Determination of the Structure of the Atmosphere between 90 and 250 km by means of Contaminant Releases at Woomera, May 1968

Gravity wave saturation in the middle atmosphere: A review of theory and observations

Abstract: This paper provides a review of recent advances in our understanding of gravity wave saturation in the middle atmosphere. A brief discussion of those studies leading to the identification of gravity wave effects and their role in middle atmosphere dynamics is presented first. This is followed by a simple development of the linear saturation theory to illustrate the principal effects. Recent extensions to the linear saturation theory, including quasi-linear, nonlinear, and transient effects, are then described. Those studies addressing the role of gravity wave saturation in the mean circulation of the middle atmosphere are also discussed. Finally, observations of gravity wave motions, distribution, and variability and those measurements specifically addressing gravity wave saturation are reviewed.

Seasonal variation of turbulent energy dissipation rates at high latitudes as determined by in situ measurements of neutral density fluctuations

Abstract: In the last 6 years a series of 22 sounding rockets were launched in order to investigate the dynamic state of the mesosphere and lower thermosphere. Of these flights, 19 were performed at high latitudes, either from the Andoya Rocket Range (17 flights) in northern Norway (69°N) or from Esrange (2 flights) in northern Sweden (68°N). An ionization gauge mounted on board these sounding rockets measured neutral density fluctuations down to very small spatial scales of a few meters. During several flights, small-scale density fluctuations were found in layers of a few kilometers thickness. Subsequent analysis of these fluctuations indicates that they were caused by turbulent motions. The high resolution of these measurements makes it possible to unambiguously deduce turbulent energy dissipation rates e from the spectra of the relative density fluctuations. The e profiles and the corresponding heating rates obtained at high latitudes show a significant and systematic seasonal variation: Whereas in winter the turbulent heating rates are comparatively small (typically 0.1 K/d and 1-2 K/d below and above ∼ 75 km, respectively) in the entire mesosphere and lower thermosphere, much stronger mean values of ∼10-20 K/d are observed around the summer mesopause (∼90 km). During none of the seven summer flights have we detected any noticeable turbulence in the middle and lower mesosphere. Turbulence is confined to a relatively small height region of 78-97 km during summer but covers the entire mesosphere from 60 to 100 km during winter. From our measurements we arrive at the curious conclusion that turbulent heating in the mesosphere is strongest at the coldest part of the atmosphere, namely, at the polar mesopause in summer. Our observations imply that turbulent heating is an important contribution to the energy budget of the upper mesosphere in summer, whereas it is presumably negligible in the entire mesosphere in winter. Mean turbulent velocities w turb and mean turbulent diffusion coefficients K do not exhibit such a distinct seasonal variation. In the lower and upper mesosphere, typical values for w turb are 0.3 and 1-3 m/s, respectively, and typical values for K are 4 and 100 m2/s, respectively. The seasonal variation of mean profiles of turbulent parameters as obtained by high-resolution in situ techniques puts a serious constraint on models dealing with the energy budget of the upper atmosphere, in particular the parameterization of subgrid process in terms of mean state quantities, e.g., used in gravity wave breaking scenarios.

Winds and shears in the mesosphere and lower thermosphere: Results from four decades of chemical release wind measurements

Abstract: [1] Since 1958, over 400 chemical tracer measurements have been made of the wind profiles in the upper mesosphere and lower thermosphere. The measurements cover a wide range of latitudes, longitudes, seasons, and local times. The results from the analysis of the data set show that a wind maximum in the altitude range between 100 and 110 km is a consistent feature of the observations at midlatitudes and low latitudes. The wind speed associated with the maximum exceeds 100 m s−1 in over 60% of the observations. Large shears are found, especially on the bottomside of the wind maximum, and the observed shears are often large enough to be either unstable or approaching instability. The wind maximum and shear features are characteristics of the overall data set and are not tied to particular observation sites, particular seasons, or special conditions.

Turbulence in the region 80―120 km

Abstract: Measurements of turbulent energy dissipation rates and eddy diffusion coefficients have been collated, and mean height profiles of fundamental turbulence parameters in the region 80–120 km are presented.

A comparison of wind observations of the upper thermosphere from the dynamics explorer satellite with the predictions of a global time-dependent model

Abstract: Seven polar passes of the NASA Dynamics Explorer 2 (DE-2) satellite during October and early December 1981 have been used to examine the high-latitude circulation in the upper thermosphere. Vector winds along the satellite track are derived by appropriate merging of the data from the remote-sensing Fabry-Perot interferometer (meridional wind) and the in situ wind and temperature spectrometer (zonal wind) and are compared with the predictions of a three-dimensional, time-dependent, global model of the thermosphere. Major features of the experimental winds, such as the mean day to night circulation caused by solar u.v. and e.u.v. heating, augmented by magnetospheric processes at high latitude and the sharp boundaries and flow reversals imposed on thermospheric winds by momentum transfer (ion drag) from the magnetosphere, are qualitatively explained by a version of the global model using a semi-empirical global model of polar electric fields (Volland Model 2 or Heppner Model A) and a model of global electron density which excludes the effects of high-latitude geomagnetic processes. A second version of the global dynamic model includes a theoretical model of the high-latitude ionosphere which is self-consistent and reflects the enhancement of ionization due to magnetospheric phenomena acting in addition to solar e.u.v. photo-ionization, including the interactive processes which occur between ionization and high latitude ion convection and thermospheric winds. This second dynamical model shows an improved comparison with the structure and magnitude of polar cap and auroral oval winds at times of other than extremely low geomagnetic activity, when the first model appears a better match. An improved empirical description of the complex magnetospheric processes exciting the thermosphere in the vicinity of the dayside polar cusp and an empirical description of storm-time electric fields will be required for a quantitative explanation of the polar thermospheric winds during geomagnetic substorm events.

Molecular theory of gases and liquids

Abstract: Molecular theory of gases and liquids , Molecular theory of gases and liquids , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

The theory of homogeneous turbulence

Abstract: Preface 1 Introduction 2 Mathematics representation of the field of turbulence 3 The kinematics of homogeneous turbulence 4 Some linear problems 5 General dynamics of decay 6 The universal equilibrium theory 7 Decay of the energy-containing eddies 8 The probability distribution of u(x) Bibliography of research on homogeneous turbulence Index

Variations in exospheric density at heights near 1100km, derived from satellite orbits

Abstract: In an earlier paper, values of exospheric density were obtained from the orbit of Echo 2 for the years 1964–1965. The results indicated a semi-annual variation in density by a factor of between 2 and 3, considerably larger than predicted by existing atmospheric models. These studies have now been extended to the beginning of 1967, using both Echo 2 and Calsphere 1, to show how the density is responding to increasing solar activity. Variations in density during 1964 have been analysed in more detail. The long-term variation associated with the solar cycle and the short-term variations associated with magnetic and solar disturbances agree with the variations expected on the basis of current models. The semi-annual variation is persisting to higher levels of solar activity, and although its amplitude is diminishing the factor of variation was still 1.6 in 1966.