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NASA NTRS1984

The zonally averaged circulation, temperature, and compositional structure of the lower thermosphere and variations with geomagnetic activity

A zonally averaged chemical-dynamical model of the thermosphere is used to examine the effect of high-latitude particle and Joule heating on the neutral composition, temperature, and winds at solstice for solar minimum conditions. The meridional circulation forced by solar heating alone is a summer-to-winter flow, with a winter enhancement in atomic oxygen. The high-latitude heat sources drive mean circulation cells that reinforce the solar-driven circulation in the summer hemisphere and oppose this circulation in the winter hemisphere. The changes in wind and temperature caused by the high-latitude heat sources increase the relative concentration of N2 and O2 in the high-latitude upper thermosphere and decrease the O concentration in the high-latitude lower thermosphere. For prolonged moderate levels of geomagnetic activity the peak atomic oxygen density in the polar regions can decrease by factors of 2-3 from geomagnetic quiet conditions.

Roble, R. G.↗

NASA NTRS1984

The earth's thermospheric superrotation from Dynamics Explorer 2

Neutral thermospheric zonal winds measured from the Wind and Temperature Experiment on Dynamics Explorer 2 have been analyzed to provide a measure of the superrotation of the neutral thermosphere. The data were averaged in local time in 10 degree bands from the equator to 40 degrees latitude. The results show an average wind velocity on the order of 18 m/s to the East at the equator and a westward wind of the same magnitude at mid latitudes. Within the range of altitudes of the data base (200 to 700 km) no evidence of any significant altitude dependence was found. Averaging our data over the latitude bands studied indicates a net corotation of the thermosphere. This result is not in agreement with the superrotation value reported by King-Hele and Walker on the basis of satellite drag analysis.

Wharton, L. E.↗

NASA NTRS1984

Observations of the dynamics of the polar thermosphere

The dynamics of the polar thermosphere are examined by using observations made from the Dynamics Explorer 2 satellite. The results used in this study were obtained primarily from the Fabry-Perot interferometer (FPI) and the wind and temperature spectrometer (WATS) during the time period from September 1981 through January 1982. Two primary geophysical conditions were examined: these were the southern summer and the northern winter polar regions. The results support the conclusion that above 60 degrees of latitude the neutral winds are strongly controlled by ion/neutral frictional momentum transfer resulting from magnetospheric convection. This implies that the natural coordinate system within which to display the neutral winds in the high polar thermosphere is magnetic. The collected observations of this study were used to assess the validity of two of the large thermospheric general circulation models. The result of this assessment was that the models reasonably represent the vector winds at high altitudes but do not, at present, accurately simulate the thermodynamics of that regime.

Hays, P. B.↗

NASA NTRS1984

Coupling Between the Thermosphere and the Stratosphere: the Role of Nitric Oxide

In order to understand the lower ionosphere and its probable control by dynamical processes, the behavior of nitric oxide below 100 km was investigated. A two dimensional model with coupled chemical and dynamical processes was constructed. Calculations based on the model reveal that the chemical conditions at the stratopause are related to the state of the thermosphere. This coupling mechanism can be partly explained by the downward transport of nitric oxide during the winter season, and consequently depends on the dynamical conditions in the mesosphere and in the lower thermosphere (mean circulation and waves). In summer, the photodissociation of nitric oxide plays an important role and the thermospheric NO abundance modulates the radiation field reaching the upper stratosphere. Perturbations in the nitric oxide concentration above the mesopause could therefore have an impact in the vicinity of the stratopause.

Brasseur, G.↗

NASA NTRS1985

The westward thermospheric jet-stream of the evening auroral oval

Ground-based and satellite measurements of the thermospheric wind in jet-streams during the evening auroral oval are analyzed, in order to study the geophysical mechanisms of thermospheric wind generation. Numerical simulations using a global, three-dimensional, time-dependent model of thermospheric dynamics were compared with the satellite data, and the results are discussed in detail. The wind distribution during the storm is shown in a series of color plates.

Rees, D.↗

NASA NTRS1985

Thermospheric hydrogen - The long-term solar influence

Atmospheric Explorer C and E satellite data are employed for a long-term analysis of the behavior of thermospheric hydrogen with respect to the 11 yr solar cycle. The data covered the period 1974-79 (increasing solar activity) and comprised in situ ionospheric (F region) and neutral atmospheric data. The data were analyzed statistically to characterize low latitude hydrogen behavior, e.g., the diurnal variation and mean concentration over the 5 yr data sampling period. Both the mean and daily maximum/minimum ratio (DMMR) varied with the solar F index. The escaping flux of H ions became a contant around 1000 K. Increasing thermospheric temperatures lowered the DMMR value. However, the DMMR values calculated were consistently large enough to require inclusion of neutral winds and/or diurnal variations in charge exchange fluxes moving in and out of the plasmasphere in any model for thermospheric hydrogen behavior.

Breig, E. L.↗

NASA NTRS1985

The thermosphere as a sink of magnetospheric energy - A review of recent observations of dynamics

It is pointed out that the past few years have seen an unprecedented influx of new experimental information on the dynamics of the neutral upper atmosphere of the earth. Vector wind measurements provide new information for studies of the thermospheric response to magnetospheric forcing. This response occurs through the medium of convecting ionospheric ions set into motion by electric fields of magnetospheric origin. The ultimate sink for much of the energy and momentum coming from the magnetosphere is the neutral thermosphere whose dynamics have, in the past, received far less attention than their ionospheric counterpart because of basic experimental limitations. In this paper, a review is provided of the progress made in the last few years on the basis of the Dynamics Explorer neutral wind observations, taking into account the coupling between the magnetosphere and the thermosphere via the ionosphere.

Killeen, T. L.↗

NASA NTRS1985

Effects of breaking gravity waves on the chemical composition of the mesosphere and lower thermosphere

Eddy diffusion and momentum forcing in the mesosphere and lower thermosphere arise principally from the effects of breaking gravity and waves. The propagation and dissipation of gravity waves depend strongly on season and latitude because of its relationship to the background zonal wind. These processes have significant effects on the transport of chemical species in that region, so that observations of variations in chemical constituents provide useful tracers for dynamical theory. A parameterization of gravity wave propagation and dissipation was incorporated in the coupled dynamical-chemical model to study these effects. Several easily observable airglow features of the mesosphere-thermosphere region are shown to provide sensitive indications of gravity wave influence. In particular, theoretical predictions of large seasonal and latitudinal variations in the atomic oxygen green line, the Meinal bands of excited OH, and the O3 densities inferred from O2 (delta supra 1 sub g) emission at altitudes from 75 to 90 km are shown to be in remarkably good agreement with observations. Implications of these sensitive chemical tracers for the dynamics of the mesosphere-lower thermosphere region and its relationship to gravity waves are discussed.

Solomon, S.↗

NASA NTRS1986

Applications of a Venus thermospheric circulation model

A variety of Pioneer Venus observations suggest a global scale, day-to-night Venus thermospheric circulation. Model studies of the dynamics and energetics of the Venus thermosphere are presented in order to address new driving, mixing and cooling mechanisms for an improved model simulation. The adopted approach was to reexamine the circulation by first using a previous two dimensional code to quantify those physical processes which can be inferred from the Pioneer Venus observations. Specifically, the model was used to perform sensitivity studies to determine the degree to which eddy cooling, eddy or wave drag, eddy diffusion and 15 micrometer radiational cooling are necessary to bring the model temperature and composition fields into agreement with observations. Three EUV heating cases were isolated for study. Global temperature and composition fields in good agreement with Pioneer data were obtained. Large scale horizontal winds 220 m/s were found to be consistent with the observed cold nightside temperatures and dayside bulges of O, CO and CO2. Observed dayside temperatures were obtained by using a 7 to 19% EUV heating efficiency profile. The enhanced 15 micrometer cooling needed for thermal balance is obtained using the best rate coefficient available for atomic O collisional excitation of CO2(0,1,0). Eddy conduction was not found to be a viable cooling mechanism due to the weakened global circulation. The strong 15 micrometer damping and low EUV efficiency imply a very weak dependence of the general circulation to solar cycle variability. The NCAR terrestrial thermospheric general circulation model was adapted for Venus inputs using the above two dimensional model parameters, to give a three dimensional benchmark for future Venus modelling work.

Bougher, S. W.↗

NASA NTRS1986

A three dimensional model of the Venusian thermosphere with superrotation

An improved three dimensional spectral model of the thermosphere of Venus is described. The model solves the Navier-Stokes equations and includes nonlinear effects for an arbitrary number of atmospheric species. A two dimensional axisymmetric model of the superrotation of the thermosphere is also presented. This model addresses the Pioneer-Venus mission finding, which suggested the thermospheric rotation rate to be much higher than that of the planet as seen from the asymmetric distribution of hydrogen and helium. Both models include the effects of an anisotropic eddy diffusion that is consistent with atmospheric mixing length theory.

Stevens-Rayburn, D. R.↗

NASA NTRS1987

Deuterium in the daytime thermosphere

Ion concentration measurements for H(+) and D(+) from the magnetic ion mass spectrometer on the Atmosphere Explorer C satellite are used, in conjunction with other atmospheric data, to determine the concentrations of H and D in the nonpolar daytime thermosphere. The ratio of the observed D(+) to H(+) concentrations has essentially the same height dependence in the 300 to 800-km region as expected for their neutral counterparts, even in the presence of ion temperature gradients and probable large vertical ion fluxes. Rapid charge exchange with atomic oxygen ensures that D/H is about equal to D(+)/H(+) at the lower altitudes where the derived D to H concentration ratio is a factor of about 6 larger than its sea level value, for an exospheric temperature of 930 K. This relative enhancement of deuterium arises from the fact that hydrogen more readily escapes the earth, and a large vertical gradient in the H concentration relative to its diffusive equilibrium value is necessary to drive this upward flux through the lower thermosphere. If these planetary losses of hydrogen are much greater than those associated with evaporative escape, as is the current view, then correspondingly larger deuterium loss rates are also likely in order that the thermospheric D/H ratio not increase well above the observed value. The absolute winter daytime concentration of deuterium at 300 km is found to be 210 + or - 50 atoms/cu cm.

Breig, E. L.↗

NASA NTRS1988

Non-thermal distribution of O(1D) atoms in the night-time thermosphere

The 6300 A O(1D-3P) emission has been used for many years to remotely monitor the thermospheric temperature from the Doppler width of its line profile. The O(1D) atoms in the nighttime thermosphere are initially produced by the dissociative recombination of O2(+) ions with kinetic energy much greater than the thermal energy of the ambient neutrals. The validity of the technique to monitor neutral ambient temperature by measuring O(1D) 6300 A emission depends on the degree of thermalization of the O(1D) atoms. The object of this study is to calculate the velocity distribution of the O(1D) atoms and to examine the effect of nonthermal distribution on the nighttime thermospheric neutral temperature determined.

Yee, Jeng-Hwa↗

NASA NTRS1988

Venus mesosphere and thermosphere. III - Three-dimensional general circulation with coupled dynamics and composition

The three-dimensional structure and circulation of Venus' upper mesosphere and thermosphere is examined by means of a modification of NCAR's earth thermosphere general circulation model, using the parameterizations from an earlier two-dimensional Venus model that included eddy diffusion and wave drag. Many of the observed Venus thermosphere features are found to be reproduced by the model, which thereby serves as a benchmark on which to incorporate additional minor constituents and test new self-consistent parameterizations for wave drag and superrotation.

Bougher, S. W.↗

NASA NTRS1988

Thermosphere dynamics - Contributions from the first 5 years of the Dynamics Explorer program

Instrumentation flown on the Dynamics Explorer 2 (DE-2) spacecraft enabled the dynamics of both the neutral and the ionized components of the earth's upper atmosphere to be monitored over the lifetime of the spacecraft, from August 1981 to February 1983. The direct measurements of global thermospheric vector neutral winds and ion drifts were supplemented by observations of neutral and ionic constituent abundances and temperatures, precipitating particle fluxes and pitch angle distributions, electric and magnetic fields, and global-scale auroral luminosity distributions. Analysis of this unique and comprehensive global data base has led to advances in our understanding of the manner in which the earth's thermosphere responds dynamically to the insertion of energy and momentum. We review the scientific progress achieved over the past 5 years through the efforts of members of the Dynamics Explorer science team and the other interested scientists who have used the DE data base to investigate the dynamical response of the thermosphere.

Killeen, T. L.↗

NASA NTRS1989

Lower thermosphere (80-100 km) dynamics response to solar and geomagnetic activity: Overview

The variations of solar and geomagnetic activity may affect the thermosphere circulation via plasma heating and electric fields, especially at high latitudes. The possibility exists that the energy involved in auroral and magnetic storms can produce significant changes of mesosphere and lower thermosphere wind systems. A study of global radar measurements of winds at 80 to 100 km region revealed the short term effects (correlation between wind field and geomagnetic storms) and long term variations over a solar cycle. It seems likely that the correlation results from a modification of planetary waves and tides propagated from below, thus altering the dynamical regime of the thermosphere. Sometimes the long term behavior points rather to a climatic variation with the internal atmospheric cause than to a direct solar control.

Kazimirovsky, E. S.↗

NASA NTRS1989

On the mechanisms responsible for high-latitude thermospheric composition variations during the recovery phase of a geomagnetic storm

The causal mechanisms for the recovery of the perturbed high-latitude thermospheric composition to the unperturbed state in the period following a geomagnetic storm are investigated. Model runs of the NCAR thermosphere/ionosphere GCM (TIGCM) and thermosphere GCM are used to calculate the averaged mass mixing ratio variations and the forcing terms responsible for these variations during the recovery phase of a geomagnetic storm. High latitude compositional recovery is found to occur in the NCAR TIGCM on a time scale of about 12 hr to 1 day. This time scale is in agreement with previously observed time scales for typical poststorm F region electron density recoveries. Neither molecular diffusion nor large-scale horizontal advection is the dominant process in determining the compositional state during the recovery period. Thermospheric compositional recovery at high geomagnetic latitudes is driven primarily by vertical advection.

Burns, A. G.↗

NASA NTRS1989

Numerical simulations of the seasonal/latitudinal variations of atomic oxygen and nitric oxide in the lower thermosphere and mesosphere

A 2-Dimensional zonally-averaged thermospheric model and the global University College London (UCL) thermospheric model have been used to investigate the seasonal, solar activity and geomagnetic variation of atomic oxygen and nitric oxide. The 2-dimensional model includes detailed oxygen and nitrogen chemistry, with appropriate completion of the energy equation, by adding the thermal infrared cooling by O and NO. This solution includes solar and auroral production of odd nitrogen compounds and metastable species. This model has been used for three investigations; firstly, to study the interactions between atmospheric dynamics and minor species transport and density; secondly, to examine the seasonal variations of atomic oxygen and nitric oxide within the upper mesosphere and thermosphere and their response to solar and geomagnetic activity variations; thirdly, to study the factor of 7 to 8 peak nitric oxide density increase as solar F sub 10.7 cm flux increases from 70 to 240 reported from the Solar Mesospheric Explorer. Auroral production of NO is shown to be the dominant source at high latitudes, generating peak NO densities a factor of 10 greater than typical number densities at low latitudes. At low latitudes, the predicted variation of the peak NO density, near 110 km, with the solar F sub 10.7 cm flux is rather smaller than is observed. This is most likely due to an overestimate of the soft X-ray flux at low solar activity, for times of extremely low support number, as occurred in June 1986. As observed on pressure levels, the variation of O density is small. The global circulation during solstice and periods of elevated geomagnetic activity causes depletion of O in regions of upwelling, and enhancements in regions of downwelling.

Rees, D.↗

NASA NTRS1990

Thermospheric gravity waves - Observations and interpretation using the transfer function model (TFM)

This paper presents some numerical experiments performed with the TFM to study the various wave components excited in the auroral regions that propagate through the thermosphere and lower atmosphere, and to demonstrate the properties of realistic source geometries. The model is applied to the interpretation of satellite measurements, and gravity waves seen in the thermosphere of Venus are discussed. Gravity waves are prominent in the terrestrial thermosphere polar region and can be excited by perturbations in Joule heating and Lorentz force due to magnetospheric processes. Observations from the Dynamics Explorer-2 satellite are used to illustrate the complexity of the phenomenon and to review the TFM that is utilized.

Mayr, H. G.↗