Long term trends in the temperature of the mesosphere/lower thermosphere region: 2. Solar response

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2011ja016766, 2011 Long term trends in the temperature of the mesosphere/lower thermosphere region: 2. Solar response Gufran Beig 1 Received 18 April 2011; revised 5 August 2011; accepted 8 August 2011; published 18 October [1] Understanding trends in any atmospheric quantity typically requires the ability to distinguish between naturally occurring processes that result in trends, such as the 11 year solar cycle, and potential anthropogenic secular trends that occur simultaneously. After the review of mesospheric and lower thermospheric temperature response to solar activity by Beig et al. (2008), a few new results along with some modified results by revisiting the older data sets have been reported recently. Main improvement is due to the length of data series and amount of data which have been accounted in recent years. This article summarizes the progress made in the field of temperature variability due to changing solar activity as reported recently. Recent investigations revealed that the solar signal becomes stronger with increasing latitude in the mesosphere. Temperature response to solar activity at the lower part of mesopause region is around a few degrees per 100 solar flux units (sfu), which becomes stronger (4 5 K/100 sfu) in the upper part of this region in both hemispheres. The overall global picture indicates that the solar signal in the mesopause region temperature in the Northern Hemisphere is relatively stronger in recent time in a majority of locations compared to results reported in earlier reviews. Citation: Beig, G. (2011), Long term trends in the temperature of the mesosphere/lower thermosphere region: 2. Solar response, J. Geophys. Res., 116,, doi: /2011ja Introduction 1 Indian Institute of Tropical Meteorology, Pune, India. Copyright 2011 by the American Geophysical Union /11/2011JA [2] In any atmospheric quantity, especially temperature, normally all kind of variabilities like natural, episodic, and anthropogenic signals are present. The real challenge is to separate out different components of variabilities and ultimately to distinguish between naturally occurring processes such as the 11 year solar cycle and potential anthropogenic processes that may be occurring simultaneously. The solar response in temperature, if not properly filtered out, is still one of the major sources of variation which may interfere with the detection of human induced temperature trends for the middle atmosphere [Beig, 2000; Beig et al., 2003] and will have strong implication in the quantification of global change signals. It is found that solar activity has decreased in the second half of the 20th century and continued to decrease even in the beginning of the 21st century. It is believed that the effect of solar activity on solar cycle time scales can be modulated when long term trends are computed in the upper atmosphere. In the case of the mesosphere/ lower thermosphere (MLT), the solar variability presents quite a large signal over a relatively short period of time that, in principle, may be observed and quantified. Incoming solar radiation provides the external forcing for the Earthatmosphere system. It is believed that the essentially permanent changes arising in several mesospheric parameters due to human activities are weaker, whereas periodic changes due to variations in solar activity are comparatively stronger [Beig, 2000]. The study related to the influence of solar activity on the vertical structure of temperature and its separation from global change signals has always been a challenge. To investigate the temperature variabilities in the MLT region, a panel called Mesospheric Temperature Trend Assessment (MTTA) was established under the auspices of a joint working group of the International Association of Geomagnetism and Aeronomy (IAGA) and International Commission for Middle Atmosphere (ICMA) on trends. The outcome of this panel resulted in two overview papers. The first paper focused mainly on long term trends arising because of anthropogenic activities at the ground but also provided a brief account of changes in temperature arising because of natural variability [Beig et al., 2003]. The other paper in this direction exclusively addressed the overview of the temperature response in the mesosphere and lower thermosphere to solar activity [Beig et al., 2008]. [3] Recently, the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite has provided a high quality data set relating to the atmospheric state parameters (temperature, pressure, and density), composition and thermodynamic properties [Mlynczak et al., 2007]. This data, as well as those from other satellites and sensors, started to facilitate a detailed and quantitative assessment of 1of7

2 Figure 1. Temperature response to the 11 year solar cycle as a function of altitude from different studies for mesosphere in the tropics. the MLT response to the solar cycle. These observations are being used to develop the understanding of the atmosphere which may be reflected in the physics incorporated into numerical models [Mlynczak et al., 2010]. [4] Two review papers by Beig et al. [2003, 2008] concluded that the positive signal in the annual mean solar response of the MLT regions is positive has an amplitude of a few degrees per solar cycle, and in some cases no significant solar signal is found. However, the magnitude of solar response in recent literature is reported to be different as compared to the results reported earlier in the 1970s and 1980s. Thereafter and during the past few years, quite a few new results and updates were also reported [Remsberg, 2007, 2008, 2009; Fadnavis et al., 2011; Laštovička et al., 2006, 2008; G. Beig et al., Intercomparison of decadal solar signal in mesospheric ozone and temperature obtained by HALOE satellite data and HAMMONIA model, submitted to Journal of Geophysical Research, 2011]. This article presents an update on the MLT region temperature arising because of changes in the solar activity and mainly discusses those results which are not reported in earlier reviews. For convenience, the whole region from 50 to 100 km is referred to as the mesosphere and lower thermosphere (MLT) region. The region from 50 to 79 km will be referred to as the mesosphere, and the region from 80 to 100 km will be referred to as the mesopause region. In addition, parts of the mesosphere are referred to as the lower mesosphere (50 70 km) and the upper mesosphere (70 79 km). Similarly, we classified the lower mesopause region (80 90 km) and upper mesopause region ( km). We discuss the lower thermosphere only as an upper boundary rather than in its own right. 2. Solar Response in the Mesosphere [5] We discuss here mainly those results in detail which were either not included or could not be reported until the time of the publication of the baseline review papers [Beig et al., 2003, 2008]. Hence, for all those results which are reported but not discussed here in detail, readers are referred to Beig et al. [2003, 2008]. Various results related to solar response in temperature reported by various authors in the literature are mixed in terms of scaling. Some authors report the change per solar cycle and some report the change per 100 F 10.7 solar flux units (sfu). In this paper, an attempt has been made to discuss all results as reported originally by different authors, but while illustrating them in Figure 1 for comparison, identical scaling is used, and hence, an attempt has been made to scale down the results to Kelvin per 100 sfu. Some new results pertaining to middle latitudes are also published recently, but the majority of new results are reported for the tropical region. Hence, while this paper discusses in the text all of the results, results pertaining only to the tropics are illustrated in figure form (Figure 1). [6] There were series of papers published during recent years based on the analysis of complete temperature time series ( ) obtained by Halogen Occultation Experiment (HALOE) on board Upper Atmosphere Research Satellite (UARS) [Remsberg, 2007, 2008, 2009; Fadnavis et al., 2011; Beig et al., submitted paper, 2011, and references therein]. In this series of recent publications, a reanalysis of temperature time series was conducted with longer and complete time series. Remsberg [2007, 2008] reported the solar coefficient in zonally averaged temperature versus pressure profile data provided by the full 14+ years of HALOE. The data was based on 95,900 sunrise (SR) plus sunset (SS) measured profiles. The analyses accounted for the first order autoregression term properly, leading to seasonal and longer period terms of larger amplitude. Terms are reported for 10 wide latitude zones from 60 S to 60 N and for pressure levels from 2 to hpa. Analyses are also conducted for some additional levels, which are 1, 0.5, 0.7, 0.15, 0.07, and hpa. A total of 208 separate time series were analyzed. Their results were coherent with latitude and pressure altitude. The findings from the reanalyses are claimed to be now more definitive because of a proper accounting for the effects of autoregression for the adjacent points of the time series and because of the addition of several more years of data. It is claimed by the author that for the first time, interannual terms have been obtained from a single satellite data set covering such a large region of the mesosphere and for a time span of more than a decade. Remsberg [2007] made adjustments for any slight mismatches with the phase of the solar flux. The solar terms, direct, maximum minus minimum values for temperature, are found to be of the order of K in the middle mesosphere, which then increase to about K in the upper mesosphere. Average profiles of the solar response are larger for the middle latitudes than for the low latitudes and may be indicative of added effects due to solar related wave activity at the middle latitudes. In the analysis reported later by Remsberg [2008], a simple solar cycle like term of 11 year period was fitted to the time series residuals after accounting for the seasonal and interannual terms. Remsberg [2008] has reported a highly significant solar cycle (SC) response for both the upper mesosphere and the upper stratosphere. The phases of these SC like terms were checked for their continuity with latitude and pressure altitude; the larger amplitude responses are directly in phase with that of standard proxies for the solar 2of7

3 flux variations. The analyzed, maximum minus minimum, responses at low latitudes are of order K, while at middle latitudes they are as large as 3 K in the upper mesosphere. The diagnosed solar cycle response from HALOE for the middle to upper mesosphere at middle latitudes were found to be larger than what was simulated with most models, perhaps an indication of decadal scale dynamical forcing that is not being simulated as well as opined by the author. Remsberg [2009] has again reanalyzed the HALOE data sets and binned the data according to 10 wide latitude zones from 40 S to 40 N and at 10 altitudes from 43 to 80 km. This paper mainly focused on comparing results with other available solar responses obtained by lidar recently and highlighted results for limited latitude bins, unlike the previous two publications [Remsberg, 2007, 2008]. Although all the recent publications from the group [Remsberg, 2007, 2008, 2009] properly accounted for the effects of autoregression for the adjacent points of the time series, there are differences in the results reported within these papers. Reasons for these differences are not properly explained and hence are not very clear at this stage. Although the present review includes discussion of the results from all the above papers, it is stressed that the latest publication [Remsberg, 2009] may be considered as the most updated work of this group. However, this paper does not discuss results in detail for several additional latitude bins as compared to Remsberg [2008]. In that case, we have considered results for those additional latitudes reported by Remsberg [2008] as most updated. [7] Figure 1 compares all the results on temperature response to solar activity change in the mesosphere of tropics which are reported after the work by Beig et al. [2008]. The solar cycle response in temperature by Remsberg [2009] is found to be 0.6 K at 50 km, which becomes even weaker between 50 and 70 km and then increases to 1.2 K at 75 km and 1.8 K at 80 km with a 2 sigma error of around ±0.5 K near the latitude bin of 20 N ± 5 N. Li et al. [2008] analyzed the data obtained at Hawaii (19.5 N) from lidar during and reported a solar cycle response of 0.4 K at 50 km and 0 K at 65 km, which is in agreement with HALOE results. However, there is a disagreement above 65 km where lidar data show either a negative response or a negligible response between 67 and 80 km, whereas HALOE data show a stronger solar response of K. The reason for this discrepancy is not yet clear. The difference cannot be attributed to the time difference in time series which are almost overlapping. Some differences may arise because of the fact that lidar data are for a point location, whereas HALOE analysis is performed for a broader latitude bin with zonally averaged data. Similar is the case when Remsberg [2009] results are compared with those of Sridharan et al. [2009] for a latitude around 10 N ± 5 N. Sridharan et al. [2009] have reported a positive solar response of K with a standard deviation of ±0.5 K between 50 and 60 km at Gadanki (13.5 N), which is close to the HALOE results. The Gadanki data reported a changeover of the sign of solar response at 65 km from positive to negative which becomes relatively stronger with height and becomes 1.0 K at 70 km but with an uncertainty of ±0.9 K. This is in disagreement with the positive response reported by a majority of the profiles shown in Figure 1 for the upper mesosphere. The short length of time series, biases in lidar data in the upper mesosphere (if not accounted for properly) may be responsible for this unusual negative solar response obtained by Sridharan et al. [2009]. Hence, such results should be viewed with caution as the standard deviation is also very high, which tends to make the response almost insignificant. Beig and Fadnavis [2009] have revisited the longest temperature records available for the equatorial region using the rocketsonde from Thumba, India (8 N), during the period They have used the up todate multifunctional regression model in the light of current understanding on correction factors. Beig and Fadnavis [2009] have reported a transition of negative solar response in the stratosphere to positive solar response in the mesosphere at around 52 km. They reported a positive solar response of the order of +0.7 ± 0.3 K/100 sfu at 55 km which remains consistent between 0.5 and 1.0 K/100 sfu until about 70 km. Solar coefficient falls off with height above 70 km, and value touches as high as 2.6 ± 0.85 K/100 sfu at 75 km. Although there is a significant improvement in the method of analysis adopted by Beig and Fadnavis [2009] in this work as compared to earlier work carried out using the same data set [Mohanakumar, 1995], results above 70 km should be viewed with caution as rocketsonde measurements are marred with some biases above 70 km. Fadnavis et al. [2011] provided the seasonal solar response in the HALOE data for the tropics (0 N 30 N) of the Northern Hemisphere. Their result indicates a temperature response to solar variability as K/100 sfu below 70 km and 1 K/100 sfu above 70 km in the tropics. Temperature solar response shows seasonality in the mesosphere. In the discussion that follows, we group the results by season (winter as December February, spring as March May, summer as June August, and autumn as September November). In general, positive response is observed during all seasons throughout the northern tropics except during summer. Significant positive temperature response is observed during winter and autumn in the lower mesosphere and during summer and spring in the upper mesosphere. Temperature response varies from 0.1 to 1.2 K/100 sfu during most of the seasons except during summer, where it varies between 0.4 K/100 sfu and 1.2 K/100 sfu. All the above results pertain to tropics and are cited from the paper of Fadnavis et al. [2011]. Golitsyn et al. [2006] have consolidated Russian results from their earlier analysis [Golitsyn et al., 1996] on the basis of the rocket data from Volgograd (49 N) and data from different airglow emissions obtained at Abastumani (42 N) and Zvenigorod (56 N), covering two activity cycles ( ). They obtained the minimal solar response at heights km with a value of +2 ± 0.4 K/100 sfu for winter and 1 ± 0.4 K/100 sfu for summer. There are differences in seasonal variability of solar response (even in the sign of response) reported by Golitsyn et al. [2006] and that of Fadnavis et al. [2011], but results are for different latitude bins pertaining to midlatitudes and tropics, respectively, and hence, a direct comparison is not practical as differences may be attributed to latitudinal variability. [8] Very recently, Beig et al. (submitted manuscript, 2011) have compared the solar response in temperature obtained by 3 D model HAMMONIA with that of HALOE analysis performed with broader latitude bins to increase the sampling points and to make the data more homogeneous for better statistical robustness. They have used the multiple 3of7

4 more suitable for capturing the latitudinal variability. However, in both the cases, the zonally averaged HALOE data are used because of limited sampling (SR and SS) to provide sufficient data for statistical analysis, and hence, HALOE results in the strict sense cannot be considered as representative of a particular point location. In addition, the statistical technique applied by the Remsberg group has autoregression problems in their earlier publications reported until 2005 [Remsberg and Deaver, 2005] which is later rectified in subsequent reanalyses as reported in their recent publications [Remsberg, 2007, 2008, 2009]. Batista et al. [2009] reported the solar cycle response in lidar temperature time series at 23 S but did not find any significant values for the altitudes of km. The responses near 23 S from HALOE are also no greater than about 0.5 K for km [Remsberg, 2008]. The solar signal in temperature obtained by lidar data of Haute Provence Observatorie [Keckhut et al., 2005] reported a stronger positive response for the midlatitude (44 N) compared to low latitudes. Keckhut et al. [2005] reported a solar response of 0.5 K at 50 km, which becomes 3 K at 78 km. Figure 2. Temperature response to solar cycle in the Northern Hemisphere for the mesopause region. regression analysis and reported the data for tropics (0 30 ) and midlatitudes (40 60 ) for both the Northern and Southern Hemispheres. Results of Beig et al. (submitted manuscript, 2011) indicate a temperature solar response of K/100 sfu below 70 km and 1 K/100 sfu above 70 km in the tropics (both hemispheres). In the 40 N 60 N region, a temperature response of K/100 sfu is obtained in the middle mesosphere. The inferred solar signal in the temperature for the midlatitude Southern Hemisphere (40 S 60 S) is found to be K/100 sfu, but in most cases, the signal is not significant. Slight differences in the solar response reported by Remsberg [2008, 2009] and Beig et al. (submitted manuscript, 2011) using the same set of HALOE data may be attributed to the choice of size of the latitude bin for which analysis is performed and subsequent statistical analysis. There are distinct advantages and disadvantages related to the choice of latitude bins in HALOE data. If longer latitude spans or bins are taken (as done by Beig et al. (submitted manuscript, 2011) and Fadnavis et al. [2011]), then the number of sampling points increases, which in turn makes the statistical analysis more robust, but results are representative of a broader range. On the other hand, smaller latitude bins (as adopted by Remsberg [2008, 2009]) are 3. Solar Response in the Mesopause Region 3.1. Northern Hemisphere [9] The majority of the results discussed in this section are obtained from satellite data or from measurements of OH airglow which correspond to nominal altitudes of 87 km. All of the results which are already discussed by Beig et al. [2003, 2008] are either referred to here very briefly or are not discussed. Figure 2 shows the annual mean solar response in temperature (K/100 sfu) as reported in the recent literature for the mesopause region during the past 2 3 decades for the Northern Hemisphere. [10] Offermann et al. [2010] used OH airglow measurements during the period over Wuppertal (51 N, 7 E) to derive the temperature near the mesopause region that is homogeneous. The data is reported to agree well with satelliteborne observations from Sounding of the Atmosphere Using Broadband Emission Radiometry and hence are considered to be robust. The data from the second OH measurement instrument operated at the station of Hohenpeißenberg (48 N, 11 E) since 2003 have been combined with the above Wuppertal data and analyzed. The combined data made the coverage of 80% and hence increased the time resolution. A significant solar response of 3.5 ± 0.2 K/100 sfu is reported by Offermann et al. [2010]. It is stressed by them that the derived solar flux sensitivity of temperature depends significantly on the length and the date of the time interval used. Results can be very different if different phases of the solar cycle are chosen and if the time interval is too short. She et al. [2009] have analyzed temperature data between 85 and 105 km on the basis of 894 nights of Na lidar observation obtained during the period over Fort Collins (41 N, 105 W). This study focuses on the impact of volcanic eruption in temperature and stressed that one should account for the volcanic term in the statistical model to filter out any kind of influence it can have in the detection of solar response. Now that the volcanic free period is much longer than a solar cycle, She et al. [2009] expressed more confidence in their analysis and reported a solar response as 5.6 ± 1 K/100 sfu between 85 and 90 km, which becomes 4of7

5 5 K/100 sfu at 101 km and 4 K/100 sfu at 104 km. However, the results obtained by using the nonlinear approach of She et al. [2009] to take into account the Pinatubo eruption should be viewed with caution. If it is not used, then the magnitude of solar response is likely to reduce, and we could find a good agreement with other results reported recently for the midlatitudes [Offermann et al., 2010; Pertsev and Perminov, 2008]. Earlier, She and Krueger [2004], using the same data set but with shorter time series have reported a solar response of the order of 2.9 ± 1.5 K/decade at the mesopause region. Espy et al. [2011] have used a 10 year time series of July mesospheric temperatures derived from the hydroxyl (OH) nightglow. These measurements of Espy et al. [2011] were made from Stockholm, Sweden (59.5 N, 18.2 E), during the summers of 1991 and 1993 and then continuously from 1993 through After 1998, the instrument was moved to Onsala, Sweden (57.4 N, 11.9 E), and data collection continued through They have performed a regression analysis between the individual nightly mean temperatures and the corresponding F 10.7 flux (in solar flux units) and reported a solar attributable signal of 2.0 ± 0.4 K/100 sfu. [11] Remsberg [2007, 2008, 2009] have mainly reported the HALOE data analysis for mesosphere as reported in section 2 but also provided the solar response at the highest pressure level of 0.07 hpa. The solar cycle responses in temperature by Remsberg [2009] at 80 km are found to be 1.9 K with a standard deviation of around ±0.5 K near the latitude bin of 20 N ± 5 N and 2.8 ± 0.44 K near the latitude bin of 45 N ± 5 N. Beig et al. (submitted manuscript, 2011) have used the broader latitude bin of HALOE data and also reported a solar signal of 1.0 ± 0.5 K/100 sfu around 80 km in the tropics and 0.7 ± 0.7 K/100 sfu for the midlatitude. The latter response for midlatitude is found to be insignificant. Li et al. [2008] analyzed the data obtained by lidar measurements at Mauna Loa (19.5 N) during and found negligible solar response, but the data length was just about a decade long and did not cover even one solar cycle. It may be argued that the response is negligible for a specific location as obtained by lidar data of Li et al. [2008], but the response increases to 1 K if a larger bin (0 30 ) is considered (Beig et al., submitted manuscript, 2011) in HALOE and it further becomes stronger if latitude span is reduced to 10 [Remsberg, 2009]. Alternatively, it may also be stated that lidar results of Li et al. [2008] are averages of measurements obtained over several nighttime hours, whereas the HALOE results are strictly from its SR and SS measurements. The difference in solar signal between lidar and HALOE data may be due to the decadal or solar cycle effect in the tidal forcing at low latitudes, where tidal amplitudes are large [Remsberg, 2008]. Midlatitude solar response of 3 K obtained by lidar at Observatorie Haute Provence, France (44 N) [Keckhut et al., 2005], is found to be in good agreement with Remsberg [2009]. It should be kept in mind that results of Keckhut et al. [2005] are based on the longer period of observations from 1979 to 2001 and include around 12 years of data (1979 onward) which are obtained earlier than 1991, when HALOE started to produce the data. Hervig and Siskind [2006] have used the HALOE VPMC data set for the period for the latitude range, which provides the only long term H 2 O measurements at PMC altitudes and latitudes. They reported the solar cycle temperature variation of 4 5 K at 80 km. For the higher latitudes, Hervig and Siskind [2006] found no statistically significant phase lag in the temperature cycle in either hemisphere. Pertsev and Perminov [2008] have reanalyzed the Russian data of Zvenigorod (56 N, 37 E) for the years and reported an annual mean response of 4.5 ± 0.5 K/100 sfu, which is a little higher than previous results partly reported earlier by Golitsyn et al. [2006] from the same site. Recent analysis [Pertsev and Perminov, 2008] also indicates a strong seasonality in the solar response whichgoesashighas7.5kinthewinter.golitsyn et al. [2006] have earlier consolidated Russian results from their earlier analysis [Golitsyn et al., 1996] on the basis of the rocket data from Volgograd (49 N) and data from different airglow emissions obtained at Abastumani (42 N) and Zvenigorod (56 N), covering two activity cycles ( ) at the altitude of 87 km. They reported the annual mean response to be around 2.7 ± 1.7 K/100 sfu at the mesopause region Southern Hemisphere [12] Figure 3 shows the annual mean solar response in temperature (K/100 sfu) as reported in the recent literature after Beig et al. [2008] for the mesopause region during the past 2 3 decades for the Southern Hemisphere. The most significant results in the Southern Hemisphere reported recently are based on the observations of the hydroxyl nightglow emission with a scanning spectrometer at Davis station, Antarctica (68 S) [French, 2010] for the period from 1995 to This is based on the longest series of 15 years of observations of the hydroxyl nightglow emission in the Southern Hemisphere for winter season. A multivariate fit of the solar flux (F 10.7 ) index term fitted to the above data yields a solar cycle coefficient of 4.2 ± 0.8 K/100 sfu for the winter mean temperature. A peak correlation coefficient, where the F 10.7 index leads OH temperature by 160 days, is also found, indicating that a delayed response to solar forcing is dominant in this region. This point is apparently no longer fully maintained by them in their recent publication [French and Klekociuk, 2011]. Earlier, French et al. [2005] presented a solar response amplitude of 5 K/100 sfu which was nondiscernable at that time and was based on shorter length of the data series as compared to the above. [13] Beig et al. (submitted manuscript, 2011) have used the broader latitude bin of HALOE data and reported a solar signal of 1.0 K/100 sfu with a 2 sigma error of ±0.6 K around 80 km in the tropics (0 S 30 S) and 0.7 ± 0.7 K/100 sfu for the midlatitude (40 S 60 S) of the Southern Hemisphere, which is insignificant. Azeem et al. [2007] have reported the response of mesospheric temperature at 87 km to F 10.7 radio flux determined from the 11 year time series of OH rotational temperature at the South Pole. Multiple linear regression technique is used to fit the solar cycle variation to the deseasonalized OH rotational temperature data set. The estimated amplitude of solar cycle variation in the South Pole OH rotational temperature data set reported by Azeem et al. [2007] is found to be of the order of 4 ± 1 K/100 sfu. The solar response amplitude estimated by Azeem et al. [2007] is about one third that of the temperature increase reported earlier by Hernandez [2003], who suggested a 13 K value per 100 sfu at the South Pole. The study by Hernandez 5of7

6 Scheer, 2002]. A positive response of 3.3 ± 0.3 K/100 sfu is reported when temporal trend is not fitted, but if temporal trend is simultaneously fitted, then solar response reduces to 1.32 ± 0.3 K/100 sfu. Figure 3. Temperature response to solar cycle response in the Southern Hemisphere for the mesopause region. [2003], however, did not account for the variations in the amplitude of various periodic terms, and thus, the estimate of 13 K could potentially contain inherent biases. [14] Remsberg [2008], using the time series of HALOE SR and SS data, have made analysis for their seasonal, interannual, SC like, and trend terms from 60 S to 60 N and from 2 to hpa. The SC like terms are generally in phase with the solar flux forcing in the tropics. There is an increasing SC like response from low to middle latitudes of the upper mesosphere that is presumed to be due to decadalscale, dynamical processes that are also in phase with the solar forcing [Remsberg, 2008]. The SC like term at the pressure level of hpa is found to vary from 1.2 K at 40 S to 2.5 K at 60 S in the midlatitude, whereas it is found to be from 1.2 K at 10 S to 2 K at 30 S for the tropics. The slight difference in the results with Beig et al. (submitted manuscript, 2011) is mainly attributed to the selection of latitudinal bands which is quite narrow (10 ) in the case of Remsberg [2008], whereas it is broad (20 30 wide) for Beig et al. (submitted manuscript, 2011). The mean solar cycle effect in O 2 rotational temperatures measured at El Leoncito (32 S, 69 W) [Scheer et al., 2005] is consistent with the range of upper limits estimated earlier [Reisin and 4. Conclusions and Future Work [15] Although the overall conclusion on the temperature response to solar activity has not changed significantly from the earlier reviews, quite a few additional fine features have emerged internally. Recent investigations revealed that the solar signal in the lower mesosphere is around 0 1 K/100 sfu in the tropics, which is quite consistent among results reported by different authors, but there is a scatter in the upper mesospheric results. Solar signal becomes stronger with increasing latitude in the mesosphere, and hence, the magnitude of midlatitude response is relatively strong. The magnitude of solar response obtained by lidar data is found to be little different as compared to satellite results, especially in the upper part of the mesosphere. The differences may be attributed to the fact that lidar results are averages of measurements obtained over several nighttime hours, whereas the HALOE results are strictly based on the data obtained for sunrise and sunset time only. The differences may also be due to decadal or solar cycle effect in the tidal forcing at tropics, where tidal amplitudes are large. In the Northern Hemisphere, temperature response to solar activity at the mesopause region is around 2 3 K/100 sfu 80 km for midlatitude, which becomes slightly stronger with height and touches 4 5 K above 85 km. The solar response in the Northern Hemispheric mesopause region is found to be generally stronger in a majority of the cases as compared to the response reported in the earlier review [Beig et al., 2008]. [16] The solar response appears to be relatively weaker for the tropical region, but as not many results are available for the low latitude mesopause region, it is premature to conclude something firmly at this stage. In the Southern Hemisphere, temperature response to solar activity at the lower mesopause region is found to be 1 2 K/100 sfu, which becomes stronger ( 4 K/100 sfu) at the OH airglow height. [17] Looking forward as an opportunity, the data from the TIMED satellite are providing a good opportunity to explore the structure of the MLT region. A sufficiently large and good quality data set relating to the atmospheric state parameters, composition, and thermodynamic properties is now becoming available from different satellites and sensors and also from several ground based measurements. These data sets, if analyzed systematically, would enable a detailed and quantitative assessment of the MLT response to the solar cycle and are likely to provide answers to many unresolved issues. Because the solar cycle is a well known, long term influence on the MLT, it must be well understood and, subsequently, well modeled in order to assess and ascribe other changes to anthropogenic origin. Qian et al. [2006, 2011] showed that cooling trends should be larger at solar minimum because of the larger relative role of CO 2 radiative cooling compared to the nitric oxide (NO) radiative cooling. This is confirmed by SABER measurements showing that global integrated NO radiative power decreased by almost an order of magnitude from 2002 to 2009, while 6of7

7 CO 2 radiative power decreased only by 35% [Mlynczak et al., 2010]. [18] Looking forward as a challenge, there are many unresolved compelling scientific questions that need to be answered. The major challenge is in the interpretation of the various reported results which are diverse and even indicate latitudinal and longitudinal variability in solar response. The intervention of dynamics through mediation by planetary waves further compounds the picture, which is likely to become clearer only after more results on the long term solar response become available. Hence, this topic remains a problem to be explored more rigorously in the future. [19] Acknowledgments. The author is grateful to the temperature trend community for providing their valuable input. The author is grateful to Juergen Scheer for helpful discussion in all aspects of this paper and to the director, IITM, for the encouragement for this project work. 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