- Email: [email protected]
On thermosphere-mesosphere-stratosphere coupling
Available Pergamon
online at www.sciencedirect.com SCIENCE
www.elsevier.com/locate/asr
doi:
DIRECT.
lO.l016/SO273-1177(03)00672-O
ON THERMOSPHERE-MESOSPHERE-STRATOSPHERE
COUPLING
D. K. Chakrabarty Centerfor Environment Survey, Vi&anagar Sociev 29/251, Ahmedabad 380015, India
ABSTRACT
To study the long-term trend of coupling between two regions, we have chosen a speciesthat travels from one region to another. Nitric oxide is one such specieswhich is produced in the thermosphere and is transported downwards. Evidences point out that there is increase in this transport rate. As a result, a decreasein the lower thermosphere and a possible increase in the mesospherein [NO] are expected. A decreasein [NO] has indeed been reported in the lower thermosphere. However, a study of electron density data of Thumba (9“N, 77”E) at 8Okm where NO is ionized by Ly-a, does not show any increase in [NO], rather it shows a decreasing trend. Thermospheric nitric oxide could also penetrate to the upper stratosphere. In this region G is photo-chemically controlled. One of the major lossesof 03 in this region is through NO. A decrease in [NO] in this region will decreasethe loss rate of OSand hence increase [a]. Ozone is being measured at many places in India by Dobson spectrophotometer. The nearest place to Thumba where 03 is being measured is Kodaikanal (lo’%, 77”E). An examination of Umkehr 03 data of Kodaikanal shows that at layers-7 and 9 i.e. in the upper stratosphere, there is an increasing trend of OS. It appears, therefore, that there is a decreasing trend of NO in the lower thermosphere, mesosphere and upper stratosphere. Though there is an increasing trend in the coupling strength of this region, the decreasing trend in NO is explainable by the decreasingtrend in its production rate. 0 2003 COSPAR. Published
by Elsevier Ltd. All rights reserved.
INTRODUCTION
The distribution of minor gases depends on the transport/coupling processesbetween different regions of the atmosphere. Changes in several parameters of our atmosphere have taken place during the past half a century (see for example a series of papers published in Adv. Space Res., 20(11), 1997, in Long-term Changes and Trends in the Atmosphere, New Age, Ed. Gufiw Beig, 2001 aud Phys. Chem. Earth, 27(6-S), 2002). It is quite possible that long-term changes in the transport/coupling processes between different layers of the atmosphere have taken place during this period. A decreasing trend in temperature iu the lower thermosphere by different magnitudes has been seen at many places (see above cited references), though at some places no trend has been found (Bit&r et al., 2002). A decreasein temperature would enhance the intensity of eddy diffusion. The trends of [Ar]/[ N2] ratio and wind parameters also suggest that the transport processescould indeed have got intensified (Danilov, 2001). In this work we have examined the long-term trend of transport parameter in thermosphere-mesosphere-stratosphere. To study the long-term trend of coupling between two regions, one can examine the behavior of a species that travels from one region to another. Nitric oxide is one such species which is produced iu the thermosphere and travels downwards. We have, therefore, chosen this species and have examined its long-term behavior in the thermosphere, mesosphere and stratosphere. ANALYSIS
AND RESULTS
To study the long-term trend of nitric oxide a large number of observations at one place by the same technique for a long period of time is needed. This is not available. In the D-region around 8Okm, electrons are produced by the interaction of solar Ly-a with NO. Hence, the production rate of electrons, q, can be written as: q = c. PO]
Adv. SpaceRes. Vol. 32, No. 9, pp. 1685-1688.2003 Q 2003 COSPAR. Published by Elsevier Ltd. All rights Printed in Great Britain 0273-1177/$30.00 + 0.00
------ (1)
reserved
1686
D. K. Chakraharty
where [NO] is the number density of nitric oxide and c is a function of Ly-a flux, solar zenith angle, ionization crosssection of NO, absorption cross-sectionand number density of 02. Also for daytime steady state condition, at this altitude where negative ions are negligible, production rate of electrons can be equated to its loss rate as: 4 = adf.[e]2 ---
(2)
where czcsis called effective electron loss-coefficient. Combining (1) and (2) one can write: [NO] = (a,ff lc). [e]’ ----
(3)
For simplicity we have taken a ee /c = 1. At the equatorial station, Thumba, a large number of electron density measurements by rocket are available (Subbaraya et al., 1983; Chakrabarty et al., 1989, ISRO-IMAF’, 1993). They are mostly around 45” solar zenith angle and are measured by the probe and propagation experiment techniques. The values of NO obtained from these electron density measurements using (3) are shown in Figure 1. Two things are clear from this figure. One is the solar activity dependence and the other is the long-term trend To Iiuther examine the dependence of NO on solar activity, we have plotted the values of 10.7cm solar flux for the period 1965-1987 in Figure 1. A comparison of NO values with 10.7cm solar flux shows that there is an increasein PO] with the increase of solar activity level. The linear correlation coefficient is found to be 0.75. Another parameter which might a&ct our study is temperature. In this study we have not considered any variation of temperature. A decrease in temperature could be taking place at this altitude. The decrease of temperature would increase a effand hence should decrease [e]. However, it has been shown earlier (Chakmbarty, 1997) that a decreasein [e] due to a decreasein temperature is marginal. Another parameter which could have a&&xl our result is the long-term variation of Ly-a. Since, however, the magnitude of this variation is not properly known we have not considered this variation However, there is an increase of Ly-a flux by a factor of 1.5 from solar minimum to solar maximum condition But this variation will not tiect the long-term variation NO seen in Figure 1. We have, therefore, not considered any variation of Ly-cr flux in our study. Although there is scatter in data, yet a decreasingtrend of NO with time is clearly seenin Figure 1.
A l
l
----
1965
1970
1975
THUMBA
(80
10.7 Cm SOLAR
1980
Km DATA) FLUX
1985
YEAR FQ. 1. Nitric oxide density obtained at Thumba from rocket measurements of electron density horn 1965 to 1986 for 80 km. War flux at 10.7cm wavelength for the corresponding period is also shown by dashed lines (right hand side scale). In the upper stratosphere 03 is photo-chemicalIy controlled. But one of the major losses of 03 in this region is through NO. A decmase in [NO] will decreasethe loss rate of 03 and hence increase [OJ. In Figure 2 we have plotted the Umkehr 03 data of Kodaika& for Umkehr layers-7 and 9 corresponding to the upper stratosphere. An increasing trend of 03 is indeed seenin this region implying thereby that there is a decreasing trend of NO in the upper stratosphere.
Thermosphere-Mesosphere-Stratosphere
Coupling
Illll/l/l,/L//I1IIIIl(,//(/
I
120
-0
8:
240 Months .
r),
Fig. 2. Plot of Umkehr stratosphere.
ozone
1687
data with time obtained
Kodaikanai hyer No. 7
at Kodaikanal
360
L--L--iio
I /
for layers 9 and 7 corresponding
to upper
DISCUSSION At altitudes where the ionization of NO plays the dominant role in the total ionization rate, the trend in [e] would be the same as the trend of [NO]. Friedrich and Torkar (2001) have compiled 119 rocket electron density profiles from nonauroral stations of the world to determine the long-term trends of electron densities. They find a negative trend of 0.7% p.a., signifying thereby a decreasing trend of NO. However, in view of large scatter in the data, they consider this finding as insignificant. This also agrees with the radio wave absorption data of Nestorov et al. (1991). Danilov (2000) on the other hand studied the ratio of a modeled electron density and the observed rocket electron density values at mid- and low latitude together from 1959 to 1990 and reported an increasing trend of electron density, signifying thereby an increasing trend of NO. The molecules of NO are formed in the lower thermosphere. They are transported downward by eddy dif&sion. Kalgin (1998) analyzed the rocket mass-spectrometer data of [Ar]/[N2] ratio from 19664991at altitudes 105-135 km and concluded that there is a positive trend of the eddy diffusion intensity. The reason of this trend is related to temperature trend in 80-120 km. Portnyagin and Merzlyakov (1998) also reported significant trends in the wind parameters. These trends of temperature, [Ar]4N2] ratio and wiud parameters all suggest that the downward transport processesmight be getting intensi&d (Danilov, 2001). As a consequence,there would be more transport of NO from the lower thermosphere leading to a decrease in [NO] in this region. A long-term decreasing trend of f,E observed by many (see e.g. Bremer, 2001 and the references therein) and a long-term decreasing trend of [NO+]/ [02c] ratio observed by Danilov (1997) and Danilov and Smirnova (1997) have shown that there is, indeed a decreasingtrend of [NO] in the lower thermosphere. The excessNO (the amount of NO due to the increasing trend of transport parameter) going out from the lower thermosphere enters the mesosphere. According to Danilov (2001) this increasesthe density of NO in the mesosphere and this also increasesthe electron concentration at the altitudes where the NO ionization plays the dominant role in the total ionization rate of the mesosphere. But then how do we explain the decrease in [NO] in the upper stratosphere? While
D. K. Chakrabarty
1688
coming down from the thermosphere why this excessNO should stop in the mesosphere? Why should it not go further down and enter the upper stratosphere? From the above it is to be noted that there is a decreasing trend of NO in the lower thermosphere, mesosphereand upper stratosphere. The molecules of NO are produced in the lower thermosphere through meta-stable odd-nitrogen speciesN(2D) and NeS) in the following way: N(‘D)+@+NO+O
These meta-stable species-areproduced in dissociation of Nz by XUV photons (800-1000~) or through photo-electron impact and auroral activity. A long-term decreasein 02 and NZ density has been predicted by many (Keating et al., 2000; Roble and Dickinson 1989). Ifthere is a decrease in 02 density then, from the above reactions, it should be obvious that there will be a decreasein production of NO in the thermosphere. This decreasedproduction of NO in the thermosphere would lead to a decreased NO density in the lower thermosphere, mcsosphere and upper stratosphere. It appears, therefore, that in the whole region from lower thermosphere to upper stratosphere there is a decreasing trend of NO and this trend is due to its decreasingtrend of production rate and not due to the intensification of thermosphere-mesospherestratosphere coupling process.
Bittner, M., D. Gffermann, H. -H. Graef, M. Danner, and ,K. Hamilton, An 18 year time seriesof OH temperatures and middle atmosphere decadal variations, J. Atmos. sbl. Terr. Phys., 64,1147-l 166,2002. Bremer, J., Trends in the thermosphere derived from global ionosonde observations,Adv. Space Rex, 28(7), 997-1006, 2001. Chakrabarty, D. K., Mesopause scenario on doubling of CQ, Adv. Space Res., 20(11), 2117-2125,1997. Chakrabarty, D, K., S. V. Pakhomov, and G. Beig, Variation of D-region nitric oxide density with solar activity and seasonat the dip equator, HandbookforAUP, 29,108-l 11,1989. Danilov, A D., Do ionospheric trends indicate to the greenhouse effect?, Adv. Space Rex, 28(7),987-996,200l. Danilov, A D., New ideas on the D-region modeling, Adv. Space Res., 25 (l), 5-14,200O. Danilov, A D., Long-term changesof the mesosphere and lower thermosphere temperature and composition, Adv. Space Res. 20(11), 2137-2147,1997. Danilov, A D., and N. V. Smimiva, Long-term trends of the ion composition of the E-region, Geomugnetism and Aemnomiu (in Russian), 37(4), 35-41, 1997. Friedrich, M. and K. M. To&u, Long-term trends and other residual features of the lower ionosphere, Proc. 15* ESA Sym. on European Rocket and Balloon Programs and Related Research,Biarritz, France, 28-3 1 May 2001, ESA SP471, pp. 357-362, August 2001. ISRG-IMAP, Rocket-borne experiments for D and E-region ionization campaign, Indian SpaceResearchGrganization, Bangalore, Sci. Rep., ISRG-IMAP-SR-41-93, 1993. Kalgin, Yu. A, Dynamical aspect of long-term trend of the neutral atmosphere composition at turbopause region, in Proceedings of tie International Workshop: Cooling and Sinking of the Middle and Upper Atmosphere, Moscow, July 6-10, 1998, p. 26, 1998. Keating, G. M., R. H. Tolson, and M. S. Bradford, Evidence of long-term global decline in the Earth’s thermospheric densitiesapparently related to anthropogenic effects, Geophys Res. Letts., 27(10), 1523-1526,200O. Nestorov, G., D. Pancheva,and A D. Dan&v, Climatic changes of the ionospheric radio wave absorption in SW region, Geomagnetism undAeronomiu (in Russian), 31(6), 1070-1075,199l. Portnyagm, Yu. I,, and E. G. Merzlyalcov, Long-term variability of the wind regime parameters the lower thermosphere of moderate latitude based on long wind measurements, in Proceedings of the International Workshop: Cooling andsinking of theMidle and UpperAtmosphere, Moscow, July 6-10,1998, p. 25, 1998. Roble, R G., and R. E. Dickinson, How will changesin carbon dioxide and metbane modify the mean structure of the mescsphereand thermosphere? Geophys. Res.Lett., 6(12), 1441-1444,1989. Subbaraya, B. H., S. Prakash,and S. P. Gupta, Electron densities in the equatorial lower ionosphere from the Laugmuir probe experiments conducted at Thumba during the year 1%6-l 978, Indian Space ResearchOrgamzation, Bangalore, Sci. Rep. SR-15/83,1983. E-mail addressof D. K. Chakrabarty [email protected] Manuscript received 24 December 2002; revised 26 March 2003, accepted27 March 2003.
online at www.sciencedirect.com SCIENCE
www.elsevier.com/locate/asr
doi:
DIRECT.
lO.l016/SO273-1177(03)00672-O
ON THERMOSPHERE-MESOSPHERE-STRATOSPHERE
COUPLING
D. K. Chakrabarty Centerfor Environment Survey, Vi&anagar Sociev 29/251, Ahmedabad 380015, India
ABSTRACT
To study the long-term trend of coupling between two regions, we have chosen a speciesthat travels from one region to another. Nitric oxide is one such specieswhich is produced in the thermosphere and is transported downwards. Evidences point out that there is increase in this transport rate. As a result, a decreasein the lower thermosphere and a possible increase in the mesospherein [NO] are expected. A decreasein [NO] has indeed been reported in the lower thermosphere. However, a study of electron density data of Thumba (9“N, 77”E) at 8Okm where NO is ionized by Ly-a, does not show any increase in [NO], rather it shows a decreasing trend. Thermospheric nitric oxide could also penetrate to the upper stratosphere. In this region G is photo-chemically controlled. One of the major lossesof 03 in this region is through NO. A decrease in [NO] in this region will decreasethe loss rate of OSand hence increase [a]. Ozone is being measured at many places in India by Dobson spectrophotometer. The nearest place to Thumba where 03 is being measured is Kodaikanal (lo’%, 77”E). An examination of Umkehr 03 data of Kodaikanal shows that at layers-7 and 9 i.e. in the upper stratosphere, there is an increasing trend of OS. It appears, therefore, that there is a decreasing trend of NO in the lower thermosphere, mesosphere and upper stratosphere. Though there is an increasing trend in the coupling strength of this region, the decreasing trend in NO is explainable by the decreasingtrend in its production rate. 0 2003 COSPAR. Published
by Elsevier Ltd. All rights reserved.
INTRODUCTION
The distribution of minor gases depends on the transport/coupling processesbetween different regions of the atmosphere. Changes in several parameters of our atmosphere have taken place during the past half a century (see for example a series of papers published in Adv. Space Res., 20(11), 1997, in Long-term Changes and Trends in the Atmosphere, New Age, Ed. Gufiw Beig, 2001 aud Phys. Chem. Earth, 27(6-S), 2002). It is quite possible that long-term changes in the transport/coupling processes between different layers of the atmosphere have taken place during this period. A decreasing trend in temperature iu the lower thermosphere by different magnitudes has been seen at many places (see above cited references), though at some places no trend has been found (Bit&r et al., 2002). A decreasein temperature would enhance the intensity of eddy diffusion. The trends of [Ar]/[ N2] ratio and wind parameters also suggest that the transport processescould indeed have got intensified (Danilov, 2001). In this work we have examined the long-term trend of transport parameter in thermosphere-mesosphere-stratosphere. To study the long-term trend of coupling between two regions, one can examine the behavior of a species that travels from one region to another. Nitric oxide is one such species which is produced iu the thermosphere and travels downwards. We have, therefore, chosen this species and have examined its long-term behavior in the thermosphere, mesosphere and stratosphere. ANALYSIS
AND RESULTS
To study the long-term trend of nitric oxide a large number of observations at one place by the same technique for a long period of time is needed. This is not available. In the D-region around 8Okm, electrons are produced by the interaction of solar Ly-a with NO. Hence, the production rate of electrons, q, can be written as: q = c. PO]
Adv. SpaceRes. Vol. 32, No. 9, pp. 1685-1688.2003 Q 2003 COSPAR. Published by Elsevier Ltd. All rights Printed in Great Britain 0273-1177/$30.00 + 0.00
------ (1)
reserved
1686
D. K. Chakraharty
where [NO] is the number density of nitric oxide and c is a function of Ly-a flux, solar zenith angle, ionization crosssection of NO, absorption cross-sectionand number density of 02. Also for daytime steady state condition, at this altitude where negative ions are negligible, production rate of electrons can be equated to its loss rate as: 4 = adf.[e]2 ---
(2)
where czcsis called effective electron loss-coefficient. Combining (1) and (2) one can write: [NO] = (a,ff lc). [e]’ ----
(3)
For simplicity we have taken a ee /c = 1. At the equatorial station, Thumba, a large number of electron density measurements by rocket are available (Subbaraya et al., 1983; Chakrabarty et al., 1989, ISRO-IMAF’, 1993). They are mostly around 45” solar zenith angle and are measured by the probe and propagation experiment techniques. The values of NO obtained from these electron density measurements using (3) are shown in Figure 1. Two things are clear from this figure. One is the solar activity dependence and the other is the long-term trend To Iiuther examine the dependence of NO on solar activity, we have plotted the values of 10.7cm solar flux for the period 1965-1987 in Figure 1. A comparison of NO values with 10.7cm solar flux shows that there is an increasein PO] with the increase of solar activity level. The linear correlation coefficient is found to be 0.75. Another parameter which might a&ct our study is temperature. In this study we have not considered any variation of temperature. A decrease in temperature could be taking place at this altitude. The decrease of temperature would increase a effand hence should decrease [e]. However, it has been shown earlier (Chakmbarty, 1997) that a decreasein [e] due to a decreasein temperature is marginal. Another parameter which could have a&&xl our result is the long-term variation of Ly-a. Since, however, the magnitude of this variation is not properly known we have not considered this variation However, there is an increase of Ly-a flux by a factor of 1.5 from solar minimum to solar maximum condition But this variation will not tiect the long-term variation NO seen in Figure 1. We have, therefore, not considered any variation of Ly-cr flux in our study. Although there is scatter in data, yet a decreasingtrend of NO with time is clearly seenin Figure 1.
A l
l
----
1965
1970
1975
THUMBA
(80
10.7 Cm SOLAR
1980
Km DATA) FLUX
1985
YEAR FQ. 1. Nitric oxide density obtained at Thumba from rocket measurements of electron density horn 1965 to 1986 for 80 km. War flux at 10.7cm wavelength for the corresponding period is also shown by dashed lines (right hand side scale). In the upper stratosphere 03 is photo-chemicalIy controlled. But one of the major losses of 03 in this region is through NO. A decmase in [NO] will decreasethe loss rate of 03 and hence increase [OJ. In Figure 2 we have plotted the Umkehr 03 data of Kodaika& for Umkehr layers-7 and 9 corresponding to the upper stratosphere. An increasing trend of 03 is indeed seenin this region implying thereby that there is a decreasing trend of NO in the upper stratosphere.
Thermosphere-Mesosphere-Stratosphere
Coupling
Illll/l/l,/L//I1IIIIl(,//(/
I
120
-0
8:
240 Months .
r),
Fig. 2. Plot of Umkehr stratosphere.
ozone
1687
data with time obtained
Kodaikanai hyer No. 7
at Kodaikanal
360
L--L--iio
I /
for layers 9 and 7 corresponding
to upper
DISCUSSION At altitudes where the ionization of NO plays the dominant role in the total ionization rate, the trend in [e] would be the same as the trend of [NO]. Friedrich and Torkar (2001) have compiled 119 rocket electron density profiles from nonauroral stations of the world to determine the long-term trends of electron densities. They find a negative trend of 0.7% p.a., signifying thereby a decreasing trend of NO. However, in view of large scatter in the data, they consider this finding as insignificant. This also agrees with the radio wave absorption data of Nestorov et al. (1991). Danilov (2000) on the other hand studied the ratio of a modeled electron density and the observed rocket electron density values at mid- and low latitude together from 1959 to 1990 and reported an increasing trend of electron density, signifying thereby an increasing trend of NO. The molecules of NO are formed in the lower thermosphere. They are transported downward by eddy dif&sion. Kalgin (1998) analyzed the rocket mass-spectrometer data of [Ar]/[N2] ratio from 19664991at altitudes 105-135 km and concluded that there is a positive trend of the eddy diffusion intensity. The reason of this trend is related to temperature trend in 80-120 km. Portnyagin and Merzlyakov (1998) also reported significant trends in the wind parameters. These trends of temperature, [Ar]4N2] ratio and wiud parameters all suggest that the downward transport processesmight be getting intensi&d (Danilov, 2001). As a consequence,there would be more transport of NO from the lower thermosphere leading to a decrease in [NO] in this region. A long-term decreasing trend of f,E observed by many (see e.g. Bremer, 2001 and the references therein) and a long-term decreasing trend of [NO+]/ [02c] ratio observed by Danilov (1997) and Danilov and Smirnova (1997) have shown that there is, indeed a decreasingtrend of [NO] in the lower thermosphere. The excessNO (the amount of NO due to the increasing trend of transport parameter) going out from the lower thermosphere enters the mesosphere. According to Danilov (2001) this increasesthe density of NO in the mesosphere and this also increasesthe electron concentration at the altitudes where the NO ionization plays the dominant role in the total ionization rate of the mesosphere. But then how do we explain the decrease in [NO] in the upper stratosphere? While
D. K. Chakrabarty
1688
coming down from the thermosphere why this excessNO should stop in the mesosphere? Why should it not go further down and enter the upper stratosphere? From the above it is to be noted that there is a decreasing trend of NO in the lower thermosphere, mesosphereand upper stratosphere. The molecules of NO are produced in the lower thermosphere through meta-stable odd-nitrogen speciesN(2D) and NeS) in the following way: N(‘D)+@+NO+O
These meta-stable species-areproduced in dissociation of Nz by XUV photons (800-1000~) or through photo-electron impact and auroral activity. A long-term decreasein 02 and NZ density has been predicted by many (Keating et al., 2000; Roble and Dickinson 1989). Ifthere is a decrease in 02 density then, from the above reactions, it should be obvious that there will be a decreasein production of NO in the thermosphere. This decreasedproduction of NO in the thermosphere would lead to a decreased NO density in the lower thermosphere, mcsosphere and upper stratosphere. It appears, therefore, that in the whole region from lower thermosphere to upper stratosphere there is a decreasing trend of NO and this trend is due to its decreasingtrend of production rate and not due to the intensification of thermosphere-mesospherestratosphere coupling process.
Bittner, M., D. Gffermann, H. -H. Graef, M. Danner, and ,K. Hamilton, An 18 year time seriesof OH temperatures and middle atmosphere decadal variations, J. Atmos. sbl. Terr. Phys., 64,1147-l 166,2002. Bremer, J., Trends in the thermosphere derived from global ionosonde observations,Adv. Space Rex, 28(7), 997-1006, 2001. Chakrabarty, D. K., Mesopause scenario on doubling of CQ, Adv. Space Res., 20(11), 2117-2125,1997. Chakrabarty, D, K., S. V. Pakhomov, and G. Beig, Variation of D-region nitric oxide density with solar activity and seasonat the dip equator, HandbookforAUP, 29,108-l 11,1989. Danilov, A D., Do ionospheric trends indicate to the greenhouse effect?, Adv. Space Rex, 28(7),987-996,200l. Danilov, A D., New ideas on the D-region modeling, Adv. Space Res., 25 (l), 5-14,200O. Danilov, A D., Long-term changesof the mesosphere and lower thermosphere temperature and composition, Adv. Space Res. 20(11), 2137-2147,1997. Danilov, A D., and N. V. Smimiva, Long-term trends of the ion composition of the E-region, Geomugnetism and Aemnomiu (in Russian), 37(4), 35-41, 1997. Friedrich, M. and K. M. To&u, Long-term trends and other residual features of the lower ionosphere, Proc. 15* ESA Sym. on European Rocket and Balloon Programs and Related Research,Biarritz, France, 28-3 1 May 2001, ESA SP471, pp. 357-362, August 2001. ISRG-IMAP, Rocket-borne experiments for D and E-region ionization campaign, Indian SpaceResearchGrganization, Bangalore, Sci. Rep., ISRG-IMAP-SR-41-93, 1993. Kalgin, Yu. A, Dynamical aspect of long-term trend of the neutral atmosphere composition at turbopause region, in Proceedings of tie International Workshop: Cooling and Sinking of the Middle and Upper Atmosphere, Moscow, July 6-10, 1998, p. 26, 1998. Keating, G. M., R. H. Tolson, and M. S. Bradford, Evidence of long-term global decline in the Earth’s thermospheric densitiesapparently related to anthropogenic effects, Geophys Res. Letts., 27(10), 1523-1526,200O. Nestorov, G., D. Pancheva,and A D. Dan&v, Climatic changes of the ionospheric radio wave absorption in SW region, Geomagnetism undAeronomiu (in Russian), 31(6), 1070-1075,199l. Portnyagm, Yu. I,, and E. G. Merzlyalcov, Long-term variability of the wind regime parameters the lower thermosphere of moderate latitude based on long wind measurements, in Proceedings of the International Workshop: Cooling andsinking of theMidle and UpperAtmosphere, Moscow, July 6-10,1998, p. 25, 1998. Roble, R G., and R. E. Dickinson, How will changesin carbon dioxide and metbane modify the mean structure of the mescsphereand thermosphere? Geophys. Res.Lett., 6(12), 1441-1444,1989. Subbaraya, B. H., S. Prakash,and S. P. Gupta, Electron densities in the equatorial lower ionosphere from the Laugmuir probe experiments conducted at Thumba during the year 1%6-l 978, Indian Space ResearchOrgamzation, Bangalore, Sci. Rep. SR-15/83,1983. E-mail addressof D. K. Chakrabarty [email protected] Manuscript received 24 December 2002; revised 26 March 2003, accepted27 March 2003.