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Advances in Space Research 35 (2005) 1951–1956

Seasonal variation in the solar diurnal tide and its possibleinfluence on the atmospheric sodium layer

B.R. Clemesha *, D.M. Simonich, P.P. Batista, H. Takahashi

Aeronomy Division, Instituto Nacional de Pesquisas Espaciais, Avenida dos Astronautas, 1758, Sao Jose do Campos 12227-010 SP, Brazil

Received 5 October 2004; received in revised form 17 February 2005; accepted 18 February 2005

Abstract

In a paper published 12 years ago, we showed that the height of the atmospheric sodium layer at our location is about 1 km lower

in November than at any other time of the year. We also showed that the decrease in height of the sodium layer was accompanied by

an increase in the intensity of the OI 557.7 nm and OH(9,4) band airglow emissions. At that time we suggested that this behavior

could be the result of large scale convective transport in the MLT region. We have now had the opportunity to compare the diurnal

variations of the sodium layer and airglow emissions with the tidal winds measured by meteor radar over the past 5 years. We find

that the amplitude of the diurnal tide is much smaller in November than at other times of the year. Since most of the sodium mea-

surements and all of the airglow observations are for night-time conditions only, a change in the amplitude of the 24-h tide could

strongly influence the average measured sodium and airglow parameters. It is shown that the observed changes in the tidal winds are

qualitatively consistent with the sodium measurements, but the amplitude of the observed height change is much greater than would

be expected from the tidal winds.

� 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Upper atmosphere; Atmospheric tides; Sodium layer

1. Introduction

Twelve years ago we published a paper (Clemesha

et al., 1992) in which we showed that the height of the

sodium layer observed at our location, Sao Jose dosCampos (23�S, 46�W), reaches a sharp minimum in

November. This effect was observed in the annual vari-

ation of the layer centroid height averaged over nearly

20 years of somewhat sparse data. At the same time of

year we found a sharp peak in the OH(9,4) band airglow

and a broader maximum in the OI 557.7 nm emission.

The airglow emissions were measured at Cachoeira

Paulista, located about 100 km NE of our lidar site.At the time we suggested that these effects could be

0273-1177/$30 � 2005 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/j.asr.2005.02.081

* Corresponding author. Tel.: +55 12 3945 6953; fax: +55 12 3945

6952.

E-mail address: bclem@laser.inpe.br (B.R. Clemesha).

the result of downwelling associated with the large-scale

meridional circulation. Our suggestion was that the

downward wind would cause a lowering of the sodium

layer and an increase in the atomic oxygen concentra-

tion, leading to the observed increase in the airglowintensities. The fact that we now have nearly 5 years

of winds data obtained from a Skiymet meteor radar in-

stalled at Cachoeira Paulista prompts us to re-examine

this explanation.

2. Measurements and discussion

Figs. 1 and 2, in our 1992 paper, show the sodium

layer centroid height and the airglow intensity variations

respectively. Fig. 1 of the present paper shows the up-

dated annual variation in centroid height, incorporating

measurements made up to 2003. The centroids plotted in

ved.

1 2 3 4 5 6 7 8 9 10 11 12

90.5

91.0

91.5

92.0

92.5

Cen

troi

d H

eigh

t (km

)

Month

Fig. 1. Annual variation of the sodium layer centroid height.

75

80

85

90

95

100

105

110

0.000 0.005 0.010 0.015 0.020

November

Annual Mean

Relative Sodium Concentration

Hei

ght (

km)

Fig. 2. Height distribution of atmospheric sodium.

1 2 3 4 5 6 7 8 9 10 11 12100

200

300

400

OI 5577

Inte

nsity

(R

ayle

ighs

)

Month

1 2 3 4 5 6 7 8 9 10 11 12

300

400

500

600 O2

Inte

nsity

(R

ayle

ighs

)

Month

1 2 3 4 5 6 7 8 9 10 11 12

120013001400150016001700

OH(6,2)

Inte

nsity

(R

ayle

ighs

)

Fig. 3. Annual variations in airglow intensities.

1952 B.R. Clemesha et al. / Advances in Space Research 35 (2005) 1951–1956

this figure are monthly averages based on data acquired

between 19:00 and 22:00 local time. We restricted the

analysis to this time period to avoid aliasing between an-

nual and diurnal variations and, in any case, most of our

data is acquired during this interval. To compute the

monthly means we first computed daily average centroid

heights for all available data from 1972 to 2003, exclud-

ing profiles which showed the presence of sporadic so-dium layers. We then used these daily averages to

generate monthly means, and subsequently used these

to compute average centroid heights for each calendar

month. Note that the precision of measurement of the

daily mean centroid heights is extremely high, of the or-

der of a few meters, but the geophysical noise is much

greater than this. The error bars shown in Fig. 1 repre-

sent the standard error of the mean, computed as thestandard deviation of the monthly averages divided by

the square root of the number of monthly mean cent-

roids included in each average. The November mini-

mum is even more marked in Fig. 1 than in our

original paper. The centroid height of the layer is

approximately 1 km lower in the November average

than at any other time of the year. The vertical height

distributions for November and for the average of the

rest of the year, shown in Fig. 2, make it clear that the

change in centroid height is mainly a result of a loweringof the entire layer. New airglow variations, based on

observations from Cachoeira Paulista, made between

1987 and 2000 are shown in Fig. 3. The plots in Fig. 3

are based on nightly average airglow intensities, and

no attempt has been made to avoid aliasing effects of ti-

dal oscillations. As in the case of Fig. 1, the error bars

represent the standard error of the mean. As can be seen

from this figure, the OH(6,2), OI 557.7 nm and O2

Atmospheric band emissions all show maximum inten-

sity in November, confirming our previous study. It

should be noted, however, that at other times of the year

the airglow variations are quite different to those of the

sodium centroid height, so we cannot be sure that the

November peaks in airglow are related to the minimum

in sodium layer height.

If the November lowering of the sodium layer is theresult of advection by a vertical wind, then it might be

possible to identify a related seasonal behavior in the

meteor winds. In principle it is possible to measure the

vertical wind component by the meteor radar technique.

Unfortunately, the precision of measurement achievable

in practice is inadequate to determine the real vertical

velocities which have magnitudes of only a few cm/s.

1 2 3 4 5 6 7 8 9 10 11 12

-15

-10

-5

0

5

10

15

20

25

Meridional

Zonal

Pre

vaili

ng W

ind

(m/s

)

Month

Fig. 4. Annual variations of the zonal and meridional prevailing winds

for Cachoeira Paulista.

B.R. Clemesha et al. / Advances in Space Research 35 (2005) 1951–1956 1953

However, if this wind forms part of a meridional cell,

then the horizontal wind should also be expected to

show an effect. Fig. 4 shows the annual variations of

Fig. 5. Meridional tide measured at Cachoeira Paulista. Velocities are in m/s

component of the meridional wind averaged from 80 to 100 km.

both the meridional and zonal prevailing winds, aver-

aged over 80–100 km, based on meteor radar data taken

between 1999 and the end of 2003 at Cachoeira Paulista.

The data plotted in this figure represent the constant

term in a harmonic analysis of contiguous 10-day inter-

vals, in which constant, 24- and 12-h components werefitted to the hourly wind values determined for 3-km

height intervals from 80 to 100 km. As can be seen from

this figure, there is no evidence of any effect compatible

with the observed Na layer and airglow variations. A

general analysis of the mean winds and tides observed

at Cachoeira Paulista can be found in Batista et al.

(2004).

Although the prevailing wind shows no effect relevantto the lowering of the Na layer in November, we find

that the meridional component of the diurnal tide does.

Fig. 5 shows the amplitude and phase of the diurnal

meridional tide measured at Cachoeira Paulista for

more than 4 years. The data plotted in this figure are

based on the same harmonic analysis as was used to

and phase is days. The lowest panel shows the amplitude of the diurnal

1 2 3 4 5 6 7 8 9 10 11 120

10

20

30

40

50

GSWM Meridional Wind for 90.6 kmVel

ocity

(m

/s)

Month

0 1 2 3 4 5 6 7 8 9 10 11 12

102030405060

Diurnal Tide

Height AveragedMeridional Wind

Win

d A

mpl

itude

(m

/s)

Month

Fig. 7. Annual variations of the amplitude of the meridional 24-h tide:

upper panel – wind measured at Cachoeira Paulista; lower panel –

GSWM for 90.6 km at 24�S.

800

1954 B.R. Clemesha et al. / Advances in Space Research 35 (2005) 1951–1956

determine the prevailing winds. It is clear from this fig-

ure that the tide exhibits a sharp minimum in amplitude

in November of each year, and at the same time there is

a large and rapid change in phase. This behavior sug-

gests that the lowering of the sodium layer in November

might be the result of a change in the diurnal variationof the height of the layer, caused by the vertical compo-

nent of the diurnal tide. This could come about because

the sodium measurements analyzed were restricted to

the time interval 19:00–22:00 LT. If the diurnal variation

in the height of the layer causes it to be above the diur-

nal average at this time of day, then a decrease in the

amplitude of the tide will cause a decrease in layer height

at the time of our measurements. We can investigate thispossibility by examining the experimentally determined

diurnal variation in the height of the sodium layer, or

via a model vertical tidal wind.

We have measured the diurnal variation in the atmo-

spheric sodium layer at our location (Clemesha et al.,

1982, 2002; Batista et al., 1985), although this variation

most probably combines both dynamical and photo-

chemical effects. Measuring the diurnal variation in thesodium layer is difficult because it involves making mea-

surements during daytime, when the background noise

is very high. For this reason we have adequate data only

during winter, although we do have a few measurements

for other times of year. Consequently, we do not know if

the diurnal variation decreases in November. Fig. 6

shows the average diurnal variation in the centroid

height for our location, based mainly on measurementsmade in June, July and August. The shaded area in this

figure shows the time period over which those measure-

ments included in the seasonal variation analysis were

made. The average height over this time period is about

300 m above the diurnal average, only about a third of

the observed November decrease, although it is in the

right sense.

As we have already pointed out, the meteor radar isunable to measure vertical winds with the required pre-

cision, but we can compare our measurements with the

0 6 12 18 2490.5

91.0

91.5

92.0

92.5

93.0

Hei

ght (

km)

Local Time (Hours)

Fig. 6. Diurnal variation in the sodium layer centroid height measured

at Sao Jose dos Campos for winter conditions.

GSWM winds (Hagan et al., 2001). First, in Fig. 7, we

compare the annual variation in the measured meridio-

nal tide with the GSWM tide taken from the GSWM

website. In general the agreement is quite good, except

for a phase difference of about 1 month in the semi-an-

nual oscillation. We also note that although the mea-surements show a very sharp minimum at the end of

November, and a less sharp minimum in December/Jan-

uary, the model tide shows only a single broader mini-

mum in January. In view of the fairly good agreement

in the diurnal meridional tide, there is some justification

in using the model vertical wind to see whether the ob-

served lowering of the Na layer can be explained in this

way.In Fig. 8 we show the average diurnal oscillation in

the height of an air parcel at around 90.6 km, based

0 6 12 18 24

-400

-200

0

200

400

600

GSWM June - August AverageHei

ght D

ispl

acem

ent (

m)

Local Time (Hours)

Fig. 8. Diurnal variation in the height of an air parcel at 90.6 km

calculated on the basis of the GSWM vertical wind.

B.R. Clemesha et al. / Advances in Space Research 35 (2005) 1951–1956 1955

on the GSWM data for June–August for 24�S. This fig-ure was obtained by numerically integrating the GSWM

diurnal and semidiurnal vertical velocities for 90.6 km.

Somewhat surprisingly this is almost 180� out of phasewith the experimentally observed variation in the height

of the sodium layer. We can also use the GSWM verticalwind to see how the mean height between 19:00 and

22:00 LT would be expected to vary during the year.

To do this we computed, for each month of the year,

the mean diurnal variations in the height of an air par-

cel, as in Fig. 8. For each month, we then subtracted

the diurnal average from the mean height between

19:00 and 22:00 LT. The results are shown in Fig. 9.

The seasonal variation in height is much less than theexperimentally observed variation, and shows minima

in October and January rather than November. We

are forced to the conclusion, then, that a change in the

amplitude of the vertical tidal wind does not provide a

very satisfactory explanation for the sharp decrease in

height of the sodium layer in November. On the other

hand, it seems unlikely that the November changes in

the meridional wind and the sodium layer centroidheight are unrelated.

At first sight one might suspect that the November

effect could be related to the ‘‘Springtime Transition

in Atomic Oxygen’’ (Shepherd et al., 1999) or the

‘‘Equinox Transition’’ described by Shepherd et al.

(2004). However, these transitions appear to separate

distinct winter and summer regimes, whereas the

November effect is a transitory phenomenon lastingfor not more than one month. Furthermore, according

to Shepherd et al. (2004), these effects do not extend to

low latitudes and we should not expect to see them at

23�S.The November increase in the oxygen-related airglow

emissions suggests that the observed November effect in-

1 2 3 4 5 6 7 8 9 10 11 12

-400

-300

-200

-100

0

100

Dis

plac

emen

t at 1

9-22

LT

(m

)

Month

Fig. 9. Annual variation in the height of an air parcel in the region of

90.6 km, relative to the diurnal mean, calculated for 19–22 LT on the

basis of the GSWM vertical wind.

volves an increase in the concentration of atomic oxy-

gen. This would be consistent with a photochemically

induced increase in sodium on the bottomside of the

layer, and thus a lowering of its center of mass. On

the other hand, such a photochemical effect would also

be expected to result in increased sodium abundanceand a change in the shape of the layer. Neither of these

effects are observed in practice.

3. Conclusions

Continuing measurements of the vertical distribution

of atmospheric sodium at Sao Jose do Campos confirmour earlier finding that the sodium layer is approxi-

mately 1 km lower in November than during the rest

of the year. Airglow observations made at a nearby site

show that the OI 557.7 nm, O2 Atmospheric band and

the OH (6,2) band emissions reach maximum values in

November. Nearly 5 years of meteor winds measure-

ments, made at the same site as the airglow observa-

tions, show a sharp minimum in the meridionalcomponent of the diurnal tide, occurring in November

each year. A phase change of nearly 12 h is associated

with the minimum amplitude. The prevailing winds

show no rapid changes at this time of year. Although

it seems highly likely that the changes in the tidal ampli-

tude and the lowering of the sodium layer are connected,

we are unable to achieve quantitative agreement be-

tween the observed lowering of the layer, the observeddiurnal variation in the layer height and the GSWM

modeled vertical wind. It is always possible, of course,

that the near simultaneous November changes in the

meridional tide and the sodium height are merely a coin-

cidence, and our original suggestion that the observed

effects are the result of the large-scale meridional circu-

lation is correct. The November effect does not seem

to be related to the Springtime or Equinox transitionsreported by Shepherd et al. (1999, 2004).

Acknowledgments

We are grateful to Maura Hagan for the model winds

data taken from the GSWM website. This research has

received financial support from the Fundacao de Am-paro a Pesquisa do Estado de Sao Paulo and the Conse-

lho Nacional de Desenvolvimento Cientıfico e

Tecnologico.

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