Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 193–199www.elsevier.nl/locate/jastp

Doppler velocities obtained by the EISCAT VHF radar and thedynasonde during the PMSE95 campaign

C.C. Leea, J.Y. Liua;b; ∗, C.J. Pana, C.H. LiuaaInstitute of Space Science, National Central University, Chung-Li 32054, Taiwan

bCenter for Space and Remote Sensing Research, National Central University, Chung-Li 32054, Taiwan

Received 7 September 1999; accepted 28 November 1999

Abstract

In this paper, we examine the Doppler velocities simultaneously derived from the EISCAT VHF radar and the dynasonde,under the PMSE conditions. Spectral and �ltering analyses are applied to study the velocities in detail. It is found thatthe high- and the low-frequency motions derived from the two radars correlate di�erently, high for low-frequency andlow for high-frequency component. The agreement=discrepancy between the two velocities are interpreted in terms of radarscattering=operation principles and characteristics of acoustic gravity waves. c© 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Doppler velocity; PMSE; VHF radar; Dynasonde

1. Introduction

The vertical motion of the atmosphere=ionosphere isan important dynamical quantity. The vertical (Doppler,line-of-sight) velocities contain contributions from meanwind, wave, turbulence and layer motion. In the last severalyears, the VHF radars have been making direct measure-ments of vertical motion at many locations in the polarmesosphere summer echoes (PMSE) observation. The ver-tical velocities of the PMSE were deduced by using thePoker Flat radar (50 MHz) (Cho and Morley, 1995), byusing the SOUSY radar (53.5 MHz) (Czechowsky andR�uster, 1997), and by using the EISCAT VHF radar (224MHz) (R�ottger, 1994; Palmer et al., 1996). Moreover, theHF (8–9 MHz) radar was also applied to observe the PMSEechoes and calculate velocity (Karashtin et al., 1997). Thevertical velocities in these papers were derived from theDoppler shift of the Fourier-transformed complex data(Woodman, 1985).

∗ Correspondence address: Institute of Space Science,National Central University, Chung-Li 32054, Taiwan. Tel.: +886-3-4227151 ext. 5763; fax: +886-3-4224394.E-mail address: jyliu@jupiter.ss.ncu.edu.tw (J.Y. Liu).

In this paper, to further understand the vertical motionsof the PMSE, Doppler velocities simultaneously observedby the EISCAT VHF radar and the HF dynasonde duringthe PMSE95 campaign are investigated. It is interesting tocompare Doppler velocities, since the radar wavelengths,beamwidths and the possible scattering mechanisms for thetwo radars are signi�cantly di�erent. Spectral and �lteringanalyses are applied on the two velocities to examine thehigh- and low-frequency motions in detail. The discrepancyin the two high-frequency motions and the similarity in thetwo low-frequency motions of the two radars are discussed,and possible causes for the agreement=discrepancy of thetwo motions are proposed. Moreover, two particular periodsof the low-frequency motions in velocities, echo amplitude,and echo locations of the dynasonde are examined, and themechanism of focusing and defocusing are investigated anddiscussed in detail.

2. Experimental description

Both, the VHF radar and the dynasonde are collocated atthe EISCAT Ramfjordmoen site (69◦35′N; 19◦14′E) nearTroms�, Norway. The data to be presented here were col-lected by the EISCAT VHF radar at 224 MHz and the dyna-

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194 C.C. Lee et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 193–199

sonde at �xed frequency of 7.953 MHz during the PMSE95campaign, between 1024 and 1330 UT on July 20, 1995.The EISCAT VHF radar antenna is a parabolic cylinder

extending over 120 m in the zonal east–west direction and40 m in the meridional direction. During our experiment thetransmission was at 1 MW peak power through the easternhalf of the antenna (60 m×40 m) and this half as well as thewestern half were used for receiving in two channels. Thehalf power beamwidth for each of the receiving antennas isabout 1:2◦ in the west–east direction. The antenna beamswere pointed vertically for both transmitting and receivingduring the observation period. Detailed description of thecon�gurations of the EISCAT VHF radar can be found inLa Hoz et al. (1989). The range resolution is 300 m andthe altitude coverage is 81.45–89.85 km, covering about therange where PMSE occurred most frequently (Ecklund andBalsley, 1981; Hoppe et al., 1988).The dynasonde was used together with part of the EIS-

CAT heating facility (Rietveld et al., 1993) to transmit at a�xed sounding frequency (7.953 MHz) with a peak power50 kW. Note that this transmitting power is about one or-der of magnitude larger than the standard peak power of thedynasonde. The antenna system gain is about 24 dB, whichcorresponds to ±14:5◦ north–south (N–S) and ±3:0◦ east–west (E–W) beamwidths. The receiving antennas were 22m long dipoles, which have very little gain. A 30 �s trans-mission pulse and 60 kHz receiving bandwidth were em-ployed together with an inter-pulse period (IPP) of 20 ms, asampling interval of 10 �s and an altitude range between 77and 105.5 km with a range resolution of 1.5 km. The pulseset of the dynasonde was composed of four pulses, which

were received using two phase-matched receivers and fourspaced antennas; and for each pulse set there are eightcomplex signals. Note that the distance between the tworadars is less than 500 m, and therefore the scattering vol-ume of the EISCAT radar is in fact covered by that of thedynasonde.These received signals from the two radars are sampled

at many heights, averaged and Fourier analyzed to yield aDoppler spectrum. The �rst three moments of the spectrumyield the signal strength, mean Doppler frequency (veloc-ity), and spectrum width (Woodman, 1985). General discus-sions of radar technology and their clear-air measurementcapabilities may be found in the work of R�ottger and Larsen(1990). Meanwhile, the echo locations of the dynasonde arederived by using Pitteway and Wright (1992).

3. Data analyses and results

Fig. 1 illustrates the range–time intensity (RTI) plotsof the returned power of the EISCAT VHF radar and thedynasonde. Strati�ed structure can be seen over an alti-tude range from 81.5 to 89.0 km. For further study, theDoppler velocities for the EISCAT VHF radar and thedynasonde coincidentally at 84.5 km are investigated andcompared. Examinations of ionograms during the observa-tion period indicate as expected that the plasma frequenciesin the PMSE region are much lower than the �xed sound-ing frequency 7.953 MHz. This indicates that the echomechanism of the dynasonde is mainly scattering, not totalre ection.

Fig. 1. Range-time-intensity plot observed from the EISCAT VHF radar and dynasonde, July 20, 1995.

C.C. Lee et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 193–199 195

Fig. 2. Comparison of the Doppler velocities between the VHF radar and dynasonde at 84.5 km.

The data of the EISCAT VHF radar and the dynasondeare both analyzed by the �rst moment method. The meanDoppler velocity (every 40 s) of the EISCAT VHF radaris derived from one spectrum (64-point) after 20 times ofincoherent integration, while the velocity (every 40.96 s)of the dynasonde is deduced from one spectrum (64-point)corresponding to one channel of each pulse after 8 times ofincoherent integration. Note that the mean Doppler velocityof the dynasonde is obtained by averaging the summationof the 8-channel velocities. Fig. 2 illustrates the Dopplervelocities of the EISCAT VHF radar and the dynasonde at84.5 km, where the sense of the Doppler velocities is towardradar. The correlation coe�cient 0.46 shows that the twovelocities at this height are moderately correlated.For further studies, spectral and �ltering analyses are ap-

plied to the Doppler velocities of two radars. Fig. 3 showsthat a period of about 30 min simultaneously appears in thespectra of the two velocities. Then a high-pass �lter and alow-pass Chebyshev �lter with a common cuto� period of20 min are utilized to isolate the high- and low-frequencymotions from the mean Doppler velocities of the EISCATVHF radar and the dynasonde shown in Fig. 2, respectively.Fig. 4 displays the high-frequency motions of the two ve-locities and the associated correlation coe�cient at 84.5 km.The coe�cient 0.14 indicates that the high-frequency mo-tions of the two velocities are nearly uncorrelated. On theother hand, the low-frequency motions of the two velocitiesare displayed in Fig. 5. The coe�cient 0.80 suggests thatthe two low-frequency motions are well correlated. More-over, using Fisher’s z-transformation with a normal appr-oximation (David, 1938), 95% corresponding con�denceinterval (CI) is obtained as [0:87; 0:70]. This con�rmsthat the two low-frequency motions of the two radars arein better agreement for motions with periods greater than20 min.We examine the relationship between the low-frequency

components (motions) in the echo amplitude and the associ-ated velocity of the two radars. It is found that the coherencevalue of the dynasonde is 0.79 and a tendency for the veloc-ity tends to lead the echo amplitude by about 105◦, whilethe coherence value of the VHF radar is 0.36 and no phasedi�erence can be evaluated. We then further examine themeasurements observed by the dynasonde. Figs. 6a–d illus-

Fig. 3. The spectra of the two Doppler velocities at 84.5 km.

trate the low-frequency components in the velocities, echoamplitudes, and E–W as well as N–S locations at 84.5 km,respectively. Fig. 7 shows that the period of 50 min concur-rently appears in variations of the amplitude, velocity andN–S location, while the period of 30 min occur in those ofthe velocity and E–W location.

4. Discussion and conclusion

We have presented an analysis of Doppler velocities, de-rived by the �rst moment method, simultaneously obtainedfrom the EISCAT VHF radar and the dynasonde during thePMSE conditions. The results show the velocities from theEISCAT VHF radar and the dynasonde correlate moder-ately. To further understand the correlation, we applied thelow- and high-pass �lters on the two velocities. Both high-and low-frequency motions of the two velocities at the oneheight are correspondingly compared. Nearly no correlationbetween the two high-frequency motions with period shorterthan 20 min is found, while good correlation existed between

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Fig. 4. Comparison of the high-frequency motions between the two radars by a high-pass �lter with 20 min cuto�.

Fig. 5. Comparison of the low-frequency motions between the two radars by a low-pass �lter with 20 min cuto�. The con�dence intervalis displayed at 84.5 km.

Fig. 6. (a) Doppler velocity; (b) echo amplitude; (c) E–Wecho-location; and (d) N–S echo-location in the low-frequencymotions of the dynasonde.

Fig. 7. The spectra of the Doppler velocity, echo amplitude, E–Wecho-location, and N–S echo location of the dynasonde.

the low-frequency motions measured by the two radars.To understand this observed di�erence, let us consider thedi�erences of the two radars.The �rst di�erence is that the EISCAT VHF radar at

224 MHz senses refractive index irregularities (scatterers)of size 67 cm or so, while the dynasonde at 7.953 MHz issensitive to variations of the refractive index of scale size

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about 19 m. The second di�erence is the scattering vol-umes of the two radars at the observational height of 86 km:300 m× 1:8 km for the VHF radar and 1; 500 m× 21:8 kmfor the dynasonde. Because of the smaller scattering vol-ume and shorter horizontal spread of the beam for the VHFradar, it is more likely that the background wind �eld canbe assumed to be constant spatially within the radar scat-tering volume. Of course, there is also a random compo-nent due to turbulence. The 67 cm scatterers move withthis wind �eld resulting in the observed Doppler frequencyspectrum. It is reasonable to expect for this case that thetime series of the Doppler velocity obtained by analyzingthe Doppler spectrum represent the time variation of thevertical motion of the region occupied by the scattering vol-ume. For 7.953 MHz dynasonde, the situation is quite dif-ferent. Because of the much larger scattering volume andgreater horizontal dimension of the beam, the backgroundwind �eld within the radar scattering volume could actu-ally vary spatially. If this is the case, following the discus-sion by Liu and Pan (1993), it seems to be reasonable toexpect that the observed Doppler motion is the weightedaverage of the inhomogeneous wind �eld within the beam.The averaging process is weighted because the scatteringechoes from di�erent regions of beam depend on the an-tenna pattern as well as the possible non-homogeneous sta-tistical nature of scattering processes for 7.953 MHz radarsignal. Therefore, the time series of the Doppler veloc-ity obtained by the dynasonde is the time variation ofthe vertical motion averaged (weighted) over the radarvolume.We now come back to the comparison of the time series

of motions observed by the two radars. If the backgroundwind �eld is homogeneous spatially across the scatteringvolumes of both radars, then the time series of the motionsobserved by the two radars should correlate well. This seemsto be the case for the low-frequency components of the ob-served data. We note that for typical medium-sized acousticgravity waves of periods 20–60 min, the horizontal wave-lengths are of the order of several hundreds of kilometers(Williams, 1996). For these waves, the spatial variationswithin the beams for both radars can be neglected, hencethe good correlation of the motions. For the shorter period(higher frequency) motions, however, it is apparent thatthe corresponding spatial variations of the wind are also ofsmaller horizontal scales. Therefore, within the wider beamof the dynasonde the background wind may not be homo-geneous, even though for the VHF radar, the wind may stillbe considered as constant within the narrower beam. There-fore, the weighted-average motion observed by the dyna-sonde could be quite di�erent from the motion observed bythe VHF radar. Therefore, obviously the correlation will below.Meanwhile, we found that the radar beamwidth could

strongly a�ect the phase relationship between the radar pa-rameters. Due to the smaller scattering volume and shorterhorizontal spread of the beam, the e�ects of focusing and de-

focusing for the VHF radar are not signi�cant. Although theechoing layer can be tilted up and down by gravity waves,due to the amplitude of the up and down motion being muchsmaller than the altitude of the echoing layer (about 85 km),the altitude variation of a certain echoing layer should notsigni�cantly modify its associated echo amplitude. Thesemay be the reasons why no clear phase relation, a coher-ence value 0.36, between variations in echo amplitude andDoppler velocity of the VHF radar is found. In contrast,for the wider beam of the dynasonde, the echo amplitudesigni�cantly depends on the geometry of the echoing layer(convex or concave). Fig. 8a illustrates that the echo ampli-tude increases and decreases accordingly due to the focusingand defocusing. Meek and Manson (1992) examined thatthe relationship between the sharpness of layer and strengthof signal of the MF radar during AGW conditions in detail.They found that when the radius of curvature (Rc) of thelayer is greater than the height (Ho) of the layer (see theirFig. 4), the returned signals were focusing. Consequently,for wider beam observations the focusing and defocusingplay important roles.Liu and Berkey (1993) studied the ionospheric parame-

ters derived from the dynasonde and showed that there is aintrinsic ±90◦ phase di�erence between the Doppler veloc-ity and echo amplitude oscillations. An estimation of crosscorrelation between the parameters shown in Figs. 6a and bafter a 35 min �ltering reveals that oscillations in the veloc-ity lag those of the echo amplitude by 105◦. If the Dopplervelocity is de�ned as the rate of change of height, we can�nd the relationship between variations in Doppler velocity,echo amplitude and layer height. Fig. 8a illustrates a sketchthat due to focusing and defocusing, variations in the echoamplitude (or height) follow those in the Doppler velocityby 90◦. It is obvious that the phase shift 105◦ between thevelocity and amplitude oscillations observed by the dyna-sonde (see Fig. 8b) agrees with the phase relation betweenthe two parameters due to focusing and defocusing.Moreover, the oscillations of wave should concurrently

appear in various dynasonde parameters (Liu and Berkey,1993). Fig. 7 reveals that in layer structures of the PMSE,a wave motion could also cause echo-location oscillations.The period of 30 min simultaneously observed in variationsof the velocity and E–W location, and 50 min found in thoseof the velocity and the N–S location suggest that the twoperiods possibly result from two di�erent sources. The ar-rows in Figs. 6c and d indicate that 30 min wave propagatesfrom west to east, while the 50 min wave travels from northto south.Thus, applying the fundamental principle for MST radar

scattering and operation, we have attempted to interpret thedata of simultaneous observation of vertical mesosphericmotions during the PMSE conditions. The conclusion is thatfor HF and=or MF radars with wild beamwidth, care must betaken in interpreting the data. In many cases, the observedmotion is actually the weighted average of the inhomoge-neous wind �eld within the beam.

198 C.C. Lee et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 193–199

Fig. 8. (a) The relationship between the variations in the Doppler velocity, echo amplitude, and the height of the scattering layer.(b) Magni�ed plots of the Doppler velocity and amplitude after a 35 min low-pass �ltering from Figs. 6a and b, respectively.

Acknowledgements

The EISCAT Scienti�c Association is supported byCNRS (France), MPF (Germany), NIPR (Japan), NFR(Sweden), PPARC (UK), RCN (Norway), and SA (Fin-land). The authors appreciate the valuable discussionswith Dr. J. R�ottger and Dr. M. T. Rietveld. The re-search has partially been supported by the grant NSC88-2111-M-008-008-AP9. The authors would like to thankProf. Y. I. Chen, Institute of Statistics, National CentralUniversity for providing valuable information on derivingthe approximate con�dence interval for correlation coe�-cient; and Prof. L.-C. Tsai, Center for Space and RemoteSensing Research, National Central University for usefuldiscussion. The authors wish to thank the referees for usefulsuggestions.

References

Cho, J.Y.N., Morley, R.L., 1995. PMSE dependence on long-periodvertical motions. Geophysical Research Letters 22, 1197–1200.

Czechowsky, P., R�uster, R., 1997. VHF radar observations ofturbulent structures in the polar mesopause region. AnnalesGeophysicae 15, 1028–1036.

David, F.N., 1938. Tables of the Correlation Coe�cient. In:Hartley, H.O., Pearson, E.S. (Eds.), Reprinted in BiometrikaTables for Statistics, 1996, Vol. I. 3rd Edition. CambridgeUniversity Press, Cambridge.

Ecklund, W.L., Balsley, B.B., 1981. Long-term observations of theArctic mesosphere with the MST radar at Poker Flat. Alaska.Journal of Geophysical Research 86, 7775–7780.

Hoppe, U.-P., Hall, C., R�ottger, J., 1988. First observations ofsummer polar mesospheric backscatter with a 224-MHz radar.Geophysical Research Letters 15, 28–31.

Karashtin, A.N., Shlyugaev, Y.V., Abramov, V.I., Belov, I.F.,Berezin, I.V., Bychkov, V.V., Eryshev, E.B., Komrakov, G.P.,1997. First HF radar measurements of summer mesopauseechoes at SURA. Annales Geophysicae 15, 935–941.

La Hoz, C., R�ottger, J., Rietveld, M.T., Wannber, G., Franke,S. J., 1989. The status and planned developments of EISCATin mesosphere and D-region experiments. In: Liu, C.H.,Edwards, B. (Eds), Handbook for MAP. Vol. 28, pp. 476–488.SC OSTEP Secretariat, University of Illinois.

Liu, J.Y., Berkey, F.T., 1993. Oscillations in ionospheric virtualheight, echo amplitude and Doppler velocity: theory andobservations. Journal of Geomagectism and Geoelectrics 45,207–217.

Liu, C.H., Pan, C.J., 1993. New observational techniques forstudying the dynamics of the middle atmosphere using theChung-Li Radar. Journal of Atmospheric and Terrestrial Physics55, 1055–1066.

C.C. Lee et al. / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 193–199 199

Meek, C.E., Manson, A.H., 1992. Angle-of-arrival oscillations inthe mesosphere as seen by medium frequency (MF) radar.Journal of Atmospheric and Terrestrial Physics 54, 277–293.

Palmer, J.R., Rishbeth, H., Jones, G.O.L., Williams, P.J.S., 1996.A statistics study of polar mesosphere summer echoes observedby EISCAT. Journal of Atmospheric and Terrestrial Physics 58,307–315.

Pitteway, Ml L.V., Wright, J.W., 1992. Toward an optimumreceiving array and pulse set for the dynasonde. Radio Science27, 481–490.

Rietveld, M.T., Kohl, H., Kopka, H., Stubbe, P., 1993. Introductionto ionospheric heating at Troms� — I. Experimental overview.Journal of Atmospheric and Terrestrial Physics 55, 577–599.

R�ottger, J., 1994. Polar mesosphere summer echoes: dynamics andaeronomy of the mesosphere. Advance in Space Research 14,123–137.

R�ottger, J., Larsen, M.F., 1990. UHF=VHF radar techniques foratmospheric research and wind pro�ler applications. In: Atlas, D.(Ed.), Radar in Meteorology. American Meteorological Society,Boston, MA.

Williams, P.J.S., 1996. Tides, atmospheric gravity waves andtravelling disturbances in the ionosphere. In: Kohl, H., R�uster,R., Schlegel, K. (Eds.), Modern Ionospheric Science. EuropeanGeophysical Society, Katlenburg-Lindau.

Woodman, R.F., 1985. Spectral moment estimation in MST radars.Radio Science 20, 1185–1195.