J. M. Picone

Global morphology of infrasound propagation

[i] Atmospheric sound waves in the 0.02-10 Hz region, also known as infrasound, exhibit long-range global propagation characteristics. Measurable infrasound is produced around the globe on a daily basis by a variety of natural and man-made sources. As a result of weak classical attenuation (∼0.01 dB km -1 at 0.1 hz), these acoustic signals can propagate thousands of kilometers in tropospheric, stratospheric, and lower thermospheric ducts. To model this propagation accurately, detailed knowledge of the background atmospheric state variables, the global winds and temperature fields from the ground to ∼170 km, is required. For infrasound propagation calculations, we have developed a unique atmospheric specification system (G2S) that is capable of providing this information. Using acoustic ray tracing methods and detailed G2S atmospheric specifications, we investigate the major aspects of the spatiotemporal variability of infrasound propagation characteristics.

Atomic oxygen in the thermosphere during the July 13, 1982, solar proton event deduced from far ultraviolet images

The far ultraviolet (FUV) analysis technique of Strickland et al. [this issue] is used to infer height-integrated O/N2 column densities from O I-130.4-nm FUV dayglow images taken by the Dynamics Explorer 1 (DE 1) spin scan auroral imager (SAI) [Frank et al., 1981] during a large solar proton event that occurred on July 13, 1982. The FUV results show that O/N2 decreases in the northern hemisphere as a result of seasonal and storm time effects. Furthermore, these decreases differ significantly from corresponding predictions made by the MSISE-90 model of Hedin et al. [1991]. The FUV-derived O/N2 column densities are compared to in situ volume density and neutral temperature ([O], [N2], Tn) measurements made from the Dynamics Explorer 2 (DE 2) satellite [Carignan et al., 1981; Spencer et al., 1981]. The comparisons are made using two techniques, In the first approach, ground truth estimates of O/N2 column densities are made from the in situ measurements. In the second approach, [O] volume densities are inferred from the FUV measurements of O/N2 column density and directly compared to direct DE 2 neutral atmosphere composition spectrometer (NACS) measurements of [O]. While some discrepancies exist, the results show that reasonable estimates of the atomic oxygen volume density [O] during the July 13, 1982, solar proton event could be made using the FUV images.

Middle and upper thermospheric odd nitrogen: 1. A new analysis of rocket data

We evaluate the NRLMSISE-00 model for calculations of odd nitrogen (NO, N(4S)) in the middle to upper thermosphere (z = 140-250 km). NRLMSISE-00 incorporates new data on O 2 that improves the agreement between odd nitrogen models and data significantly. In particular, the photochemical calculation that uses NRLMSISE-00 predicts a NO solar cycle variation that is significantly less than previous calculations and that agrees well with the NO observations. This agreement is consistent with the inference from FUV solar occultation data that the O 2 abundance above 140 km varies weakly with solar activity and that the O 2 vertical profile at solar maximum is sensitive to other factors besides molecular diffusion. Residual discrepancies remain with the comparison of calculated to observed N( 4 S), which may be due to a combination of theoretical deficiencies and uncertainties in the observations.

Enhanced empirical models of the thermosphere

Abstract The Naval Research Laboratory (NRL) has embarked on a development program to upgrade empirical models of the neutral upper atmosphere (thermosphere and upper mesosphere) and to apply these models to scientific and engineering problems. The program focus has been the Mass Spectrometer — Incoherent Scatter Radar (MSIS) model of composition and temperature. The new NRLMSIS model, due for release in 2000, has ingested additional data sets, including, for the first time, the drag and accelerometer data of Jacchia and others. The formulation in the lower thermosphere now has improved flexibility, and a new species, “anomalous oxygen,” allows for appreciable O + and hot atomic oxygen contributions to the total mass density at high altitudes. A new full disk proxy for the solar chromospheric EUV driver of thermospheric variability is available for studies in combination with the NRLMSIS model and database. This will determine the value of augmenting F 10.7 in the model formulation — a longstanding issue. Whereas F 10.7 correlates more closely with coronal EUV flux, chromospheric fluxes provide the primary thermospheric heating.

Similarity transformation-based analysis of atmospheric models, data, and inverse remote sensing algorithms

The similarity transformation (ST) defines a new class of robust and stable parametric functions with embedded physical shape information to optimize flexibility in fitting or inverting data. The similarity transformation also permits the extraction of information on the shape of a particular class of physical functions, thereby providing the basis for comparing alternative models and for analyzing the information content of data. We employ these properties of similarity transformations to study differences between state-of-the-art physics-based atmospheric models (the thermosphere ionosphere electrodynamic general circulation model, or TIEGCM) and empirical atmospheric models (Mass Spectrometer Incoherent Scatter, or MSIS) and to investigate the universality of these models; we examine the role of noise in determining acceptable resolution for faithful retrieval of physical properties; and we measure the performance of MSIS-based forward models for inversion of ultraviolet remote sensing of the neutral upper atmosphere. The similarity transform method proves to be a valuable new tool for identifying common and discrepant properties of the models. Further, the ST method shows that TIEGCM and MSISE-90 profiles embody similar shape information and that a suitable ST parameterization can be constructed that approximates profiles from either model to within a few percent accuracy.

Spectral fitting applications: improved calibration and radiometric accuracy of EUV/FUV sensors

Techniques, applications, and results will be discussed with respect to the Low Resolution Airglow/Auroral Spectrograph and Special Sensor Ultraviolet Limb Imager instruments aboard current and upcoming satellite remote sensing missions.

Dynamic modeling of the solar atmosphere

Over the past 2 decades our observational understanding of the outer layers of the solar atmosphere has increased dramatically. We now know that the solar chromosphere, transition region, and corona are highly structured and dynamic. Magnetic fields, rooted in the subphotospheric layers, provide the structure that channels the plasma in the upper layers of the solar atmosphere.