Gravity wave saturation and eddy diffusion in the middle atmosphere

A discussion is given of gravity wave saturation and its relation to eddy diffusion in the middle atmosphere. Attention is focused on the saturation process and some of its observable manifestations. It does not serve as a review of all related work. Although a theoretical point of view is taken, the emphasis is on which wave parameters need be measured to predict quantitatively the influence of gravity waves on eddy transport. The following considerations are stressed: the variation of spectra with observation time T; that eddy diffusivities are determined by velocity spectra; the anisotropic nature of diffusivity; a unified approach to saturation; an attempt to make eddy diffusivity more precise; the relationship between eddy diffusivity and wave dissipation.The subjects of ‘wave drag’ (momentum flux deposition) and heat flux need only be treated briefly, because they are related to eddy diffusivity in simple ways. Consideration is also given to two different theoretical mechanisms of wave saturation—wave induced convective instability and strong nonlinear wave interactions. The saturation theory is then used to predict a globally averaged height profile of vertical diffusivity in the middle atmosphere. This calculation shows that gravity waves are a major contributor to eddy diffusion from heights of 40–110 km, and that they are significant down to 20 km. A more detailed calculation of wave induced eddy diffusion, including latitudinal and seasonal variations, can be made if wave velocity spectra become available. The paper closes with recommendations for future research.     

On the estimation of eddy diffusion coefficient in the mesosphere from partial reflection technique

Estimations of the eddy diffusion coefficient in the mesosphere are reported using observations from the Partial Reflection Technique at Buckland Park, South Australia (35°S, 139°E). The method of estimation is approximately the same as reported by Schlegel et al. (1977) with one difference i.e. in the present work the medium is considered to be moving and the effect due to horizontal winds have been incorporated. It is found that the inclusion of horizontal winds decreases the value of the eddy diffusion coefficient and the decrease is more marked as the horizontal velocity increases.     

Semi-theoretical global models of the eddy diffusion coefficient based on satellite data

Long periodic density variations of the thermospheric constituents can be described by combined variations of two parameters: exospheric temperature and turbopause height. Two global models of thermospheric composition relying on exospheric temperature and turbopause height variations were presented by the authors in a previous paper. In this paper the turbopause height changes and their interpretation in terms of the eddy diffusion coefficient are discussed. The determination of the eddy diffusion coefficient is not unique as some assumptions have to be made with respect to the shape of the height profile of the eddy diffusion coefficient. Two models are presented: one for a height independent and one for a height dependent profile of the eddy diffusion coefficient. The models are discussed and compared with other results.     

Storm-time variation of foE at a low latitude station

foE changes during geomagnetic storms are studied for Ahmedabad. Individual storms show erratic effects. The average curves show a possible 5–10% decrease in the post-Main Phase Onset period and a somewhat larger decrease after about 40 storm hours.     

Seasonal and latitudinal variations of eddy diffusion coefficient in the mesosphere and lower thermosphere

Two approaches to turbulence parameter determination around the turbopause are discussed. It is shown that there is a contradiction in the results concerning seasonal variations of the eddy diffusion coefficient between direct measurements and estimations based on minor constituents. Taking account of the vertical mean transport in the continuity equations for the above constituents might eliminate that contradiction. Results of the re-examination of the authors' data on the Ar to N₂ ratio published earlier are presented. The results show stronger turbulence in winter than in summer, the amplitude of the effect increasing towards higher latitudes.     

Subsidiary maxima in late evening NmF2 at a low latitude station at solar maximum

At solar maximum during the late evening hours (2100–2400 LT), N_mF2 at Tahiti frequently does not decrease monotonically but exhibits temporary subsidiary maxima. Thus, in 1980, of 66 evening periods for which good data were available, 20 showed monotonie decreases but the remainder exhibited such subsidiary maxima. In summer the subsidiary maxima correspond to hmF2 significantly lower than the monotonie decreases. This lower hmF2 during subsidiary maxima corresponds to a weakening or reversal of the equatorward neutral wind, accompanied by an increase in the flux from the equatorial fountain. In winter the subsidiary maxima are fully accounted for by increases in the flux from the fountain effect, probably due to increases in the strength of the equatorial electrojet.     

F-region neutral winds from ionosonde measurements of hmF2 at low latitude magnetic conjugate regions

The behavior of the F2-peak height difference Δh_mF2, between low latitude magnetic conjugate points, is known to be governed by thermospheric winds blowing along the magnetic meridian. Ground based ionosonde measurements of h_mF2, at two pairs of magnetic conjugate stations, have been analysed in conjunction with the results of a realistic dynamic computer model of the tropical ionospheric F-region, to determine thermospheric wind velocities. The behavior of monthly average values of the sum, at conjugate points, of the thermospheric horizontal wind velocity component in the magnetic meridian, at low latitudes, has been inferred for months of solstice and equinox, as well as for periods of low and high solar activity.     

A new aspect of daily variations of the geomagnetic field at low latitude

Data from the unique network of low latitude geomagnetic observatories in India extending from the dip equator to the northern focus of the Sq current system have shown a new type of Sq current distribution different from those associated with the normal or the counter electrojet currents. On 3 December 1985 both the horizontal as well as the vertical components of the geomagnetic field at Annamalainagar showed maximum values around the midday hours. The abnormal feature described seems to be rather a rare phenomenon. The solar daily range of H field is found to be fairly constant from the dip equator up to about 12° dip latitude, suggesting the complete absence of the equatorial enhancement of ΔH, typical of the equatorial electrojet. The cancellation of the equatorial electrojet is suggested to be caused by a westward flowing current system much wider than the conventional equatorial electrojet. This additional current system could be due to the excitation of certain tidal modes at low latitudes on such abnormal days.     

Trimpi events on low latitude paths: An investigation of gyroresonance interactions at low L-values

We report on Trimpi events observed at Durban (L = 1.69, 29°53′S, 31°00′E) and investigate the efficacy of gyroresonance scattering in precipitating electrons into the atmosphere at low L (<2). The rate of occurrence of Trimpis at Durban is less than one per day. Our observations include a number of daytime events on OMEGA signals from La Reunion. Using the full relativistic equations of motion, a test particle simulation is employed to find the region in parameter space where large pitch angle scattering occurs. We find that at low L the conditions for pitch angle scattering are less favourable than at higher L (L ∼ 4). Resonant electrons have high (relativistic) energies, interaction times are of the order of milliseconds (T_i ∼ 5 ms) and large wave amplitudes (B_w ∼ 200 pT) are required at whistler frequencies to produce pitch angle changes of greater than 1°. Large pitch angle scattering is needed near Durban since particles near the loss cone will have been lost in the South Atlantic Geomagnetic Anomaly. We note that the radio frequencies transmitted into the magnetosphere from lightning are too low to give effective electron scattering at low L. We suggest an explanation for the low rate of occurrence of Trimpis at Durban.     

Lunar tides in the middle atmosphere from NIMBUS 5 data

Lunar tides in temperature have been determined at stratospheric heights from about 2 yr of radiance measurements by the NIMBUS 5 satellite. The tides have an amplitude of order 0.1 K, and results are presented for the variations with height and latitude.     

Comparison of one year of data on geomagnetic pulsations at geostationary orbit and at a low latitude ground station

In spite of several satellite-ground comparisons of pulsation data, many questions remain open for future investigation. This paper reports on a comparison of the satellite ATS 6 and Nagycenk data (L ∼ 1.9) on pulsation occurrence, activity, period and switches. This low latitude ground station sees a lot of activity which is less evident at L ∼ 6.6, i.e. these pulsations are due to amplification in the inner magnetosphere. The ATS harmonic structure is shown to have little influence on the ground activity. The inner magnetospheric amplification is changing and is influenced by solar wind velocity. The switches confirm that a large part of the two pulsation activities are of different origin, supposedly at least partly from a Kelvin-Helmholtz source at L ∼ 6.6 and from the upstream source at L ∼ 1.9.     

Vertical movements in the middle atmosphere derived from foil cloud experiments

Results obtained on vertical velocities of air in the mesosphere are presented which were measured by small foil clouds tracked by radar at Andenes (69°) during January and February 1984. The results (typically ± 4–6 m s⁻¹, up to 10 m s⁻¹, and oscillatory in nature) are in good agreement with those obtained by ground-based remote sensing methods. Supplementary observation techniques of the radar return signal show that the interactions between background wind and waves quite often cause small-scale flow separation effects which escape detection when conventional radar tracking is the sole source of information.     

Rocket altitude atmospheric X-rays from magnetospheric electrons at low and middle latitudes

Like auroral electrons, quasitrapped magnetospheric electrons mirroring in the upper atmosphere at low and middle latitudes will generate X-rays by the bremsstrahlung and K_α line excitation processes. These atmospheric X-rays may contribute a diffuse background to rocket-borne astronomy experiments launched from White Sands, New Mexico and Kauai, Hawaii. Calculations of the atmospheric X-ray spectrum at these launch sites based on observations of quasitrapped electrons indicate that this locally generated flux is comparable to the reported soft X-ray flux below a few keV.     

Comprehensive meteorological modelling of the middle atmosphere: a tutorial review

This paper reviews the current state of comprehensive, three-dimensional, time-dependent modelling of the circulation in the middle and upper atmosphere from a meteorologist's perspective. The paper begins with a consideration of the various components of a comprehensive model (or general circulation model, GCM), including treatments of processes that can be explicitly resolved and those that occur on scales too small to resolve (and that must be parameterized). The typical performance of GCMs in simulating the tropospheric climate is discussed. Then some important background on current ideas concerning the general circulation of the stratosphere and mesosphere is presented. In particular, the transformed-Eulerian mean flow formalism, the role of vertically-propagating internal gravity waves in driving the large-scale circulation, and the notion of a stratospheric surf zone are all briefly reviewed. Using this background as a guide, some middle atmospheric GCM results are discussed, with a focus on simulations made recently with the GFDL ‘SKYHI’ troposphere-stratosphere-mesosphere GCM. The presentation attempts to emphasize the interaction between theory and comprehensive modelling. Many theoretical notions cannot be confirmed in detail from observations of the real atmosphere due to the various limitations in the observational methods, but can be very completely examined in GCMs in which every atmospheric variable is known perfectly (within the limits of the numerical methods). It will be shown that our understanding of both the role of gravity waves in the general circulation and the nature of the stratospheric surf zone has benefited from analysis of GCM results.From the point of view of the upper atmosphere, one of the most interesting aspects of GCMs is their ability to generate a self-consistent field of upward-propagating gravity waves. This paper concludes with a discussion of the gravity wave field in the middle atmosphere of GCMs. Comparisons of the explicitly-resolved gravity wave field in the SKYHI model with observations are quite encouraging, and it seems that the model is capable of producing a gravity wave field with many realistic features. However, the simulated horizontal spectrum of the eddy momentum fluxes associated with the waves is quite shallow, suggesting that much of the spectrum that is important for maintaining the mean circulation is not explicitly resolvable in current GCMs. A brief discussion of current efforts at parameterizing the mean flow effects of the unresolvable gravity waves is presented.     

Numerical simulation of the penetration and reflection of a whistler beam incident on the lower ionosphere at very low latitude

By the full-wave algorithm with Fourier synthesis, 3-D propagation of a whistler beam incident on the pre-dawn lower ionosphere at very low latitude is numerically investigated. Processes of transmission, reflection, and coupling with the Earth-ionosphere waveguide are discussed via the wave energy and polarisation distributions and their dependence on the wave parameters and the ionospheric profile (such as the E_s-layer). It is shown that the dominant wave above 90 km altitude has the propagation characteristics of the magneto-ionic whistler mode, and absorption, spreading, reflection and mode conversion mainly occur at, and are greatly affected by, the bottom of the ionosphere. It is found that the transmitted energy density along the Earth's surface is reduced by 20 dB or more. Beam transmission loss varies asymmetrically with the incident angle, but changes little with the frequency. In the region 150 km (for 5 kHz) away from the ‘exit area’ where whistlers emerge, the bearing measurements using ground-based VLF direction-finders may be in error because direction-finding algorithms assume plane wave propagation. Only a small portion (about −25 dB at 5 kHz) of the incident energy is reflected up to an altitude of 150 km, and major reflection takes place in a small range of altitude at the bottom of the ionosphere with little spreading and lateral shift with respect to the incident beam. Reflection is enhanced considerably at lower frequency. Our results also suggest that an E_s-layer or an ionospheric gradient refracting waves to higher latitudes would be favorable factors for multi-hop echoes to be received on the ground.     

Ray-tracing the paths of very low latitude whistler-mode signals

VLF whistler-mode signals with very low group delays (75–160 ms) received at night in Dunedin, N.Z., from the 23.4 kHz MSK transmissions of NPM, Hawaii (21.5°N, 158°W), are explained by ray-tracing along unducted paths. The typical vertical and horizontal electron density gradients of the night equatorial ionosphere are found to be sufficient to explain not only the typical group delays but also their decrease during the night and the typical frequency shifts observed on these signals. An important feature appears to be the decreasing starting and finishing latitudes (and the decreasing maximum height of the path) during the course of the night. The amplitude of the signals in relation to the expected collisional absorption in the ionosphere is discussed. A simple but quite accurate analytical expression suitable for ray-tracing is derived for the night electron density in the height range 170–1400 km, based on non-isothermal diffusive equilibrium and O⁺/O friction.     

Climatology of gravity waves in the middle atmosphere

An attempt is made at the statistical analysis of small-scale disturbances in the stratosphere and mesosphere with the aid of meteorological rocket observations at many stations from 77°N to 8°S for several years.By applying a high-pass filter to daily rocket data in the height range 20–65 km, wind and temperature fluctuations with characteristic vertical scales close to or less than 10 km are obtained, which are considered to be due to internal gravity waves. Results are expressed in terms of parameters which tend to emphasize smallscale vertical fluctuations and which should provide qualitative measures of gravity wave activity.It is found that the gravity wave activity shows a notable annual cycle in higher latitudes with the maximum in wintertime, while it shows a semiannual cycle in lower latitudes with the maxima around equinoxes. It is also found from the standard deviation around the monthly mean that the temporal variability of gravity waves is very large.     

Climatology of the middle atmosphere of the Southern hemisphere

Main features of spatial distribution and time variations of meteorological parameters in the Southern hemisphere at altitudes 25–80 km are reviewed on the basis of zonal empirical models revised in 1982. Meridional distribution and seasonal variations are analysed. For comparison purposes with the Northern hemisphere, a model developed by Cole and Kantor in 1978 is used. It is revealed that the compilation of new models of the Southern hemisphere atmosphere has not resulted in substantial revision of hemispheric-structure outlined earlier in studies conducted in the Central Aerological Observatory. Meridional distribution of the parameters in summer is characterized by higher values of temperature, pressure and density gradients in the stratosphere of the Southern hemisphere than in that of the Northern hemisphere. This resulted in greater advancement of the core of the summer-time easterly (low towards the equator in the Southern hemisphere than in its northern counterpart. In winter, meridional temperature gradients in the middle stratosphere are greater in the Southern hemisphere than those in the Northern hemisphere, however, rapid attenuation of the gradients with height is observed in the Southern hemisphere, and above 35–40 km they become negative near 50–60°S, in contrast to positive values typical for the Northern hemisphere stratosphere. In the wind field, specific features of the Southern hemisphere westerly flow are high intensity and relatively low altitude of the maximum speed (as compared to the Northern hemisphere).The phases of the annual temperature wave at low latitudes are similar south and north of the equator; south of 30°S a reversal of the phase is observed. The semi-annual oscillation of temperature and wind is less pronounced in middle and high latitudes of the Southern hemisphere than in the Northern hemisphere.The origin of the primary differences between the hemispheres is related mainly to lower intensity of large-scale stratospheric processes in the Southern hemisphere as compared to those in the Northern hemisphere, which is confirmed by values of the standard deviation of the parameters in the two hemispheres.In summer, temperature and pressure fields based on satellite data are symmetric relative to the poles in both hemispheres. In winter, the distortion of the mean pressure field in the mesosphere may be as great in the Southern as in the Northern hemisphere.     

On retrieving Rossby waves in the middle atmosphere from measurements of thermal outgoing radiation

The first step is made in elaborating special methods to retrieve the planetary-scale waves for the stratosphere and mesosphere from measurements of thermal outgoing radiation. The method is adapted for the nadir sounding of Rossby normal modes of the Lamb wave type in the 15 μm CO₂ band. The main formulae are presented in a dimensionless form. The proposed method consists of EOF filtering to extract a wave-induced signal and of Hermite polynomial expansions to describe the vertical structure of the wave. The accuracy of the retrievals is estimated; it is dependent on the duration of the record as well as on the number of channels. The method is able to provide a higher accuracy than currently available methods.     

Evaluation of electron and ion densities in the middle atmosphere from rocket-borne blunt probes

Detailed consideration has been given to the determination of electron number densities from conductivity data gathered by rocket-borne blunt probes in the middle atmosphere, and the intercomparison of these electron densities with those derived from other diagnostics. A definition of the difficulty of electron density determination from rocket-borne probes is presented. Also, the procedures for the determination of ion densities from blunt probe data in the middle atmosphere are critically evaluated. General aspects of particle collection by supersonic probes are compared with those of subsonic probes. It is noted that strong (× 10) compression regions will form in front of supersonic probes at altitudes up to 100 km, and the altered electron attachment rates could significantly affect indicated electron and negative ion concentrations. A summary of new analysis for determining electron densities from negative conductivities taken with a subsonic blunt probe is presented and the analysis is applied to data on several days where intercomparisons are possible. Blunt probe data from 31 January 1972 and 5 December 1972 (WI),² and 2 October 1975 and 29 September 1977 (WSMR)³ are reduced to predict electron density profiles. In the region of intercomparison, there is general agreement in the electron density predictions. The indications of electron density at altitudes below 70 km are new, and predict a region of moderately enhanced densities down to 45 km.