摘要:

The Lower Thermosphere Coupling Study (LTCS) is a coordinated investigation of the lower thermosphere (90–150 km) combining observational (optical and radar) and numerical modeling efforts with the ultimate goal of better understanding the dynamic and electrodynamic processes coupling the mesosphere, lower thermosphere, and upper thermosphere regions. The study, which is being conducted under the auspices of the National Science Foundation CEDAR (Coupling, Energetics, and Dynamics of Atmospheric Regions) Program, is intended to address such questions as, What is the relative importance of upward propagating tides, planetary waves, and gravity waves, related accelerations of the mean flow, and heating and momentum sources of magnetospheric origin, in determining neutral dynamical fields and ionospheric structures as a function of height between 90 and 150 km? How do these different spatial and temporal scales of motion interact with each other, and how do they influence the vertical, seasonal‐latitudinal, and diurnal variations in turbulent diffusivity and transport of minor chemical constituents? What are the relative roles of magnetospheric currents and the ionospheric wind dynamo in determining ionospheric electric fie

ISSN:0148-0227    

DOI:10.1029/90JA02415

年代:1991

数据来源: WILEY

摘要:

The incoherent scatter radars at Arecibo (18.3°N), Millstone Hill (42.6°N), and Sondrestrom (67°N) simultaneously observed the thermal structure and dynamics of the Earth's lower thermosphere during the period September 21–25, 1987, as part of the Lower Thermosphere Coupling Study. Plasma spectral measurements were used to derive ion temperatures and horizontal neutral wind velocities in the altitude region 90–140 km, and semidiurnal harmonics were determined from the data sets. The daily mean temperatures were found to be within 40°K at the three stations, and the largest semidiurnal temperature amplitudes, exceeding 70°K above 115 km, were recorded at Arecibo. Mean winds were about ±30 m/s at most altitudes, shifting from eastward to westward directions at 115 km, and the semidiurnal amplitudes were in the range 20–70 m/s. Phase variations with height reveal reasonable consistency among the three stations, within 3–4 hours, and indicate an average vertical wavelength of 40–70 km. However, a strong variation is seen in the vertical structure in the 110‐ to 130‐km region, suggesting the presence of different tidal modes. Comparison with the most current tidal propagation model by Forbes and Vial (this issue) indicates that the model tends to underestimate the amplitudes for this time period, particularly above 110 km. By virtue of the latitudinal distribution of the three radars their coordinated observations of the lower thermosphere serve to constrain numerical models and improve our understanding of this important yet complex regio

ISSN:0148-0227    

DOI:10.1029/90JA01529

年代:1991

数据来源: WILEY

摘要:

The incoherent scatter radar located at Søndre Strømfjord, Greenland, obtainedEandFregion 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 obser

ISSN:0148-0227    

DOI:10.1029/90JA01803

年代:1991

数据来源: WILEY

摘要:

Ion‐neutral collision frequencies determined from measurements obtained by the incoherent scatter radar located at Søndre Strømfjord, Greenland, have been used to derive lower thermospheric neutral densities during the first Lower Thermosphere Coupling Study (LTCS 1), September 21–26, 1987. Periods of Joule and particle heating which might disturb theEregion thermal equilibrium were systematically eliminated. The mean profile of neutral density for the period is in good agreement with the mass spectrometer incoherent scatter 1986 (MSIS‐86) model between 92 and 104 km. A tendency to overestimate collision frequencies above 105 km may arise from range‐smearing effects. The results of a tidal analysis performed on the neutral density between 92 and 109 km show that the amplitudes of the diurnal and semidiurnal components of the tides are approximately equivalent. The observations are generally in better agreement with the MSIS‐86 predictions than with the thermosphere‐ionosphere general circulation model (TIGCM) simulation of the LTCS 1 interval. The observed phase of the diurnal component is approximately constant with height above 98 km and is in close agreement with the MSIS‐86 model phases; however, the TIGCM diurnal phases are shifted by 6–8 hours to later local times. The phase of the semidiurnal tide is in good agreement with predictions of the MSIS‐86 model and the TIGCM simulation of this interval, except near 98 km. The observed semidiurnal phase is also consistent with previous high‐latitude results (Kirkwood, 1986). The relative amplitude of the observed semidiurnal oscillation is up to 15% larger than that previously observed at the European Incoherent Scatter facility but is consistent with the amplitudes presented in an earlier study of Millstone Hill measu

ISSN:0148-0227    

DOI:10.1029/90JA02126

年代:1991

数据来源: WILEY

摘要:

The incoherent scatter radar located at Søndre Strømfjord, 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 obtainedEandFregion 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 convergi

ISSN:0148-0227    

DOI:10.1029/90JA02314

年代:1991

数据来源: WILEY

摘要:

Winds and tides were measured by nine MLT (mesosphere, lower thermosphere) radars with locations between 70°N and 78°S, including an equatorial station at Christmas Island, 2°N (Avery et al., 1990). The mean winds were eastward (westward) in the northern (southern) hemisphere mesosphere, consistent with midwinter circulations. For the 12‐hour (semidiurnal) tide, observations and the model of Forbes and Vial (1989) were in generally good agreement: in both cases northward components were closer to being in phase in the two hemispheres, and winter wavelengths were shorter than those of the mid‐latitude summer. Major differences were large (small) amplitudes at 70°N for model (observations); and poor agreement of equatorial tidal profiles. For the 24‐hour (diurnal tide), the radar observations and model of Forbes and Hagan (1988) were in useful agreement in the summer hemisphere. However, the short (long) wavelengths at mid (high) latitudes of the model's winter hemisphere were not observed during LTCS (Lower Thermosphere Coupling Study) 2, nor in climatologies for December. Suggestions as to the reasons for this disparity are

ISSN:0148-0227    

DOI:10.1029/90JA01755

年代:1991

数据来源: WILEY

摘要:

The meteor wind radar at Durham and the incoherent scatter radar at Millstone Hill, which are closely located at 43°N, 71°W, were used simultaneously during the first campaign of the Lower Thermosphere Coupling Study in September 1987 to map the structure of the semidiurnal tidal wind from 80 km to 135 km. The results from the two techniques are consistent and indicate overall continuity in the observed propagation of the semidiurnal tide in both amplitude and phase. Northward winds gradually increase from about 30 m/s at 80 km to 60 m/s above 110 km, while the eastward winds are near 20 m/s below 100 km and reach a maximum of 70 m/s at 115 km. Phase variations indicate a vertical wavelength of the propagating tidal mode of about 100 km at all altitudes for the eastward component, but the incoherent scatter results suggest a shorter wavelength for the northward component above 100 km. The results obtained during the campaign are similar to climatological averages obtained from nonsimultaneous data sets from the two radars. Comparison with numerical model results from Forbes and Vial (this issue) indicates that the model underestimates the wind amplitudes and reaches the maximum value about 10 km below the observed peak; the phase variations lag the observations by 2 hours but indicate an overall structure with altitude that is very similar to the dat

ISSN:0148-0227    

DOI:10.1029/90JA02186

年代:1991

数据来源: WILEY

摘要:

The thermosphere (≥ 90 km) is dynamically coupled to the mesosphere and lower atmospheric regions by gravity waves, tides, and planetary waves which propagate upward from below and often undergo dissipation in the 80 to 150‐km region. This paper focuses on the mesosphere‐thermosphere coupling due to the solar semidiurnal atmospheric tide during the first Lower Thermosphere Coupling Study (LTCS) campaign interval of September 21–25, 1987. During this period, determinations of semidiurnal eastward (u) and northward (υ) velocity components and temperature (δT) are available within the 80 to 150‐km region from a total of 13 incoherent scatter, meteor, and spaced antenna radars. A new method is developed and reported here for assimilating such data sets into a self‐consistent global empirical description of the tidal dynamics between 80 and 150 km. Using a set of numerically generated ‘Hough mode extension’ (HME) functions foru, v,and δTcorresponding to the (2,2), (2,3), (2,4), and (2,5) semidiurnal tidal modes, a least squares fit to the radar data is performed. The fit results in a single (complex) normalizing factor for each HME from which globally continuous and internally consistent semidiurnal oscillations inu, v,and δT(as well as vertical velocity and density) can be derived. The quality of fit to these data from various height regions including both northern and southern hemispheres establishes the internal consistency between these measurements as well as the applicability of the method to future data sets. The resulting empirical model reproduces the data significantly better than recent theoretical models, suggesting use of the method for deriving tidal boundary conditions for thermospheric general

ISSN:0148-0227    

DOI:10.1029/90JA01768

年代:1991

数据来源: WILEY

摘要:

Theoretical predictions of global monthly and seasonally averaged semidiurnal temperatures and winds between 80 and 150 km are presented and compared with seasonal means from measurements at the Arecibo, Chatanika, Millstone Hill, and Sondrestrom incoherent scatter radars. The model values above 100 km are obtained by utilizing a set of tabulated ‘Hough mode extension’ functions to extrapolate the (2,2), (2,3), (2,4), and (2,5) propagating tidal modes at 100 km from a spectral model (not valid in the E region due to the way molecular dissipation is parameterized) to higher altitudes and by superimposing a component driven by in‐situ excitation (important mainly above 140 km). The monthly simulations demonstrate the close coupling that exists between seasonal and latitudinal structures and argue for an observational program emphasizing adequate seasonal and latitudinal coverage. Despite constraints imposed by the distributions of existing data with respect to month and year, comparisons with the model reveal several trends that appear to be real: (1) the model amplitudes are usually 20–50% smaller than those observed; (2) vertical wavelengths are well‐predicted by the model, but model phases generally lead the observations at middle and low latitudes by approximately 2–3 hours at all heights for both winds and temperatures; (3) semidiurnal temperatures at middle and low latitudes, in both the observations and model, indicate significant seasonal changes in the phase of the temperature oscillation, but not in the winds; and (4) the seasonal trends in the model and observations are consistent, with “summer leading winter” at Millstone Hill, while the reverse is true at Arecibo. The sources of the absolute amplitude and phase discrepancies in the model, which are probably connected with the adopted mean wind and thermal excitation specifications, need to be investigated. In addition, a concerted effort must be made within the Coupling Energetics and Dynamics of Atmospheric Regions (CEDAR) program to better delineate the seasonal‐latitudinal thermal structure and dynamics of the E region, following the example of the extensive F region observational data base which has been accumulated ove

ISSN:0148-0227    

DOI:10.1029/90JA02187

年代:1991

数据来源: WILEY

摘要:

The MSIS‐86 empirical model has been revised in the lower thermosphere and extended into the mesosphere and lower atmosphere to provide a single analytic model for calculating temperature and density profiles representative of the climatological average for various geophysical conditions. Tabulations from the Handbook for MAP 16 are the primary guide for the lower atmosphere and are supplemented by historical rocket and incoherent scatter data in the upper mesosphere and lower thermosphere. Low‐order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, and simplified local time and longitude variations. While month to month details cannot be completely represented, lower atmosphere temperature data are fit to an overall standard deviation of 3 K and pressure to 2%. Comparison with rocket and other data indicates that the model represents current knowledge of the climatological average reasonably well, although there is some conflict as to details near the mesopa

ISSN:0148-0227    

DOI:10.1029/90JA02125

年代:1991

数据来源: WILEY