Engineering topics

Killeen, T. L.

Publications and source records attributed to Killeen, T. L..

At least 37 records · Page 2

NASA NTRS1992

Thermospheric heating away from the auroral oval during geomagnetic storms

The data on the morphology of the changes in the heating of thermosphere in the middle and high latitudes during geomagnetic storms, collected by the DE-2 satellite between July 1981 and February 1983 are compared with the NCAR-Thermosphere-Ionosphere General Circulation Model simulation of the November 24, 1982 storm on a one-on-one basis for an individual orbit in the middle of this storm. Good agreement was found for the winter hemisphere. A thermodynamic diagnostic processor was then used to investigate the mechanism by which geomagnetic storms cause temperature increases at lower latitudes. It was found that the pattern of heating during geomagnetic storms is complex, indicative of the complex nature of the physical processes that alter the thermal structure of the thermosphere at these times. Model predictions indicated that the greatest temperature increase at 40 deg N occurs in the morning, in agreement with results of Proelss (1984).

Burns, A. G.↗

NASA NTRS1992

Neutral winds in the lower thermosphere from Dynamics Explorer 2

Doppler line profile measurements of the OI lambda 557.7 nm 'green line' emission, made by the Fabry-Perot interferometer on Dynamics Explorer 2, have provided altitude profiles of the meridional component of the lower thermospheric neutral wind. The wind inversion technique of Nardi has been used to extract the neutral wind profiles from the line-of-sight measurements. Individual lambda 557.7 nm Doppler line profiles and inverted volume-emission-rate and neutral-wind profiles are presented. Neutral wind measurements from the about 120 km altitude level, obtained on multiple orbital passes over the summer hemisphere polar region, have been merged to produce a synthesized averaged 'vector' wind field in geomagnetic coordinates. The wind pattern exhibits a region of anticyclonic vorticity in the daytime sector of the magnetic polar cap. Averaged winds of about 300 m/sec in the equatorial direction are observed in the early morning sector. The satellite winds are in reasonable agreement with the predictions of the NCAR and UCL thermosphere general circulation models, with significant regional discrepancies evident in both magnitude and direction.

Killeen, T. L.↗

NASA NTRS1992

The equatorial neutral thermospheric response to geomagnetic forcing

Thermospheric temperature and neutral density measurements from Dynamics Explorer 2 (DE 2) have been used to study the time-dependent response of the equatorial thermosphere to geomagnetic forcing. Data taken following the onsets of several different geomagnetic storms have been used in a superposed-epoch scheme to investigate the characteristic response of the density and temperature structure of the equatorial thermosphere at about 350 km altitude in the morning and evening local-time sectors. Two travelling waves, one generated at northern high latitudes and one at southern high latitudes, pass through the equatorial thermosphere, with wave crests crossing the equator at a time lag of 3-6 hrs after the storm onset. Following the passage of these travelling waves, the densities and temperatures in the evening sector return relatively rapidly to their undisturbed state, whereas those in the morning sector have significant variations at longer lag times. In particular, significant increases in neutral density and temperature occur in the morning sector about 12 hrs after the onset of the storm.

Burns, A. G.↗

NASA NTRS1992

Changes of neutral composition in the thermosphere

An overview is given of the changes in neutral composition that occur in the upper thermosphere during geomagnetic storms. Emphasis is given to solar EUV radiation, upward propagating tides and gravity waves, and coupling between magnetosphere and the ionosphere/thermosphere as the sources of the compositional changes. Present understanding of the poststorm recovery is summarized.

Burns, A. G.↗

NASA NTRS1992

The effect of solar cycle variations on the thermosphere

A review of current knowledge of the effect of solar activity on the thermosphere is presented. Data obtained by the Dynamics Explorer 2 spacecraft and model results from the NCAR Thermospheric GCM, which demonstrate the effect of increased solar and geomagnetic activity on composition, temperature and the dynamic structure of the upper atmosphere are discussed. It is shown how subtle change in the IMF can affect the form of the neutral gas circulation at high latitudes, with important implications for polar orbiting spacecraft.

Mccormac, F. G.↗

NASA NTRS1991

On the dynamics and composition of the high-latitude thermosphere

An overview is presented of some of the more recent progress made in the study of the dynamics and composition of the high-latitude thermosphere at the University of Michigan, with emphasis on the interpretation of experimental measurements made from the DE-2 spacecraft and from ground-based observatories in Greenland. Attention is focused on perturbations to the solar-driven upper thermospheric winds, temperatures, and compositional structures due to forcing associated with high-latitude ionosphere-thermosphere coupling processes. The nature of the experimental measurements made on DE-2 is discussed. The vector spherical harmonic model, some sample results, and experiment-model comparisons are described using both satellite and ground-based measurements of high-latitude thermosphere dynamics. Considerations pertaining to compositional perturbations at high latitudes are presented.

Killeen, T. L.↗

NASA NTRS1991

Vorticity and divergence in the high-latitude upper thermosphere

Measurements made from the Dynamics Explorer-2 satellite in November 1981 through January 1982 and November 1982 through January 1983 have been analyzed to determine the divergence and the vertical component of vorticity of the high-latitude neutral wind field in the upper thermosphere for quiet (Kp not greater than 3) and active (Kp between 3+ and 6) geomagnetic conditions and for both Northern (winter) and Southern (summer) Hemispheres. This analysis provides the first experimental determination of the large-scale vorticity and divergence patterns in the polar thermosphere and provides insight into the relative strengths of the different sources of momentum and energy responsible for driving the winds. The principal findings from this work include the following: the mean neutral wind pattern is dominated by rotational flow rather than by divergent flow, with a typical vorticity:divergence ratio of about 2:1 for active conditions and about 4:1 for quiet conditions. Comparison of the divergence and vorticity patterns for quiet and active conditions indicates that the divergent component of the neutral flow intensifies more significantly with increasing geomagnetic activity than does the rotational component.

Thayer, J. P.↗

NASA NTRS1991

Ionospheric storm effects at subauroral latitudes - A case study

An attempt is made to classify ionospheric storm effects at subauroral latitudes according to their presumed origin. The storm of December 7/8, 1982, serves as an example. It is investigated using ionosonde, electron content, and DE 2 satellite data. The following effects are distinguished: (1) positive storm effects caused by traveling atmospheric disturbances, (2) positive storm effects caused by changes in the large-scale thermospheric wind circulation, (3) positive storm effects caused by the expansion of the polar ionization enhancement, (4) negative storm effects caused by perturbations of the neutral gas composition, and (5) negative storm effects caused by the equatorward displacement of the trough region.

Proelss, G. W.↗

NASA NTRS1991

The influence of IMF B(y) on the high-latitude thermospheric circulation during northward IMF

This paper uses the Dynamics Explorer data set, together with a sufficient quantity of corresponding IMF data, to establish correlations among the observed relationships between the IMF B(y) and the high-latitude thermospheric circulation during periods of northward IMF. Statistical evidence is presented that illustrates a relationship between the respective magnitudes of the B(z) and B(y) components of the IMF and the occurrence of sunward winds in the polar cap. The ion-neutral coupling processes responsible for the observed effects are discussed together with the general implications of the results.

Mccormac, F. G.↗

NASA NTRS1991

Thermospheric dynamics, energetics, and composition at auroral latitudes

Recent experimental results from the Dynamics Explorer-2 (DE-2) spacecraft and theoretical calculations from the NCAR-TGCM (thermospheric general circulation models) are discussed. DE-2 observations show that the close coupling exists between the magnetosphere and the thermosphere, with magnetospheric energy and momentum sources playing a key role in establishing the thermospheric dynamical and thermal structure via ion-neural collisions. The tight ion-neutral momentum coupling results in the neutral-wind pattern which is strongly dependent on the level of geomagnetic activity and the orientation of the interplanetary magnetic field. Thermospheric temperatures are directly controlled by the Juole and particle heat sources associated with auroral latitudes. It is concluded that the various dynamical, energetic, and compositional variations associated with auroral processes are highly coupled with each other.

Killeen, T. L.↗

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

Combining electric field and aurora observations from DE 1 and 2 with ground magnetometer records to estimate ionospheric electromagnetic quantities

Global distribution of electric fields and currents in the high-latitude ionosphere was estimated using data from the ground-based network of magnetometers and from nearly simultaneous observations with DE 1 and DE 2 satellites. The electric field and current distributions at high altitudes were calculated from instantaneous ionospheric conductivity (estimated from the DE 1 auroral data), using the Kamide et al. (1981) magnetogram inversion technique; an optimum conductivity was then chosen iteratively so that the resultant electric fields would become consistent with electric field deduced from ion drifts measured along the DE-2 orbit. It is demonstrated that, when analyzing the large-scale electrodynamics of individual substorms, statistical conductivity models are not fully adequate for use with the magnetogram inversion technique.

Kamide, Y.↗

NASA NTRS1989

Processes responsible for the compositional structure of the thermosphere

The relative importances of the various physical and chemical mechanisms that force changes in neutral thermospheric composition for a given geophysical situation were quantified using a diagnostic postprocessor analysis package in conjunction with runs of the NCAR thermospheric general circulation model (TGCM) that was extended to include the terms of the neutral composition equation. New information was obtained on the causal mechanisms responsible for changes in the concentrations of the three neutral species, O2, O, and N2, whose time-dependent mass mixing ratios were calculated within the TGCM. Principal results calculated for F-region altitudes are described, and thermospheric compositions calculated using the NCAR-TGCM model are compared with the predictions of the empirical thermospheric model of Hedin (1987).

Burns, A. G.↗

NASA NTRS1988

Empirical global model of upper thermosphere winds based on atmosphere and dynamics explorer satellite data

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been used to generate an empirical wind model for the upper thermosphere, analogous to the MSIS model for temperature and density, using a limited set of vector spherical harmonics. The model is limited to above approximately 220 km where the data coverage is best and wind variations with height are reduced by viscosity. The data base is not adequate to detect solar cycle (F10.7) effects at this time but does include magnetic activity effects. Mid- and low-latitude data are reproduced quite well by the model and compare favorably with published ground-based results. The polar vortices are present, but not to full detail.

Hedin, A. E.↗

NASA NTRS1988

Polar cap diurnal temperature variations - Observations and modeling

High-spectral-resolution measurements of the O(1D) emission line, performed with the Fabry-Perot interferometer (Thule, Greenland), were used to observe the diurnal variation of the thermospheric temperature in the geomagnetic polarcap at solar minimum (January 1987). By tracing the trajectory of a parcel backward in time and space from the Thule location, it is shown that the observed diurnal temperature variation is due to the degree of solar heat input that a parcel experiences en route to the polar cap, and the route a parcel takes through the polar cusp. It is also shown that the hydrodynamic variations in the winter high-latitude regions from solar maximum to solar minimum are insufficient to mask the thermodynamical effects associated with the offset of the geographic and geomagnetic poles.

Mccormac, F. G.↗

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 NTRS1988

On the relationship between dynamics of the polar thermosphere and morphology of the aurora - Global-scale observations from Dynamics Explorers 1 and 2

The effect of the neutral-air circulation in the high-latitude F region on the spatial distribution of the aurora is investigated on the basis of simultaneous global-scale auroral images and neutral-wind vectors obtained over the northern polar cap by the DE 1 and DE 2 satellites in December 1981. The results are presented graphically, along with DE 2 data on the composition, ion densities, and cross-track ion drift of the neutral winds and ISEE-3 IMF measurements, and analyzed. It is found that the large-scale features of the neutral circulation follow the substorm-driven expansion and contraction of the auroral oval, and that the drag effect from ions convecting sunward extends to much lower latitudes and has a more complex morphology than that predicted by GCMs. A number of asymmetries in the flow patterns are considered in detail.

Killeen, T. L.↗

NASA NTRS1988

Ionosphere-thermosphere momentum coupling at solar maximum and solar minimum from DE-2 and AE-C data

DE-2 and AE-C measurements of plasma and neutral densities were used to derive time constants for momentum transfer (MT) to neutrals from ions in the high-latitude thermosphere. The MT time constants for solar cycle maximum (DE-2) and for solar cycle minimum (AE-C) were averaged and binned according to geomagnetic latitude and LT to provide a quantitative measure for the tightness of ion-neutral momentum coupling (MC) in the 250-350-km altitude range. Comparisons with results obtained using the Chiu and MSIS-83 empirical models have provided an indication of the accuracy with which thermospheric general circulation models quantitatively reproduce the MC between ions and neutrals in the high-latitude F-region.

Ponthieu, J. J.↗