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.
Based on zonally averaged TIDI meridional wind data from one yaw period (2004079 -140) during equinox, we examine the latitudinal and altitudinal distribution of the migrating diurnal and semi-diurnal tides using least squares fitting method to provide a global view of these tidal waves. The TIDI results are compared with Global Scale Wave Model 00 output for the month of April. The diurnal tide amplitude distribution and are in a good agreement. The TIDI results show a lower peak altitude (97 km) while GSWM00 peaks at 102 km. The vertical wavelength from the TIDI is about 20 km while the model predicts 25 km. The semi-diurnal tide comparisons are also reasonable. Both model and TIDI data show peak amplitudes at 45° latitude. The TIDI meridional winds compare very well with ground based meteor radar measurements at Maui. The zonal wind discrepancies are not unexpected.
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, O 2 (0-0), O 2 (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 ±45° and ±135° 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.
The incoherent scatter radar located at S0ndre Str0m0ord, Greenland, obtained E and F region measurements during the first Lower Thermosphere Coupling Study (LTCS 1), September 21-26, 1987. Lower thermospheric neutral winds deduced from these measurements show that the neutral dynamics are influenced by both tidal oscillations and magnetospheric forcing. During an interval which was relatively quiet geomagnetically, September 23-24, a semidiurnal oscillation dominated the neutral motion. The model equinox tidal amplitudes and phases of Forbes (1982) for the diurnal tide are roughly in agreement with the observed diurnal oscillation for the first four days of the experiment. Vertical variations in the observed diurnal phases are consistent with the results of Forbes and Hagan (1988) and may provide evidence of dissipation of the propagating (1, 1) tidal mode. The semidiurnal motion observed during this period is not well represented by the recent theoretical results for the amplitude and phase of the semidiurnal tide (Forbes and Vial, this issue). Neutral winds obtained during a geomagnetically active interval, September 25-26, displayed a flow pattern that was significantly distorted from that observed during the preceding, relatively quiet interval. Although the changes in the zonal winds throughout this active interval were consistent with the direction of the ion drag force at 115 km and above, the variations in the meridional winds suggest that other forces, such as pressure gradients driven by Joule heating, need to be considered to explain the observations. ß , ß . ß , ß , ß ß ß ß ß ß ß -0.02
The incoherent scatter radar located at S0ndre Str0mfjord, Greenland (67øN, 51øW, 74.5øA) and the EISCAT incoherent scatter facility located in northern Scandinavia (69.5øN, 19øE, 66.3øA) 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 I 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. and therefore geographically controlled, can be expected to be similar, assuming that longitudinal effects are negligible. This paper presents a comparison of the neutral winds derived from the measurements obtained at the EISCAT and Sondrestrom radars during the period of simultaneous observations. A companion paper presents discussion of the measurements obtained at Sondrestrom alone [Johnson, this issue]. The following section briefly outlines the data acquisition modes utilized at the two facilities (see Johnson [this issue] for further details). An analysis of the deduced neutral winds follows, focusing on both the average flow and tidal structures observed during the interval as well as on the response of the neutral winds to an interval of impulsive forcing. Comparisons with appropriate theoretical models are introduced in the discussion of the radar results.
Abstract. The Global-Scale Wave Model (GSWM) is a steady-state two-dimensional linearized model capable of simulating the solar tides and planetary waves. In an e ort to understand the capabilities and limitations of the GSWM throughout the upper mesosphere and thermosphere a comparative analysis with observational data is presented. A majority of the observational data used in this study was collected during the World Day campaign which ran from 20 January to 30 January 1993. During this campaign data from 18 ground-based observational sites across the globe and two instruments located on the UARS spacecraft were analyzed. Comparisons of these data with the simulations from the GSWM indicate that the GSWM results are in reasonable agreement with the observations. However, there are a number of cases where the agreement is not particularly good. One such instance is for the semidiurnal tide in the northern hemisphere, where the GSWM estimates may exceed observations by 50% . Through a number of numerical simulations, it appears that this discrepancy may be due to the eddy di usivity pro®les used by the GSWM. Other di erences relating to the diurnal tide and the quasi-two-day wave are presented and discussed. Additionally, a discussion on the biases and aliasing di culties which may arise in the observational data is also presented.
It is conceivable that the solvent residence time is slightly shorter in regions near polymer chain ends because their larger flexibility engenders faster local segmental motion.However, that this could be (if at all) the sole reason for the observed effect is conjecture. The mobility data do not indicate that, at the higher polymer concentrations, the system passes through a glass transition, but our measurements are not accurate enough to make a definite decision. We have Properties of Poly(dimethylsiloxane) Networks 41 shown, on the basis of a temperature study, that the number of polymer segments necessary to permit slippage of the (low molecular weight) chains is not exceedingly higher than that involved in solvent diffusion.
A set of tabulated functions called 'Hough Mode Extensions' (NM&), which represent numerical extensions of classical Hough modes into the viscous regime of the thermosphere, are used to leastsquares fit a climatological data base of tidal measurements. The data base consists of monthly average vertical profiles of semidiurnal amplitudes and phases at I7 radar sites accessing some part of the 80-I 50 km height region. The radars are distributed between 78 S and 70 N latrtude, 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), (?,3). (2.4) and (2.5) after their classical Hough function designatioI]s. Once the complex coefficients are derived, re~onstru~lion by weighted superposition of the HMEs results in globally continuous specifications of semidiurnal horizontal and vertical wind, temperature. pressure, and density throughout the X0-150 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 field5 from the F~KHES and VIAL [(IY89) J. NII~MJS. /err. Ph~~s. St, 6491 numerical tidal model.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.