Lidar observations of a horizontal variation in the atmospheric sodium layer

Journal

of Atmosphrric andTensniolPhysics. Vol.39,pp.1405-1409. Pergamon Press,1977. Printed in Northern Ireland

Lldar observations

of a horizontal

atmospheric

sodium

variationin the

layer

L. THOMAS, A. J. GIBSON and S. K. BHATTACHARVVA SRC., Appleton Laboratory, Ditton Park, Slough SL3 9JX, UK (Received I March 1977; in revised

Abstract-A steerable

form 19 May 1977)

laser radar system has been used to measure the distributions

of sodium

between 80 and 105 km by observations of resonance scattering in three directions from Winkfield (51.4”N, 0.7”W). Simultaneous observations of Rayleigh scattering from heights of 20 to 35 km were used to normalise the sodium concentration. Sequences of measurement at zenith angles of 30” have permitted a comparison to be made of the sodium concentrations at three locations separated horizontally by distances of about 80 km. The results for two nights in August 1976 showed clear evidence of a horizontal variation near the peak of the layer, the concentration changing by about 50% between locations. The results for a third night did not show such a variation near the peak of the layer, the major differences being observed at greater heights. 1. lNTRODUClTON

Observations of airglow emissions, both from the ground and from aircraft have provided evidence of horizontal variations at mesospheric heights. Early studies of the OI(‘D-‘S) emission at 557.7 nm by ROACH et al. (1958) indicated emission cells of dimensions 2000-3000 km, and more recent studies by BARAT et al. (1972) have shown evidence of irregularities of much smaller dimensions. Ground-based observations of hydroxyl emissions have indicated horizontal variations in intensity (PETERSON and KIEFFAKER, 1973) and in rotational temperature (KRASSOVSKY and SHEFOV, 1965). The fluctuations in these parameters have been associated with atmospheric gravity waves (KRASSOVSKY and SHEFOV, 1975) and the characteristic sizes observed from the ground (MOREELS and HERSE, 1977) and from aircraft (CRAWFORD et al., 1975) are consistent with such waves. Shortterm changes in the height distribution of sodium have been observed by laser radar (SANDFORD and GIBSON, 1970; HAKE et al., 1972; BLAMONT et al., 1972) but the measurements have been limited to the vertical direction. Recently, evidence of a small-scale horizontal variation of the sodium layer has been obtained using a steerable laser system (THOMAS et al., 1976). These results referrred to the form of the height distribution of sodium at two horizontal locations separated by about 15 km and contained no information on relative concentrations at these locations. In the present study an examination has been made of the spatial variation over distances of about 80 km, as indicated by comparisons of sodium concentrations in the 80105 km height range measured at zenith angles of

30” in the directions NNE, SE and W from the observing site at Winkfield (51.4”N, 0.7”W), during three nights in August 1976. 2.

AND EXPE RIMRNTAL

PROCEDURE

The lidar system incorporated a flashlamppumped dye laser and a receiving telescope mounted parallel to each other in a trailer, with a steerable plane mirror used for both transmission and reception mounted about 10 m from the trailer. The laser was similar to that described previously (GIBSON, 1972) and was tuned to the sodium D2 line (589 nm) by two optically-contacted air-spaced Fabry-Perot etalons and a temperature-controlled solid etalon used as output reflector. The resulting linewidth was comparable with that of the D, line at mesospheric temperatures. Tuning of the laser was checked by means of a 1 m grating spectrograph and also by observing resonance scattering of part of the laser beam in a cell of sodium vapour heated to about 140°C. To reduce the fire hazard associated with the dye solvent (GRANT and HAWLEY, 1975) a water-isopropanol mixture was used, which resulted in a laser output energy per pulse of about 100 mJ. The receiver consisted of a 0.3 m dia. Newtonian telescope with an aperture in the focal plane defining a beam divergence variable between 1 and 10 mrad, an interference filter of 1 nm bandwidth centred on 589 nm, and a thermo-electrically cooled S-20 prismatic-cathode Venetian-blind photomultiplier. The laser firing was synchronized with a rotating shutter mounted near the focal plane of the receiver telescope so that light scattered from ranges less than about 15 km was excluded.

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EQUIPMENT

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L. THOMAS,A. J. GIBSON and S. K. BHAITACHARYYA

The signal from the photomultiplier after amplification and discrimination was recorded as a function of range in a 60-channel photon counting system. Channels of 1 km width over the range 90-120 km were used to record the count from the sodium layer, and 2 km channels over the range 20-40 km were used to record a signal due mainly to Rayleigh scattering for the purpose of normalising the sodium measurements. Checks were made to ensure that the variation over the range 2040 km was consistent with this form of scattering from an appropriate model atmosphere (COLE et al., 1965). This was necessary to prevent uncertainties arising from a spatial variation of aerosol scattering. Channels corresponding to ranges of 140 to 256 km, from which no detectable scattering was expected, were used to determine the background count. The mount for the plane mirror was fitted with a system of motors, clutches and brakes controlled from the trailer to provide finely controlled rotation about axes approximately parallel and perpendicular to the axis of the laser. Rotation about each axis was coupled to a precision potentiometer to enable the orientation of the mirror to be monitored. To ensure accurate conversion of range to height, direct checks of the mirror angles were made with a clinometer during observing periods, between runs of laser firings, and the actual directions of the beam after reflection from the mirror were measured after the observing periods with a theodolite and pentaprism. In the present experiment the lidar beam was directed successively to three angular positions approximately 30” from the zenith, this cycle of observations being repeated immediately. The corresponding sampling positions in the sodium layer are represented in Fig. 1, the separations referring to a height of 90 km near the level of maximum sodium concentration. Runs of about 500 laser pulses, occupying periods of 6-15 min, were taken in each direction. These periods represented a compromise between the need to achieve statistically significant measurements over a substantial part of the sodium layer and the necessity to minimise the effects of variations with time. 3.

EXPERIMENTAL RESULTS

The observations of the height distributions measured sequentially in the three directions are illustrated by the first cycle of measurements carried out on the night of 16 August 1976, shown in Fig. 2. A distinction is drawn between the results for the W, NNE and SE directions, the corresponding start

,NNE

Fig. 1. Locations at 90 km height of measurements made at zenith angles of approximately 30” at Winkfield.

times for the measurements being 2250, 2302 and 2314 UT respectively; at each height the standard errors for the three directions are similar but for clarity only those for SE are shown. The simultaneous measurements for 23-35 km height range used to normalise the sodium results are shown in Fig. 3, together with the height variation deduced for Rayleigh scattering from the standard atmosphere for middle-latitude summer conditions (COLE et al., 1965). It is seen that these measurements are consistent with Rayleigh scattering. Mie scattering from aerosols having a uniformly mixed height distribution could have contributed but measurements using two wavelengths at the U.K. Meteorological Office have shown that this contribution was less than 10% @LINGO, private communication, 1977). Figure 2 shows that there was a significant difference between the sodium concentrations near the layer peak, amounting to a 50% increase between the results for the NNE and W bearings. The sequence of distributions deduced for each bearing showed small changes with time. Within each cycle the measurements for the NNE bearing were intermediate in time between those for W and SE, and all three cycles of measurements carried out showed the same overall result as Fig. 2. It is concluded that the differences between the distributions for the three bearings, specifically the lower peak concentrations observed on the NNE bearing, did not arise from the time variation of a

1407

Lidar observations of a horizontal variation

w

1

NNE

l

SE

0

2 i

.o

90-

P

I L

I

I Relative

3 sodurn

10 concentration

Fig. 2. The height distributions of sodium measured on 16 August 1976 during a single cycle of observation at the locations shown in Fig. 1.

WA NNE 0 SE 0

IOO-

00

I L

I

Relot~ve

3 sodium

J IO concentration

Fig. 4. The mean height distributions of sodium measured on 16 August 1976 deduced from three cycles of observation at the locations shown in Pig. 1.

Fig. 3. Variations of signals from the height range 2235 km measured simultaneously with the results shown in Fig. 2 and used to normalise the sodium concentrations. horizontally uniform layer but represented a horizontal variation in the sodium layer. The results for the three cycles of measurement are summarised in Fig. 4 which shows the mean distributions deduced for each bearing. It is seen that in addition to the difference already noted in the vicinity of the layer peak, a marked horizontal variation is also indicated between about 84 and 86 km, the concentrations for NNE again being smaller. The corresponding results deduced from measurements on the morning of 19 August 1976 are shown in Fig. 5. This presents the mean distributions deduced for each of the three directions from

two cycles of measurements during the period 0037 to 0150 UT. The most obvious differences occurred near and immediately below the layer maximum, with the concentrations again increasing by about 50% between the NNE and SE or W locations. On the third night of observation, 23124 August 1976, three cycles of measurements were completed but during the second and third cycles the signals received in the height range 20-35 km showed marked discrepancies from the height variation expected for Rayleigh scattering, and corresponding uncertainties arose in the normalisation of the simultaneous sodium measurements. The results obtained for each direction from the first cycle of measurements, in the period 2322 to 2345 UT on 23 August, are shown in Fig. 6; again the measurements in the NNE direction were intermediate in time between those in the other two

1408

L. THOMAS,A. J.

GIBSON and S. K. BHATTACHARYYA

results is the larger concentrations observed in the SE direction near 98 km and higher. 4. CONCLUSIONS

Laser radar measurements of atmospheric sodium during three nights in August 1976 have provided evidence of a horizontal variation in the layer. The most noticeable effect occurred near the peak of the layer, between about 88 and 96 km. A change in sodium concentration of about 1.5 : 1 was observed over horizontal distances of about 80 km, and this scale of variation is generally consistent with those identified previously in airglow features. However, in the present investigation the same form of variation was observed on two nights.

._ Relative sad&n cancentratian

Fig. 5. The mean height distributions of sodium measured on 19 August 1976 deduced from two cycles of observation at the locations shown in Fig. 1. directions. Although differences layer peak, the values observed tion are not consistently low. noticeable difference between

Acknowledgements-Valuable contributions to the design and construction of the laser radar system were made by M. C. W. SANDFORDand E. HAMMOND.The work described above was carried out at the Science Research Council’s Appleton Laboratory and is published by the permission of the Director.

are seen near the in the NNE direcInstead, the most the three sets of

105-

:*

IOO-

.

w

A

NNE SE

0

.

95E r f :90I

I

05-

. I

80’

I

I

I

I

3 Relative sodun

I

I,,,,

IO concentration

30

Fig. 6. The height distribution of sodium measured on 23 August 1976 during a single cycle of observation at the locations shown in Fig. 1.

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Lidar observations of a horizontal variation GRANT W. B. and HAWLEY J. G. HAKE R. D., ARNOLD D. E., JACKSOND. W., EVANS W. E., FICKLM B. P. and LONG R. A. KRASSOVSKYV. I. and SHEFOVN. N. KRASSOVSKYV. I. and SHEFOVN. N. MOREELS G. and HERSE M. PETERSONA. W. and KIEFFAKERL. M. ROACH F. E., TANBERG-HANSSENE. and MEGILL L. R. SANDFORDM. C. W. and GIBSONA. J. THOMAS L. GIBSON A. J. and BHATTACHARYYAS. K.

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Space Sci. Rev. 4, 176. Awls Geoohvs. 32. 43. Planet. Spice’ Sci. k, 265. Nature, Land. 242, 321. J. atmos. [err. Phys. 13, 113.

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