral band 90 min±130 min for thethree observing positions and across F-regionaltitudes indicates intensi®cation of the dis-turbances at altitudes <300 km at the timesof sunset and sunrise terminator passage

Table 1. Cross-correlation coe�cient for positions Z, W, and Nnear the time of ST passage

h, km W-Z N-Z

196 0.80 0.60217 0.83 0.75238 0.95 0.91259 0.90 0.87280 0.84 0.63301 0.62 0.45322 0.40 0.38

Table 2. TID group velocity (Vg) and direction of motion(c elevation angle and / azimuth)

h, km Vg, m/s c, deg /, deg

217 475 0 101238 343 0 98259 268 0 96280 268 0 96301 236 0 120

Table 3. TID phase velocity and direction of propagation of thestrongest (2-h) component

h, km Vph, m/s, c, deg /, deg

217 550 )3 17238 193 62 253259 268 51 212280 165 27 )49

V. G. Galushko et al.: AGW/TIDs generated by solar terminator 825

indicate that the observed TIDs are associated with theST. First, the group velocity, Vg, is close to the terminatorvelocity at ionospheric heights �Vt �360 m sÿ1�. Second,propagation is directed to the west (i.e., perpendicular tothe terminator). Our results indicate that the STgenerates low frequency disturbances of large spatialscale. Figure 5 shows that TIDs with spatial wave-lengths from hundreds to a thousand kilometers aredetected most frequently. This implies that the observedTIDs are connected with internal waves belonging to thegravity part of AGW. The observations support theo-retical predictions of the expected spatial scale of ST-generated TIDs, in keeping with the model of STin¯uence on the ionosphere described in Somsikov(1983).

It is known (Williams, 1996) that under atmosphericconditions AGW phase velocity and group velocity areperpendicular to each other. The angle between Vg andVph obtained in these experiments may be derivedthrough a comparison of Table 2 and Table 3, whoseresults are given in Table 4. It can be seen that over theheight range 217±259 km, the angle between Vg and Vph

is close to 90°. This fact indicates that the observeddisturbances are of AGW origin.

An interesting result concerns the observation ofhorizontal motion for the TIDs (Table 2). In accordancewith the accepted theory, only a wave which has afrequency close to the Brunt-VaÈ isaÈ laÈ frequency is able topropagate horizontally in the atmosphere. FollowingHargreaves (1979), the angle of elevation for AGWpropagation can be derived as

c � atan�x2B=x

2 ÿ 1�1=2 �2�where x is the frequency of the AGW, xB is the Brunt-VaÈ isaÈ laÈ frequency. Consequently, if x� xB (which isthe case) then c � 90� and the wave under considerationshould propagate vertically. This contradiction proba-bly arises from the fact that expression (2) applies toAGWs (i.e., to neutral gas variations), while chargedcomponent variations (TIDs) were measured in theexperiments. The relationship between TIDs and AGWsis complicated by the necessity of taking into accountthe in¯uence of the Earth's magnetic ®eld on the motionof electron density structure in the ionosphere.

AGW parameters are closely related to the charac-teristics of the atmosphere. For instance, the velocity ofsound in the ionosphere can be found from the ratio ofphase to group velocity observed in such experiments. Anew method for upper atmosphere/ionosphere diagnos-

Table 4. Angle between TID group and phase velocities

h, km 217 238 259 280D/, deg 84 115 105 137

Fig. 5. Vph versus K has been derived for the most powerful spectral components of the electron density variations for di�erent heights. Thesefollow the dispersion law for ionospheric wavelike processes and is evident that for longer wavelengths, the wave moves faster

826 V. G. Galushko et al.: AGW/TIDs generated by solar terminator

tics using wavelike disturbances generated by the mov-ing solar terminator is suggested, based on the interpr-etation of such results.

5 Conclusions

The generation of ionospheric disturbances by themoving solar terminator (ST) has been con®rmed withthe aid of the three-position incoherent scatter radarexperiments and the use of special data-processingalgorithms. It was shown that the strongest componentsof TIDs associated with the ST have temporal periods ofabout 2 h. The TIDs were shown to move horizontally,perpendicular to the terminator, and the occurrence ofdominant horizontal motion seems to contradict theaccepted theory of AGW/TID propagation, indicating aneed for additional investigations, both theoretical andexperimental. The use of ST-generated disturbances asprobe signals is suggested as a new technique foratmospheric diagnostics.

Acknowledgements. The authors wish to thank the colleagues ofthe Millstone Hill Observatory who have taken part in theseexperiments. Millstone Hill radar observations are supported bythe US National Science Foundation through cooperative agree-ment with the Massachusetts Institute of Technology.

Topical Editor D. Alcayde thanks R. D. Hunsucker and K.Hocke for their help in evaluating this paper.

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