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Laser radar measurements of the atmospheric sodium layer
Journal of Atmosyhxic
ancl Terrwtriul Physiw1070, Vol. 32,pp.1423-1430. Per&won Press. Printed In Northern Irelrrnd
Laser radar rn~~~erne~~
of the atmosphe~c sodium layer
3%. C. W. SANDFORD and A. J. GIBSOX Rrdio and Space Research Station, Ditton Park, Slough, Ducks., England (Beceived26 January 1970) Abstract-Regular measurements of the sodium layer at 90 km height, during twilight and throughout the night, by moans of a laser radar are reported. The main features observed are night-time abundances similar to those at twilight, topside scale heights of 2-3 km and fluetuations corresponding to patches of sodium oxtending over horizontal distances of lo-200 km, The observations arc discutised in relation to theories of the sodium layer and euggoetions for future experiments are mado. 1. INTRODUCTION LAYER of free sodium atoms in the atmosphere at a height of about 90 km has been studied for over 30 yr by means of resonance scattering of sunlight, observed from the ground first at twilight and more recently in daytime (see reviews by C~IA~~~R~I~, 1961; HUNTEN, 1967). Rocket measurements have also recently been made in daytime (HUNTEN and WALLACE, 1967; DOWAHCE and MEIES, 1967). It is not certain whether the origin of the sodium is the sea or meteors, although currently a meteoric origin seems generally favoured. A number of theories for the production of the layer have been proposed but, on account of our poor knowledge of various reaction rates and inadequate measurement of certain variables, it does not seem possible to decide which mechanisms are dominant. GADSDEX (1968) drew attention to the complete lack of data on the night-time sodium layer. Rocket measurements of the sodium night-glow have been made (GREER and BEST, 1967), but the theories of its production and its precise relation to the free sodium cannot be considered to have been confirmed (BATES, 1964). Laser probing offers the possibility of measuring the sodium directly at night. At the Radio and Space Research Station a laser radar system was developed to study the upper atmosphere by Rayleigh scattering (BAIN and SANDFORD, 1966) and the possibility of studying minor constituents by resonance scattering was considered. It seemed that sodium would be the easiest constituent to measuro, and the first observations were made in 1968 (BOWMAN et al., 1969, 1970). These preliminary measurements established the feasibility of the technique and showed that the night-time abundance was not greatly different from that given by the observations of twilight and daytime airglow. Recently a more powerful laser radar was brought into regular operation at Winkfield (0.7”W, 51*4*N), and observations are now made on nearly every clear night. Although at least a year will be required before a complete picture of the nighttime behaviour is obtained, it seems wo~hwhile to present the initial results as some interesting conclusions ~811bc drawn and a number of suggestions can be made for future experiments of this type. A description of the techniques of tho measurement and calibration is at present, in preparation, and the equipment is a development of that previously described, I423
THE
1424
M. C. W. SANDFOHDand A. J. GIBSON
(GIBSON, 1969). Briefly, the method involves measuring the relative backscattered intensity of a laser light pulse as it propagates vertically upwards. At 30 km the signal is due to Rayleigh scattering by 0, and N,, and, as the density is known quite accurately at 30 km, this can bc used to calibrate the equipment. When the laser wavelength is tuned to that of one of the sodium U lines, 589 nm, very strong scattering is produced by the free sodium atoms, and accurate height profiles can be obtained by averaging the return from several laser pulses to reduce statistical fluctuations due to the limited photon count. All results here are quoted directly in terms of number densities of sodium atoms. The accuracy of the calibration is about * 10 per cent. However, a systematic error will be introduced if there is appreciable scattering by dust at 30 km. The true sodium densities would then be higher, but the error is unlikely to be more than 20 per cent.
2. RESULTS AND DISCUSSION Regular observations with a fully-calibrated system began on 14 July 1969, and have been made on most nights when there have been breaks of sufficient duration in the cloud. 2.1 Irregular nocturnal variations
One main feature disclosed is the considerable irregularity of the layer. During most of the night of 718 August 1969, observations were taken in sets of 25 shots extending over 10 min. From each of these sets an average height distribution has been deduced, a typical example being shown in Fig. 1, with standard errors due to the limited photon count. Figure 2(a) shows isopleths of sodium density as a function of height and time deduced from the height distributions. An idea of the errors in the heights of the isopleths can be obtained from the errors shown in Fig. 1. As a result, it can be confidently said that most of the detail evident in Fig. 2(a) represents real changes in sodium densities rather than statistical fluctuations in the photon count. At 0020, because it was evident that the bottom of the layer had fallen, an additional counting channel at 82-84 km was brought into operation in order to plot isoplcths below 84 km. It seems likely that the fluctuations are mainly due to irregularities of sodium density which are carried past the observation point by the motion of the atmosphere. If we assume for the wind the mean value at 90 km given by the model of GROVES (1969) i.e. 25 msec-l towards the N.E., then each 10 min set of observations consisted of 25 samples each of a circular area of about 20 m dia. spread along a line 15 km long. The marked region of constant sodium density between 2320 and 0110 located between 86 and 92 km, containing a tendency towards a minimum at 89 km and a secondary maximum at 87 km, would occupy a length of 165 km in the N.E.-S.W. direction. Some of the smaller features such as the peak at 2105 at 93 km occupy only 20 or 30 km. With finer time resolution one could doubtless see still smaller features, although there is some evidence, from column number densities (abundances) taken on other nights with 3 min resolution, that fluctuations are smaller for shorter time periods, except possibly during twilight. The isopleths for the night 22/23 July 1969 (Fig. 2(b)) show a very large change commencing at about 0200, the sodium abundance increasing by about a factor of
1425
Laser rrtdar measurements of the atmospheric sodium layer
’
0 82
84
’
86
’
t
88
90 Height
t
93
’
94
’
96
’
98
’
;OO
(km)
Fig. 1. Average distribution of sodium density with height for 25 shots between 0053 and 0103 UT, 8 August 1969.
lb0 98 96-
Na density (109mw3)
94 z ” 92 IE *; 90T 8%-
x 0
5 4
:
:
l
I
86-
82 20
2 21
22
I 23
01 24 UT (hr) Fig.
02
03
04
2(a).
4 in about 1 hr. This is the greatest change found in any of the data, Figure Z(b) umfortunately contains a larger amount of error due to statistical fluctuations in the photon count than Fig. 2(a), for example, the structure around 98 km between 2300 and 2400 is probably due to this. Patches of thin cloud were present and reduced the signal, particularly around 0300. However, since the presence of cloud reduces the
6
M. C. W. SAHDFORDand A. J. C&BON
1426
04
L
,
I
I
1
I
02. 03 24 O! U T (hr) Fig. 2. IeopIeths of sodium density during night, (a) 7/8 August 1969. (b) 22123 July 1969. 21
‘22
23
signal from the sodium layer and the 25-35 km calibration signal in the same ratio, it should not produce any systematic error in the measurement, and this has been conmmed by the lack of correlation between the presence of cloud and the apparent abundance of sodium. The interpretation of the fluctuations as a spatial variation has been made before. SWINGS and NICOLET (1949) first noted the patchiness of the sodium twilight glow. Geographic irregularities of the sodium night glow have been reported by NASYROV (1967), but this may relate to the distribution of some reacting constituent other than sodium. The spatial nature of the fluctuations could be confirmed, and any time fluctuations could be measured, by operating two or more laser radars from the same site. These would sample regions of the layer separated by up to about 100 km, or, with more powerful equipment, even further. Such an experiment could also be regarded as a method of measuring winds at this altitude.
Because of the irregular variations described in Section 2.1, measured changes of abundance during the night that might be used to test particular theories of the sodium layer can be studied only by taking averages over a large number of nights. Satisfactory measurements were made during evening twilight on 8 nights, beginning at solar depression angles of 5” or less, for a period of at least 100 min. (Twilight abundance measurements by means of sunlight are usually made at solar depressions of about 6+5”, and height measurements at angles between 7 and 10.) The data for each night were normalized to the average abundance during the 100 min period. The means over the 8 nights for each 10 min period are shown in Fig. 3(a) together with the standard errors. There is slight evidence for an increase of about 20 per cent in the abundance during the period O-30 min after solar depression of 6’, but
1427
Laser radar me~urements of the ~tmo~pheriesodium layer
I*3
I.3-
I.2E f 2 8
e(’ OQ to
I.I0
I
.z I.0z ul _g 0.9 ti cl
-20
0
20
Time (min) after
I
I
I
4O 6” Solar Depression
40
60
80
Solar Depression of 6O
20
22
24 UT
02
04
(hr)
J
6” IO” Angles
Fig. 3. Relative sodium abundances, (a) averaged over 8 evening twilights 20 July-7 Augnst 1969. (b) averaged over 14 nights 14 July-4 September 1969.
the significance level is only 5 per cent. A similar analysis for the whole night (data grouped in hourly periods, Fig. 3(b)) shows no significant variations from the mean. Observations during morning twilight have been made on only a few occasions and, at this time of year, are frequently prevented by mist and fog, so that a detailed analysis of morning twilight effects is not yet possible. According to the photochemical theory of HITXT (1966) the ratio n(O)/n(O,) at the height of the sodium layer is considerably greater during daytime than at night, and this would be expected to cause a corresponding variation in the abundance of sodium (HUXTEN, 1967). Since photodissociation of ozone can be caused by all wavelengths, Iz,up to l-18 pm, it would be expected that such changes would occur when the solar depression angle is greater than 5”, e.g. sunset at 90 km for il N O-6 pm occurs when the solar depression angle is about 10’ (THOMAS and BOWMAN, 1969). The measurements in Fig. 3 suggest that photochemical changes when solar excitation ceases have little effect on the sodium abundance, e.g. a rapid removal of sodium by oxidation which might result from an increase in ozone concentration is not a dominant process. 2.3 Changes of
abundance from night to night
The data for each night were averaged over 1 hr periods, and the mean of these was taken to provide a time-averaged column number density on each night (Fig. 4). These measurements agree quite well in magnitude with twilight values at this latitude and season (HUFITEN, 1967), and suggest that the seasonal trend towards the winter maximum, well-known from twilight values, is present also in the night-time values. One might expect the laser measurements to exhibit
1428
M.
fg
C.
W.
IL
0 Observations during 3 or more hours d Observations during 2 or fewer hours
* z oIO
1 I5
” 20
”
25
30
July Fig.
and A. J. GIBSOX
SANDFORD
”
4
9
14
” 19 24
’
29
August
“1 3
8
13
1 18
September
4. Night-to-night variation of column number density, July-September 1969.
somewhat smoother trends than the twilight observations, which are averaged over only about 10 min but large night-to-night changes still occur. The largest change, on 5 August, is shown in more detail in Pig. 5, where hourly averages are plotted. Z.~~~~S~T~~U~~O~ witJ4heig7Lt In Fig. 6 two typical height distributions, averaged over about 4 hr, are presented, The contribution to the standard error due to the limited photon count is about 3 per cent at the peak of the layer, but the true standard errors may be slightly larger
-
I
4th
7th 6th 5th Date (August1969)
8th
Fig. 6. Hourly averages of column number density on successive nights 3-8 August 1969.
Laser radar measurements of the atmospheric sodium layer
IO*
I
80
I
I
I
I
I
I.
I
I
I
I
1129
1
84
88 92 96 100 10% Height (km) Fig. 6. Average distributions of sodium density with height for two nights A 314 September 1969, 0000-0400 UT. 0 6/7 September 1969,1900-2300 UT.
because the height distribution is not constant over the period of the observations. A double-peaked height profile is a common shape. A smooth single-peaked layer occurred infrequently. The steep gradient of the bottomside of the layer may easily be explained in terms of ozone concentration increasing rapidly with decreasing height (HUNTEN, 1967). The topside density is found to fall off with a scale height between 2 and 3 km. This remarkably steep gradient was first found in daytime rocket measurements (HUNTEN and WALLACE, 1967; DONAHUE and MEIER, 1967), which in 3 flights yielded scale heights in the range 2.5-3 km, and this necessitated the development of new theories for sodium. Refinements in the twilight photometry technique have recently led to estimates of the scale height as less than 4.5 km with no lower limit (Gault and Rundle, 1969). Thus it seems that a small topside scale height is the normal situation throughout the day and night. The data so far obtained are not sufficient to discriminate between the recently proposed theories of the sodium layer. According to HANSON and DONALDSON(1967) there is a convective cycle involving upward diffusion of free sodium and loss by photo-ionization to produce the steep gradient in the topside, followed by downward transport of the ions by winds blowing across the magnetic field and recombination in a small height region. This theory suggests that the topside scale height should increase during the night when photo-ionization ceases. For a typical value of the vertical eddy diffusion coefficient, K, of 500 m2 set -l (HUNTEN, 1967), the diffusion time, H2/K, for a scale height, H, of 3 km, would be about 5 hr. Thus the present observations of steep topside gradients throughout the night suggest a lower value of the vertical eddy diffusion coefficient, and are consistent with the conclusion of LAYZER and BEDIN~ER (1969) that there is no turbulence in the height range 80-140 km.
1430
M. C.
W. SANDFORDs.nd A. J. GIBSON
An ~ternative theory by GADSDEN(1968) eo~iders ablation from meteors taking place with a very sudden increase in the rate of sodium production below 100km. This theory, however, suggests a decline in sodium production during the night, and production lo-20 km higher in the predawn period. It is not known how the loss mechanisms vary during the night, but no indication of increased production at higher levels has been found in the data. 3. CONCLmION The abundance of sodium at night has been found to be similar to that during twilight. No trends are noted during the night apart from a possible 20 per cent increase in about 30 min after a solar depression of 6’ in the evening. The height ~strib~tion exhibits very small topside scale heights in the range 2-3 km. Considerable irregula~ties in the layer exist and may extend over horizontal distances in the range IO-200 km. Extended measurements of the present type are likely to yield much information which will aid the elucidation of the chemical and transport mechanism affecting the sodium distribution, and it would be desirable for measurements to be carried out at different latitudes and longitudes. Extension of the measurements further into daytime conditions should be undertaken. Use of a steerable or multiple beam system could yield information on horizontal movements. ~c~ow~ed~e~e~~-The authors aoknowlodguthe sssistsnee of Mr. P. G. L. THOXASin ceng out the observations. The work described was et-m&d out at the Radio and Space Research Station of the Science Research Council ad this paper is published with the permission of the Director. REFERENCES BAIN W. C. and BATES D. R. BOWMAN M. R., SA~RORD M. BOWNAN M. It., S_mm+om M.
SANDFORDEII.C. W. GIBSON A. J. and C. W. GIBSON A. J. snd C. W.
1966
1964 1969 1970
J. Atmosph. Tew. Phys. 28,543. Disc. Par&~ 8%. 81, 21. Nature,Lord. !%!I, 466. Proc. IERX Conf. on Lasersano?OptoElectrode, Soutbmptm Radio electron Eng4- 3s, 29. Pfqsice of thesAurora ad A~rg~w, pp.
CKAFd~EKLAIN J. W.
1961
DONAEW~:T. M. snd MEIER R. R. GADSPEN M. GAULT W. A. snd Rvxom H. N. GXBSONA. J. GREERR. H. G. and BEST G. T. GROVESG. V. HANSON W. B. snd DONALDSONJ. S. HUNT B. G. HVNFEN D. M. HUNTBN D. M. and WA&LACEL. LAYZICRD. and BEDIXGERJ. F. NASYROV G. A.
1967 1968 1969 1969 1967 1969 1967 1966 1967 1967 1969 196’7
J. geop%s. Rec. la, 2803. J. Atmoq&. Tew. Phys. g@, 151. Gasa,J. P&e. 47,&X J. Soi. In&., Seti 2, 2, 802. P&net. Spas Sci. 15, 1857. J. Br. Isterplalaet. Sot. W, 285 J. geophye. Rec. V2, 8513. J. geophye. Res. 71, 1385.
Swmos B. and NICO~T M. THOMASL. and BOWMAN M. R.
1949 1969
A&o&s. J. 169, 327. J. Atmoeph. Tew. Php.
444-472. AcsdemioPfaay,New York.
SpaceS& Rev. 6, 493. J. geophp. Rec. 72, 09. Planet. Space Sci. 17, 1891.
Aurwae and Airglow (Edited by V. I. BRASOVSKIX),pp. 8-10. NASA l&h-
nice1?.%an&tionF.530.
31, 1311.
ancl Terrwtriul Physiw1070, Vol. 32,pp.1423-1430. Per&won Press. Printed In Northern Irelrrnd
Laser radar rn~~~erne~~
of the atmosphe~c sodium layer
3%. C. W. SANDFORD and A. J. GIBSOX Rrdio and Space Research Station, Ditton Park, Slough, Ducks., England (Beceived26 January 1970) Abstract-Regular measurements of the sodium layer at 90 km height, during twilight and throughout the night, by moans of a laser radar are reported. The main features observed are night-time abundances similar to those at twilight, topside scale heights of 2-3 km and fluetuations corresponding to patches of sodium oxtending over horizontal distances of lo-200 km, The observations arc discutised in relation to theories of the sodium layer and euggoetions for future experiments are mado. 1. INTRODUCTION LAYER of free sodium atoms in the atmosphere at a height of about 90 km has been studied for over 30 yr by means of resonance scattering of sunlight, observed from the ground first at twilight and more recently in daytime (see reviews by C~IA~~~R~I~, 1961; HUNTEN, 1967). Rocket measurements have also recently been made in daytime (HUNTEN and WALLACE, 1967; DOWAHCE and MEIES, 1967). It is not certain whether the origin of the sodium is the sea or meteors, although currently a meteoric origin seems generally favoured. A number of theories for the production of the layer have been proposed but, on account of our poor knowledge of various reaction rates and inadequate measurement of certain variables, it does not seem possible to decide which mechanisms are dominant. GADSDEX (1968) drew attention to the complete lack of data on the night-time sodium layer. Rocket measurements of the sodium night-glow have been made (GREER and BEST, 1967), but the theories of its production and its precise relation to the free sodium cannot be considered to have been confirmed (BATES, 1964). Laser probing offers the possibility of measuring the sodium directly at night. At the Radio and Space Research Station a laser radar system was developed to study the upper atmosphere by Rayleigh scattering (BAIN and SANDFORD, 1966) and the possibility of studying minor constituents by resonance scattering was considered. It seemed that sodium would be the easiest constituent to measuro, and the first observations were made in 1968 (BOWMAN et al., 1969, 1970). These preliminary measurements established the feasibility of the technique and showed that the night-time abundance was not greatly different from that given by the observations of twilight and daytime airglow. Recently a more powerful laser radar was brought into regular operation at Winkfield (0.7”W, 51*4*N), and observations are now made on nearly every clear night. Although at least a year will be required before a complete picture of the nighttime behaviour is obtained, it seems wo~hwhile to present the initial results as some interesting conclusions ~811bc drawn and a number of suggestions can be made for future experiments of this type. A description of the techniques of tho measurement and calibration is at present, in preparation, and the equipment is a development of that previously described, I423
THE
1424
M. C. W. SANDFOHDand A. J. GIBSON
(GIBSON, 1969). Briefly, the method involves measuring the relative backscattered intensity of a laser light pulse as it propagates vertically upwards. At 30 km the signal is due to Rayleigh scattering by 0, and N,, and, as the density is known quite accurately at 30 km, this can bc used to calibrate the equipment. When the laser wavelength is tuned to that of one of the sodium U lines, 589 nm, very strong scattering is produced by the free sodium atoms, and accurate height profiles can be obtained by averaging the return from several laser pulses to reduce statistical fluctuations due to the limited photon count. All results here are quoted directly in terms of number densities of sodium atoms. The accuracy of the calibration is about * 10 per cent. However, a systematic error will be introduced if there is appreciable scattering by dust at 30 km. The true sodium densities would then be higher, but the error is unlikely to be more than 20 per cent.
2. RESULTS AND DISCUSSION Regular observations with a fully-calibrated system began on 14 July 1969, and have been made on most nights when there have been breaks of sufficient duration in the cloud. 2.1 Irregular nocturnal variations
One main feature disclosed is the considerable irregularity of the layer. During most of the night of 718 August 1969, observations were taken in sets of 25 shots extending over 10 min. From each of these sets an average height distribution has been deduced, a typical example being shown in Fig. 1, with standard errors due to the limited photon count. Figure 2(a) shows isopleths of sodium density as a function of height and time deduced from the height distributions. An idea of the errors in the heights of the isopleths can be obtained from the errors shown in Fig. 1. As a result, it can be confidently said that most of the detail evident in Fig. 2(a) represents real changes in sodium densities rather than statistical fluctuations in the photon count. At 0020, because it was evident that the bottom of the layer had fallen, an additional counting channel at 82-84 km was brought into operation in order to plot isoplcths below 84 km. It seems likely that the fluctuations are mainly due to irregularities of sodium density which are carried past the observation point by the motion of the atmosphere. If we assume for the wind the mean value at 90 km given by the model of GROVES (1969) i.e. 25 msec-l towards the N.E., then each 10 min set of observations consisted of 25 samples each of a circular area of about 20 m dia. spread along a line 15 km long. The marked region of constant sodium density between 2320 and 0110 located between 86 and 92 km, containing a tendency towards a minimum at 89 km and a secondary maximum at 87 km, would occupy a length of 165 km in the N.E.-S.W. direction. Some of the smaller features such as the peak at 2105 at 93 km occupy only 20 or 30 km. With finer time resolution one could doubtless see still smaller features, although there is some evidence, from column number densities (abundances) taken on other nights with 3 min resolution, that fluctuations are smaller for shorter time periods, except possibly during twilight. The isopleths for the night 22/23 July 1969 (Fig. 2(b)) show a very large change commencing at about 0200, the sodium abundance increasing by about a factor of
1425
Laser rrtdar measurements of the atmospheric sodium layer
’
0 82
84
’
86
’
t
88
90 Height
t
93
’
94
’
96
’
98
’
;OO
(km)
Fig. 1. Average distribution of sodium density with height for 25 shots between 0053 and 0103 UT, 8 August 1969.
lb0 98 96-
Na density (109mw3)
94 z ” 92 IE *; 90T 8%-
x 0
5 4
:
:
l
I
86-
82 20
2 21
22
I 23
01 24 UT (hr) Fig.
02
03
04
2(a).
4 in about 1 hr. This is the greatest change found in any of the data, Figure Z(b) umfortunately contains a larger amount of error due to statistical fluctuations in the photon count than Fig. 2(a), for example, the structure around 98 km between 2300 and 2400 is probably due to this. Patches of thin cloud were present and reduced the signal, particularly around 0300. However, since the presence of cloud reduces the
6
M. C. W. SAHDFORDand A. J. C&BON
1426
04
L
,
I
I
1
I
02. 03 24 O! U T (hr) Fig. 2. IeopIeths of sodium density during night, (a) 7/8 August 1969. (b) 22123 July 1969. 21
‘22
23
signal from the sodium layer and the 25-35 km calibration signal in the same ratio, it should not produce any systematic error in the measurement, and this has been conmmed by the lack of correlation between the presence of cloud and the apparent abundance of sodium. The interpretation of the fluctuations as a spatial variation has been made before. SWINGS and NICOLET (1949) first noted the patchiness of the sodium twilight glow. Geographic irregularities of the sodium night glow have been reported by NASYROV (1967), but this may relate to the distribution of some reacting constituent other than sodium. The spatial nature of the fluctuations could be confirmed, and any time fluctuations could be measured, by operating two or more laser radars from the same site. These would sample regions of the layer separated by up to about 100 km, or, with more powerful equipment, even further. Such an experiment could also be regarded as a method of measuring winds at this altitude.
Because of the irregular variations described in Section 2.1, measured changes of abundance during the night that might be used to test particular theories of the sodium layer can be studied only by taking averages over a large number of nights. Satisfactory measurements were made during evening twilight on 8 nights, beginning at solar depression angles of 5” or less, for a period of at least 100 min. (Twilight abundance measurements by means of sunlight are usually made at solar depressions of about 6+5”, and height measurements at angles between 7 and 10.) The data for each night were normalized to the average abundance during the 100 min period. The means over the 8 nights for each 10 min period are shown in Fig. 3(a) together with the standard errors. There is slight evidence for an increase of about 20 per cent in the abundance during the period O-30 min after solar depression of 6’, but
1427
Laser radar me~urements of the ~tmo~pheriesodium layer
I*3
I.3-
I.2E f 2 8
e(’ OQ to
I.I0
I
.z I.0z ul _g 0.9 ti cl
-20
0
20
Time (min) after
I
I
I
4O 6” Solar Depression
40
60
80
Solar Depression of 6O
20
22
24 UT
02
04
(hr)
J
6” IO” Angles
Fig. 3. Relative sodium abundances, (a) averaged over 8 evening twilights 20 July-7 Augnst 1969. (b) averaged over 14 nights 14 July-4 September 1969.
the significance level is only 5 per cent. A similar analysis for the whole night (data grouped in hourly periods, Fig. 3(b)) shows no significant variations from the mean. Observations during morning twilight have been made on only a few occasions and, at this time of year, are frequently prevented by mist and fog, so that a detailed analysis of morning twilight effects is not yet possible. According to the photochemical theory of HITXT (1966) the ratio n(O)/n(O,) at the height of the sodium layer is considerably greater during daytime than at night, and this would be expected to cause a corresponding variation in the abundance of sodium (HUXTEN, 1967). Since photodissociation of ozone can be caused by all wavelengths, Iz,up to l-18 pm, it would be expected that such changes would occur when the solar depression angle is greater than 5”, e.g. sunset at 90 km for il N O-6 pm occurs when the solar depression angle is about 10’ (THOMAS and BOWMAN, 1969). The measurements in Fig. 3 suggest that photochemical changes when solar excitation ceases have little effect on the sodium abundance, e.g. a rapid removal of sodium by oxidation which might result from an increase in ozone concentration is not a dominant process. 2.3 Changes of
abundance from night to night
The data for each night were averaged over 1 hr periods, and the mean of these was taken to provide a time-averaged column number density on each night (Fig. 4). These measurements agree quite well in magnitude with twilight values at this latitude and season (HUFITEN, 1967), and suggest that the seasonal trend towards the winter maximum, well-known from twilight values, is present also in the night-time values. One might expect the laser measurements to exhibit
1428
M.
fg
C.
W.
IL
0 Observations during 3 or more hours d Observations during 2 or fewer hours
* z oIO
1 I5
” 20
”
25
30
July Fig.
and A. J. GIBSOX
SANDFORD
”
4
9
14
” 19 24
’
29
August
“1 3
8
13
1 18
September
4. Night-to-night variation of column number density, July-September 1969.
somewhat smoother trends than the twilight observations, which are averaged over only about 10 min but large night-to-night changes still occur. The largest change, on 5 August, is shown in more detail in Pig. 5, where hourly averages are plotted. Z.~~~~S~T~~U~~O~ witJ4heig7Lt In Fig. 6 two typical height distributions, averaged over about 4 hr, are presented, The contribution to the standard error due to the limited photon count is about 3 per cent at the peak of the layer, but the true standard errors may be slightly larger
-
I
4th
7th 6th 5th Date (August1969)
8th
Fig. 6. Hourly averages of column number density on successive nights 3-8 August 1969.
Laser radar measurements of the atmospheric sodium layer
IO*
I
80
I
I
I
I
I
I.
I
I
I
I
1129
1
84
88 92 96 100 10% Height (km) Fig. 6. Average distributions of sodium density with height for two nights A 314 September 1969, 0000-0400 UT. 0 6/7 September 1969,1900-2300 UT.
because the height distribution is not constant over the period of the observations. A double-peaked height profile is a common shape. A smooth single-peaked layer occurred infrequently. The steep gradient of the bottomside of the layer may easily be explained in terms of ozone concentration increasing rapidly with decreasing height (HUNTEN, 1967). The topside density is found to fall off with a scale height between 2 and 3 km. This remarkably steep gradient was first found in daytime rocket measurements (HUNTEN and WALLACE, 1967; DONAHUE and MEIER, 1967), which in 3 flights yielded scale heights in the range 2.5-3 km, and this necessitated the development of new theories for sodium. Refinements in the twilight photometry technique have recently led to estimates of the scale height as less than 4.5 km with no lower limit (Gault and Rundle, 1969). Thus it seems that a small topside scale height is the normal situation throughout the day and night. The data so far obtained are not sufficient to discriminate between the recently proposed theories of the sodium layer. According to HANSON and DONALDSON(1967) there is a convective cycle involving upward diffusion of free sodium and loss by photo-ionization to produce the steep gradient in the topside, followed by downward transport of the ions by winds blowing across the magnetic field and recombination in a small height region. This theory suggests that the topside scale height should increase during the night when photo-ionization ceases. For a typical value of the vertical eddy diffusion coefficient, K, of 500 m2 set -l (HUNTEN, 1967), the diffusion time, H2/K, for a scale height, H, of 3 km, would be about 5 hr. Thus the present observations of steep topside gradients throughout the night suggest a lower value of the vertical eddy diffusion coefficient, and are consistent with the conclusion of LAYZER and BEDIN~ER (1969) that there is no turbulence in the height range 80-140 km.
1430
M. C.
W. SANDFORDs.nd A. J. GIBSON
An ~ternative theory by GADSDEN(1968) eo~iders ablation from meteors taking place with a very sudden increase in the rate of sodium production below 100km. This theory, however, suggests a decline in sodium production during the night, and production lo-20 km higher in the predawn period. It is not known how the loss mechanisms vary during the night, but no indication of increased production at higher levels has been found in the data. 3. CONCLmION The abundance of sodium at night has been found to be similar to that during twilight. No trends are noted during the night apart from a possible 20 per cent increase in about 30 min after a solar depression of 6’ in the evening. The height ~strib~tion exhibits very small topside scale heights in the range 2-3 km. Considerable irregula~ties in the layer exist and may extend over horizontal distances in the range IO-200 km. Extended measurements of the present type are likely to yield much information which will aid the elucidation of the chemical and transport mechanism affecting the sodium distribution, and it would be desirable for measurements to be carried out at different latitudes and longitudes. Extension of the measurements further into daytime conditions should be undertaken. Use of a steerable or multiple beam system could yield information on horizontal movements. ~c~ow~ed~e~e~~-The authors aoknowlodguthe sssistsnee of Mr. P. G. L. THOXASin ceng out the observations. The work described was et-m&d out at the Radio and Space Research Station of the Science Research Council ad this paper is published with the permission of the Director. REFERENCES BAIN W. C. and BATES D. R. BOWMAN M. R., SA~RORD M. BOWNAN M. It., S_mm+om M.
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