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Mesopause Structure from Thermosphere, Ionosphere JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D09102, doi:10.1029/2006JD007711, 2007 Mesopause structure from Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) observations Jiyao Xu,1 H.-L. Liu,1,2 W. Yuan,1 A. K. Smith,3 R. G. Roble,2 C. J. Mertens,4 J. M. Russell III,5 and M. G. Mlynczak4 Received 29 June 2006; revised 14 November 2006; accepted 17 December 2006; published 2 May 2007. [1] Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/ Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) temperature observations are used to study the global structure and variability of the mesopause altitude and temperature. There are two distinctly different mesopause altitude levels: the higher level at 95–100 km and the lower level below 86 km. The mesopause of the middle- and high-latitude regions is at the lower altitude in the summer hemisphere for about 120 days around summer solstice and is at the higher altitude during other seasons. At the equator the mesopause is at the higher altitude for all seasons. In addition to the seasonal variation in middle and high latitudes, the mesopause altitude and temperature undergo modulation by diurnal and semidiurnal tides at all latitudes. The mesopause is about 1 km higher at most latitudes and 6–9 K warmer at middle to high latitudes around December solstice than it is around June solstice. These can also be interpreted as hemispheric asymmetry between mesopause altitude and temperature at solstice. Possible causes of the asymmetry as related to solar forcing and gravity wave forcing are discussed. Citation: Xu, J., H.-L. Liu, W. Yuan, A. K. Smith, R. G. Roble, C. J. Mertens, J. M. Russell III, and M. G. Mlynczak (2007), Mesopause structure from Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) observations, J. Geophys. Res., 112, D09102, doi:10.1029/2006JD007711. 1. Introduction Zahn et al., 1996; Yu and She, 1995; She et al., 1993] but exist at only a few sites. [2] The mesopause, defined as the temperature minimum [3] A 10-year climatology from lidar measurements indi- that separates the middle atmosphere from the thermosphere, cates that there are two distinct mesopause altitudes, one at is the coldest region in the terrestrial atmosphere. The mean about 100 km during winter and the other at 86 km during mesopause location and temperature are established by summer [von Zahn et al., 1996; She et al., 1993; Yu and radiative and dynamical processes [e.g., Leovy, 1964; Holton, She, 1995; Leblanc et al., 1998; She and von Zahn, 1998]. 1983] and also display large variability due to gravity waves, These observations, however, were from separate local sites planetary waves and tides. Both lower atmospheric and based on a limited number of observation days, and most thermospheric processes can contribute to the variability. were taken during nighttime. The temperature profile is the most important parameter for [4] The NASA Thermosphere, Ionosphere, Mesosphere, characterizing this transition from the mesosphere to the Energetics, and Dynamics (TIMED) satellite was launched thermosphere; two ground-based methods that are currently in December 2001, carrying four instruments for investi- used for determining it are in situ rocket measurement [e.g., gating the region between 60 and 180 km. The Sounding of Lu¨bken and von Zahn, 1991; Fritts et al., 2004; Lu¨bken and the Atmosphere Using Broadband Emission Radiometry Mu¨llemann, 2003; Lu¨bken, 1999] and sodium lidar measure- (SABER) instrument obtains global profiles of temperature ments. The lidars have very high vertical and temporal and other parameters [Russell et al., 1999; Mertens et al., resolution in the altitude region of 80–110 km [e.g., von 2001]. In this paper we analyze the global altitude and temperature of the mesopause and their latitudinal, 1State Key Laboratory of Space Weather, Chinese Academy of seasonal and diurnal variations as measured by SABER. Sciences, Beijing, China. Simulations with the thermosphere-ionosphere-mesosphere 2High Altitude Observatory, National Center for Atmospheric Research, electrodynamics–global climate model (TIME-GCM) help Boulder, Colorado, USA. 3 in the interpretation of the results and the implications for Atmospheric Chemistry Division, National Center for Atmospheric mesospheric dynamics. Research, Boulder, Colorado, USA. 4NASA Langley Research Center, Hampton, Virginia, USA. 5Department of Physics, Hampton University, Hampton, Virginia, USA. 2. Data Processing Copyright 2007 by the American Geophysical Union. [5] The data used in this paper for calculating the global 0148-0227/07/2006JD007711 distribution of the mesopause altitude are temperature pro- D09102 1of11 D09102 XU ET AL.: MESOPAUSE STRUCTURE D09102 Figure 1. The global distribution of mesopause (a) altitude (units are km) and (b) temperature (units are K), for a 60-day period centered around March based on 4-year-averaged Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) temperature. files from the SABER instrument onboard the TIMED marked as missing were used in this study. In other words, satellite. The height of the TIMED orbit is about 625 km, we did no extra screening to remove profiles or individual the inclination is 74.1°, and the period is about 1.6 h. Because temperature measurements that were outliers. TIMED is a slowly precessing satellite, it takes about 60 days [8] In order to analyze the mesopause altitude for the full to complete a full 24-hour coverage of local time. 24-hours local time, we apply 60-day windows on the [6] SABER measures temperature from the lower strato- SABER data. Averages over shorter periods include only sphere to the lower thermosphere by limb observation of a subset of local times, which can affect the mean altitude CO2 infrared emission. The inversion takes into account and temperature of the mesopause. Analysis labeled by a departures from local thermodynamic equilibrium (LTE) particular month includes 30 days data in that month plus and variations in CO2 mixing ratio (measured by SABER 15 days before and after the month. The data used extend during daytime). Mertens et al. [2004] present comparisons from February 2002 through February 2006, with averaging of preliminary SABER observations to in situ falling sphere periods centered on each month. measurements at high latitudes. The comparisons indicate [9] For averaging over all local times, the mesopause that the mesopause temperatures are similar between the two temperatures and altitudes were first sorted into the 24 local but that the SABER mesopause altitude is lower than that time bins and averaged. Then values for all local time bins determined by the falling sphere method by about 2–4 km. that were not empty were averaged. Note that a few bins This difference persists in more recent versions of the around local noon were never filled. This introduces a bias SABER retrieval (Version 1.06). The exact cause of this into the diurnal means, especially in the tropics where the discrepancy is not clear, though recent study suggests that diurnal temperature perturbations are largest. non-LTE retrieval with additional CO2 v-v exchange pro- cesses can bring the SABER summer mesopause altitudes 3. SABER Mesopause Altitude and Temperature closer to ground base observations [Kutepov et al., 2006]. The present study uses Version 1.06 Level 2A data. Four [10] In this study we are going to focus only on the years of SABER data from February of 2002 to February of climatological mean temperature structure and its variation 2006 are used in this paper. with solar local time. We thus average the temperature along [7] The SABER temperature inversion provides profiles the latitude circle for each local time hour and each latitude on pressure levels. The effective field of view of the bin over the 60-day period. This averaging will remove or SABER instrument gives a vertical resolution of about reduce the nonmigrating tidal components, but the migrat- 2km[Mertens et al., 2001]. Altitude information for each ing components will remain. The confidence level of the temperature measurement is a standard data product that averaged temperature can be estimated. At June solstice at was obtained by integrating the temperature hydrostatically the mesopause for the local time bin 0–1 hour, for example, from a reference position in the stratosphere, using NCEP the analysis error estimates for a confidence level of 95% analyses. In our analysis, the mesopause location for each are 2.4 K, 2.4 K, and 5.8 K for the latitudes of 0°,45°N, and profile is identified by determining the minimum tempera- 75°N, respectively. The mesopause altitude and temperature ture. Mesopause temperature and altitude data are binned in are then calculated from these averaged profiles. It should 10 degrees of latitude (from 90°S–90°N) and 1 hour of be noted here that, in this analysis, we take the minimum local time, and the binning is performed every 5 degrees . temperature of the vertical profile as the mesopause. When There are two hours of missing data around noon in almost more than one local minimum is present in a profile, as can every latitude [Zhu et al., 2005]; at high latitudes, the length happen in the presence of tides or other waves, the smallest of the missing midday measurements becomes longer: up to among these local minima determines the mesopause. 3–4 hours varying with season. The SABER data not Figures 1 and 2 give the results for the March and 2of11 D09102 XU ET AL.: MESOPAUSE STRUCTURE D09102 Figure 2. (a, b) Similar to Figure 1 but for September. September equinoxes for the 4-year-averaged results. The night. The nighttime mesopause positions are in good mesopause altitude is between 95 and 100 km around qualitative agreement with the bimodal mesopause altitudes equinoxes, but dips below 90 km for a few hours at low derived from nighttime potassium lidar measurements in latitudes during the morning and at middle latitudes during June of 1996 [vonZahnetal.,1996],exceptthatthe night.
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