First Principles Modeling

ars_thumbA first principles physics model is one that seeks to calculate a physical quantity starting directly from established laws of physics without making assumptions such as empirical or fitted parameters. The most common example is ab initio calculations of electronic structure of atoms and materials using Schrödinger's equation within a set of approximations that do not include fitting the model to experimental data. The value of this approach is the calculation of physical quantities with no input parameters, or a minimal set, that provides an estimate of the true state of the physical system. This often provides an accurate initial approximation from which inferences can be made for physical applications.

CPI has developed a number of first-principle physics models for radiation propagation through or generated by the atmosphere, including those for the lower atmosphere for target signature modeling and scene generation, and for the upper atmosphere for space weather applications. This software includes community standards such as the MOSART, TERTEM, AURIC, and B3C models which are used by Government and industrial researchers and analysts, as well as development and use of ionospheric models. Applications include describing aurora, predicting optical backgrounds, calculating atmospheric transmission and emission for remote sensing, and forecasting the space weather environment in the ionosphere. CPI has significant capabilities in five major areas.

B3C Auroral Electron/Proton/H-atom Transport Model
CPI's auroral model (Boltzmann 3-Constituent, or B3C) has been used extensively by CPI scientists and others within the auroral community to investigate emission and ionization characteristics in theoretical studies, data analyses, and development of remote sensing algorithms. Pure electron, pure proton/H atom, and mixed electron/proton/H atom aurora may be addressed by the model. Particle transport calculations are performed that provide altitude profiles of particle fluxes versus energy and pitch angle for specifying excitation and ionization rates. These rates, in turn, are used to calculate column emission rates for a variety of optical emission features and density profiles of chemically active species (neutral and ion).

Approximately thirty papers have been published in peer-reviewed journals (mostly in the Journal of Geophysical Research). Categories include:

B3C has played a central role in algorithm development for the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) auroral products and their validation for Defense Meteorological Satellite Program (DMSP) satellites F16 to F18. Validation activities include the use of monoenergetic yields obtained from series of B3C runs to rapidly map coincident particle data (from the SSJ/5 sensor) to column emission rates in statistical studies for SSUSI-SSJ/5 comparisons. Products obtained from the above algorithm work include parameters characterizing particle precipitation and the associated auroral E-layer, namely the ionosphere between ~90 and 180 km. The images below provide samples of data used by our algorithms. Shown is DMSP satellite F16 SSUSI data from two of the three channels which provide the needed inputs for product generation. LBHL refers to emission from N2 between ~165 and 180 nm, while Ly α refers to emission at 121.6 nm produced by proton aurora and the hydrogen geocoronal background. The third channel (not shown), LBHS, refers to emission between ~140 to 153 nm. The differences in the atmospheric absorption and emission between the two channels make it possible to monitor the intensity and spectral hardness of the precipitation.

Far ultraviolet auroral images from the SSUSI sensor on DMSP satellite F16 in two of SSUSI's five spectral channels (LBHL refers to emission from N2 between ~165 and 180 nm while Ly α refers to emission at 121.6 nm produced by proton aurora and hydrogen geocoronal background). Data from these channels and from a third (LBHS between ~140 to 153 nm) are used to specify B3C-based products which quantify particle precipitation and auroral E-layer parameters.

AURIC Upper Atmosphere Dayside and Nightside Model
CPI has a long heritage in first-principles modeling of processes occurring in the dayside thermosphere. Such modeling has been used in theoretical investigations to better understand basic processes, data analysis (rocket and satellite), simulation of sensor data, and development of remote sensing algorithms. CPI's current capability is embodied in the Atmospheric Ultraviolet Radiance Integrated Code (AURIC) model that is described in detail by Strickland et al. (1999). AURIC was developed with Air Force funding through the Phillips Laboratory, Hanscom AFB (currently part of AFRL) with the emphasis on re-engineering of existing CPI research codes. Many enhancements were made including a more comprehensive chemistry model (for neutral and ionospheric species), new radiative transfer capabilities, the option of performing photoelectron energy degradation with or without transport, updates to electron impact cross sections (Majeed and Strickland, 1997; Strickland et al., 1997), and the addition of new emission features. AURIC is currently in use by a number of organizations such as AFRL, NRL, and APL. (The model and a user's manual may be downloaded from our Products page.) Examples of applications papers using AURIC are Siskind et al. (1995), Swaminathan et al. (1998), Strickland, et al. (1997), Strickland et al. (2004), and Strickland et al. (2007).

Examples of AURIC's capabilities is the figure below showing the abundance of O relative to that of N2 in a highly disturbed thermosphere. The image was generated by applying the AURIC-based remote sensing algorithm to far ultraviolet imaging data from the Global Ultraviolet Imager (GUVI) sensor on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. Like SSUSI on DMSP, GUVI provides images in five spectral channels. Data used to produce the image below come from the 135.6 and LBHS channels which are dominated by O and N2 emission, respectively. The extreme behavior shown here arises from a major geomagnetic disturbance commencing on the previous day.

Abundance of O relative to that of N2 in a highly disturbed thermosphere.


AURIC also models nightglow in the form of O atom recombination (this is important in the vicinity of 85 km) and O+ recombination (important at higher altitudes). Examples of nightglow spectral radiances may be seen in Strickland et al. (1999).

Ionospheric Modeling
CPI has been active in both the development and use of first principles ionospheric models. For example, under Phase II of an SBIR contract, CPI added light ions (H+ and He+) to AFRL's Global Theoretical Ionospheric Model (GTIM) to allow limited modeling of the plasmasphere and a more realistic topside ionosphere. In addition to using this to develop CPI's Parameterized Appleton Anomaly Model (PAAM), CPI has used this model to conduct space weather research. This activity includes modeling storm time foF2 observed by ionosondes while varying the thermospheric composition in accordance with the O/N2 ratio obtained from the analysis of DE-1 images. The AURIC and B3C models also have ionosheric components for E-region modeling. Illustrations of modeling results from AURIC may be seen in the figure below. The auroral E-region validations under the DMSP SSUSI F16-F18 programs for sensor calibration/validation made exclusive use of B3C for SSUSI product generation.

Preflare (Nov 3 2003) and flare (Nov 4 2003) electron densities (panel d) derived with the AURIC model. Corresponding ion product rates appear in panel c. Ionization arises from two sources: 1) photoionization and 2) ionization by photoelectrons. Illustrations of spectral characteristics of photoelectrons are given in panels a (source functions) and b (fluxes) under preflare and flare conditions. The altitude for illustrating photoelectron behavior is 120 km. See Strickland et al., [2007] for further discussion of these results.

Atmospheric Radiative-Transfer Modeling
CPI scientists have extensive experience in accurately and realistically modeling the propagation of electromagnetic radiation through the atmosphere. CPI's depth of expertise is reflected in CPI having primary responsibility for developing Moderate Spectral Atmospheric Radiance and Transmittance Code (MOSART), a U.S. Department of Defense standard code, that accurately and realistically calculates the atmospheric transmission and radiance along sensor-target and sensor-background lines-of-sight paths and optical radiance backgrounds against which targets are detected by sensor systems. This capability to calculate atmospheric transmission and radiation in the ultraviolet (UV) through microwave spectral regions (0.2 μm to infinity or 0 - 50,000 cm-1) is used to support both scene and signature simulations, particularly for infrared (IR) signature studies. MOSART is built upon decades of development of first-principles modeling codes that began in the mid-1970s, and that finally resulted from the combining of the Atmospheric Propagation and Radiative Transfer (APART) and MODTRAN® codes in Dec 1995, as a result of funding through the Air Force Phillips Laboratory (AFPL). Subsequent development of MOSART is continuing under funding from the U.S. Naval Research Laboratory (NRL) and the Missile Defense Agency (MDA). Substantial independent verification and validation (IV&V) has been performed with MOSART using measurements conducted at the Atmospheric Radiation Measurement (ARM) Program at Lamont, Oklahoma.

Validation Using ARM Measurements of Direct Solar Irradiance

Validation Using ARM Measurements of Diffuse Solar Irradiance

Validation for Direct Normal Solar Irradiance

Cornette, W.M., P.K. Acharya, and G.P. Anderson (1994) Using the MOSART Code for Atmospheric Correction. Invited Paper. 1994 International Geoscience and Remote Sensing Symposium Proc. pp. 215-219

Cornette, W.M., S.J. Westmoreland, P.K. Acharya, A. Berk, D.C. Robertson, G.P. Anderson, W.A.M. Blumberg, and L.S. Jeong, (1998) The MOSART Code. Proc. NATO Research and Technology Organization, Sensors and Electronics Panel, Symposium on E-O Propagation, Signature, and System Performance Under Adverse Meteorological Conditions, Considering Out-of-Area, Operations, Napoli, Italy.

Cornette, W.M. and J.M. Goldspiel, (2010) MOSART V3.0:  A Four-Dimensional Radiative Environment Prediction Tool. Proc IEEE-GRSS/AFRL Atmospheric Transmission Modeling Conference, Lexington,

Majeed, T.; and Strickland, D. J. (1997), New Survey of Electron Impact Cross Sections for Photoelectron and Auroral Electron Energy Loss Calculations, J. Phys. Chem. Ref. Data, 26(2), 335, doi:10.1063/1.556008.

Siskind, David E.; Strickland, D. J.; Meier, R. R.; Majeed, T.; Eparvier, F. G. (1995), On the Relationship Between the Solar Soft X Ray Flux and Thermospheric Nitric Oxide: An Update with an Improved Photoelectron Model, J. Geophys. Res., 100(A10), pp. 19687–19694, doi:10.1029/95JA01609.

Strickland, D. J.; Majeed, T.; Evans, J. S.; Meier, R. R.; Picone, J. M. (1997), Analytical representation of g factors for rapid, accurate calculation of excitation rates in the dayside thermosphere, J. Geophys. Res., 102(A7), pp. 14485–14498, doi:10.1029/97JA00943.

Strickland, D. J., J. Bishop, J.S. Evans, T. Majeed, P.M. Shen, R.J. Cox, R. Link, R.E. Huffman (1999), Atmospheric Ultraviolet Radiance Integrated Code (AURIC): theory, software architecture, inputs, and selected results, Journal of Quantitative Spectroscopy & Radiative Transfer, 62(6), 689-742, doi:10.1016/S0022-4073(98)00098-3,.

Strickland , D. J., R. R. Meier, R. L. Walterscheid, J. D. Craven, A. B. Christensen, L. J. Paxton, D. Morrison, and G. Crowley (2004), Quiet‐time seasonal behavior of the thermosphere seen in the far ultraviolet dayglow, J. Geophys. Res., 109, A01302, doi:10.1029/2003JA010220.

Strickland, D. J., et al. (2007), Constraining and validating the Oct/Nov 2003 X-class EUV flare enhancements with observations of FUV dayglow and E-region electron densities, J. Geophys. Res., 112, A06313, doi:10.1029/2006JA012074.

Swaminathan, P. K.; Strobel, D. F.; Kupperman, D. G.; Kumar, C. Krishna; Acton, L.; DeMajistre, R.; Yee, J.-H.; Paxton, L.; Anderson, D. E.; Strickland, D. J.; Duff, J. W. (1998), Nitric oxide abundance in the mesosphere/lower thermosphere region: Roles of solar soft X rays, suprathermal N(4S) atoms, and vertical transport, J. Geophys. Res., 103(A6), pp. 11579–11594, doi:10.1029/97JA03249.