This paper presents an UltraViolet-Visible (UV-Vis) spectral radiance simulation capability for Non-Local Thermodynamic Equilibrium (non-LTE) conditions, consisting of a full line-by-line (LBL) radiative transfer (RT) algorithm and a UV-Vis signatures library. Results are presented for two example scenarios where strong UV-Vis emissions arise, an atmospheric high altitude auroral event and a High Explosive (HE) detonation.
Optimal interpretation of remote sensing imagery requires characterizing the atmospheric composition between a sensor and the area it is observing. Timely estimates of atmospheric temperature, water vapor, and other constituents from the ground to the edge of the space environment are not always readily available. In those cases, we must supplement our knowledge of the atmosphere’s composition to fill in any gaps in knowledge and empirical models of the atmosphere are useful tools for this purpose. The Standardized Atmosphere Generator (SAG) was constructed is one such empirical. It has been designed to allow all the major known, systematic variability in the atmosphere and may be used to generate atmospheric profile from the ground to 300 km consistent with user-specified temporal, geophysical, and geographical information Output provides reasonable estimates for temperature, pressure, and densities of atmospheric constituents and can be directly incorporated into radiative transfer forward models or retrieval algorithms. SAG draws upon a number of existing empirical atmospheric models and ensures consistency of output between them. It can be used either as a stand-alone interactive program or scripted for batch execution and assist in determining atmospheric attenuation, refraction, scattering, chemical kinetic temperature profiles, and a host of other naturally occurring processes. Here, we will discuss the capabilities and performance of the SAG model for a variety of applications including its interactive and batch processing use. We will also demonstrate the physical realism of SAG through a small number of relevant use cases.
KEYWORDS: Clouds, Monte Carlo methods, 3D modeling, Atmospheric modeling, Sun, Image fusion, Sensors, Point spread functions, Atmospheric sensing, Transmittance
A calculation method has been developed for rapidly synthesizing radiometrically accurate ultraviolet through longwavelengthinfrared spectral imagery of the Earth for arbitrary locations and cloud fields. The method combines cloudfree surface reflectance imagery with cloud radiance images calculated from a first-principles 3-D radiation transport model. The MCScene Monte Carlo code [1-4] is used to build a cloud image library; a data fusion method is incorporated to speed convergence. The surface and cloud images are combined with an upper atmospheric description with the aid of solar and thermal radiation transport equations that account for atmospheric inhomogeneity. The method enables a wide variety of sensor and sun locations, cloud fields, and surfaces to be combined on-the-fly, and provides hyperspectral wavelength resolution with minimal computational effort. The simulations agree very well with much more time-consuming direct Monte Carlo calculations of the same scene.
This paper presents results that demonstrate the auroral modeling capabilities of the Air Force Research Laboratory
(AFRL) SAMM2 (SHARC And MODTRAN® Merged 2) radiance code. A scene generation capability is obtained by
coupling SAMM2 with a recently developed Clutter Region Atmosphere and Scene Module (CRASMO), which
provides an approach for rapid generation of time sequences and images of radiance clutter. Modeled results will be
compared to data collected by the Midcourse Space Experiment (MSX)1 in the IR and UV-visible spectral regions during
an auroral event on November 10, 1996.
The paper is organized as follows. We first present a brief history of the AFRL SHARC/SAMM codes, leading up to the
current version, SAMM2 v.2. The SAMM2 UV-visible auroral kinetic model will then be described, followed by a
comparison of modeled results to the MSX data.
We investigated the contributions of the hydroxyl (OH) airglow to the illumination of resident space objects.
During nighttime, in a moonless sky, the airglow is the largest contributor to the sky brightness in the visible
(vis), the near-infrared (NIR) and short-wave infrared (SWIR) spectral region. The dominant contributors to
the airglow are vibrationally excited hydroxyl radicals, OH(ν). The radicals are formed in vibrational states
up to υ=9 by the reaction of hydrogen atoms with ozone. The strong emissions, known as Meinel emissions,
are sequences with σν= 1-6. Emissions with υ = 3, 4, 5 and 6 occur in the visible and NIR between .4 and 1.0
µm. From 1.0 to 2.5 µm there are very strong emissions from the δν= 2 sequences. The σν= 1 emissions
extend into the thermal infrared to 4.5 μm. In this work, we considered four band passes, a vis-NIR band
pass, two SABER band passes centered at 1.6 and 2.0 μm, respectively, and a broad band pass around 2.7
µm. SAMM2 was utilized to compute spectra and line of sight radiances. We used line of sight (LOS)
radiances to compute the irradiance on a space object that was taken as a flat plate with a Lambertian surface
reflectance. Profiles of irradiance versus orientation were calculated. The OH airglow will illuminate a facet
even if it is pointing somewhat upward. However, the irradiance in the 2.7 μm band pass comes almost
entirely from the atmosphere in the low altitude and the earth emission.
A new correlated-k algorithm has recently been incorporated into SAMM-2, the Air Force Research Laboratory
background radiance and transmission code. SAMM-2 incorporates all of the major components necessary for
background scene generation at all altitudes: atmospheric characterization, solar irradiance, molecular chemical kinetics
and molecular spectroscopic data. The underlying physical models are applicable for both low-altitude local
thermodynamic equilibrium (LTE) conditions as well as high-altitude non-LTE (NLTE) conditions. Comprehensive
coverage in the .4 to 40 micron (250 to 25,000 wavenumber) wavelength region for arbitrary lines-of-sight (LOS) in the
0 to 300 kilometer altitude regime is provided. A novel 1 cm-1 resolution correlated-k algorithm has been developed in
order to provide the orders-of-magnitude increase in computational efficiency when compared to the existing SAMM-2
line-by-line (LBL) algorithm and applicable to both LTE and NLTE atmospheric conditions. The SAMM-2 correlated-k
algorithm processes molecular lines at runtime by reading line center information from the HITRAN 2000 database and
computing statistical cumulative probability distributions within a spectral interval under the presumption of a Voigt line
shape profile. This algorithm is useful for treating atmospheric phenomena at all altitudes requiring a spectrally
monochromatic treatment of the atmospheric transmission and/or radiance, including multiple scattering or atmospheric structure.
Assuming large signal-to-noise ratio and using the rotationally resolved fundamental vibration-rotation band emission from NO near 5.3 μm we propose a scheme for remotely sensing temperature above the altitudes where the 15 μm emission from CO2 becomes very weak. We also find that the rotationally resolved 5.3 μm emission can be used to remotely sense N(4S) atom, O2, and O densities in the terrestrial thermosphere -- this being the only method for remotely sensing the first two species.
Radiation transport modulates the spatial frequencies of atmospheric structures, acting as a low pass filter, which causes the power spectra of the accumulated radiance to have different power spectral slopes than the underlying atmospheric structure. Additional effects arise because of the non-stationarity of the atmosphere. The SHARC atmospheric radiance code is used to model both non- stationarity of the atmosphere. The SHARC atmospheric radiance code is used to model both equilibrium and non- equilibrium radiance and radiance fluctuation statistics. It predicts two dimensions. Radiance spatial covariance functions and power spectral densities, PSDs. Radiance power spectral slopes for paths through isotropic Kolmogorov turbulence are predicted to vary from -5/3 to -8/3 depending on the length of the path through the turbulence. The input gravity wave 3D covariances and PSDs of atmospheric temperature are consistent with current gravity wave theory, having vertical and horizontal power spectral indices of -3 and -5/3, respectively. Altitude profiles of variances and correlation lengths account of the non-stationary of the gravity wave structure in the atmosphere. The radiance covariance and PSD power spectral slopes differ from the atmospheric gravity wave temperature model values of -3 and -5/3. These modulations depend on LOS orientations, and scale lengths of the sampled altitudes along the LOS.
Classical trajectory calculations have been performed to determine the rotational distribution of vibrationally excited nitric oxide from collisions with atomic oxygen. The reaction occurs on two electronic potential energy surfaces which must be considered for a realistic description of the O+NO collision dynamics. The results, which have been statistically averaged over both electronic potential energy surfaces, are in good agreement with the available experimental data for vibrational relaxation of NO(v less than or equal to 9), as well as the temperature dependence of NO(v equals 1). The state-to-state relaxation rate coefficients involve the formation of long-lived collision complexes and indicate statistical behavior in O+NO collisions. The present study confirms earlier analysis that the NO(v equals 1) rotational distributions can indeed by described by a Maxwell-Boltzmann distribution, albeit with a rotational temperature of approximately 75% of the initial translational temperature. Thus, it appears possible to establish a lower bound to, and an estimate of, the nighttime quiescent terrestrial thermosphere by measuring the rotational envelope of the 5.3 micrometer emission from NO.
Atmospheric infrared radiance fluctuations result from fluctuations in the density of atmospheric species, individual molecular state populations, and kinetic temperatures and pressures along the sensor line of sight (LOS). The SHARC-4 program models the atmospheric background radiance fluctuations. It predicts a two dimensional radiance spatial covariance function from the underlying 3D atmospheric structures. The radiance statistics are non-stationary and are dependent on bandpass, sensor location and field of view (FOV). In the upper atmosphere non-equilibrium effects are important. Fluctuations in kinetic temperature can result in correlated or anti-correlated fluctuations in vibrational state temperatures. The model accounts for these effects and predicts spatial covariance functions for molecular state number densities and vibrational temperatures. SHARC predicts the non-equilibrium dependence of molecular state number density fluctuations on kinetic temperature and density fluctuations, and calculates mean LOS radiances and radiance derivatives. The modeling capabilities are illustrated with sample predictions of MSX like experiments with MSX sensor bandpasses, sensor locations and FOV. The model can be applied for all altitudes and arbitrary sensor FOV including nadir and limb viewing.
This paper describes the development of a new version of the SHARC code, SHARC-3, which includes the ability to simulate changing atmospheric conditions along the line-of-sight (LOS) paths being calculated. SHARC has been developed by the U.S. Air Force for the rapid and accurate calculation of upper atmospheric IR radiance and transmittance spectra with a resolution of better than 1 cm-1 in the 2 to 40 micrometers (250 to 5,000 cm-1) wavelength region for arbitrary LOSs in the 50 - 300 km altitude regime. SHARC accounts for the production, loss, and energy transfer processes among the molecular vibrational states important to this spectral region. Auroral production and excitation of CO2, NO, and NO+ are included in addition to quiescent atmospheric processes. Calculated vibrational temperatures are found to be similar to results from other non-LTE codes, and SHARC's equivalent-width spectral algorithm provides very good agreement with much more time-consuming `exact' line-by-line methods. Calculations and data comparisons illustrating the features of SHARC-3 are presented.
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