MODTRAN4, version 2, will soon be released by the U.S. Air Force Geophysics Laboratory; it is an extension of the MODTRAN4, v1, atmospheric transmission, radiance and flux model developed jointly by the Air Force Research Laboratory / Space Vehicles Directorate (AFRL / VS) and Spectral Sciences, Inc. The primary accuracy improvements in MODTRAN4 remain those previously published: (1) the multiple scattering correlated-k approach to describe the statistically expected transmittance properties for each spectral bin and atmospheric layer, and (2) the Beer-Lambert formulation that improves the treatment of path inhomogeneities. Version 2 code enhancements are expected to include: *pressure-dependent atmospheric profile input, as an auxiliary where the hydrostatic equation is integrated explicitly to compute the altitudes, *CFC cross-sections with band model parameters derived from pseudo lines, *additional pressure-induced absorption features from O2, and *a new 5 cm-1 band model option. Prior code enhancements include the incorporation of solar azimuth dependence in the DISORT-based multiple scattering model, the introduction of surface BRDF (Bi-directional Radiance Distribution Functions) models and a 15 cm-1 band model for improved computational speed. Last year's changes to the HITRAN database, relevant to the 0.94 and 1.13 micrometers bands of water vapor, have been maintained in the MODTRAN4,v2 databases.

MODTRAN4, the newly released version of the U.S. Air Force atmospheric transmission, radiance and flux model is being developed jointly by the Air Force Research Laboratory/Space Vehicles Directorate and Spectral Sciences, Inc. It is expected to provide the accuracy required for analyzing spectral data for both atmospheric and surface characterization. These two quantities are the subject of satellite and aircraft campaigns currently being developed and pursued by, for instance: NASA (Earth Observing System), NPOESS (National Polar Orbiting Environmental Satellite System), and the European Space Agency (GOME--Global Ozone Monitoring Experiment). Accuracy improvements in MODTRAN relate primarily to two major developments: (1) the multiple scattering algorithms have been made compatible with the spectroscopy by adopting a corrected-k approach to describe the statistically expected transmittance properties for each spectral bin and atmospheric layer, and (2) radiative transfer calculations can be conducted with a Beer-Lambert formulation that improves the treatment of path inhomogeneities. Other code enhancements include the incorporation of solar azimuth dependence in the DISORT- based multiple scattering model, the introduction of surface BRDF (Bi-directional Radiance Distribution Functions) models and 15 cm-1 band model for improved computational speed.

FLAASH (Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes) is a MODTRAN-based "atmospheric correction" software package which is being developed by the Air Force Research Laboratory, Hanscom AFB, and Spectral Sciences, Inc., to support current and planned SWIR/visible/UV hyperspectral and multispectral sensors, typically in image format. The AF intent is to provide surface reflectance and emissivity image cubes of sufficient accuracy for input into subsequent analyses of surface properties, effectively removing the atmospheric component. The main objectives are (1) accurate, physics-based descriptions of surface and atmospheric properties (such as surface albedo, relative elevation, water vapor column, aerosol and cloud optical properties, and temperatures), (2) minimal computational time requirements, and (3) interactive, user-friendly interface for generating MODTRAN4-based look-up tables. Validation and development exercises are being carried out on data from the airborne AVIRTS and HYDICE sensors, which cover the 0.4-2.5 /spl mu/m region; applications are also planned for infrared sensors. The algorithms for deriving the surface and atmospheric properties utilize the full MODTRAN4 accuracy (thermal and solar) and account for adjacency effects associated with atmospheric scattering. A new line-tail treatment and a correlated-k (CK) radiative transfer algorithm provide improved accuracy, especially under conditions of partial cloud cover and/or heavy aerosol loading.

A new, state-of-the-art atmospheric correction algorithm for the solar spectral range has been developed based on the MODTRAN4 code. The primary data products are surface reflectance spectra, column water vapor maps and relative surface elevation maps. In addition, a radiance simulation tool, an automated visibility retrieval algorithm and a spectral "polishing" algorithm are included. Validations of retrievals have been carried out by analyzing data that encompass a variety of atmospheric and surface conditions. Some results and their implications for atmospheric correction and spectroscopy are discussed.

MODTRAN4, the latest publicly released version of MODTRAN, provides many new and important options for modeling atmospheric radiation transport. A correlated-k algorithm improves multiple scattering, eliminates Curtis-Godson averaging, and introduces Beer's Law dependencies into the band model. An optimized 15 cm(-1) band model provides over a 10-fold increase in speed over the standard MODTRAN 1 cm(-1) band model with comparable accuracy when higher spectral resolution results are unnecessary. The MODTRAN ground surface has been upgraded to include the effects of Bidirectional Reflectance Distribution Functions (BRDFs) and Adjacency. The BRDFs are entered using standard parameterizations and are coupled into line-of-sight surface radiance calculations.

Atmospheric emission, scattering, and photon absorption degrade spectral imagery data and reduce its utility. We report on the use of an atmospheric compensation code for the visible and near-infrared, based on MODTRAN 4, that includes spectral analysis, accounts for interference to a given pixel by adjacent pixels, and provides a polishing routine to clear residual atmospheric spectral features common to a group of pixels. A NASA/JPL AVIRIS data sample is analyzed.

We have obtained blue photometric measurements of the N2 second positive 399.8 nm and the N+2 first negative 427.8 nm emission from sprites, elves and lightning, along with supporting video images. The pulse width and intensity results for sprites are consistent with those ofSuszcynsky et al. (1998). The red emission from sprites has been independently and unambiguously identified byHampton et al. (1996)andMende et al. (1995)as the nitrogen first positive band. The source has been attributed to electron impact excitation from low energy electrons (≈1 eV) in the sprite. The short pulse width of the 427.8 nm and 399.8 nm photometer time traces obtained in this investigation are probably not from the same source that produces the red emission. The results reported here indicate an initial energetic ionizing event sufficient to ionize and excite nitrogen followed by secondary electron processes which give rise to the dominant red emission. The photometer results for elves are consistent with the EMP mechanism suggested byInan et al. (1996). The photometer traces obtained for lightning indicate emissions consistent with a ‘continuing current’ as the charge redistributes within the thunderstorm cloud. We find that the ratio of the intensity of the 399.8 nm N2(2P) emission to that of 427.8 nm N+2(1N) emission can be used to discriminate among sprites, elves and lightning.