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An explanation of discrepancy in mesospheric temperature trends derived from ground-based LF phase-height observations
Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1727 – 1730
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An explanation of discrepancy in mesospheric temperature trends derived from ground-based LF phase-height observations J. La,stovi,cka∗ Institute of Atmospheric Physics, Academy of Science of the Czech Republic, Bo cn II, 14131 Prague, Czech Republic Received 10 December 2002; received in revised form 18 August 2003; accepted 30 March 2004
Abstract Bremer and Berger (J. Atmos. Solar Terr. Phys. 64 (2002) 805) applied a correction for trends in the NO concentration and e7 in the interpretation of trends in the low frequency (LF) phase height measurements and obtained results less consistent with model simulations as well as the observed trends in mesospheric temperatures. The correction is shown to be too large most probably due to the application of inappropriate trends in e7 of Chakrabarty (Adv. Space Res. 20 (1997) 2117), which yield a trend in electron density opposite to that which is observed. The discrepancy between the observational data and model-simulated trends of Bremer and Berger (J. Atoms. Solar. Terr. Phys. 64 (2002) 805) in the LF phase heights can be largely removed. Even more important, the trends in mesospheric temperatures inferred by Bremer and Berger (J. Atmos. Solar Terr. Phys. 64 (2002) 805) from trends in the LF phase heights without the inappropriate correction agree well with the results of analysis of a global set of results on trends in the mesospheric temperatures by Beig et al. (Rev. Geophys. 41 (2003) 1015). c 2004 Elsevier Ltd. All rights reserved. Keywords: Mesosphere; Long-term trends; LF phase heights; Temperature
1. Introduction Estimating the long-term trends of mesospheric temperature, which might be of the anthropogenic origin, is a topic of great current interest. One method of such estimating is to use the long-term continuous indirect phase re>ection height measurements in the low frequency (LF) range. Recently Bremer and Berger (2002) published the estimate of the mesospheric temperature trend based on the LF phase height measurements for an extended period from 1959 until 2000. They estimated the trend in the mean temperature over the height interval of 48–82 km from the trend of lowering of the LF phase re>ection height, i.e. of a height of constant pressure level. The lowering of the constant pressure level was estimated from the observed lowering of the LF phase re>ection height under assumption of no trend in the NO ∗
Fax: +42-2727-63745. E-mail address: [email protected] (J. La,stovi,cka).
concentration and in the e7ective coeCcient of recombination e7 , and then in another way taking into account the trends in the NO concentration from Beig (2000) and e7 from Chakrabarty (1997). The comparison with the model calculations of Bremer and Berger (2002), which take into account changes in the greenhouse gas and ozone concentration, revealed a surprising result. The agreement of the former estimate (without NO and e7 trends) with model was excellent, whereas the latter estimate (with NO and e7 trends) provided substantially larger cooling and decrease of the LF phase height than the model. Even more important, the comparison of temperature trends inferred from the LF phase re>ection height data with observational trends in the lower and middle mesosphere at middle latitudes reviewed by Beig et al. (2003) reveals evidently better agreement when the trends in NO concentration and e7 are not considered. Since a relatively consistent pattern of the temperature trends in the mesosphere is emerging (except the wintertime mesopause region), as shown by Beig et al. (2003), the
c 2004 Elsevier Ltd. All rights reserved. 1364-6826/$ - see front matter doi:10.1016/j.jastp.2004.03.012
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J. La stovi cka / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1727 – 1730
new values of temperature trend derived from phase height measurements by Bremer and Berger (2002) appear to be inappropriate. Here we try to Fnd the reason of this surprising result and derive the phase height-inferred temperature trend to values well consistent with the temperature trend pattern in the mesosphere. 2. Why is the “better” estimate worse? In the Frst glance we can expect that an estimate, which takes into account also the trend in the NO concentration and e7 , should be better. Why it is not the case? Are the applied model or majority of other measurements used to establishing the temperature trends in the mesosphere wrong? No, and therefore the problem might consist in the NO concentration and e7 trends used by Bremer and Berger (2002). There is no observational information about long-term trends in the NO concentration and e7 near 80 km. The only available observational information concerns trends in ion composition, n(NO+ ) =n(O+ ) , near 120 and 150 km based 2 on rocket measurements (Danilov and Smirnova, 1997; Danilov, 1997). Therefore it is necessary to use model estimates of these trends. They are essentially available from two authors, namely Beig (2000) and Chakrabarty (1997). They provide a similar trend for the NO concentration, but opposite trends in the electron density. The trend in e7 is explicitly provided only by Chakrabarty (1997), therefore Bremer and Berger (2002) use his trend in e7 . The analysis of the LF phase height measurements for the 162 kHz measurements along the radio path Allouis –KKuhlungsborn (path length 1023 km, mid-point 50:7◦ N, 6:6◦ E) has been made at a constant solar zenith angle of 78:4◦ . This typically corresponds to re>ection heights near 81–82 km. The standard approach assumes the radio wave re>ection at a level of constant electron density, i.e. at a constant pressure level. However, if trends in the NO concentration and e7 are present, then Eq. (9) of Bremer and Berger (2002) must be used for the “re>ecting” pressure level at the beginning (pb ) and at the end (pe ) of measuring interval pb − pe = 0:00522 (hPa) × ln{[nNO (b)e7 (e)]=[nNO (e)e7 (b)]} = 0:00522 (hPa) + ln C;
(1)
where nNO is the concentration of NO and C is the correction factor. If there is no trend in nNO and e7 , C = 1, ln(1) = 0 and, therefore, pb = pe . If there is a trend in the above quantities, then the term in brackets provides correction and pb = pe is no longer valid. For the 2 × CO2 scenario, both Beig (2000) and Chakrabarty (1997) reveal a 10 K cooling and about 40% decrease of the NO concentration near 81–82 km in agreement with Roble and Dickinson (1989). To make the comparison with the results of Bremer and Berger (2002) possible, it is necessary to transform the change of NO
concentration into trend in %/decade, as Bremer and Berger (2002) did, when they transferred the nNO decrease into a trend of −2%/decade. In a similar way they transferred Chakrabarty’s (1997) change in e7 near 81–82 km into a trend of +0:9%/decade. In the same way we can transform Chakrabarty’s (1997) decrease of electron density of 30% for at 81–82 km into a trend of −1:5%/decade. Chakrabarty’s (1997) decrease in electron density contradicts the results of both the rocket measurements (Friedrich and Torkar, 2001) and LF phase height measurements (Bremer and Berger, 2002); they both reveal a positive trend in the electron density. An overview of trends in the lower ionosphere by La,stovi,cka and Bremer (2004) conFrms the positive trends in electron density at heights of 81 –82 km. Beig (2000) obtained a small increase of electron density by 7–8% for 81–82 km and 2 × CO2 , which can be transformed into a trend of 0.4%/decade. Beig (2000) did not publish change of e7 , because it was very small, an increase of only 5%, which may be transformed into a trend of +0:25%/decade. The above values of nNO and e7 provide the correction factor in Eq. (1) C = 1:029 and 1.023 for a decade for trends based on data of Chakrabarty (1997) and Beig (2000), respectively. These values are summarized in Table 1. The “Beig” correction factor is about 80% of the “Chakrabarty” correction factor C, because C is dominated by change in nNO . It should be mentioned that Beig (2000) obtained a positive trend in electron density in qualitative agreement with observational results. We assume for the ionization rate q and the electron density N q = e7 N 2
(2)
which is physically correct, even though observational data at these heights yield an almost linear relationship due to the dependence of e7 on electron density as a consequence of ion composition changes with changing ionization rate q (e.g. La,stovi,cka, 1985). If we consider “Chakrabarty” values of trends from Table 1, the decrease of N is not compensated by the increase of e7 and the ionization rate q has to decrease according to Eq. (2). A decrease of q is consistent with a decrease of nNO , because qNO is proportional to nNO and NO plays a dominant role in ionization at heights of 81–82 km. However, observational data (Bremer and Berger, 2002; Friedrich and Torkar, 2001) provide an increase of electron density. Where is the reason for such contradiction? The reason is an increase of the intensity of ionizing radiation at a Fxed height during thermal shrinking of the mesosphere. The phase height data of Bremer and Berger (2002) yield a decrease of a level of constant pressure by about 1 km over three decades (0.93 and 1:32 km for “no trend” and “trend” scenarios, respectively, see their Table 1). For the large solar zenith angle of measurements, = 78:5◦ and heights 81–82 km, the decrease by 1 km means a decrease of pressure at a Fxed height and of the total mass above this
J. La stovi cka / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1727 – 1730
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Table 1 Long-term trends in electron density, N , nitric oxide density, nNO , e7ective recombination coeCcient, e7 , and the values of correction factor C from Eq. (1) for change over one decade, derived based on model data of Chakrabarty (1997) and Beig (2000), and on rocket data used by Friedrich and Torkar (2001)
Chakrabarty Beig Rockets
N (%/decade)
nNO (%/decade)
e7 (%/decade)
C
−1.5 +0.4 +6
−2 −2 −2
+0.9 +0.25 −1.1
1.029 1.023 1.009
height, which results in a signiFcant increase in the intensity of the ionizing Lyman-alpha radiation at the Fxed height. Let us use the standard equation for calculation of the Lyman-alpha/NO ionization rate (e.g. Sechrist Jr., 1970) with the F=F∞ -dependent Lyman-alpha absorption cross section by Hall (1972), which is important for the given heights and (La,stovi,cka, 1976). For h = 81 km, re>ection height trend −0:33 km=decade and = 78:5◦ we obtain an increase by about 13%/decade in the Lyman-alpha >ux, which in combination with the trend of −2%/decade in nNO yields a trend of +11%/decade (factor 1.11) in q. Rocket data (Friedrich and Torkar, 2001) provide for 81–82 km the trend of +6%/decade. Then Eq. (2) reveals a trend of −1:1%/decade in e7 . With such a trend in e7 we obtain from Eq. (1) the value of correction factor C = 1:009, which is only 30% of C based on “Chakrabarty’s” input data. The “rocket” values are also presented in Table 1. With the “rocket” value of C, the trends in phase re>ection heights and inferred trends in temperatures will di7er little from trends obtained without taking changes in NO and e7 into account. Moreover, the temperature trends based both on such a small value of C and on C = 0 are close to each other and agree with the observational temperature trends obtained by Beig et al. (2003). Thus the answer to the question “why is the better estimate of Bremer and Berger (2002) worse” is very probably the use of inadequate e7 trend based on the results of Chakrabarty (1997), which provide the electron density trend, which contradicts the observational trend in electron density. There might be some correction of trends in re>ection heights and inferred trends in the mesospheric temperature due to trends in nNO and e7 , but signiFcantly smaller than that obtained by Bremer and Berger (2002). There are a lot of uncertainties in the above estimates. The Lyman-alpha ionization of NO is the dominant source of ionization near 81–82 km, but with decreasing concentration of NO and decreasing relative concentration of NO+ , the role of X-ray ionization and particularly of O2 (1 g ) ionization probably increases. Models yield a 40% decrease of nNO for 2 × CO2 scenario near 81–82 km (Roble and Dickinson, 1989; Chakrabarty, 1997; Beig, 2000). There is no direct observational information about trends in nNO at these heights, and nNO plays the decisive role in correction factor C. Ob-
servations reveal a negative trend in n(NO+ ) =n(O+ ) at 120 km 2 (Danilov, 1997; Danilov and Smirnova, 1997), which is supported by a weak positive trend in foE (e.g. Danilov, 1997; Bremer, 1998). The negative trend in n(NO+ ) =n(O+ ) was in2 terpreted as a consequence of decreasing concentration of NO (Danilov, 1997) due to increasing downward transport of NO caused by cooling of the MLT region (Danilov and Smirnova, 1997). This would lead to increasing, not decreasing NO concentration in the mesopause region. However, at least in summer there is probably no cooling in the mesopause region (Beig et al., 2003) and, therefore, there is probably no e7ect of this factor on the NO concentration near 81–82 km. Thus we have no observational evidence on the behaviour of nNO near 81–82 km and we must fully rely on the model results with various uncertainties in the model input information (reaction rates, etc.). There is no direct observational evidence on trends in e7 . The heights of 81–82 km belong to the water cluster-dominated region with complicated chemistry (uncertainties in some reaction rates, etc.), thus to estimate the trend in e7 from model calculations and particularly its accuracy is problematic. Nevertheless, the temperature trends inferred from the LF phase height measurements, particularly those with small C, agree well with trends obtained by more direct methods and, therefore, seem to be realistic. Models of Chakrabarty (1997) and Beig (2000) consider only the e7ect of greenhouse gases, while Bremer and Berger (2002) showed that the inclusion of the e7ect of changes of the ozone concentration strengthens the model trends and improves much the agreement between the model and observational trends. On the other hand, the trend in the LF phase heights would be the most reliable way to how to establish the trend in electron density near 81–82 km, if the steepness of electron density proFle was reliably known and relatively stable. However, this is not the case. Nevertheless, we can estimate from LF phase heights such a trend in electron density to be positive and a few percent per decade, slightly less than rocket data trend but much larger than model trends (Table 1). Thus the rocket-based estimate of C in Table 1 is probably more realistic than model estimates. The results presented in this paper do not change the high value of the paper by Bremer and Berger (2002). They pointed out to the necessity to consider possible
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e7ects of trends in nNO and e7 in interpretation of trends in the LF phase re>ection height and, even more important, they showed that the model simulates temperature trends in the mesosphere better, when the e7ect of change of ozone concentration in the atmosphere is added to the e7ect of increasing concentration of greenhouse gases. 3. Conclusions The performed calculations are simpliFed, of course. Nevertheless, they indicate that there is a small correction to the LF phase height trend from the expected trends in the NO concentration and e7 . The larger correction of Bremer and Berger (2002) was a consequence of application of trends in the NO concentration and e7 as obtained by Chakrabarty (1997), which provide a trend in electron density, which is quite inconsistent with the observed trends in electron density. The reason of this inconsistency is most probably the inappropriate value of the e7 trend. With a more realistic correction factor C there is no more a signiFcant discrepancy between the observational data and model-simulated trends of Bremer and Berger (2002) in the LF phase heights. Moreover, the resulting trends in the mesospheric temperatures of Bremer and Berger (2002) without the inappropriate correction are in a good agreement with the results of analysis of global set of results on trends in the mesospheric temperatures by Beig et al. (2003). Thus the inconsistency in temperature trends introduced by the inappropriate correction to the LF phase height trend is largely explained and essentially removed. Acknowledgements This study was supported by the Grant No. A3042101 of the Grant Agency of the Academy of Sciences of the Czech Republic. References Beig, G., 2000. The relative importance of solar activity and anthropogenic in>uences on the ion composition, temperature,
and associated neutrals of the middle atmosphere. Journal of Geophysical Research 105, 19841–19856. Beig, G., Keckhut, P., Lowe, R.P., Roble, R.G., Mlynczak, M.G., Scheer, J., Fomichev, V.I., O7ermann, D., French, W.J.R., Shepherd, M.G., Semenov, A.I., Remsberg, E.E., She, C.Y., LKubken, F.-J., Bremer, J., Clemesha, B.R., Stegman, J., Sigernes, F., Fadnavis, S., 2003. Review of mesospheric temperature trends. Reviews of Geophysics 41 (4), 1015. doi: 10.1029/2002RG000121. Bremer, J., 1998. Trends in the ionospheric E and F regions over Europe. Annales Geophysicae 16, 986–996. Bremer, J., Berger, U., 2002. Mesospheric temperature trends derived from ground-based LF phase-height observations at mid-latitudes: comparison with model simulations. Journal of Atmospheric and Solar Terrestrial Physics 64, 805–816. Chakrabarty, D.K., 1997. Mesopause scenario on doubling CO2 . Advances in Space Research 20 (11), 2117–2125. Danilov, A.D., 1997. Long-term changes in the mesosphere and lower thermosphere temperature and composition. Advances in Space Research 20 (11), 2137–2147. Danilov, A.D., Smirnova, N.V., 1997. Long-term trends in ion composition in the E region. Geomagnetism and Aeronomy 37 (4), 35–40 (in Russian—English translation published by AGU). Friedrich, M., Torkar, K.M., 2001. Long-term trends and other residual features of the lower ionosphere. In: Proceedings of the 15th ESA Symposium European Rocket and Balloon Programs and Related Research, ESA SP-471, 357–362, ESA. Hall, J.E., 1972. Atmospheric pressure, density and scale height calculated from H Lyman-a absorption allowing for the variation in cross-section with wavelength. Journal of Atmospheric and Terrestrial Physics 34, 1337–1342. La,stovi,cka, J., 1976. On some sources of uncertainty in the Lyman-a ionization-rate calculation. Studia geophysica et geodaetica 20, 273–283. La,stovi,cka, J., 1985. On the linear electron loss process in the D region. Journal of Atmospheric and Terrestrial Physics 47, 497–498. La,stovi,cka, J., Bremer, J., 2004. An overview of long-term trends in the lower ionosphere below 120 km. Surveys in Geophysics 25, 69–99. Roble, R.G., Dickinson, R.E., 1989. How will changes of carbon dioxide and methane modify the mean structure of the mesosphere and thermosphere? Geophysical Research Letters 16, 1441–1444. Sechrist Jr., C.F., 1970. Interpretation of D-region electron densities. Radio Science 5, 663–670.
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An explanation of discrepancy in mesospheric temperature trends derived from ground-based LF phase-height observations J. La,stovi,cka∗ Institute of Atmospheric Physics, Academy of Science of the Czech Republic, Bo cn II, 14131 Prague, Czech Republic Received 10 December 2002; received in revised form 18 August 2003; accepted 30 March 2004
Abstract Bremer and Berger (J. Atmos. Solar Terr. Phys. 64 (2002) 805) applied a correction for trends in the NO concentration and e7 in the interpretation of trends in the low frequency (LF) phase height measurements and obtained results less consistent with model simulations as well as the observed trends in mesospheric temperatures. The correction is shown to be too large most probably due to the application of inappropriate trends in e7 of Chakrabarty (Adv. Space Res. 20 (1997) 2117), which yield a trend in electron density opposite to that which is observed. The discrepancy between the observational data and model-simulated trends of Bremer and Berger (J. Atoms. Solar. Terr. Phys. 64 (2002) 805) in the LF phase heights can be largely removed. Even more important, the trends in mesospheric temperatures inferred by Bremer and Berger (J. Atmos. Solar Terr. Phys. 64 (2002) 805) from trends in the LF phase heights without the inappropriate correction agree well with the results of analysis of a global set of results on trends in the mesospheric temperatures by Beig et al. (Rev. Geophys. 41 (2003) 1015). c 2004 Elsevier Ltd. All rights reserved. Keywords: Mesosphere; Long-term trends; LF phase heights; Temperature
1. Introduction Estimating the long-term trends of mesospheric temperature, which might be of the anthropogenic origin, is a topic of great current interest. One method of such estimating is to use the long-term continuous indirect phase re>ection height measurements in the low frequency (LF) range. Recently Bremer and Berger (2002) published the estimate of the mesospheric temperature trend based on the LF phase height measurements for an extended period from 1959 until 2000. They estimated the trend in the mean temperature over the height interval of 48–82 km from the trend of lowering of the LF phase re>ection height, i.e. of a height of constant pressure level. The lowering of the constant pressure level was estimated from the observed lowering of the LF phase re>ection height under assumption of no trend in the NO ∗
Fax: +42-2727-63745. E-mail address: [email protected] (J. La,stovi,cka).
concentration and in the e7ective coeCcient of recombination e7 , and then in another way taking into account the trends in the NO concentration from Beig (2000) and e7 from Chakrabarty (1997). The comparison with the model calculations of Bremer and Berger (2002), which take into account changes in the greenhouse gas and ozone concentration, revealed a surprising result. The agreement of the former estimate (without NO and e7 trends) with model was excellent, whereas the latter estimate (with NO and e7 trends) provided substantially larger cooling and decrease of the LF phase height than the model. Even more important, the comparison of temperature trends inferred from the LF phase re>ection height data with observational trends in the lower and middle mesosphere at middle latitudes reviewed by Beig et al. (2003) reveals evidently better agreement when the trends in NO concentration and e7 are not considered. Since a relatively consistent pattern of the temperature trends in the mesosphere is emerging (except the wintertime mesopause region), as shown by Beig et al. (2003), the
c 2004 Elsevier Ltd. All rights reserved. 1364-6826/$ - see front matter doi:10.1016/j.jastp.2004.03.012
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J. La stovi cka / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1727 – 1730
new values of temperature trend derived from phase height measurements by Bremer and Berger (2002) appear to be inappropriate. Here we try to Fnd the reason of this surprising result and derive the phase height-inferred temperature trend to values well consistent with the temperature trend pattern in the mesosphere. 2. Why is the “better” estimate worse? In the Frst glance we can expect that an estimate, which takes into account also the trend in the NO concentration and e7 , should be better. Why it is not the case? Are the applied model or majority of other measurements used to establishing the temperature trends in the mesosphere wrong? No, and therefore the problem might consist in the NO concentration and e7 trends used by Bremer and Berger (2002). There is no observational information about long-term trends in the NO concentration and e7 near 80 km. The only available observational information concerns trends in ion composition, n(NO+ ) =n(O+ ) , near 120 and 150 km based 2 on rocket measurements (Danilov and Smirnova, 1997; Danilov, 1997). Therefore it is necessary to use model estimates of these trends. They are essentially available from two authors, namely Beig (2000) and Chakrabarty (1997). They provide a similar trend for the NO concentration, but opposite trends in the electron density. The trend in e7 is explicitly provided only by Chakrabarty (1997), therefore Bremer and Berger (2002) use his trend in e7 . The analysis of the LF phase height measurements for the 162 kHz measurements along the radio path Allouis –KKuhlungsborn (path length 1023 km, mid-point 50:7◦ N, 6:6◦ E) has been made at a constant solar zenith angle of 78:4◦ . This typically corresponds to re>ection heights near 81–82 km. The standard approach assumes the radio wave re>ection at a level of constant electron density, i.e. at a constant pressure level. However, if trends in the NO concentration and e7 are present, then Eq. (9) of Bremer and Berger (2002) must be used for the “re>ecting” pressure level at the beginning (pb ) and at the end (pe ) of measuring interval pb − pe = 0:00522 (hPa) × ln{[nNO (b)e7 (e)]=[nNO (e)e7 (b)]} = 0:00522 (hPa) + ln C;
(1)
where nNO is the concentration of NO and C is the correction factor. If there is no trend in nNO and e7 , C = 1, ln(1) = 0 and, therefore, pb = pe . If there is a trend in the above quantities, then the term in brackets provides correction and pb = pe is no longer valid. For the 2 × CO2 scenario, both Beig (2000) and Chakrabarty (1997) reveal a 10 K cooling and about 40% decrease of the NO concentration near 81–82 km in agreement with Roble and Dickinson (1989). To make the comparison with the results of Bremer and Berger (2002) possible, it is necessary to transform the change of NO
concentration into trend in %/decade, as Bremer and Berger (2002) did, when they transferred the nNO decrease into a trend of −2%/decade. In a similar way they transferred Chakrabarty’s (1997) change in e7 near 81–82 km into a trend of +0:9%/decade. In the same way we can transform Chakrabarty’s (1997) decrease of electron density of 30% for at 81–82 km into a trend of −1:5%/decade. Chakrabarty’s (1997) decrease in electron density contradicts the results of both the rocket measurements (Friedrich and Torkar, 2001) and LF phase height measurements (Bremer and Berger, 2002); they both reveal a positive trend in the electron density. An overview of trends in the lower ionosphere by La,stovi,cka and Bremer (2004) conFrms the positive trends in electron density at heights of 81 –82 km. Beig (2000) obtained a small increase of electron density by 7–8% for 81–82 km and 2 × CO2 , which can be transformed into a trend of 0.4%/decade. Beig (2000) did not publish change of e7 , because it was very small, an increase of only 5%, which may be transformed into a trend of +0:25%/decade. The above values of nNO and e7 provide the correction factor in Eq. (1) C = 1:029 and 1.023 for a decade for trends based on data of Chakrabarty (1997) and Beig (2000), respectively. These values are summarized in Table 1. The “Beig” correction factor is about 80% of the “Chakrabarty” correction factor C, because C is dominated by change in nNO . It should be mentioned that Beig (2000) obtained a positive trend in electron density in qualitative agreement with observational results. We assume for the ionization rate q and the electron density N q = e7 N 2
(2)
which is physically correct, even though observational data at these heights yield an almost linear relationship due to the dependence of e7 on electron density as a consequence of ion composition changes with changing ionization rate q (e.g. La,stovi,cka, 1985). If we consider “Chakrabarty” values of trends from Table 1, the decrease of N is not compensated by the increase of e7 and the ionization rate q has to decrease according to Eq. (2). A decrease of q is consistent with a decrease of nNO , because qNO is proportional to nNO and NO plays a dominant role in ionization at heights of 81–82 km. However, observational data (Bremer and Berger, 2002; Friedrich and Torkar, 2001) provide an increase of electron density. Where is the reason for such contradiction? The reason is an increase of the intensity of ionizing radiation at a Fxed height during thermal shrinking of the mesosphere. The phase height data of Bremer and Berger (2002) yield a decrease of a level of constant pressure by about 1 km over three decades (0.93 and 1:32 km for “no trend” and “trend” scenarios, respectively, see their Table 1). For the large solar zenith angle of measurements, = 78:5◦ and heights 81–82 km, the decrease by 1 km means a decrease of pressure at a Fxed height and of the total mass above this
J. La stovi cka / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 1727 – 1730
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Table 1 Long-term trends in electron density, N , nitric oxide density, nNO , e7ective recombination coeCcient, e7 , and the values of correction factor C from Eq. (1) for change over one decade, derived based on model data of Chakrabarty (1997) and Beig (2000), and on rocket data used by Friedrich and Torkar (2001)
Chakrabarty Beig Rockets
N (%/decade)
nNO (%/decade)
e7 (%/decade)
C
−1.5 +0.4 +6
−2 −2 −2
+0.9 +0.25 −1.1
1.029 1.023 1.009
height, which results in a signiFcant increase in the intensity of the ionizing Lyman-alpha radiation at the Fxed height. Let us use the standard equation for calculation of the Lyman-alpha/NO ionization rate (e.g. Sechrist Jr., 1970) with the F=F∞ -dependent Lyman-alpha absorption cross section by Hall (1972), which is important for the given heights and (La,stovi,cka, 1976). For h = 81 km, re>ection height trend −0:33 km=decade and = 78:5◦ we obtain an increase by about 13%/decade in the Lyman-alpha >ux, which in combination with the trend of −2%/decade in nNO yields a trend of +11%/decade (factor 1.11) in q. Rocket data (Friedrich and Torkar, 2001) provide for 81–82 km the trend of +6%/decade. Then Eq. (2) reveals a trend of −1:1%/decade in e7 . With such a trend in e7 we obtain from Eq. (1) the value of correction factor C = 1:009, which is only 30% of C based on “Chakrabarty’s” input data. The “rocket” values are also presented in Table 1. With the “rocket” value of C, the trends in phase re>ection heights and inferred trends in temperatures will di7er little from trends obtained without taking changes in NO and e7 into account. Moreover, the temperature trends based both on such a small value of C and on C = 0 are close to each other and agree with the observational temperature trends obtained by Beig et al. (2003). Thus the answer to the question “why is the better estimate of Bremer and Berger (2002) worse” is very probably the use of inadequate e7 trend based on the results of Chakrabarty (1997), which provide the electron density trend, which contradicts the observational trend in electron density. There might be some correction of trends in re>ection heights and inferred trends in the mesospheric temperature due to trends in nNO and e7 , but signiFcantly smaller than that obtained by Bremer and Berger (2002). There are a lot of uncertainties in the above estimates. The Lyman-alpha ionization of NO is the dominant source of ionization near 81–82 km, but with decreasing concentration of NO and decreasing relative concentration of NO+ , the role of X-ray ionization and particularly of O2 (1 g ) ionization probably increases. Models yield a 40% decrease of nNO for 2 × CO2 scenario near 81–82 km (Roble and Dickinson, 1989; Chakrabarty, 1997; Beig, 2000). There is no direct observational information about trends in nNO at these heights, and nNO plays the decisive role in correction factor C. Ob-
servations reveal a negative trend in n(NO+ ) =n(O+ ) at 120 km 2 (Danilov, 1997; Danilov and Smirnova, 1997), which is supported by a weak positive trend in foE (e.g. Danilov, 1997; Bremer, 1998). The negative trend in n(NO+ ) =n(O+ ) was in2 terpreted as a consequence of decreasing concentration of NO (Danilov, 1997) due to increasing downward transport of NO caused by cooling of the MLT region (Danilov and Smirnova, 1997). This would lead to increasing, not decreasing NO concentration in the mesopause region. However, at least in summer there is probably no cooling in the mesopause region (Beig et al., 2003) and, therefore, there is probably no e7ect of this factor on the NO concentration near 81–82 km. Thus we have no observational evidence on the behaviour of nNO near 81–82 km and we must fully rely on the model results with various uncertainties in the model input information (reaction rates, etc.). There is no direct observational evidence on trends in e7 . The heights of 81–82 km belong to the water cluster-dominated region with complicated chemistry (uncertainties in some reaction rates, etc.), thus to estimate the trend in e7 from model calculations and particularly its accuracy is problematic. Nevertheless, the temperature trends inferred from the LF phase height measurements, particularly those with small C, agree well with trends obtained by more direct methods and, therefore, seem to be realistic. Models of Chakrabarty (1997) and Beig (2000) consider only the e7ect of greenhouse gases, while Bremer and Berger (2002) showed that the inclusion of the e7ect of changes of the ozone concentration strengthens the model trends and improves much the agreement between the model and observational trends. On the other hand, the trend in the LF phase heights would be the most reliable way to how to establish the trend in electron density near 81–82 km, if the steepness of electron density proFle was reliably known and relatively stable. However, this is not the case. Nevertheless, we can estimate from LF phase heights such a trend in electron density to be positive and a few percent per decade, slightly less than rocket data trend but much larger than model trends (Table 1). Thus the rocket-based estimate of C in Table 1 is probably more realistic than model estimates. The results presented in this paper do not change the high value of the paper by Bremer and Berger (2002). They pointed out to the necessity to consider possible
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e7ects of trends in nNO and e7 in interpretation of trends in the LF phase re>ection height and, even more important, they showed that the model simulates temperature trends in the mesosphere better, when the e7ect of change of ozone concentration in the atmosphere is added to the e7ect of increasing concentration of greenhouse gases. 3. Conclusions The performed calculations are simpliFed, of course. Nevertheless, they indicate that there is a small correction to the LF phase height trend from the expected trends in the NO concentration and e7 . The larger correction of Bremer and Berger (2002) was a consequence of application of trends in the NO concentration and e7 as obtained by Chakrabarty (1997), which provide a trend in electron density, which is quite inconsistent with the observed trends in electron density. The reason of this inconsistency is most probably the inappropriate value of the e7 trend. With a more realistic correction factor C there is no more a signiFcant discrepancy between the observational data and model-simulated trends of Bremer and Berger (2002) in the LF phase heights. Moreover, the resulting trends in the mesospheric temperatures of Bremer and Berger (2002) without the inappropriate correction are in a good agreement with the results of analysis of global set of results on trends in the mesospheric temperatures by Beig et al. (2003). Thus the inconsistency in temperature trends introduced by the inappropriate correction to the LF phase height trend is largely explained and essentially removed. Acknowledgements This study was supported by the Grant No. A3042101 of the Grant Agency of the Academy of Sciences of the Czech Republic. References Beig, G., 2000. The relative importance of solar activity and anthropogenic in>uences on the ion composition, temperature,
and associated neutrals of the middle atmosphere. Journal of Geophysical Research 105, 19841–19856. Beig, G., Keckhut, P., Lowe, R.P., Roble, R.G., Mlynczak, M.G., Scheer, J., Fomichev, V.I., O7ermann, D., French, W.J.R., Shepherd, M.G., Semenov, A.I., Remsberg, E.E., She, C.Y., LKubken, F.-J., Bremer, J., Clemesha, B.R., Stegman, J., Sigernes, F., Fadnavis, S., 2003. Review of mesospheric temperature trends. Reviews of Geophysics 41 (4), 1015. doi: 10.1029/2002RG000121. Bremer, J., 1998. Trends in the ionospheric E and F regions over Europe. Annales Geophysicae 16, 986–996. Bremer, J., Berger, U., 2002. Mesospheric temperature trends derived from ground-based LF phase-height observations at mid-latitudes: comparison with model simulations. Journal of Atmospheric and Solar Terrestrial Physics 64, 805–816. Chakrabarty, D.K., 1997. Mesopause scenario on doubling CO2 . Advances in Space Research 20 (11), 2117–2125. Danilov, A.D., 1997. Long-term changes in the mesosphere and lower thermosphere temperature and composition. Advances in Space Research 20 (11), 2137–2147. Danilov, A.D., Smirnova, N.V., 1997. Long-term trends in ion composition in the E region. Geomagnetism and Aeronomy 37 (4), 35–40 (in Russian—English translation published by AGU). Friedrich, M., Torkar, K.M., 2001. Long-term trends and other residual features of the lower ionosphere. In: Proceedings of the 15th ESA Symposium European Rocket and Balloon Programs and Related Research, ESA SP-471, 357–362, ESA. Hall, J.E., 1972. Atmospheric pressure, density and scale height calculated from H Lyman-a absorption allowing for the variation in cross-section with wavelength. Journal of Atmospheric and Terrestrial Physics 34, 1337–1342. La,stovi,cka, J., 1976. On some sources of uncertainty in the Lyman-a ionization-rate calculation. Studia geophysica et geodaetica 20, 273–283. La,stovi,cka, J., 1985. On the linear electron loss process in the D region. Journal of Atmospheric and Terrestrial Physics 47, 497–498. La,stovi,cka, J., Bremer, J., 2004. An overview of long-term trends in the lower ionosphere below 120 km. Surveys in Geophysics 25, 69–99. Roble, R.G., Dickinson, R.E., 1989. How will changes of carbon dioxide and methane modify the mean structure of the mesosphere and thermosphere? Geophysical Research Letters 16, 1441–1444. Sechrist Jr., C.F., 1970. Interpretation of D-region electron densities. Radio Science 5, 663–670.