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Long-term trends in the meso- and thermosphere
Adv. Space Res. Vol. 20, No. 11, pp. 2075-2083.1997 01997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/97 $17.00 + 0.00 PII: SO273-1177(97)00598-X
LONG-TERM TRENDS IN THE MESO- AND THERMOSPHERE J. Bremer Institutfiir AtmosphZirenphysik, SchloJstr. 4-6, D-18225 Kiihlungsborn, Germany
ABSTRACT Ionospheric plasma parameters are markedly controlled by the variability of the solar radiation and high energetic particle fluxes. Therefore it is necessary to remove these influences by a multiple regression analysis before longterm trends can be detected. Using this method trends have been found in LF phase height and LF absorption data. These trends can be explained by a decrease of the atmospheric pressure near the LF reflection heights (80...85 km) caused by a temperature decrease below this altitude range Qualitatively the same effect was predicted by Rishbeth (1990) on the basis of model calculations by Roble and Dickinson (1989) for reflection heights deduced from ionosonde measurements of the ionospheric E- and F-layers. Analyses of selected ionosonde standard parameters confirm the predictions, but there are also observations which do not show the expected negative trends. Also in the wind field of the upper mesosphere/lower thermosphere long-term trends could be detected. Especially the amplitudes of the tidal components have markedly been reduced during the last 30 years in all observations available on both hemispheres. 01997 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION Temperature observations near the surface of the Earth have been carefully analyzed during the last twenty years to get significant information of an expected warming due to an enhanced atmospheric greenhouse effect. Now it seems to be that the observed latest warming trends can no longer be interpreted as atmospheric variability but are related to a greenhouse effect (Hegerl et al., 1996). In the strato- and mesosphere an increasing content of greenhouse gases should reduce the atmospheric temperature due to an enhanced infrared cooling as demonstrated by different model calculations of Rind et LEI.(1990) Brasseur et al. (1990), Berger and Dameris (1993). The same effect was also predicted for the thermosphere (Roble and Dickinson, 1989). In this paper different ionospheric plasma parameters have been analyzed to detect possible long-term trends and to test if such trends could be an indication of an atmospheric greenhouse effect in the mesosphere and thermosphere. EXPERIMENTAL
RESULTS
To detect long-term trends in ionospheric plasma data it is often necessary to remove the influence of the ionizing solar radiation and high energetic particle precipitation. Therefore, we calculated for the experimental data X (monthly mean values at constant solar zenith angle or at each full hour) regressionm equations of the following 2075
2076
J. Bremer
form x,
=A+B*FlO.7
+C-Ap
(1)
Here F10.7 and Ap are monthly mean values of the solar radio flux at 10.7 cm and of the planetary geomagnetic activity index. To describe the mean variation of solar radiation it is also possible to use the relative solar sunspot number R instead of F10.7. Then we estimated the deviations of the experimental data from the corresponding theoretical values calculated after Eq. 1
Ax = Using these deviations
AX we estimated
xexp- x,
linear trends AX = a + c
-year
(3)
Such analyses were made for monthly mean values but also for seasonal or yearly mean values of AX data. Some details of the experimental data used in this paper have been summarized in Table 1.
Table I. Stations and measuring their reflection points). Station/measuring
paths used in trend analyses (The coordinates
path
Method
of the measuring
Location
paths are given for
Period
LF phase height
Sl”N,
07”E
1964 - 1995
Kalundborg-Ktihlungsbom (243 kHz) Berlin - Kuhlungsborn (177 kHz) Hoyer - Ktihlungsborn (I 28 kHz)
LF absorption LF absorption LF absorption
55”N, 53”N, 55”N,
11”E 10”E 10”E
1948 - 1995 1952 - 1995 1968 - 1991
Juliusruh Dourbes Freiburg Rome
Ionosonde Ionosonde Ionosonde Ionosonde
55”N, 50”N, 48”N, 42”N,
13”E 05”E 08”E 12”E
1957 1959 1948 1957
Collm/ Kuhlungsborn Ktihlungsborn Heiss Island Molodezhnaya Obninsk Atlanta Christchurch Saskatoon
Dl -Wind
51”N, 13”E 54”N, 12”E 54”N, 12’E 8l”N, 58”E 68”S, 45”E 55”N, 38”E 34”N, 84”W 44”S, 173”E 52”N, 107’W
Allouis-Ktihlungsborn
(162 161~)
DZWind D2-Wind D2-Wind D2-Wind DZWind MF-Wind MF-Wind
-
1995 1990 1976 1995
1964 - 1994 1976 1965 1967 1964 1975 1978 1979
-
1994 1985 1986 1981 1986 1986 1988
LF Phase Heights From daily indirect phase height measurements in the LF-range at Kuhlungsbom (for details of the measurements see Lauter et al., 1984) the ionospheric reflection height at constant solar zenith angle (x=78.5”) has been derived from observations at 162 kHz between 1964 and 1995. As shown by Taubenheim et ul.(1990) this phase height corresponds to a nearly isobaric level if the variability of the solar radiation has been removed and there is no long-term trend of nitric oxide near the mesopause level. In Fig. 1 the seasonal variation of the monthly trend parameters c after Eq. 3 are presented. As to be seen during all months negative regression coefficients have been
Long-term Trends in the Meso- and Thermosphere
2011
162 kHz
JFMAMJJASONO MONTH
Fig. 1. Seasonal variation of monthly regression coefficient c for LF phase heights after elimination of solar and geomagnetic influences. derived with significance levels of more than 99% (using Fisher’s F-parameter after Taubenheim, 1969).In Fig. 2 the LF phase heights after elimination of the Fig. 2. Seasonal and yearly trends of LF phase heights solar/geomagnetic influence are shown for different after elimination of solar and geomagnetic influences. seasons and the whole year. The individual values are presented by crosses connected with thin full lines and the linear trends calculated for the total observation period by thick full lines. As to be expected after Fig. 1 all seasonal and yearly trends in Fig. 2 are negative (significance levels in all cases >99%) with a lowering of the mean reflection height by about 0.7 . .. 1.0 km during the observation period, thus indicating a marked decrease of the atmospheric pressure level near 80 km altitude. LF Absorntion Absorption measurements in the LF range (for details of the method see Lauter et al., 1976) are carried out at Ktihlungsbom since 1948 up to now. Using again data at constant solar zenith angle (x=78.5”) a similar trend analysis has been carried out as before with the phase height data. In Fig. 3 the seasonal variation of the monthly trend parameters is shown for three different absorption measuring paths. The full symbols mark significant trends (>95%), whereas the significance level for the other months (open symbols) is less than 95%. During nearly all months we derived negative trends, only at the 243 kHz path during winter months positive trends were detected. In Fig. 4 yearly trends of the LF absorption after elimination of the solar/geomagnetic induced part are presented. The negative trends on the two paths at 177 kHz and 128 kHz are significant (significance level >99%), the small negative trend at 243 kHz, however, not. This behaviour is caused by the fact that the seasonal trends are different, during winter months positive and during summer month negative as to be seen in Fig. 3. Combining only summer months we found again a significant negative trend. As will be discussed in more detail below the observed negative trends of the LF absorption 4 can be explained by a pressure decrease at the reflection height of about 85 km. Ionosonde
data in Figs. 3 and
Data
Basing on the model calculations sphere (-10 K) and thermosphere
of Roble and Dickinson (1989) who deduced a marked cooling of the meso(-50 K) due to an assumed doubling of the greenhouse gases CO? and CH,,
J. Bremer
2078
::k=----J 0.0
-Z
i
a
o
6.1
-02
Rishbeth (1990) and Rishbeth and Roble (1992) predicted a lowering of the F2-layer peak height by 1.5 ... 20 km, a smaller lowering of the E-layer peak height (-2.5 km), no significant changes of the peak electron densities in the F2- and E-layers and an increase of the peak electron density of the Fl-layer. Using ionosonde observations between 1957 and 1995 at Juliusruh trend analyses have been carried out with different standard parameters. In Fig. 5 the seasonal variation of the derived trend parameters are presented for M30OOF2, hmF2, h’F, h’E, foF2, foF1 and foE. hmF2 was estimated from M3000F2 after the well known formula derived by Shimazaki (1955)
hmF2 =
JFMAMJJASOND
MONTH
Fig. 3. Seasonal variation of monthly regression coefficients c for LF absorption at 3 different measuring paths after elimination of solar and geomagnetic influences.
1490
M3OOOF2 + AM
- 176
(4)
with the correction term AM=O. As shown by Bremer (1992) the trends derived from hmF2 values do not markedly depend on the AM value used. The full symbols in Fig. 5 mark significant trends (>95%). The different height values show negative trends during nearly all months. Whereas four of the negidtive hmF2 trends are significant with more than 95% (two of them >99%), all monthly trends of h’F and h’E values are negative with a significance level of more than 95%, most of them even more than 99%. In contrast to the height values nearly all peak electron densities of the E-, Fl- and F2-layers characterized by the critical frequencies of these layers do not show significant trends. Only the foF1 values during May have a significant positive trend (>95%). In Fig. 6 yearly trends of the thermospheric reflection height par&meters after elimination of the solar/geomagnetic influence are shown. All trends are significant with more than 99%. hmF2, h’F and h’E decrease by about 7 . .. 10 km during the observation period between 1957 and 1995. The corresponding yearly trends of the critical frequencies not shown here are very small and all not significant as to be expected after the results of Fig. 5. Meso-/Lower
,.,
1
1950
7995
1960
1965
1970
1975
1990
199s
IS90
!999
YEW
Fig. 4. Yearly trends of LF absorption at 3 different after elimination of solar and geomagnetic influences.
measuring
paths
Thermosphere
Wind
By means of ionospheric drift measurements in the LF range (Dl method) at Kiihlungsbom and Collm between 1964 and 1995) and radar meteor wind measurements (D?
2079
Long-term Trends in the Meso- and Thermosphere
7
a
0.002
% 0
0.000
i
!
,
i
t-
._.,.
/-
_. ,1 /
F % B 3 0 L=
1
0.002 -
B r 0
0.000 0.002 -
!
__
.,
r %
0.000 -
P 2 0
-0002 -0.004
’ J
F
M
A
M
,
,
J
JASOND
,
,
,
,
/ ’
MONTH
Fig. 5. Seasonal variation of monthly regression coefficients c for different ionosonde data of the station Juliusruh after elimination of solar and geomagnetic influences.
hmF2
method) at Kiihlungsbom between 1976 and 1994 the wind field of the meso-/lower thermosphere region has continuously been observed. Unfortunately the exact height of these observations is not known. Since 1982 this height is measured for the ionospheric drift observations at Collm (Kiirschner, 1981). However, to avoid inconsistencies in the time-series these results are not used here. We assume a mean height of 95 km. The mean zonal and meridional winds as well as the amplitudes of the semidiurnal and diurnal tidal components have been analyzed for long-term variations. As discussed in detail by Bremer et al., ( 1997) the solar activity influence on the wind field is not very strong. Therefore, it is not necessary to remove the solar/geomagnetic influence before linear trends can be analyzed. In Fig. 7 we show the results of yearly mean values of the zonal wind from our measurements at Ktihlungsbom and Collm together with other observations (most radar meteor measurements, but also results of MF radars) at different stations on both hemispheres. We used here yearly mean values as these values should be nearly no influenced by possible long-term trends of the mean reflection height (Bremer er al., 1997). As can be seen from the results shown in Fig. 7 six of seven observations show negative trends, four of them are significant with at least 95%. Also in the meridional wind trends can be detected (not shown here), six of seven observations are positive but only one of them with a significance level greater than 99%. The most convincing trends have been detected, however, in the amplitudes of the tidal waves presented in Fig. 8. All trends derived are negative and most of them with a significance level of more than 95% or even 99%. Only the trends of the very short data series at Christchurch are not significant. The excellent agreement of all trends indicates that the decrease of the tidal wind components seems to be a global phenomenon. DISCUSSION
The trends detected in the LF phase heights are a strong indication that the atmospheric pressure near 80 km decreased during the last 30 years more or less continuously. Using the monthly trend values of the phase heights shown in Fig.1 (c = -21...-38 m/year) and a small Fig. 6. Yearly trends of different ionospheric height parameters of the station trend near 50km (-10 m/year of the Juliusruh after elimination of solar and geomagnetic influences. 1 hPa level (1994) from data series between and for a point 50N, IOE) column-mean temperature of the mesosphere (50-80km) of -0.09...-0.22 K/year can be derived. A different analysis by Taubenheim and Entzian (1997) using also phase
JASSR20-11-S
2080
J. Bremer
\
MOLOOEZHNAYA
KX?1
OSNINSK (D2)
KUEHLUNGSSORN @2)
iO.a
ATIANTA. (D2) h
0.a -1o.c 10x 5.C
lsao
198.5
19so
1995
YEAR
Fig. 7. Yearly trends of the zonal wind different stations.
near
95 km altitude at
height data (10 day means for summer months) results in even larger negative trends (-0.47 K/year if again a negative trend of about -10 m/year for the 1 hPa level is assumed). These temperature trends are markedly more pronounced than those of model calculations by Rind er al. (1990), Brasseur et ~1. (1990) and Berger and Dameris (1993) with trends of -0.02...-0.06 K/year near 65 km. The explanation of the observed negative trends in the LF absorption is more complicated as this parameter depends mainly on the electron density and the collision frequency profiles. If there are changes of the atmospheric pressure then both components will be influenced. By means of model calculations (Bremer, 1993) it could however be shown that the influence of pressure changes upon the collision frequency is the dominating factor to explain the variations of LF absorption shown in Figs. 3 and 4. As the collision frequency below about 100 km altitude is directly proportional to the atmospheric pressure (Bain and Harrison, 1972) the negative, trends of LF absorption can be explained by a pressure decrease near and below 85 km altitude. Only the positive absorption trends detected at the highest frequency investigated (243 kHz) during winter months in Fig. 3 cannot be caused by a simple pressure decrease. This frequency is reflected at the lower border of the winter anomaly region where the structure of the atmosphere is markedly more complex than during other seasons due to a highly variable amount of excessive nitric oxide (Taubenheim, 1983).
CHRiSTCHURCH !MFJ
Fig. 8. Yearly trends of the semidiumal 95 km altitude at different stations.
and diurnal
tidal wind components
near
The decrease of different thermospheric reflection heights hmF2, h’F, h’E observed by ionosonde measurements at Juliusruh is in qualitative agreement with the model calculations of Roble and Dickinson (1989), Rishbeth ( 1990) and Rishbeth and Roble ( 1992). Also the fact that no significant trends in foE and foF2 could be found agrees with the theoretical predictions of these authors. The foF1 increase expected by Rishbeth and Roble
Long-term Trends
in the Meso-and Thermosphere
o.“~
-0.2
-0.4
!L
;’
-0.4 ’
Y
0.0
-
DOURBES
(1959 - 1990)
FREIBURG
(1948 - 1976)
0
-0.4
‘I 1
,
I
I
,
,
,
)
,
I
,
ri
2
3
4
5
6
7
8
9
IO
11
12
2081
(1992) at mid-latitudes, however, could only partly be found (only during one month). To check the results of the trends detected in the Juliusruh data also data of other stations have been analyzed. In Fig. 9 some examples of h’F trends at different stations are shown. In all data of the stations Juliusruh, Dourbes, Freiburg and Rome we got negative trends during all months. Full symbols characterize again significant trends. We found, however, also stations without clear trends in h’F (e.g. Poitiers, Kiew) or even with positive trends (Kaliningrad, Slough, Lannion). The reason for these discrepancies is not quite clear. Compared with the variations of the ionosonde parameters in dependence on solar activity the amplitude of the trends is very small. Therefore, a detection of such small trends needs a very stable equipment over a very long time interval and no essential changes of the evaluation algorithms. Discontinuities due to such technical changes can prevent the detection of small trends. Therefore, it is often difficult to use foreign data if information about technical changes are not available. Our own data of Juliusrnh, where we can grantee stable technical conditions during the observation period between 1957 and 1995, qualitatively confirm the theoretical prediction by Rishbeth (1990) that in the thermosphere long-term trends may exist which could be caused by an atmospheric greenhouse effect.
MONTH
The results of the negative pressure trends in the mesosphere (Figs. l-4) are in qualitative agreement with lidar measurements of the vertical distribution of atmospheric sodium near 93 km altitude. Observations of Clemesha et al., (1992) in Brazil show a clear negative trend of the Na layer (-49 m/year) between 1972 and 1987. Analyses of rocket and radiosonde soundings between 1964 and 1990 at five different stations by Kokin and Lysenko (1994) gave however markedly stronger negative temperature trends in the mesosphere (-0.3... -1.0 K/year). The maximum negative trends were. detected at the upper boundary of reliable temperature data at 75 km. Using lidar observations in the south of France between 1979 and 1991 Keckhut et al. (1995) derived also clear negative temperature trends in the mesosphere (-0.3... -0.4 K/year between 60 and 75 km). At the upper part of their observations near 80 km the trends become positive. However, at this height the confidence of these trend results is poor due to different measuring problems (noise, sky background, initialization). From ground based OH* observations at Wuppertal, Germany (5 1ON, 7”W) Offermann and Graef (1992) derived near 86 km also positive trends (0.9 K/year). These results are, however, in contrast to Russian OH* observations (Semenov, 1996) between 1957 and 1994 with clear negative trends (-0.7 K/year). Other trend observations of this height region are the number of observed noctilucent clouds (NLC) published by Gadsen (1990). He found positive trends which could be interpreted by an atmospheric cooling of 7 K during the last 20 to 30 years or by an increase of the water vapour content at the NLC region near 83 km. Summarizing there are a lot of theoretical and experimental data of long-term trends. However, the experimental trends are often essentially more pronounced (rocket and lidar results, phase height column temperature trends) than the model results of the greenhouse effect. In the region near 80 .., 86 km there are also some contrary observations which require further investigations.
Fig. 9. Seasonal variation of monthly regression coefficient c for h’F data of different ionosonde stations after elimination of solar and geomagnetic influences.
Whereas the negative trends of the ionospheric reflection heights in the meso- and thermosphere are interpreted here by a general cooling of the strato-, meso- and thermosphere the explaination of the observed trends in the
2082
J. Bremer
wind field near 95 km altitude (Figs. 7 and 8) is more difficult. After 3D-model calculations of Berger and Dameris (1993) and Berger (1994) an increase of CO, induces a temperature decrease in the strato- and mesosphere but nearly no changes of the mean circulation and only small changes of the tidal components. Also with the tidal model of Forbes (1982a, b) the negative trends of the tidal amplitudes cannot be explained (Portnyagin er al., 1993). Changes of the atmospheric ozone content may play an important role. Ross and Waterscheid (1991) deduced by model calculations that the stratospheric ozone decrease causes a reduction of the tidal amplitudes due to reduced thermal forcing and may also influence the mean circulation. After Ebel (1996) also long-term variations of tropospherically induced gravity waves could play an essential role. CONCLUSIONS Basing on long-term can be made.
observations
of different ionospheric
and atmospheric
parameters
the following
conclusions
The detection of long-term trends in ionospheric plasma parameters is only possible if - homogeneous data series are available without discontinuities caused by changes of the technical equipment or the evaluation algorithms and - the marked solar/geomagnetic induced variations have carefully been removed. Trends in LF phase heights, LF absorption and reflection heights derived from ionosonde observations can be explained by an atmospheric pressure decrease in the meso- and thermosphere. The quantitative discrepancies between experimental results and model calculations of the greenhouse effect, some experimental trends are up to one order of magnitude greater, require further theoretical as well as experimental investigations to decide if the observed trends are mainly caused by an atmospheric greenhouse effect in the upper atmosphere or if other processes are more important. Trends in the wind field at the mesosphereflower atmospheric greenhouse effect. The ozone depletion waves may play an important role.
thermosphere region cannot be explained by a simple and possible trends in the tropospherically induced gravity
ACKNOWLEDGMENTS This work was supported by the Bundesministerium under contract 07VKOl/l.
fur Bildung, Wissenschaft,
Forschung
und Technologie,
Bonn,
REFERENCES Bain, W. C., and M. D. Harrison, Model ionosphere for D-region at summer noon during sunspot maximum, Proc ZEE, 119, 790-796 (1972). Berger, U., Numerische Simulation klimatologischer Prozesse und thermischer Gezeiten in der mittleren Atmosphare, fvlitteilungen aus dem Znstitut fiir Geophysik und Meteorologic der Universikit zu K&z, edited by A. Ebel et al., Koln (1994). Berger, U., and M. Dameris, Cooling of the upper atmosphere due to CO, increases: a model study, Ann. Geophysicae, 11, 809-S 19 (1993). Brasseur, G., M. H. Hitchmann, S. Walters, M. Dymek, E. Falise, and M. Pirre, An interactive chemical dynamical radiative two-dimentional model of the middle atmosphere, J. geophys. Rex 95, 5639-5655 (1990). Bremer, J., Ionospheric trends in mid-latitudes as a possible indicator of the atmospheric greenhouse effect, J. atmos. terr. Phys. 54, 1505-1511 (1992). Bremer, J., Sakulare Variationen im Plasma der ionosphfrischen D-Region, Kleinheubacher Berichte 36, 379-388 (1993). Bremer, J., R. Schminder, K. M. Greisiger, P. Hoffmann, D. Ktirschner, and W. Singer, Solar cycle dependence
Long-term Trends in the Meso- and Thermosphere
2083
and long-term trends in the wind field of the mesosphere/lower thermosphere, J. atmos. terr. Phys., accepted (1997). Clemesha, B, R., D. M. Simonich, and P. P. Batista, A long-term trend in the height of the atmospheric sodium layer: Possible evidence for global change, Geophys. Res. Len. 19, 457-460 (1992). Ebel, A., private communication (1996). Forbes, J. M., Atmospheric Tides 1. Model description and results for the solar diurnal component, J. geophys. Res. 87, 5222-5240 (1982a). Forbes, J. M., Atmospheric Tides 2. The solar and lunar semidiurnal components, J. geophys. Res. 87, 5241-5252 (1982b). Gadsen, M., A secular change in noctilucent cloud occurrence, J. atmos. terr. Phys., 52, 247-251 (1990). Hegerl, G. C., H. von Starch, K. Hasselmann, B. D. Santer, U. Cubasch, and P. D. Jones, Detecting greenhousegas-induced climate change with an optimal fingerprint method, J. Climate 9, 2281-2306 (1996). Keckhut, P., A. Hauchecorne, and M. L. Chanin, Midlatitude long-term variability of the middle atmosphere: Trends and cyclic and episodic changes, J. geophys. Res. 100, 18887-18897 (1995). Kokin, G. A., and E. V. Lysenko, On temperature trends of the atmosphere from rocket and radiosonde data, J. utmos. terr. Phys. 56, 1035-1040 (1994). Kiirschner, D., Methodical aspects and new tests for determining the reflection height of sky waves in the long wave range at oblique incidence using amplitude-modulated long-wave transmitters, Gerlands Beitr. Ceophys. 90, 285-294 (1981). Lauter, E. A., J. Taubenheim, and G. von Cossart, Monitoring of middle atmosphere processes by means of ground-based low-frequency radio wave sounding of the D-region, J. atmos. terr. Phys. 46, 775-780 (1984). Lauter, E. A., J. Bremer, G. Entzian, and K. Sprenger, Method A3(b): Oblique incidence field strength observations on frequencies in and below the MF broadcasting band, Report UAG-57, Edited by K. Rawer, WDC-A for Solar-Terrestrial Physics, Boulder/Colorado, USA, pp. 147- 163 (1976). Offermann, D., and H. H. Graef, Messungen der OH*-Temperatur, Promet 22, 125-128 (1992). Portnyagin, Yu. I., J. M. Forbes, G. J. Fraser, R. A. Vincent, S. K. Avery, I. A. Lysenko, and N. A. Makarov, Dynamics of the Antarctic and Arctic mesosphere and lower thermosphere regions- 2. The semidiurnal tide, J. atmos. terr. Phys. 5.5, 843-855 (1993). Rind, D., R. Suozzo, N. K. Balachandran, and M. J. Prather, Climatic change and middle atmosphere. Part I: The doubled CO, climate, J. atmos. Sciences 47, 475-494 (1990). Rishbeth, H., A greenhouse effect in the ionosphere ?, Planet Space Sci. 38, 945-948 (1990). Rishbeth, H., and R. G. Roble, Cooling of the upper atmosphere by enhanced greenhouse gases - Modelling of thermospheric and ionospheric effects, Planet. Space Sci. 40, loll- 1026 (1992). Roble, R. G., and R. E. Dickinson, How will changes in carbon dioxide and methan modify the mean structul-e of the mesosphere and thermosphere, Geophys. Res. Lett. 16, 1441-1444 (1989). Ross, M. N., and R. L. Walterscheid, Changes in the solar forced tides caused by stratospheric ozone depletions, Geophys. Rex Lett. 18, 420-423 (1991). Semenov, A. I., The thermal regime of the lower thermosphere by the emission measurements during the recent decades, Geomagnetism and aeronomy (in Russian) 36, 90-97 (1996). Shimazaki, T., World wide daily variations in the height of the maximum electron density in the ionospheric F2 layer, J. Radio Res. Labs., Japan 2, 85-97 (1955). Taubenheim, J., Statistische Auswertung geophysikalischer und meteorologischer Daten, Akad. Verlagsgesellschaft Geest & Portig K.-G., Leipzig (1969). Taubenheim, J., Meteorological control of the D region, Space Sci. Rev. 34, 397-411 (1983). Taubenheim, J., G. von Cossart, and G. Entzian, Evidence of CO,-induced progressive cooling of the middle atmosphere derived from radio observations, Adv. Space Res. 10, (10) 17 1-( 10) 174 (1990). Taubenheim, J., and G. Entzian, Long-term decrease of mesospheric temperatur, 1963-1995, inferred from radiowave reflection heights, Adv. Space Res., this issue (1997). Taubenheim, J., K. Berendorf, W. Kri.iger, and G. Entzian, Height dependence of long-term temperature trends in the middle atmosphere, Paper presented at the EGS X1X General Assembly, Session ST3, Grenoble (1994).
LONG-TERM TRENDS IN THE MESO- AND THERMOSPHERE J. Bremer Institutfiir AtmosphZirenphysik, SchloJstr. 4-6, D-18225 Kiihlungsborn, Germany
ABSTRACT Ionospheric plasma parameters are markedly controlled by the variability of the solar radiation and high energetic particle fluxes. Therefore it is necessary to remove these influences by a multiple regression analysis before longterm trends can be detected. Using this method trends have been found in LF phase height and LF absorption data. These trends can be explained by a decrease of the atmospheric pressure near the LF reflection heights (80...85 km) caused by a temperature decrease below this altitude range Qualitatively the same effect was predicted by Rishbeth (1990) on the basis of model calculations by Roble and Dickinson (1989) for reflection heights deduced from ionosonde measurements of the ionospheric E- and F-layers. Analyses of selected ionosonde standard parameters confirm the predictions, but there are also observations which do not show the expected negative trends. Also in the wind field of the upper mesosphere/lower thermosphere long-term trends could be detected. Especially the amplitudes of the tidal components have markedly been reduced during the last 30 years in all observations available on both hemispheres. 01997 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION Temperature observations near the surface of the Earth have been carefully analyzed during the last twenty years to get significant information of an expected warming due to an enhanced atmospheric greenhouse effect. Now it seems to be that the observed latest warming trends can no longer be interpreted as atmospheric variability but are related to a greenhouse effect (Hegerl et al., 1996). In the strato- and mesosphere an increasing content of greenhouse gases should reduce the atmospheric temperature due to an enhanced infrared cooling as demonstrated by different model calculations of Rind et LEI.(1990) Brasseur et al. (1990), Berger and Dameris (1993). The same effect was also predicted for the thermosphere (Roble and Dickinson, 1989). In this paper different ionospheric plasma parameters have been analyzed to detect possible long-term trends and to test if such trends could be an indication of an atmospheric greenhouse effect in the mesosphere and thermosphere. EXPERIMENTAL
RESULTS
To detect long-term trends in ionospheric plasma data it is often necessary to remove the influence of the ionizing solar radiation and high energetic particle precipitation. Therefore, we calculated for the experimental data X (monthly mean values at constant solar zenith angle or at each full hour) regressionm equations of the following 2075
2076
J. Bremer
form x,
=A+B*FlO.7
+C-Ap
(1)
Here F10.7 and Ap are monthly mean values of the solar radio flux at 10.7 cm and of the planetary geomagnetic activity index. To describe the mean variation of solar radiation it is also possible to use the relative solar sunspot number R instead of F10.7. Then we estimated the deviations of the experimental data from the corresponding theoretical values calculated after Eq. 1
Ax = Using these deviations
AX we estimated
xexp- x,
linear trends AX = a + c
-year
(3)
Such analyses were made for monthly mean values but also for seasonal or yearly mean values of AX data. Some details of the experimental data used in this paper have been summarized in Table 1.
Table I. Stations and measuring their reflection points). Station/measuring
paths used in trend analyses (The coordinates
path
Method
of the measuring
Location
paths are given for
Period
LF phase height
Sl”N,
07”E
1964 - 1995
Kalundborg-Ktihlungsbom (243 kHz) Berlin - Kuhlungsborn (177 kHz) Hoyer - Ktihlungsborn (I 28 kHz)
LF absorption LF absorption LF absorption
55”N, 53”N, 55”N,
11”E 10”E 10”E
1948 - 1995 1952 - 1995 1968 - 1991
Juliusruh Dourbes Freiburg Rome
Ionosonde Ionosonde Ionosonde Ionosonde
55”N, 50”N, 48”N, 42”N,
13”E 05”E 08”E 12”E
1957 1959 1948 1957
Collm/ Kuhlungsborn Ktihlungsborn Heiss Island Molodezhnaya Obninsk Atlanta Christchurch Saskatoon
Dl -Wind
51”N, 13”E 54”N, 12”E 54”N, 12’E 8l”N, 58”E 68”S, 45”E 55”N, 38”E 34”N, 84”W 44”S, 173”E 52”N, 107’W
Allouis-Ktihlungsborn
(162 161~)
DZWind D2-Wind D2-Wind D2-Wind DZWind MF-Wind MF-Wind
-
1995 1990 1976 1995
1964 - 1994 1976 1965 1967 1964 1975 1978 1979
-
1994 1985 1986 1981 1986 1986 1988
LF Phase Heights From daily indirect phase height measurements in the LF-range at Kuhlungsbom (for details of the measurements see Lauter et al., 1984) the ionospheric reflection height at constant solar zenith angle (x=78.5”) has been derived from observations at 162 kHz between 1964 and 1995. As shown by Taubenheim et ul.(1990) this phase height corresponds to a nearly isobaric level if the variability of the solar radiation has been removed and there is no long-term trend of nitric oxide near the mesopause level. In Fig. 1 the seasonal variation of the monthly trend parameters c after Eq. 3 are presented. As to be seen during all months negative regression coefficients have been
Long-term Trends in the Meso- and Thermosphere
2011
162 kHz
JFMAMJJASONO MONTH
Fig. 1. Seasonal variation of monthly regression coefficient c for LF phase heights after elimination of solar and geomagnetic influences. derived with significance levels of more than 99% (using Fisher’s F-parameter after Taubenheim, 1969).In Fig. 2 the LF phase heights after elimination of the Fig. 2. Seasonal and yearly trends of LF phase heights solar/geomagnetic influence are shown for different after elimination of solar and geomagnetic influences. seasons and the whole year. The individual values are presented by crosses connected with thin full lines and the linear trends calculated for the total observation period by thick full lines. As to be expected after Fig. 1 all seasonal and yearly trends in Fig. 2 are negative (significance levels in all cases >99%) with a lowering of the mean reflection height by about 0.7 . .. 1.0 km during the observation period, thus indicating a marked decrease of the atmospheric pressure level near 80 km altitude. LF Absorntion Absorption measurements in the LF range (for details of the method see Lauter et al., 1976) are carried out at Ktihlungsbom since 1948 up to now. Using again data at constant solar zenith angle (x=78.5”) a similar trend analysis has been carried out as before with the phase height data. In Fig. 3 the seasonal variation of the monthly trend parameters is shown for three different absorption measuring paths. The full symbols mark significant trends (>95%), whereas the significance level for the other months (open symbols) is less than 95%. During nearly all months we derived negative trends, only at the 243 kHz path during winter months positive trends were detected. In Fig. 4 yearly trends of the LF absorption after elimination of the solar/geomagnetic induced part are presented. The negative trends on the two paths at 177 kHz and 128 kHz are significant (significance level >99%), the small negative trend at 243 kHz, however, not. This behaviour is caused by the fact that the seasonal trends are different, during winter months positive and during summer month negative as to be seen in Fig. 3. Combining only summer months we found again a significant negative trend. As will be discussed in more detail below the observed negative trends of the LF absorption 4 can be explained by a pressure decrease at the reflection height of about 85 km. Ionosonde
data in Figs. 3 and
Data
Basing on the model calculations sphere (-10 K) and thermosphere
of Roble and Dickinson (1989) who deduced a marked cooling of the meso(-50 K) due to an assumed doubling of the greenhouse gases CO? and CH,,
J. Bremer
2078
::k=----J 0.0
-Z
i
a
o
6.1
-02
Rishbeth (1990) and Rishbeth and Roble (1992) predicted a lowering of the F2-layer peak height by 1.5 ... 20 km, a smaller lowering of the E-layer peak height (-2.5 km), no significant changes of the peak electron densities in the F2- and E-layers and an increase of the peak electron density of the Fl-layer. Using ionosonde observations between 1957 and 1995 at Juliusruh trend analyses have been carried out with different standard parameters. In Fig. 5 the seasonal variation of the derived trend parameters are presented for M30OOF2, hmF2, h’F, h’E, foF2, foF1 and foE. hmF2 was estimated from M3000F2 after the well known formula derived by Shimazaki (1955)
hmF2 =
JFMAMJJASOND
MONTH
Fig. 3. Seasonal variation of monthly regression coefficients c for LF absorption at 3 different measuring paths after elimination of solar and geomagnetic influences.
1490
M3OOOF2 + AM
- 176
(4)
with the correction term AM=O. As shown by Bremer (1992) the trends derived from hmF2 values do not markedly depend on the AM value used. The full symbols in Fig. 5 mark significant trends (>95%). The different height values show negative trends during nearly all months. Whereas four of the negidtive hmF2 trends are significant with more than 95% (two of them >99%), all monthly trends of h’F and h’E values are negative with a significance level of more than 95%, most of them even more than 99%. In contrast to the height values nearly all peak electron densities of the E-, Fl- and F2-layers characterized by the critical frequencies of these layers do not show significant trends. Only the foF1 values during May have a significant positive trend (>95%). In Fig. 6 yearly trends of the thermospheric reflection height par&meters after elimination of the solar/geomagnetic influence are shown. All trends are significant with more than 99%. hmF2, h’F and h’E decrease by about 7 . .. 10 km during the observation period between 1957 and 1995. The corresponding yearly trends of the critical frequencies not shown here are very small and all not significant as to be expected after the results of Fig. 5. Meso-/Lower
,.,
1
1950
7995
1960
1965
1970
1975
1990
199s
IS90
!999
YEW
Fig. 4. Yearly trends of LF absorption at 3 different after elimination of solar and geomagnetic influences.
measuring
paths
Thermosphere
Wind
By means of ionospheric drift measurements in the LF range (Dl method) at Kiihlungsbom and Collm between 1964 and 1995) and radar meteor wind measurements (D?
2079
Long-term Trends in the Meso- and Thermosphere
7
a
0.002
% 0
0.000
i
!
,
i
t-
._.,.
/-
_. ,1 /
F % B 3 0 L=
1
0.002 -
B r 0
0.000 0.002 -
!
__
.,
r %
0.000 -
P 2 0
-0002 -0.004
’ J
F
M
A
M
,
,
J
JASOND
,
,
,
,
/ ’
MONTH
Fig. 5. Seasonal variation of monthly regression coefficients c for different ionosonde data of the station Juliusruh after elimination of solar and geomagnetic influences.
hmF2
method) at Kiihlungsbom between 1976 and 1994 the wind field of the meso-/lower thermosphere region has continuously been observed. Unfortunately the exact height of these observations is not known. Since 1982 this height is measured for the ionospheric drift observations at Collm (Kiirschner, 1981). However, to avoid inconsistencies in the time-series these results are not used here. We assume a mean height of 95 km. The mean zonal and meridional winds as well as the amplitudes of the semidiurnal and diurnal tidal components have been analyzed for long-term variations. As discussed in detail by Bremer et al., ( 1997) the solar activity influence on the wind field is not very strong. Therefore, it is not necessary to remove the solar/geomagnetic influence before linear trends can be analyzed. In Fig. 7 we show the results of yearly mean values of the zonal wind from our measurements at Ktihlungsbom and Collm together with other observations (most radar meteor measurements, but also results of MF radars) at different stations on both hemispheres. We used here yearly mean values as these values should be nearly no influenced by possible long-term trends of the mean reflection height (Bremer er al., 1997). As can be seen from the results shown in Fig. 7 six of seven observations show negative trends, four of them are significant with at least 95%. Also in the meridional wind trends can be detected (not shown here), six of seven observations are positive but only one of them with a significance level greater than 99%. The most convincing trends have been detected, however, in the amplitudes of the tidal waves presented in Fig. 8. All trends derived are negative and most of them with a significance level of more than 95% or even 99%. Only the trends of the very short data series at Christchurch are not significant. The excellent agreement of all trends indicates that the decrease of the tidal wind components seems to be a global phenomenon. DISCUSSION
The trends detected in the LF phase heights are a strong indication that the atmospheric pressure near 80 km decreased during the last 30 years more or less continuously. Using the monthly trend values of the phase heights shown in Fig.1 (c = -21...-38 m/year) and a small Fig. 6. Yearly trends of different ionospheric height parameters of the station trend near 50km (-10 m/year of the Juliusruh after elimination of solar and geomagnetic influences. 1 hPa level (1994) from data series between and for a point 50N, IOE) column-mean temperature of the mesosphere (50-80km) of -0.09...-0.22 K/year can be derived. A different analysis by Taubenheim and Entzian (1997) using also phase
JASSR20-11-S
2080
J. Bremer
\
MOLOOEZHNAYA
KX?1
OSNINSK (D2)
KUEHLUNGSSORN @2)
iO.a
ATIANTA. (D2) h
0.a -1o.c 10x 5.C
lsao
198.5
19so
1995
YEAR
Fig. 7. Yearly trends of the zonal wind different stations.
near
95 km altitude at
height data (10 day means for summer months) results in even larger negative trends (-0.47 K/year if again a negative trend of about -10 m/year for the 1 hPa level is assumed). These temperature trends are markedly more pronounced than those of model calculations by Rind er al. (1990), Brasseur et ~1. (1990) and Berger and Dameris (1993) with trends of -0.02...-0.06 K/year near 65 km. The explanation of the observed negative trends in the LF absorption is more complicated as this parameter depends mainly on the electron density and the collision frequency profiles. If there are changes of the atmospheric pressure then both components will be influenced. By means of model calculations (Bremer, 1993) it could however be shown that the influence of pressure changes upon the collision frequency is the dominating factor to explain the variations of LF absorption shown in Figs. 3 and 4. As the collision frequency below about 100 km altitude is directly proportional to the atmospheric pressure (Bain and Harrison, 1972) the negative, trends of LF absorption can be explained by a pressure decrease near and below 85 km altitude. Only the positive absorption trends detected at the highest frequency investigated (243 kHz) during winter months in Fig. 3 cannot be caused by a simple pressure decrease. This frequency is reflected at the lower border of the winter anomaly region where the structure of the atmosphere is markedly more complex than during other seasons due to a highly variable amount of excessive nitric oxide (Taubenheim, 1983).
CHRiSTCHURCH !MFJ
Fig. 8. Yearly trends of the semidiumal 95 km altitude at different stations.
and diurnal
tidal wind components
near
The decrease of different thermospheric reflection heights hmF2, h’F, h’E observed by ionosonde measurements at Juliusruh is in qualitative agreement with the model calculations of Roble and Dickinson (1989), Rishbeth ( 1990) and Rishbeth and Roble ( 1992). Also the fact that no significant trends in foE and foF2 could be found agrees with the theoretical predictions of these authors. The foF1 increase expected by Rishbeth and Roble
Long-term Trends
in the Meso-and Thermosphere
o.“~
-0.2
-0.4
!L
;’
-0.4 ’
Y
0.0
-
DOURBES
(1959 - 1990)
FREIBURG
(1948 - 1976)
0
-0.4
‘I 1
,
I
I
,
,
,
)
,
I
,
ri
2
3
4
5
6
7
8
9
IO
11
12
2081
(1992) at mid-latitudes, however, could only partly be found (only during one month). To check the results of the trends detected in the Juliusruh data also data of other stations have been analyzed. In Fig. 9 some examples of h’F trends at different stations are shown. In all data of the stations Juliusruh, Dourbes, Freiburg and Rome we got negative trends during all months. Full symbols characterize again significant trends. We found, however, also stations without clear trends in h’F (e.g. Poitiers, Kiew) or even with positive trends (Kaliningrad, Slough, Lannion). The reason for these discrepancies is not quite clear. Compared with the variations of the ionosonde parameters in dependence on solar activity the amplitude of the trends is very small. Therefore, a detection of such small trends needs a very stable equipment over a very long time interval and no essential changes of the evaluation algorithms. Discontinuities due to such technical changes can prevent the detection of small trends. Therefore, it is often difficult to use foreign data if information about technical changes are not available. Our own data of Juliusrnh, where we can grantee stable technical conditions during the observation period between 1957 and 1995, qualitatively confirm the theoretical prediction by Rishbeth (1990) that in the thermosphere long-term trends may exist which could be caused by an atmospheric greenhouse effect.
MONTH
The results of the negative pressure trends in the mesosphere (Figs. l-4) are in qualitative agreement with lidar measurements of the vertical distribution of atmospheric sodium near 93 km altitude. Observations of Clemesha et al., (1992) in Brazil show a clear negative trend of the Na layer (-49 m/year) between 1972 and 1987. Analyses of rocket and radiosonde soundings between 1964 and 1990 at five different stations by Kokin and Lysenko (1994) gave however markedly stronger negative temperature trends in the mesosphere (-0.3... -1.0 K/year). The maximum negative trends were. detected at the upper boundary of reliable temperature data at 75 km. Using lidar observations in the south of France between 1979 and 1991 Keckhut et al. (1995) derived also clear negative temperature trends in the mesosphere (-0.3... -0.4 K/year between 60 and 75 km). At the upper part of their observations near 80 km the trends become positive. However, at this height the confidence of these trend results is poor due to different measuring problems (noise, sky background, initialization). From ground based OH* observations at Wuppertal, Germany (5 1ON, 7”W) Offermann and Graef (1992) derived near 86 km also positive trends (0.9 K/year). These results are, however, in contrast to Russian OH* observations (Semenov, 1996) between 1957 and 1994 with clear negative trends (-0.7 K/year). Other trend observations of this height region are the number of observed noctilucent clouds (NLC) published by Gadsen (1990). He found positive trends which could be interpreted by an atmospheric cooling of 7 K during the last 20 to 30 years or by an increase of the water vapour content at the NLC region near 83 km. Summarizing there are a lot of theoretical and experimental data of long-term trends. However, the experimental trends are often essentially more pronounced (rocket and lidar results, phase height column temperature trends) than the model results of the greenhouse effect. In the region near 80 .., 86 km there are also some contrary observations which require further investigations.
Fig. 9. Seasonal variation of monthly regression coefficient c for h’F data of different ionosonde stations after elimination of solar and geomagnetic influences.
Whereas the negative trends of the ionospheric reflection heights in the meso- and thermosphere are interpreted here by a general cooling of the strato-, meso- and thermosphere the explaination of the observed trends in the
2082
J. Bremer
wind field near 95 km altitude (Figs. 7 and 8) is more difficult. After 3D-model calculations of Berger and Dameris (1993) and Berger (1994) an increase of CO, induces a temperature decrease in the strato- and mesosphere but nearly no changes of the mean circulation and only small changes of the tidal components. Also with the tidal model of Forbes (1982a, b) the negative trends of the tidal amplitudes cannot be explained (Portnyagin er al., 1993). Changes of the atmospheric ozone content may play an important role. Ross and Waterscheid (1991) deduced by model calculations that the stratospheric ozone decrease causes a reduction of the tidal amplitudes due to reduced thermal forcing and may also influence the mean circulation. After Ebel (1996) also long-term variations of tropospherically induced gravity waves could play an essential role. CONCLUSIONS Basing on long-term can be made.
observations
of different ionospheric
and atmospheric
parameters
the following
conclusions
The detection of long-term trends in ionospheric plasma parameters is only possible if - homogeneous data series are available without discontinuities caused by changes of the technical equipment or the evaluation algorithms and - the marked solar/geomagnetic induced variations have carefully been removed. Trends in LF phase heights, LF absorption and reflection heights derived from ionosonde observations can be explained by an atmospheric pressure decrease in the meso- and thermosphere. The quantitative discrepancies between experimental results and model calculations of the greenhouse effect, some experimental trends are up to one order of magnitude greater, require further theoretical as well as experimental investigations to decide if the observed trends are mainly caused by an atmospheric greenhouse effect in the upper atmosphere or if other processes are more important. Trends in the wind field at the mesosphereflower atmospheric greenhouse effect. The ozone depletion waves may play an important role.
thermosphere region cannot be explained by a simple and possible trends in the tropospherically induced gravity
ACKNOWLEDGMENTS This work was supported by the Bundesministerium under contract 07VKOl/l.
fur Bildung, Wissenschaft,
Forschung
und Technologie,
Bonn,
REFERENCES Bain, W. C., and M. D. Harrison, Model ionosphere for D-region at summer noon during sunspot maximum, Proc ZEE, 119, 790-796 (1972). Berger, U., Numerische Simulation klimatologischer Prozesse und thermischer Gezeiten in der mittleren Atmosphare, fvlitteilungen aus dem Znstitut fiir Geophysik und Meteorologic der Universikit zu K&z, edited by A. Ebel et al., Koln (1994). Berger, U., and M. Dameris, Cooling of the upper atmosphere due to CO, increases: a model study, Ann. Geophysicae, 11, 809-S 19 (1993). Brasseur, G., M. H. Hitchmann, S. Walters, M. Dymek, E. Falise, and M. Pirre, An interactive chemical dynamical radiative two-dimentional model of the middle atmosphere, J. geophys. Rex 95, 5639-5655 (1990). Bremer, J., Ionospheric trends in mid-latitudes as a possible indicator of the atmospheric greenhouse effect, J. atmos. terr. Phys. 54, 1505-1511 (1992). Bremer, J., Sakulare Variationen im Plasma der ionosphfrischen D-Region, Kleinheubacher Berichte 36, 379-388 (1993). Bremer, J., R. Schminder, K. M. Greisiger, P. Hoffmann, D. Ktirschner, and W. Singer, Solar cycle dependence
Long-term Trends in the Meso- and Thermosphere
2083
and long-term trends in the wind field of the mesosphere/lower thermosphere, J. atmos. terr. Phys., accepted (1997). Clemesha, B, R., D. M. Simonich, and P. P. Batista, A long-term trend in the height of the atmospheric sodium layer: Possible evidence for global change, Geophys. Res. Len. 19, 457-460 (1992). Ebel, A., private communication (1996). Forbes, J. M., Atmospheric Tides 1. Model description and results for the solar diurnal component, J. geophys. Res. 87, 5222-5240 (1982a). Forbes, J. M., Atmospheric Tides 2. The solar and lunar semidiurnal components, J. geophys. Res. 87, 5241-5252 (1982b). Gadsen, M., A secular change in noctilucent cloud occurrence, J. atmos. terr. Phys., 52, 247-251 (1990). Hegerl, G. C., H. von Starch, K. Hasselmann, B. D. Santer, U. Cubasch, and P. D. Jones, Detecting greenhousegas-induced climate change with an optimal fingerprint method, J. Climate 9, 2281-2306 (1996). Keckhut, P., A. Hauchecorne, and M. L. Chanin, Midlatitude long-term variability of the middle atmosphere: Trends and cyclic and episodic changes, J. geophys. Res. 100, 18887-18897 (1995). Kokin, G. A., and E. V. Lysenko, On temperature trends of the atmosphere from rocket and radiosonde data, J. utmos. terr. Phys. 56, 1035-1040 (1994). Kiirschner, D., Methodical aspects and new tests for determining the reflection height of sky waves in the long wave range at oblique incidence using amplitude-modulated long-wave transmitters, Gerlands Beitr. Ceophys. 90, 285-294 (1981). Lauter, E. A., J. Taubenheim, and G. von Cossart, Monitoring of middle atmosphere processes by means of ground-based low-frequency radio wave sounding of the D-region, J. atmos. terr. Phys. 46, 775-780 (1984). Lauter, E. A., J. Bremer, G. Entzian, and K. Sprenger, Method A3(b): Oblique incidence field strength observations on frequencies in and below the MF broadcasting band, Report UAG-57, Edited by K. Rawer, WDC-A for Solar-Terrestrial Physics, Boulder/Colorado, USA, pp. 147- 163 (1976). Offermann, D., and H. H. Graef, Messungen der OH*-Temperatur, Promet 22, 125-128 (1992). Portnyagin, Yu. I., J. M. Forbes, G. J. Fraser, R. A. Vincent, S. K. Avery, I. A. Lysenko, and N. A. Makarov, Dynamics of the Antarctic and Arctic mesosphere and lower thermosphere regions- 2. The semidiurnal tide, J. atmos. terr. Phys. 5.5, 843-855 (1993). Rind, D., R. Suozzo, N. K. Balachandran, and M. J. Prather, Climatic change and middle atmosphere. Part I: The doubled CO, climate, J. atmos. Sciences 47, 475-494 (1990). Rishbeth, H., A greenhouse effect in the ionosphere ?, Planet Space Sci. 38, 945-948 (1990). Rishbeth, H., and R. G. Roble, Cooling of the upper atmosphere by enhanced greenhouse gases - Modelling of thermospheric and ionospheric effects, Planet. Space Sci. 40, loll- 1026 (1992). Roble, R. G., and R. E. Dickinson, How will changes in carbon dioxide and methan modify the mean structul-e of the mesosphere and thermosphere, Geophys. Res. Lett. 16, 1441-1444 (1989). Ross, M. N., and R. L. Walterscheid, Changes in the solar forced tides caused by stratospheric ozone depletions, Geophys. Rex Lett. 18, 420-423 (1991). Semenov, A. I., The thermal regime of the lower thermosphere by the emission measurements during the recent decades, Geomagnetism and aeronomy (in Russian) 36, 90-97 (1996). Shimazaki, T., World wide daily variations in the height of the maximum electron density in the ionospheric F2 layer, J. Radio Res. Labs., Japan 2, 85-97 (1955). Taubenheim, J., Statistische Auswertung geophysikalischer und meteorologischer Daten, Akad. Verlagsgesellschaft Geest & Portig K.-G., Leipzig (1969). Taubenheim, J., Meteorological control of the D region, Space Sci. Rev. 34, 397-411 (1983). Taubenheim, J., G. von Cossart, and G. Entzian, Evidence of CO,-induced progressive cooling of the middle atmosphere derived from radio observations, Adv. Space Res. 10, (10) 17 1-( 10) 174 (1990). Taubenheim, J., and G. Entzian, Long-term decrease of mesospheric temperatur, 1963-1995, inferred from radiowave reflection heights, Adv. Space Res., this issue (1997). Taubenheim, J., K. Berendorf, W. Kri.iger, and G. Entzian, Height dependence of long-term temperature trends in the middle atmosphere, Paper presented at the EGS X1X General Assembly, Session ST3, Grenoble (1994).