Some aspects of Doppler radar measurements of the mean and fluctuating components of the wind field in the upper middle atmosphere

The basic assumptions made when a Doppler radar is used to measure the mean and fluctuating components of the wind field in the middle atmosphere with various beam configurations are examined. Particular reference is made to the measurement of the various components of the Reynolds stress tensor associated with short period internal gravity waves. It is shown that it is not generally possible to measure the upward flux of horizontal momentum with the conventional Doppler radar beam configuration in the upper middle atmosphere and that an optimum beam configuration is that in which beams are directed at +θ,0 and − θ to the zenith in both the zonal and meridional planes. This allows five of the six components of the Reynolds stress tensor (all those except the horizontal transport of momentum) to be obtained directly from the mean square radial velocities. In addition, the mean wind components and, in principle, the horizontal divergence and stretching deformations may be obtained. The power spectrum of the horizontal velocity may also be calculated using only the assumption that the statistics of the motions are horizontally homogeneous.     

Electric field measurements during the MAC/EPSILON campaign

As part of the MAC/EPSILON campaign in northern Norway during October and November 1987, five rocket-borne payloads made three-axis electric field measurements of the middle atmosphere. Flights 31.066 and 31.067 consisted of large multi-experiment packages, while the other three flights (30.036, 30.037 and 30.038) were devoted primarily to electrical measurements. Simultaneous measurements of the horizontal electric field made by flights 31.066 and 30.036 were in general agreement in their limited altitude region of overlap. A simultaneous small temporal feature was observed in both datasets. The relatively more extensive horizontal E-field datasets from 31.066 and 31.067 both exhibited a decreasing mapping function with decreasing altitude, which is an indication of the observation of fields from a local auroral patch. Small-scale variations in the horizontal fields of the flights were similar to observed wave-like variations in the neutral wind field. No unusual features were observed at high altitudes in the measured E₁ field. Two of the payloads observed small vertical layer structures between 40 and 50 km. No electric field structure was observed in association with the presence of a sudden sodium layer.     

The non-linear interaction of a gravity wave with the vortical modes

The wave-wave interaction theory has been used successfully in describing one class of weakly non-linear wave phenomena. The application of this theory to the atmosphere shows the possibilities of energy and momentum transfer among three interacting gravity waves, as well as from the gravity wave to the other modes of motion. It has been found that the non-resonant interaction of a gravity wave with two vortical modes can proceed at a reasonably rapid rate. With the gravity wave viewed as the primary wave and the two vortical modes as the secondary waves, the interaction equation can be linearized and solved. The resulting analytic formula gives the growth rate of the interaction. In the absence of the Earth's rotation, the growth is limited to a threshold effect. The theory shows that whenever the horizontal air parcel velocity of a gravity wave exceeds a factor of √2 times the horizontal trace velocity of the wave, energy and momentum transfer from the gravity wave to the vortical modes can proceed. The rotation of the Earth will blur this threshold effect by making the interaction more likely to occur. Thus, through this mechanism, a gravity wave can transfer its energy and momentum to the horizontal velocity field in the vortical mode. In this sense, the small scale vortical motions would serve as the sink of both energy and momentum of a propagating gravity wave. When scales of vortical modes reach sufficiently small values, dissipation through viscosity becomes important. At this scale and smaller, the vortical modes are damped out quickly and its energy spectrum must exhibit a sharp decay.     

Summertime stratospheric wind measurements above the South Pole

Observations of the mean wind flow and wave motions in the stratosphere at the South Pole are presented. The atmospheric motions are determined from the tracking of a high altitude, zero-pressure balloon launched from Amundsen-Scott Station during the austral summer of 1985–1986. The balloon position was precisely monitored by an optical theodolite for a large portion of the flight so that small scale motions could be resolved. The mean flow above the pole was approximately 3ms⁻¹. Atmospheric motions characteristic of internal gravity waves were observed with an intrinsic period of approximately 4.5 h and vertical and horizontal wavelengths of approximately 2.5km and 125km, respectively. The horizontal perturbation velocity of the observed waves was large compared to the mean horizontal flow velocity. The implication is that wave motions play a dominant role in the transport of stratospheric constituents in regions where the mean winds are light, such as over the South Pole during austral summer.     

Equatorial spread-F and neutral atmospheric turbulence : a review and a comparative anatomy

A number of satellite and rocket plasma density spectra obtained during equatorial spread-F conditions are presented and discussed in the light of similar measurements in the neutral atmosphere. We discuss this comparison in some detail and find both distinct similarities and subtle differences. The horizontal spectral measurements show a peak at an outer scale quite similar to the scale of the undulations caused by gravity wave interactions with the ionosphere. This feature is similar to a buoyancy subrange but it is easy to show that the amplitudes of the plasma fluctuations are too large to be directly driven by the neutral atmosphere. At intermediate scales the plasma fluctuations have a one-dimensional horizontal spectrum with a power law well described by a ($\text{−5}\text{3}$) slope. Once again it can be shown that the neutral fluid cannot be similarly structured at 400 km altitude due to the high viscosity coefficient. A plasma cascade process seems to be operating but it is not at all clear how the spectrum is formed. Furthermore, vertical power spectra seem to run the gamut in spectral form with slopes (n) varying in the range from −1 to −3. So the horizontal spectra are near universal in form, while the vertical spectra are quite variable.     

The nonlinear mechanism of gravity wave generation by meteorological motions in the atmosphere

Using asymptotic expansions of the hydrodynamic equations in the Rossby number and the method of multiple time scales, we derive approximate expressions for the inhomogeneous “forcing” terms which describe the continuous generation of inertio-gravity waves by quasi-geostrophic motions. As a result of numerical modelling applied to the evolution of tropospheric meso- and macro-scale wave sources, the values of these forcing terms are estimated. A three-dimensional numerical simulation of wave propagation from a mesometeorological tropospheric eddy into the upper atmosphere was done to estimate the gravity wave response to the sources described. The results of the calculations show that the most part of the wave energy propagates quasi-horizontally carried by two-dimensional inertio-gravity waves. At the same time, a part of the energy is transported into the upper atmosphere by internal-gravity waves which can create regions of wave disturbance in the upper atmosphere at considerable distances from the source site. The amplitudes of these waves increase with increasing intensity and decreasing time scales of the wave sources and can reach the values observed in the upper atmosphere.     

The relationship between electron temperature and electron density under quiet conditions in the auroral zone

A theoretical model is described which predicts electron temperature in the day-time F-region above EISCAT on geomagnetically quiet days, given the observed values of electron concentration, ion temperature and heat conduction, the daily average of solar radiation at 10.7cm and the MSIS-86 model of the neutral atmosphere. The values predicted by the model agree very closely with the observed temperatures, both for average conditions and for individual days. On two occasions the onset of a geomagnetic disturbance after a period of quiet conditions was accompanied by a growing divergence between the predicted and observed values, which corresponds to an additional source of electron heating such as would be provided by low energy particle precipitation.     

Shaking Table Tests of a Structure Equipped with Superelastic Dampers

The energy dissipation capacity of the NiTi alloy was evaluated as part of a series of shake table tests. A superelastic damper was developed to take advantage of the hysteretic energy dissipation associated with this type of shape memory alloy. Each device was tested at different intensity levels. A vertical steel cantilever with 600 kg mass on top was subjected to a series of ground motions with different spectral characteristics. The dampers were placed as part of a tie system, restraining the horizontal movement of the top mass. The devices showed stable hysteretic behavior allowing for energy dissipation.     

Modulation of the auroral proton spectrum in the upper atmosphere

An energetic auroral proton entering the atmosphere will alternate between being a proton and a neutral hydrogen atom by charge-exchange collisions with atmospheric constituents. This study uses a simple procedure to evaluate the energy degradation of the penetrating protons/hydrogen atoms by using semi-empirical range relations in air, and derives the particle energy variation as a function of altitude, starting from proton spectra observed from rockets above the main collision region. The main assumptions are that the geomagnetic field is homogeneous and vertical and that the pitch angle of the proton/hydrogen atom is preserved in collisions with atmospheric constituents before being thermalized. The calculations show that the incoming particle flux first loses the low energy particles at the highest pitch angles, even if the beam itself widens as it penetrates the atmosphere. The largest energy loss for particles with initial energy between 10 and 1000 keV occurs in the height interval between 100 and 125 km.     

Energy deposition by precipitating keV range protons and neutral hydrogen atoms into the equatorial atmosphere: dependence on the neutral atmosphere model

The problem of the precipitation of keV range protons and neutral hydrogen atoms into the equatorial atmosphere is studied by the Monte-Carlo technique, and the energy deposition is calculated for different atmospheric models corresponding to low, medium and high solar activity conditions. It is found that the total energy deposited is independent of the atmospheric model and is equal to 16% of the incident energy, but the energy deposition profile varies. The dependence of the profile parameters on the neutral atmospheric model is examined in detail. An empirical model of the Chapman type distribution, involving only the neutral atmospheric density, has been developed to represent the energy deposition profile for any solar activity condition.     

Electrical structure in the high-latitude middle atmosphere

Middle atmosphere electrodynamics at high latitudes differs significantly from the normally assumed picture of a passive region through which electric fields of external origin couple. Large Vm ⁻¹ electric fields, both horizontal and vertical, have been observed within bounded regions of the upper stratosphere and lower mesosphere. They seem to occur only in regions where the electrical conductivity is a few times 10⁻¹⁰ S m⁻¹ or less and appear to be current limned. While low conductivity is necessary, it is not a sufficient condition for occurrence. The observed large horizontal electric fields were found to be anticorrelated with the local neutral wind. However, a generation mechanism of these electric fields is as yet unknown but must involve space charge separation rather than dynamo effects. Large variations in the conductivity were also observed to occur with fluctuations in magnetic activity, and these were found to be consistent with measured variations in energy deposition during auroral phenomena. Theoretical concepts of mapping of electric fields downward from the thermosphere along equipotential magnetic field lines were shown to hold qualitatively in the D-region at the mV m⁻¹ level. Perturbations affecting such models were determined to be small.     

Gravity waves from O2 nightglow during the AIDA '89 campaign III: effects of gravity wave saturation

The O₂ atmospheric (0–1) night airglow emitted within the gravity wave saturation region at ∼90–100 km can serve as a means of studying the wave activity. In this analysis, the atmospheric motions were described by a mean spectral model and an algorithm was developed to infer the wave kinetic energy density and momentum flux from variations in O₂ (0–1) airglow emission rate and rotational temperature. The method was applied to eight nights of data collected by MORTI, a mesopause oxygen rotational temperature imager, during the AIDA campaign of 1989 in Puerto Rico (18°N, 67°W). The observed r.m.s. fractional fluctuations of airglow emission rate and rotational temperature were of the order of ∼0.07–0.15 and ∼0.02–0.04, respectively, and the characteristic vertical wavelengths were estimated at ∼10 2 -20 km. The inferred r.m.s. horizontal velocities and velocity variances were found to be ∼12–25 m/s and ∼150–600 m²/s², with the majority of the horizontal velocity and its variance associated with low-frequency, large-scale wave motions. The estimated momentum fluxes, mainly contributed by high-frequency, small-scale waves, were ∼2–10 m²/s². These results are in good agreement with those obtained from other measurements using different observational methods at low and mid-latitudes.     

Gravity-wave motions in the mesosphere and lower thermosphere observed at Mawson,Antarctica

We present the results of MF radar observations of mean winds and waves in the height range 78–108 km at Mawson (67°S, 63°E), Antarctica. The measurements were made in the period from 1984 to 1990. Climatologies of the prevailing zonal and meridional circulations made with a 12-day time resolution show that the mean circulation remained relatively stable over the 6 yr of observation. Climatologies of gravity-wave motions in the 1–24 h period range were also generated. These reveal that the r.m.s. amplitudes of horizontal wave motions near the mesopause (~90 km) are about 30 m s⁻¹, and that there is some anisotropy in the motions, especially at heights below 90 km. Meridional amplitudes are larger than zonal amplitudes, which suggests a preference for wave propagation in the north-south direction. Comparisons with MST radar wind observations made near the summer solstice at Poker Flat, Alaska (65°N) and at Andøya, Norway (69°N) show similarities with the Mawson observations, but the wave amplitudes and mean motions are larger in magnitude at the northern sites. This suggests hemispheric differences in wave activity that require further study.     

Constant Ductility Energy Factors for the Near-Fault Pulse-Like Ground Motions

This study is focused on the constant ductility energy factors for bilinear system under the near-fault pulse-like ground motions. The variation of energy factors is studied in consideration of the earthquake magnitude, rupture distance, damping ratios, and post-yield stiffness ratios. The results indicate that the near-fault pulse-like ground motions would increase the energy dissipation of structures. The energy factors are significantly influenced by the earthquake magnitude. The damping ratios have more obvious influences on the energy factors than the post-yield stiffness ratios. A predictive model is proposed for the application of constant ductility energy factors for near-fault pulse-like ground motions.     

Dynamical coupling processes between the middle atmosphere and lower ionosphere

A variety of dynamical processes are important in coupling motions within the middle atmosphere and lower ionosphere. These processes can generally be classified as either advective, wave-like or diffusive. Within the middle atmosphere, wave-like processes, and especially gravity waves, are of crucial importance. Turbulent diffusion and advection probably play lesser, but not insignificant, roles. However, whether these same concepts apply to coupling for regions outside the middle atmosphere—and especially between the upper middle atmosphere and the lower ionosphere—is not clear. In this paper the current knowledge about coupling processes between these important regions is reviewed. We discuss coupling processes in the middle atmosphere with the objective of examining how well these concepts may be extended to the topic of middle atmosphere to ionosphere coupling. Different approaches to understanding, and the different experimental procedures used by workers studying the middle atmosphere and ionosphere, also make comparisons difficult. The importance of the measurement of common parameters is highlighted, and the need for coordinated campaigns to examine the interactions between the regions is emphasized.     

First results with the Adelaide VHF radar: spaced antenna studies of tropospheric winds

The first results from a VHF radar of the ST type located at Buckland Park near Adelaide, Australia (35°S, 138°E), are presented. The radar is designed to be versatile and can be used to measure velocities in the lower atmosphere using both the spaced antenna (SA) and Doppler beam-swinging (DBS) techniques. Here studies of irregularities and motions made with the spaced antenna technique are discussed. It is shown that the scale of the diffraction pattern formed by the backscattered radiation varies with altitude, with the mean pattern scale being smaller in the troposphere than in the stratosphere. The observations are consistent with the backscattered energy decreasing as a function of off-vertical angle by 1.5 dB per degree in the troposphere and by about 2.8 dB per degree in the lower stratosphere. An intercomparison of zonal velocities measured with the SA and DBS methods shows good agreement. In May and August 1984 an extensive comparison was made between the velocities measured by the SA method and winds determined from over 80 balloon-borne radiosondes released from Adelaide Airport, situated some 36 km to the south of the radar. The velocities were compared on a statistical basis and showed excellent agreement, although the SA speeds tended to be 1–2 m s⁻¹ smaller in magnitude than the radiosonde velocities. Overall, the rms differences between the two sets of measurements was only 3–4ms⁻¹ throughout the troposphere, a result which is consistent with the random errors inherent in each technique, as well as the spatial separation between the radar and balloon observations. The utility of the SA method for meteorological observations is illustrated by a study of both the horizontal and vertical wind fields during the passage of a cold front made in November 1984. The high time resolution available with the radar allows detailed studies of the development of the pre-frontal jet, the wind convergence into the front and associated vertical motions.     

Estimation of the Displacement of Viscously Damped Pinching Hysteretic Structures Subjected to Near-fault Ground Motions

To fulfill a displacement-based design or response prediction for nonlinear structures, the concept of equivalent linearization is usually applied, and the key issue is to derive the equivalent parameters considering the characteristics of hysteretic model, ductility level, and input ground motions. Pinching hysteretic structures subjected to dynamic loading exhibit hysteresis with degraded stiffness and strength and thus reduced energy dissipation. In case of excitation of near-fault earthquake ground motions, the energy dissipation is further limited due to the short duration of vibration. In order to improve the energy dissipation capability, viscous-type dampers have been advantageously incorporated into these types of structures. Against the viscously damped pinching hysteretic structure under the excitation of near-fault ground motions, this study aims to develop a seismic response estimation method using an equivalent linearization technique. The energy dissipation of various hysteretic cycles, including stationary hysteretic cycle, amplitude expansion cycle, and amplitude reduction cycle, is investigated, and empirical formulas for the equivalent damping ratio is proposed. A damping modification factor that accounts for the near-fault effect is introduced and expanded to ensure its applicability to structures with damping ratios less than 5%. An approach for estimating the maximum displacement of a viscously damped pinching hysteretic structure, in which the pinching hysteretic effect of a structure and the near-fault effect of ground motions are considered, is developed. A time history analysis of an extensive range of structural parameters is performed. The results confirm that the proposed approach can be applied to estimate the maximum displacement of a viscously damped pinching hysteretic structure that is subjected to near-fault ground motions.     

Ion-neutral dynamics in the high latitude ionosphere: first results from the INDI experiment

The INDI experiment consisted of a series of joint observations between EISCAT and a Fabry-Perot interferometer (FPI) situated at Kiruna. The FPI measured the 630 nm neutral oxygen emission at eight positions on a 30° elevation circle, giving a spatial average of the neutral wind field with a time resolution of about 15 min. The radar performed a seven-position, near-meridional scan in a region common to the optical measurements. Such simultaneous observations of the ionised and neutral components of the Earth's atmosphere allow a study of the ion energy balance and the coupling between species. The first stage of the analysis was to derive the neutral wind from the EISCAT data using the simplified ambipolar diffusion and ion energy equations. This was then compared with the direct measurements from the FPI. There was good agreement between derived and measured meridional winds, but the zonal wind values, although showing the same trends, differed in magnitude by a factor of 3–5. The reasons for this are discussed. The meridional wind data was used to derive the ion-neutral collision frequency. This was a factor of 2 or 3 less than recent model values. Preliminary comparisons of the measured electron densities with the 630 nm emission intensity gave clues to the chemistry of the emission process.     

Forcing atmospheric oscillations by long-period seismic oscillations: a case study

Using synchronous measurements made by microbarograph and a seismograph with a vertical pendulum, the sign of the vertical direction of the wave energy flux in the atmosphere at the ground has been derived for oscillations in the ∼0.5–4 h period range and which occur simultaneously in the Earth and atmosphere. The seismic oscillations shown could generate atmospheric oscillations by the ‘piston’ mechanism. A change of sign of the flux direction is also observed.