A comparison of optically measured daytime OH temperatures over the tropics during solar maximum and minimum periods
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2010
Received: 4 August 2009
Accepted: 15 July 2010
Published: 28 October 2010
This paper deals with the variability of optically measured daytime OH temperatures during two different solar activity epochs, over Trivandrum (8.5°N, 76.5°E), a tropical station in India. The data used for the present study span the period of February–March, during the solar maximum year of 2001 and solar minimum year of 2005. The mean daytime temperature (180±5 K) during the 2001 study period was found to be lower than the mean temperature (195±9 K) of the 2005 study period by ~15 K. However, apart from this noticeable shift in mean temperature level, the variabilities in the temperature of both years were strikingly similar. Moreover, the wave periodicities present during these periods were also quite similar. Possible reasons for these observations were investigated in the context of the various forcings that control the energetics of the equatorial mesopause region. The observations presented in this study are unique and reveal a number of newer aspects of the energetics of the tropical mesopause.
Researchers are becoming increasingly aware of the importance of the mesopause region as they obtain more knowledge of the significance of the coupling processes between various atmospheric regions. The upper mesospheric region, especially the mesopause, is an area of considerable dynamical activity. The short-term variabilities in temperature, wind, and density of this region are dominated by gravity waves, tides, and planetary waves, which grow in amplitude as they propagate upward from the sources in the troposphere and stratosphere. In the mesopause region, the effect of tides on the temperature, wind, and density structures are especially significant because of its proximity to the turbopause. The wave activity and rapid drop in temperature make this region the most turbulent part of the atmosphere. In this context, the mesopause is of primary importance as it represents the coldest part of the atmosphere, which acts as a gateway between the lower atmosphere and the thermosphere.
The thermal structure of the mesopause region, where most of the energy deposition occurs, is mainly controlled by dynamical, chemical, and radiative processes. The mean background temperature in the mesosphere is largely determined by the radiative balance between the absorption and emission of solar radiation (Gavrilov and Roble, 1994). Below 95 km, the absorption of solar radiation is in the Hartley band (200–300 nm). Mesospheric ozone (O3) is the main source of solar heating in this region. Above 95 km, the absorption of solar UV radiation by molecular oxygen (O2) in the Schumann-Runge continuum (135–200 nm) and the Lyman-α line (121.5 nm) are the dominant heat sources. Solar heating is dominated by the absorption of solar UV radiation by O2 and O3 (Mlynczak and Solomon, 1993). An excellent description of the various factors affecting the mesopause energetics during the solar maximum and minimum periods and their individual role in controlling the energy budget of the mesopause region has been reported using the HAMMONIA chemistry climate model by Schmidt et al. (2006). Many other studies have also attempted to determine the contribution of solar heating on mesopause temperatures (Bittner et al., 2002; Offermann et al., 2004; Azeem et al., 2007). A linear relation of mesopause temperature with F10.7-cm radio flux has been established using nighttime hydroxyl (OH) measurements (Offermann et al., 2004). In a recent study by Beig and Fadnavis (2009), a positive correlation (~3 K/100 sfu) was found between the temperature and solar flux at an altitude of 60–75 km. Similarly, the modeling studies of Marsh et al. (2007) and Schmidt et al. (2006) also show a positive solar response at the altitude of the mesopause. In contrast, Bittner et al. (2002) were unable to find a conclusive correlation between OH rotational temperatures and the F10.7-cm flux; rather, they found a reasonable agreement between planetary wave activity and the general solar bipolar magnetic field (22-year Hale cycle). These studies clearly indicate the necessity of extended studies to understand this problem comprehensively.
In addition to the solar forcings, a significant part of the mesospheric energetics is contributed by exothermic reactions involving odd-oxygen and odd-hydrogen species, including the quenching of excited photolysis products (Mlynczak and Solomon, 1993) by radiative cooling associated with infrared emission of CO2 at 15 (Andrews et al., 1987; Rodgers et al., 1992). Chemical heating is dominated by the reaction O3 + H = O2 + OH* and is greater at night when the O3 concentrations are maximum. According to the calculations by Mlynczak and Solomon (1993), about 60% of the energy of this reaction is lost as heat while only 40% is released as airglow. The main source of O3 in the mesopause region is the three-body recombination of O and O2. The O is generated through the photolysis of O2 in the lower thermosphere and brought to the mesopause through downward diffusion or mean advection. A temperature enhancement of 3–4 K is accounted for during nighttime due to this chemical heating (Smith, 2004).
The dynamical parameters which influence the mesopause temperature include tides, gravity waves, and planetary waves. There have been a number of studies dealing with the effect of waves and tides on the mesosphere/lower thermosphere (MLT) region (Fritts and Vincent, 1987; Forbes et al., 1995; Hagan et al., 1999; Beig et al., 2003; French and Burns, 2004). The dynamical factors contribute almost ~30% of the total energy of this region (Beig et al., 2003). Due to the dominant tidal structures, the local time of maximum temperature varies with altitude, latitude, and season. The temperature perturbations at upper mesospheric altitudes associated with the diurnal tides are observed to maximize over the tropics and to vary somewhat regularly with season (Hagan et al., 1999). The diurnal tide is generally the largest amplitude wave in the tropical mesosphere region and has a major impact on not only the dynamics of the region, but also its composition (Akmaev and Shved, 1980). The gravity waves (GWs) propagating through this region would therefore be affected by these tidal wind and temperature fluctuations, resulting in an alteration of the gravity-wave drag in the middle atmosphere, which in turn affects the winds and temperatures. However, all these factors further depend indirectly on solar activity, and they also exhibit significant day-to-day variability. In this context, the study of mesopause energetics in terms of the various parameters those influence it, is very important.
The relative inaccessibility of the mesosphere region compared to other parts of the atmosphere, especially during daytime, has made it rather difficult for researchers to come to a conclusion on the direct solar influence on temperature. Most of the earlier measurements in this region originated from rocket-borne experiments and satellite-borne experiments, such as the wind imaging interferometer (WINDII) and the high-resolution Doppler imager (HRDI) onboard the upper atmosphere research satellite (UARS); this satellite has provided much needed global measurements in recent years (Shepherd et al., 2001 and references therein). However, ground-based Lidars and spectrometers from different locations over the globe have also enabled measurements of upper atmospheric temperatures and its temporal variabilities (Taylor et al., 2001; She et al., 2003; French and Burns, 2004). In terms of equatorial mesospheric energetics, recent observations using meteor wind radar and the unique multi-wavelength dayglow photometer (MWDPM) has already produced some interesting results (Sridharan et al., 1999; Pant et al., 2004; Vineeth et al., 2005).
The MWDPM has been operating at Trivandrum (8.5°N, 76.5°E), a near-equatorial station in India, since 2001. The study reported here is an attempt at comparing the daytime OH temperature during February-March of the solar maximum and minimum years. The solar, chemical, and dynamical factors that have a significant control over OH temperature, as discussed, were expected to be totally different in the solar maximum and minimum years. However, despite all these variabilities, the temperature exhibited a remarkable similarity during the study period (February–March) in both years. The authors feel that such repeatability in the temperatures is very important in terms of our knowledge of the mesopause region and that the scientific community should be aware of our results. An attempt was also made to characterize the important parameters (solar/dynamical) that influence the tropical mesopause temperature using the available limited set of data.
2. Experimental Details
The MWDPM is a unique instrument which can measure daytime intensities of three different wavelengths nearly simultaneously (Sridharan et al., 1998; Vineeth et al., 2005 and references therein). Daytime OH rotational temperatures were estimated by the selection of two wavelengths, 731.6 and 740.2 nm (Meinel 8-3 band). The daytime OH emission intensity measurements at the wavelengths mentioned above were then used to estimate the mesopause temperature adopting the method of Krassovsky (1972). It must be mentioned that the OH emission layer is co-located with the mesopause within the uncertainty of ±4 km on either side of the mesopause. Therefore, for most of the practical cases, the OH measured temperature could be considered as the mesopause temperature, although the height of the OH layer does exhibit some degree of seasonal variability (Liu and Shepherd, 2006). The time spans of the OH temperatures used for this study is 0900–1800 IST (Indian Standard Time) in 2001 and 0900–1700 IST for 2005. The temperatures obtained in each 10-s interval are averaged out for whole day and used to represent that day’s mean temperature. Due to various constraints in making regular observations, continuous temperature measurements were available only during days 35–75 in 2001. However, in 2005, the temperature measurements were available from January to May; thereafter continuous measurements of the dayglow emissions could not be made because of the overcast weather conditions associated with the ‘monsoon’.
3. Results and Discussion
Temperatures measured at the mesopause altitudes have generally been found to be highly variable (both spatially and temporally) from location to location. The main explanatory factor for this variability is that the temperatures are affected by many strong and competing forcings that come from both the atmosphere below (general circulation, various kinds of waves, tides, etc.) and that above (solar cycle influences, geomagnetic activity, etc.). The temperatures appear to be dependent not only on the time but also on other parameters, such as latitude, longitude, and season (Espy and Stegman, 2002; Semenov et al., 2002; Beig et al., 2003). In this context, the striking similarity in the mesopause temperatures during different solar activity epochs as presented here needs to be further investigated.
If the zonal wind, as conjectured earlier, plays a direct role in modulating the temperatures, then one would have expected the temperature trends to be very different during these two years. In contrast, the striking similarity in the temperature trends indicates the plausible role(s) of other dynamical factors, such as the gravity waves. However, it must be mentioned that the wave activity, in turn, can be modulated by the prevailing zonal wind conditions. Therefore, to investigate the possible contribution from factors such as GWs, we subjected the temperatures on each day to Morlet wavelet analysis and separated out the dominant periods. The powers of the wave periods between 0.5 and 2.5 h were averaged for each day to represent the strength of the wave activity.
Although the exact causative mechanisms of the observed temperature variability over Trivandrum have not been established in our study, the question of whether the equatorial variability is the counterpart of the variability over mid/high latitudes needs to be comprehensively studied as a part of the global mesopause dynamics.
The remarkable resemblance in the temperatures, despite of the similarities and differences in the dynamics, chemistry, and solar conditions, is quite intriguing. Since most of the optical techniques operate at nighttime, the daytime mesopause temperatures obtained using optical technique presented in this paper are unique. The repeatability of the temperatures irrespective of the background conditions has significant implications in establishing mean temperature trends of tropical mesopause region. Knowledge of these trends is important for providing insight into the MLT coupling processes and also for quantifying the consequences of unique equatorial geophysical phenomena, such as the equatorial electrojet and counter-electrojet. The observed variabilities in the equatorial mesopause temperatures, as discussed here, correspond more with the dynamical forcings than with the solar activity. However, the data used in this study are inadequate for discussing the long-term trends in mesopause temperature. These factors should be further confirmed with extended data by means of different experiments in multiple platforms.
We report here the first comparison of ‘daytime’ mesopause temperatures observed during two different solar activity epochs from equatorial latitudes. The temperatures show a remarkable similarity in terms of trend and periodicity and are consistent in both study years during the months of February-March. The similarities between temperature, zonal wind, and GW power pattern and the dissimilarity between temperature and F10.7 flux during these periods are indicative of the dominance of dynamical forcings on direct solar forcing. A systematic and significant enhancement in temperature and hydroxyl airglow intensities are observed around day 62 (approx. March 3), which is similar to the ‘springtime’ transition over the mid-latitude region. This enhancement deserves special attention and comprehensive investigation.
This work was supported by the Department of Space, Government of India. One of the authors, C. Vineeth, gratefully acknowledges the financial assistance provided by the Indian Space Research Organization through a research fellowship.
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