The SABER radiometer on the TIMED spacecraft scans the earthlimb continuously in ten channels spanning the spectrum from 1.27 to 15 μm. The signature of the diurnal tide in the equatorial region is apparent throughout the mesosphere in TIMED/SABER data, especially in the CO2 15-μm radiance profiles. In addition, layer structures are apparent in a large fraction of the both the radiance profiles and the kinetic temperature profiles derived from them. We present results showing tidal and layer features in the variation with local time and latitude of 15-μm radiance and temperature. Temperature inversion layers (TILs) are regions of extreme perturbations in the retrieved temperature profile where the temperature increases rapidly over 3-10 km range by tens of degrees K, sometimes approaching increases of 100 K, and are not represented in any existing atmospheric climatologies. Theories that have been proposed connect them with the amplitude and phase of atmospheric tides, as well as with the interactions and dissipation of atmospheric gravity waves and planetary waves. The radiance local maxima, or "knees," are more mysterious. Their occurrence is rather unpredictable and not well explained by models, although it is known that they are due to vibrational excitation of CO2 by atomic oxygen and they may have tidal connections. While they may be associated with strong TILs, the most important class occurring at tangent heights in the lower thermosphere between 100 and 115 km appear not to be simply related to local inversion layers. SABER data offers the opportunity to do the first global survey of MLT TILs, determine their spatial extent and persistence time, and develop a global climatology of their occurrence and properties.
The SABER instrument on TIMED continuously measures certain infrared limb radiance profiles with unprecedented sensitivity. Among these are emissions of CO2 ν3 at 4.3 μm, routinely recorded to tangent heights of ~140-150 km, and NO at 5.3 μm, seen to above ~200 km and ~300 km, respectively. We use these infrared channels of SABER and coincident far ultraviolet (FUV) measurements from GUVI on TIMED, to study the geometric storm of April 2002. These all give a consistent measure of auroral energy input into the lower thermosphere at high latitudes. Emission in yet another SABER channel, near 2.0 μm, correlates well with enhanced electron energy deposition. We also have, in the 5.3-μm emissions from the long-lived population of aurorally produced NO, a tracer of how this energy is transported equator-ward and released over an extended period of time, a few days. In this paper, we discuss the global patterns of energy deposition into the expanded auroral oval, its transport to lower latitudes, and its loss as revealed by the NO 5.3-μm emissions.
Analysis of 4.3-μm CO2 radiance data from the MSX (Midcourse Space Experiment) satellite has shown that gravity waves dominate the fluctuations of radiance at 4.3 μm for both earthlimb (above-the-horizon) and downlooking (below-the-horizon) lines-of-sight under a broad class of conditions. We review previous work on the spectra of known sources of gravity waves and on wave filtering mechanisms by M. J. Alexander and others, as well as the characteristics of gravity-wave power spectra. We then consider the power spectra of line-of-sight radiance fluctuations emitted and self-absorbed by an atmosphere perturbed by gravity waves, discussing the shape of the spectrum and the spectral slopes. We show examples of radiance spectra from gravity-wave-perturbed atmospheres that have two different slopes, with a steeper slope at large wavenumber, and discuss mechanisms that can account for this effect. The effect of latitude and season on the 4.3-μm fluctuations will also be considered.
The Sounding of the Atmosphere using Broadband Emission Radiometry
(SABER) experiment was launched onboard the TIMED satellite in
December, 2001. SABER is designed to provide measurements of temperature, constituents, and the key radiative and chemical sources and sinks of energy in the mesosphere and lower thermosphere (MLT). SABER measures Earth limb emission in 10 broadband radiometer channels ranging from 1.27 μm to 17 μm. Measurements are made both day and night over the latitude range from 52°S to 83°N with alternating hemisphere coverage every 60 days. In this paper we
concentrate on retrieved profiles of kinetic temperature (Tk) and CO2 volume mixing ratio (vmr), inferred from observed 15 μm and 4.3 μm limb emissions. SABER-measured limb radiances are in non-local thermodynamic
equilibrium (non-LTE) in the MLT region. The complexity of non-LTE
radiation transfer combined with the large volume of data measured
by SABER requires new retrieval approaches and radiative transfer
techniques to accurately and efficiently retrieve the data
products. In this paper we present the salient features of the
coupled non-LTE Tk/CO2 retrieval algorithm, along with preliminary results.
The nonequilibrium middle and upper atmosphere are very dynamic regions that are structured vertically and horizontally by the presence of persistent temperature inversion layers and by the passage of both atmospheric gravity waves and transient frontal disturbances or bores. The infrared emissions from this part of the atmosphere are already typically not in local thermodynamic equilibrium (LTE) and are further perturbed by the presence of this pervasive atmospheric structure. The inevitable result is highly structured atmospheric emissions that reflect the structure of the atmosphere. Understanding the structure of the atmosphere is essential to understanding the structure of the radiation that it emits. At the same time understanding how atmospheric structure perturbs atmospheric radiation provides a means to sense the perturbing atmospheric processes remotely. We examine methods to calculate the LTE/non-LTE radiative response to temporal and spatial variations of the atmosphere and give examples of applications. We also compare our results with existing field data. Finally, we discuss a proposed new NASA optical/infrared experiment (Waves Explorer) to sense atmospheric gravity waves remotely from earth orbit on a global basis and characterize their sources.
A coupled leaf/canopy radiative transfer model, LCM2, has been developed with NASA and AFRL funding to investigate the feasibility of detecting spectral signatures from within and under a vegetation canopy. The model is unique in that it features direct radiative transfer coupling between the leaf scattering elements and the canopy reflectance, through the within-leaf radiative transfer model, LEAFMOD. The leaf radiative transfer model characterizes photon scattering within a participating homogeneous leaf taking into account both biochemical composition and leaf thickness. The leaf model is incorporated as the scattering element in a canopy reflectance model (CANMOD) assuming a bi-Lambertian leaf scattering phase function. The primary distinction between conventional and canopy radiative transfer is canopy architecture, which is introduced through leaf angle distributions. A Lambertian partially reflecting background is assumed to lie beneath the canopy. The influences of biochemical composition, average leaf thickness, reflecting background, and canopy architecture on canopy reflectance can therefore be investigated. The model is extended here to include azimuthal dependence by considering collided and first collided radiances separately. The LCM2 model with the new azimuthal correction will be coupled to an atmospheric radiative transfer code, MODTRAN4, to simulate a satellite imager response at orbital altitude.
A major objective of the MSX program is to obtain global data of atmospheric and terrestrial backgrounds simultaneously in the infrared (2.6 to 28 micrometers ) and short wave (0.11 to 0.9 micrometers ) spectral regions. This paper presents an overview of the plans and capabilities to measure infrared earthlimb and terrestrial backgrounds with the cryogenic infrared MSX sensor, the spatial Infrared Imaging Telescope (SPIRIT) III sensor. Of particular importance is the characterization of the spatial-temporal structure and the global distribution of mean radiometric levels and spectral content of both quiescent atmospheric and terrestrial backgrounds as well as the backgrounds associated with aurora, mesospheric and noctilucent clouds, stratospheric warmings, wave phenomena, scatter and thermal emissions from clouds, terrain, and other sources. Experiments are planned to collect data as a function of latitude, season, diurnal conditions, magnetic activity and altitude, up to tangent heights of 300 km over a period of 18 months for the infrared (cryogen lifetime) and 5 years for the visible and ultraviolet sensors. Coordinated experiments are planned with ground sites and in conjunction with other satellites. Automated processing will provide rapid data reduction and the generation of data products.
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