R. M. Johnson

Revised global model of thermosphere winds using satellite and ground-based observations

Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground-based incoherent scatter radar and Fabry-Perot optical interferometers to generate a revision (HWM90) of the HWM87 empirical model and extend its applicability to 100 km. Comparison of the various data sets with the aid of the model shows in general remarkable agreement, particularly at mid and low latitudes. The ground-based data allow modeling of seasonal/diurnal variations, which are most distinct at mid latitudes. While solar activity variations are now included, they are found to be small and not always very clearly delineated by the current data. They are most obvious at the higher latitudes. The model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of Fabry-Perot observations.

High‐latitude lower thermospheric neutral winds at EISCAT and Sondrestrom during LTCS 1

The incoherent scatter radar located at Sondre Stromfjord, Greenland (67°N, 51°W, 74.5°Λ) and the EISCAT incoherent scatter facility located in northern Scandinavia (69.5°N, 19°E, 66.3°Λ) both obtained E and F region measurements during the first campaign of the Lower Thermosphere Coupling Study (LTCS 1, September 21–25, 1987). Neutral winds deduced from these measurements have been analyzed for their mean flow and tidal components. A number of the altitude profiles for the mean winds and the diurnal and semidiurnal wave components at the two radar locations show similar variations with height, indicating that latitudinal rather than longitudinal effects are dominant in determining the observed wind field. Diurnal tidal amplitudes and phases are reasonably well represented by theoretical model results (Forbes, 1982). The semidiurnal amplitudes and phases, although somewhat consistent between the two radars, are not well represented in equinox tidal model results (Forbes and Vial, this issue). Results from both radars indicate a vertical wavelength for the zonal semidiurnal oscillation of approximately 60 km. During a period of impulsive magnetospheric forcing (September 22–23), winds deduced from measurements at both radars show enhanced eastward flows near midnight accompanied by equatorward winds at Sondrestrom. Comparison with the results of a National Center for Atmospheric Research thermosphere-ionosphere general circulation model (TIGCM) simulation of the LTCS 1 interval shows generally better agreement with the observations at EISCAT than at Sondrestrom. During the period of activity on September 22–23 the TIGCM is reasonably successful at simulating the eastward surge near midnight in the EISCAT (but not in the Sondrestrom) observations and the equatorward flow after midnight in the Sondrestrom results. The observed winds in magnetic latitude and magnetic local time coordinates indicate the presence of anticyclonic divergent flow near dusk and cyclonic converging flow near dawn.

Observations of high‐latitude lower thermospheric winds from Thule Air Base and Søndre Strømfjord, Greenland

Lower thermospheric winds have been determined from Fabry-Perot interferometer (FPI) measurements of the Doppler shift of the 5577-A O(1S) emission over Thule Air Base (76.5°N, 69.0°W) and Sondre Stromfjord (67.0°N, 50.9°W), Greenland. These winds normally correspond to the altitude of the peak of the airglow O(1S) emission layer, near 97 km. The altitude ambiguity due to auroral contamination has been reduced by eliminating data when the intensity of the emission increases significantly. Contamination by airglow emission of 5577-A O(1S) originating from higher altitudes has been investigated by an FPI simulation code. The simulation results indicate that this latter emission may contribute an anomalous diurnal oscillation to ground-based 5577-A O(1S) FPI measurements of lower thermospheric wind. The agreement of diurnal phases between that deduced from the green-line measurements and that determined from simultaneous red-line observation supports this conclusion. The same simulation applied to observations from Sondre Stromfjord shows that the upper layer contamination is much weaker and is not serious. Significant day-to-day variation is evident in the lower thermospheric wind field. Average neutral winds are calculated, and a harmonic analysis is carried out to examine the major low-frequency wind components. The seasonal variations of these wind components are compared with radar data and model predictions. The observations are generally in good agreement with model results. The comparison between FPI and radar results also shows reasonable agreement. The semidiurnal amplitudes observed with the Sondre Stromfjord radar during the Lower Thermospheric Coupling Study (LTCS-I) and LTCS-2 periods are always greater than the climatological values obtained from averaging FPI and Chatanika radar observations. This result shows the variability that can be expected when comparing “instantaneous” estimates of tidal parameters with climatological results.

Semidiurnal tide in the 80–150 km region: an assimilative data analysis

Abstract A set of tabulated functions called ‘Hough Mode Extensions’ (HMEs), which represent numerical extensions of classical Hough modes into the viscous regime of the thermosphere, are used to least-squares fit a climatological data base of tidal measurements. The data base consists of monthly average vertical profiles of semidiurnal amplitudes and phases at 17 radar sites accessing some part of the 80–150 km height region. The radars are distributed between 78 S and 70 N latitude, and each one provides measurements of one or more of the following: eastward wind, southward wind, perturbation temperature. As a result of the fitting process, a single complex normalizing coefficient is derived for each month and for each of the four HMEs, designated (2,2), (2,3), (2,4) and (2,5) after their classical Hough function designations. Once the complex coefficients are derived, reconstruction by weighted superposition of the HMEs results in globally continuous specifications of semidiurnal horizontal and vertical wind, temperature, pressure, and density throughout the 80–150 km height region. The tidal variations in density, in particular, provide greater accuracy for several aerospace applications. The methodology developed here can also be utilized to derive tidal lower boundary conditions for Thermospheric General Circulation Models (TGCMs), or as a basis for future empirical model development. Comparisons are also made with HME coefficients and global tidal fields from the Forbes and Vial [(1989) J. atmos. terr. Phys. 51 , 649] numerical tidal model.

TIMED Doppler interferometer (TIDI)

The TIMED Doppler Interferometer (TIDI) is a Fabry-Perot interferometer designed to measure winds, temperatures, and constituents in the mesosphere and thermosphere (60 - 300 km) region of the atmosphere as part of the TIMED mission. TIDI is a limb viewer and observes emissions from OI 557.7 nm, OI 630.0 nm, OII 732.0 nm, O2(0-0), O2(0-1), Na D, OI 844.6 nm, and OH in the spectral region 550 - 900 nm. Wind measurement accuracies will approach 3 ms-1 in the mesosphere and 15 ms-1 in the thermosphere. The TIDI instrument has several novel features that allow high measurement accuracies in a modest-sized instrument. These include: an optical system that simultaneously feeds the views from four scanning telescopes which are pointed at plus or minus 45 degrees and plus or minus 135 degrees to the spacecraft velocity vector into a high-resolution interferometer, the first spaceflight application of the circle-to-line imaging optic (CLIO), and a high quantum efficiency, low noise CCD.

On the dynamics and composition of the high-latitude thermosphere

Abstract Recent experimental measurements of the dynamics of the neutral upper thermosphere have demonstrated the important roles of ion-drag and Joule heating processes in establishing the basic neutral wind morphology and controlling neutral composition, particularly in the high-latitude region. Instruments on the Dynamics Explorer-2 spacecraft (DE 2), for example, were capable of measuring the three-dimensional vector neutral wind and ion drift in the thermosphere along the orbital track, together with constituent densities and temperatures. Ground-based optical and radar measurements of winds and temperatures from observatories in Greenland have contributed additional measurements of thermospheric neutral wind velocities and ionospheric parameters. The comprehensive nature of these various data sets has enabled more stringent experimental constraints to be placed on the numerical models of the region (thermosphere-ionosphere general circulation models, TIGCMs), leading to an improved theoretical understanding of the important physical processes that control thermospheric circulation and variability. In addition, the measurements have enabled the development of semi-empirical models of thermosphere dynamics which can be used in various theoretical studies. The Vector Spherical Harmonic (VSH) model, for example, provides a description of global thermospheric state variables (wind, temperature and density), using a combination of empirical data and NCAR-TIGCM calculations. This paper presents a brief review of some of the more recent progress made in this area by the team of researchers at the University of Michigan, with emphasis on the interpretation of experimental measurements made from DE 2 and from ground-based observatories in Thule and Sondrestromfjord, Greenland. Comparisons between individual data sets from these sources and the VSH model are also presented.

Space‐time analysis of TIMED Doppler Interferometer (TIDI) measurements

A technique is developed for reducing the amount of aliasing in the spectral analysis of TIDI observations, by ingestion of ground-based data into the satellite data set. A multi-dimensional (space-time) least squares fitting approach is applied to the satellite and ground-based data to determine the aliasing spectra. The addition of ground-based data to the TIDI data set reduces the aliased components in the aliasing spectrum. For example, at 20° latitude, the combined ground-based and TIDI data set of a sampled input semidiurnal (frequency of 2 days−1) signal with zonal wavenumber 2 results in a factor of 2 reduction in the amount of power aliasing into a signal with zonal wavenumber 0 and frequency 0 days−1.

Lower-thermospheric neutral winds at high latitude determined from incoherent scatter measurements: a review of techniques and observations

Abstract Neutral winds can be deduced from E-region incoherent scatter measurements of the ion velocity, in conjunction with measurements of the electric field and knowledge of the ion-neutral collison frequency. Measurement techniques and methods used to derive lower-thermospheric neutral winds are presented and compared. The sensitivity of the results to the ion-neutral collision frequency is discussed. Observations of the lower thermosphere at mid- and high latitudes show that neutral dynamics are influenced by both tidal oscillations and magnetospheric forcing. The tidal results obtained using Mesosphere-Stratosphere-Troposphere (MST), Medium-Frequency (MF), meteor, and Incoherent Scatter (IS) radars are generally consistent, and show the dominance of semidiurnal oscillations. At Chatanika, Alaska (65°N, 65Λ), the average summer semidiurnal oscillation reaches a peak amplitude of approximately 60 m/s at 110-km altitude. Recent measurements obtained using the IS radar at Sondre Stromfjord, Greenland (67°N, 74.5Λ), show a similar strong semidiurnal component in the neutral winds. The response of the neutral winds to magnetospheric forcing is in qualitative agreement with the results of three-dimensional global model simulations of the thermosphere, which predict deviations in the normal flows as a result of frictional coupling to the convecting ions. Auroral zone measurements at Chatanika show that the response is strongest in the morning sector, where enhanced zonal flow is detected during geomagnetically disturbed conditions. Recent polar cap measurements at Sondrestrom likewise show a response to enhanced ion convection during a highly disturbed interval.

The behavior of the high-latitude F-region neutral thermosphere in relation to IMF parameters

Abstract Ground based incoherent scatter radar (ISR) and Fabry Perot interferometer (FPI) studies in the northern high latitudes during the period 1983 to 1989 have shown that the F-region neutral wind field pattern depends upon the sign of the IMF parameters. For example, the cell structure of the northern hemisphere high latitude neutral wind field during periods of low geomagnetic activity depends to a large degree upon the sign of the IMF B y parameter. Long term monitoring of the F-region thermosphere by FPI in Thule, Greenland, and by both FPI and ISR in Sondre Stromfjord, Greenland, have made it possible to produce maps of average meridional and zonal wind fields for various IMF configurations for northern high latitudes. Comparison of observations with theoretical wind field modelling, such as the Vector Spherical Harmonic model, indicates that most observed features are consistent with the models.

Semantically enhanced user modeling

Content-based implicit user modeling techniques usually employ a traditional term vector as a representation of the users interest. However, due to the problem of dimensionality in the vector space model, a simple term vector is not a sufficient representation of the user model as it ignores the semantic relations between terms. In this paper, we present a novel method to enhance a traditional term-based user model with WordNet-based semantic similarity techniques. To achieve this, we use word definitions and relationship hierarchies in WordNet to perform word sense disambiguation and employ domain-specific concepts as category labels for the derived user models. We tested our method on Windows to the Universe, a public educational website covering subjects in the Earth and Space Sciences, and performed an evaluation of our semantically enhanced user models against human judgment. Our approach is distinguishable from existing work because we automatically narrow down the set of domain specific concepts from initial domain concepts obtained from Wikipedia and because we automatically create semantically enhanced user models.