We have developed a lidar system for the measurements of the

mesospheric atomic sodium (Na) number density profile. The performances and the main characteristics of the system are evaluated showing the first observations: Na number density profiles can be obtained with a resolution of 300 m in altitude and of 200 s in time, the relative uncertainty is between 5% and 10%. These features also allow to discuss the capability of the system to detect the time and space variability of Na density profiles, which is typical of the gravity wave propagation. The preliminary analysis of the sodium profile dynamics along a 7 hours measurement session, started from 19:30UT on 10 April 1997, shows the presence of wave-like perturbations with characteristics typical of propagating gravity waves

Di Carlo, P.

Rizi, V.

Visconti, G.

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IL NUOVO CIMENTO Settembre-Ottobre 1998VOL. 21 C, N. 5Lidar observations of mesospheric sodium over Italy (*)P. DI CARLO, V. RIZI and G. VISCONTIDipartimento di Fisica, Università degli StudiVia Vetoio, Località Coppito, 67010 L’Aquila, Italy(ricevuto il 17 Aprile 1998; approvato il 21 Luglio 1998)Summary. — We have developed a lidar system for the measurements of themesospheric atomic sodium (Na) number density profile. The performances and themain characteristics of the system are evaluated showing the first observations: Nanumber density profiles can be obtained with a resolution of 300 m in altitude and of200 s in time, the relative uncertainty is between 5% and 10%. These features alsoallow to discuss the capability of the system to detect the time and space variability ofNa density profiles, which is typical of the gravity wave propagation. Thepreliminary analysis of the sodium profile dynamics along a 7 hours measurementsession, started from 19:30UT on 10 April 1997, shows the presence of wave-likeperturbations with characteristics typical of propagating gravity waves.PACS 92.60.Dj – Gravity waves, tides, and compressional waves.PACS 93.85 – Instrumentation and techniques for geophysical research.1. – IntroductionMeteoric ablations are generally accepted as the dominant source of Na and of theother alkaline metals present in the mesosphere [1]. The Na layer is present at analtitude range from 80 to 110 km and the density peak close to the mesopause (A90 km)is of the order of 103 to 104 atoms cm23 [2].Since this region is relatively inaccessible to in situ measurements, most studiesrely on remote-sensing techniques. One of the most appropriate remote-sensingtechniques for the investigation of the mesosphere characteristics is the lidar. Themeasured backscattered radiation can be discriminated in altitude and on short timeperiods. For the Na detection, the radiation source of the lidar is a laser with anemission wavelength matching up the frequency of the Na atomic transition from theground state to the first-excited state. The radiation re-emitted by the Na atoms(resonance scattering) raised at the first-excited state by the laser radiation, iscollected with a telescope. The resonance scattering is the most efficient radiation(*) The authors of this paper have agreed to not receive the proofs for correction.G Società Italiana di Fisica 541P. DI CARLO, V. RIZI and G. VISCONTI542scattering process in the atmosphere; actually its cross-section is 3 to 4 orders ofmagnitude larger than the Rayleigh or molecular cross-section. The atomic andmolecular densities of the upper atmosphere is so low that the resonance transitionlinewidth of Na is only Doppler-broadened, therefore the cross-section of theresonance scattering is [3]s(l)42pe 2mcfkln (2)Dl D kpexp2 y2 kln (2) (l2l 0 )Dl Dz2 ,(1)where e and m are, respectively, the electron charge and mass, c is the light velocity invacuum, f (40.655) is the oscillator strength, Dl D (41.245 pm) is the Doppler width athalf-height and l 0 (4588.9 nm) is the central wavelength of the transition line. Sinceresonance scattering is isotropic, the radiation from the Na atoms is emitted with thesame probability in all directions, then the differential cross-section can be written:(ds (l)OdV)4s(l)O4p .The study of the seasonal, daily and at smaller time scale variations (e.g., sporadicdevelopment of very dense narrow layers) of the Na profiles has been investigated withlidar systems [4, 5]. But the Na also is a good tracer to determine mesospheric waveactivity, i.e. the perturbations generated by atmospheric tides and gravity waves [6].The Na atoms are confined in a definite range of altitudes with large densities, and thetime scales of the diffusive and chemical processes can be assumed longer than theperiod of the wave perturbations.Lidar measurements of the Na layer are conducted in different sites in the worldsince the late ’60s, but in Southern Europe they are absent. This gap is filled up withthe development of the lidar of the University of L’Aquila. In this work we present thefeatures and the capabilities of the system and some preliminary results.2. – System configuration and performancesThis system is situated at 42.35 7N and 13.38 7E, close to L’Aquila (Italy), and is setup in a monostatic configuration. The transmitter part of the system consists of atunable dye-laser pumped by a frequency-doubled Nd:YAG-laser. The dye-laser usesas dye a solution of Rhodamine 590 and 610 and can be tuned to Na D2 line using acomputer-controller grating in the laser cavity. The emitted laser linewidth is0.05 cm21 . The Nd:YAG-laser and dye-laser main characteristics are reported intable I. 1% of laser energy is used to illuminate a hollow cathode Na lamp. The feed-back of the induced opto-Galvanic signal allows to keep the Na D2 line tuning within0.1 pm during the measurements.The receiving system uses a Cassegrain telescope coupled via a mechanical chopperand field lens to an interference filter, a photomultiplier and an electronic chain forphoton-counting. The characteristics of the receiving system are specified in table II.The electronics allows a sampling frequency of 80 Mhz, but the sampling frequency iskept below 10 Mhz, to be sure that the system is linearly responding. The mechanicalchopper, between the telescope and the field lens, prevents saturation effects by thelow-altitude molecular backscattering. The chopper is also used to trigger the laserLIDAR OBSERVATIONS OF MESOSPHERIC SODIUM OVER ITALY 543TABLE I. – Emission system.Nd: Yang-laser continuum NY80Repetition rateEnergyDivergenceEnergy stabilityPulse width10 HzA400 mJE0.5 mrad3.5%5 nsDye-laser continuum ND60Dual gratingPrecisionDivergenceLinewidthStability232400 lines/mm1 pmE0.5 mrad0.05 cm210.05 cm21 0C21 hours21TABLE II. – Receiving system.TelescopePrimary mirrorTotal focal lengthField of view50 cm (diameter)540 cm0.5 mradInterferential filterCWFWHMPeak transmission589.6 nm2.5 nmA71%PMT Thorn Emi 9202BSpectral responseQuantum efficiencyGainS2021% (peak)8 Q104with the receiving electronics. The data acquisition is performed with an EG&Gmulti-channel scaler card and a computer. The photon-counting sampling gate is 2 mslarge, corresponding to an altitude resolution of 300 m.An example of backscattering profile is reported in fig. 1; it has been obtained byaccumulating the signal from 2000 laser shots.Two altitude regions are evident, the lower one (30 to 60 km) is the signature of themolecular backscattering (below 25 km the PMT is obscured by the chopper wheel),while the higher one (between 80 and 110 km) is the resonant signal of Na atoms.P. DI CARLO, V. RIZI and G. VISCONTI544Fig. 1. – Backscattering profile obtained by accumulating the signals from 2000 laser shots.3. – Data processingBetween 80 km and 110 km, where the Na resonant scattering is dominant, thephotons received by the telescope are related to Na density according to [7]NNa (z)4hT 2 (z)s eff4pAz 2an(z)b N0 Dz ,(2)where NNa (z) is the photon number coming from the Dz layer which is centred atan altitude z , h is the optical system efficiency, T(z) is the atmospheric transmissionfactor, s eff is the effective cross-section of the resonant backscattering, A is the areaof the primary-mirror telescope, N0 the photon number emitted by the laser and an(z)bthe mean density of the Na atoms in the Dz layer. Accounting the effect of the finitelinewidth of the laser, the effective backscattering cross-section is defined:s eff4sls(l) Ne (l) dlslNe (l) dl4s 0 Dl DkDl 2L1Dl 2D!i416f exp y2 4(l 02l i )2 ln 2Dl 2L1Dl 2Dz ,(3)where s 04s(l 0 )4pe 2 fOmc is the peak value of the Na absorption cross-section (seeeq. (1)), Ne (l) is the number of photons emitted along the laser line (approximatedby a Gaussian shape) at the wavelength l , Dl L is the laser linewidth (see table I),Dl D is the Doppler width of the Na line, l 0 is the central wavelength of Na line, l iLIDAR OBSERVATIONS OF MESOSPHERIC SODIUM OVER ITALY 545and fi indicate the wavelengths and the relative strength of the hyperfine structureof the Na D2 line (i41, R , 6 ; l 02l i420.8476, 20.80513, 20.72881, 1.19498,1.22031, 1.26275 pm and fi40.03125, 0.15625, 0.43750, 0.0625, 0.15625, 0.15625, [8]).At the lower altitude range, the dominant interaction between laser radiation andatmospheric components is the molecular elastic scattering. Therefore, the lidarequation in this case is [9]NR (z)4hT 2 (z) g ds RdVhpAz 2anA (z)bN0 Dz ,(4)where anA (z)b is the mean density of the atmosphere at z altitude and (ds R OdV)p is thedifferential cross-section of molecular backscattering.Coupling eqs. (2) and (4) and scaling the NNa (z) to the NR (z) at an altitude rangewhere the molecular scattering is dominant, it is possible to eliminate the dependenceof the detected signals with the laser power fluctuations, the optical transmission of thelower atmosphere and the efficiency of the receiving system (i.e. N0 , T(z) and h) . Then,the Na density can be evaluated according toan(z)b4NNa (z) z 2( ds R OdV)ps eff O4p1k!i41k anA (zRi )bNR (zR i ) z 2R i.(5)The calibration of the Na resonant backscattering along the zR altitudes is performedbetween 35 km and 50 km; in this region the aerosol content is negligible, then thecontribution of the Mie scattering is neglected; the molecular backscattering signal isFig. 2. – The Na number density profile retrieved from the backscattering signal shown in fig. 1.The error bars indicate 3 times the standard deviation.P. DI CARLO, V. RIZI and G. VISCONTI546of the same order as the Na resonant signal and this prevents the possible conse-quences of the signal dynamic; the ratio of the atmospheric transmission functionsT 2 (zR )OT 2 (z) is close to one, since both the molecular extinction and the residualabsorption between zR and z (the Na layer altitudes) are negligible.In this sort of measurements the error sources are the uncertainty of theatmospheric density, the statistic fluctuation of the signal and the possible saturationand absorption of the Na atoms. The dominant error is the former and is roughlybetween 5% and 10% on the relative values of Na number density [10].In fig. 2 there is an example of a retrieved Na number density profile. It has beenobtained from the backscattering signal shown in fig. 1, with an altitude resolution of300 m, averaging the signals over 200 s time period.4. – Preliminary analysisOn 10 April 1997 we have collected measurements during a 7 hours long observationsession. The time series of the Na density profiles is shown in fig. 3.Time-dependent perturbations originated by the interaction of the atmosphericwave activity with the Na atoms are quite evident. Actually, at the beginning and theend of this sequence, there is a clear drift of the layer peak. The interaction or responseof the Na layer to a wave-like propagating perturbation can be determined solving thecontinuity equation:¯r Na¯t1˜KQ (r Na VK)40 ,(6)Fig. 3. – Na number density profiles plotted vs. time, during the 7 hours measurement session.The time and the altitude resolution are, respectively, 200 s and 300 m.LIDAR OBSERVATIONS OF MESOSPHERIC SODIUM OVER ITALY 547where r Na is Na density and VKis the velocity field. With the assumption that thediffusion and chemical effect are negligible and the Na layer is horizontalhomogeneous, the solution of eq. (6) for a quasi-monochromatic wave disturbance is ofthe form [6]r Na (z , t)4r 0Nagz2gH ln k11 Ag21exp k z2Hl cos (vt2kz Qz)lhk11 Ag21exp k z2Hl cos (vt2kz Qz)l ,(7)where the effect of the perturbation in the argument of r 0Na , which is the functionrepresenting the background Na profile, is evident; g is the ratio of the air specificheats, H is the height scale of the atmosphere; A , v and kz are, respectively, theamplitude, the frequency and the vertical wave number vector of the wave disturbance.If the vertical wavelength of the perturbation is small compared to the full width ofthe Na layer, eq. (7) can be approximated with the first two terms of a Taylor’s series inpower of [A Qexp [z/2H] ]. A simple Fourier transform of this expansion produces apower spectrum that, averaged over a time period t around the time t0 for eliminatingthe v-dependence, looks like [11](8) aNFNa (k)N2 b4NF0Na (k)N211y gHA2(g21)z2kg 1gH212Hh21 (k2kz )2lNF0Na (k2kz2 iO2H)N212gHA2(g21)sin (vtO2)(vtO2)Re QQ mk 1gH212H1 i(k2kz )l (F0Na (k) )* F0Na (k2kz21O2H) exp [2ivt0 ]n ,where FNa (k) and F0Na (k) are the Fourier transforms of the perturbed andunperturbed Na density profiles. This power spectrum is constituted of three terms:the first one gives a low-frequency lobe in the spectrum; the second is the spectralsignature of the gravity wave, it is proportional to the square of the wave amplitude andshows a local minimum for k4kz , i.e., a notch between two peaks; on the other hand,the third term represents the non-linear second-harmonic components of the gravitywave response [11].We have applied this analysis in a 20 min time window (between 21:31UT and21:51UT) of the 10th April 1997 observations. The width of the Na layer (A15 km) setsthe maximum vertical wavelength of the wave interacting with Na profiles (i.e. kmin40.06 km21 ); in addition, the altitude resolution (300 m) defines the minimumwavelength (i.e. kmax43.3 km21 ). The obtained mean power spectrum is shown in fig. 4,the features of a wave which interacts with the Na layer are evident: the position of thelocal minimum between the first two peaks on the side of the low-frequency lobeindicates the gravity wave number (0.21 km21 , corresponding to about 4.7 kmwavelength). The other notch between the weaker peaks at larger k values correspondsto the local minimum of the third term in eq. (8) (i.e. the non-linear interaction), and itsposition is at about twice the value of the gravity wave number (0.43 km21 ).P. DI CARLO, V. RIZI and G. VISCONTI548Fig. 4. – The average spatial power spectra calculated from the Na profiles in a period of 20 min,the wave number corresponding to the first and second harmonic of the interaction between thegravity wave and the Na layer are indicated by the arrows. The dashed line shows the limit set bythe noise.The vertical velocity of the wave can be estimated by looking at the motion ofsuccessive peaks in the Na profiles in fig. 3; the apparent phase velocity is 1.7 km/h,thus the wave period (l/c) is about 2.6 hours. This period is Doppler-shifted by thebackground wind field, and the correction for estimating the intrinsic wave period isnot trivial because it depends on mean vertical and horizontal wind velocities.5. – ConclusionsIn this work we report the status and the performances of a lidar system formesospheric Na sounding. We show that our system, based on standard techniques,has the capabilities for detecting the Na number density profile with an accuracy thatallows the study of the small-scale variability of the mesospheric Na content.Although Gibson-Wilde et al. [12] have well evidenced that there are intrinsic limitson the gravity wave information which can be extracted from Na lidar data, in apreliminary analysis of the 7 hours measurement session, we get the features ofwave-like propagating perturbation typical of gravity waves [13]. The system is inroutine operation and is now making observation during night. With an extendedcollection of measurements, we can start a finer data analysis.LIDAR OBSERVATIONS OF MESOSPHERIC SODIUM OVER ITALY 549R E F E R E N C E S[1] QIAN J. and Gardner C. S., Simultaneous LIDAR measurements of mesospheric Ca, Na,and temperature profiles at Urbana, Illinois, J. Geophys. Res., 100 (1995) 7453-7461.[2] RICHTER E. S., ROWLETT J. R., GARDNER C. S. and SECHRIST C. F. jr., LIDAR observation ofthe mesospheric Na layer over Urbana, Illinois, J. Atmos. Terr. Phys., 43 (1981) 327-337.[3] ME´GIE G. and BLAMONT J. E., Laser sounding of atmospheric sodium: interpretation interms of global atmospheric parameters, Planet. Space Sci., 25 (1977) 1093-1109.[4] SIMONICH D. M. and CLEMESHA B. R., Resonant extinction of LIDAR returns from the alkalimetal layers in the upper atmosphere, Appl. Opt., 22 (1983) 1387-1390.[5] BEATTY T. J., COLLINS R. L., GARDNER C. S., HOSTETLER C. A. and SECHRIST C. F. jr.,Simultaneous RADAR and LIDAR observations of Sporadic E and Na layers at Arecibo,Geophys. Res. Lett., 16 (1989) 1019-1022.[6] GARDNER C. S. and SHELTON J. D., Density response of neutral atmospheric layers togravity wave perturbations, J. Geophys. Res., 90 (1985) 1745-1754.[7] ME´GIE G., BOS F., BLAMONT J. E. and CHANIN M. L., Simultaneous measurements ofatmospheric Na and potassium, Planet. Space Sci., 26 (1988) 27-35.[8] MEASURES M. M. (Editor), Laser Chemical Analysis (John Wiley & Sons, New York) 1988.[9] MEASURES M. M., Laser Remote Sensing-Fundamentals and Applications (John Wiley &Sons, New York) 1984.[10] DI CARLO P., Osservazione del sodio nell’alta atmosfera con tecniche LIDAR, Tesi diLaurea, Dipartimento di Fisica, Università degli Studi, L’Aquila, 1997.[11] GARDNER C. S. and VOELZ D. G., Lidar Studies of Nighttime Na Layer over Urbana,Illinois. Gravity Waves, J. Geophys. Res., 92 (1987) 4673-4694.[12] GIBSON-WILDE D. E., REID I. M., ECKERMANN S. D. and VINCENT R. A., Simulation ofLIDAR measurements of gravity waves in the mesosphere, J. Geophys. Res., 101 (1996)9509-9522.[13] LINDZEN R. S., Turbulence and stress owing to gravity wave and tidal breakdown, J.Geophys. Res., 86 (1981) 9707-9714.

Lidar observations of mesospheric sodium over Italy

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Lidar observations of mesospheric sodium over Italy

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