Multiple-frequency studies of the high-latitude summer mesosphere : implications for scattering processes

The characteristics of polar mesosphere summer echoes (PMSE) are studied at 53.5 and 224 MHz. Observations at 2.78 MHz, simultaneous with the ones at the other two frequencies, were carefully compared for indications of PMSE, but no obvious relation was found. Relationships between relative scattering cross-section, spectral width and vertical velocity are studied for the 224 MHz radar, and observations at 53.5 MHz are compared with those at 224 MHz. Results of aspect sensitivity measurements at 53.5 MHz are presented. The implications of these characteristics for several possible scattering mechanisms are discussed. We rule out incoherent scatter and chemically induced fluctuations from the evidence that we have. In view of the extremely low temperatures near the high-latitude mesopause in summer, we discuss several scenarios involving heavy cluster ions and charged aerosol particles.     

Observational evidence of a saturated gravity wave spectrum in the mesosphere

Five vertical profiles of scalar horizontal winds have been measured at high resolution (25m) in the range from 80–95 km during the last salvo of the MAC/SINE campaign in the summer 1987 at Andenes, Northern Norway (69.3°N). Our purpose in this study is to examine the consistency of the motion spectrum with the saturated spectrum of gravity waves proposed by Smith S. A., Fritts D. C. and Van Zandt T.E., (1987, J. atmos. Sci. 44, 1404). An analysis of vertical wavenumber spectra of the five horizontal wind profiles is presented and it is found that (a) the average slope of the five vertical wavenumber spectra is −3.0 ± 0.2 for wavelengths in the range from 6.4 km to 100 m. The slope is considerably steeper than the vertical wavenumber spectra of the horizontal velocity discussed in the literature, (b) the average vertical wavenumber spectrum shows that there is excellent agreement between the observed spectrum and the saturated spectrum in both slope and amplitude, suggesting that saturation processes do indeed act to control spectral amplitudes at large wavenumbers, and (c) a dominant vertical wavelength of 6.4 km is found in the mesosphere. Taken together, our observations provide further support for the saturated spectrum theory.     

Intense turbulence in the polar mesosphere : rocket and radar measurements

Simultaneous observations of polar mesospheric summer echoes (PMSE) have been made with two different frequency radars during the launch of a sounding rocket designed to measure the fluctuations in the electron density in the same height range. The cross-section for radar backscatter deduced from the rocket probe data under the assumption of isotropic turbulence is in reasonable agreement with the measured signals at both 53.5 MHz with the mobile SOUSY radar and 224 MHz with the EISCAT VHF radar, which correspond to backscatter wavelengths of about 3 and 0.75 m, respectively. Some controversy exists over the relative roles of turbulent scatter vs specular reflections in PMSE. A number of characteristics of the data obtained in this experiment are consistent with nearly isotropic, intense meter-scale turbulence on this particular day. Since equally compelling arguments for the importance of an anisotropic-type mechanism have been presented by other experimenters studying PMSE, we conclude that both isotropic and anisotropic mechanisms must operate. We have found the inner scale for the electron fluctuation spectrum, which corresponds to the diffusive subrange for that fluid, and have compared it to the inner scale for the neutral gas. The latter was found from the Kolmogorov microscale, which in turn depends on the energy dissipation rate in the gas. We found the dissipation rate from the spectral width of the 53.5 MHz backscatter signal and from the rocket electron density fluctuation data. The diffusive subrange was found to occur at a wavelength a factor of about 10 times smaller than the viscous subrange. This corresponds to a Schmidt number of about 100. High Schmidt numbers have been reported in recent measurements of the diffusion coefficient of the electrons in this height range made with the EISCAT incoherent scatter radar. About 15 min after the rocket flight an extremely high radar reflectivity was found with the SOUSY system. We have been able to reproduce this high level theoretically by scaling the rocket data with an increase in the neutral turbulence energy dissipation rate by a factor of 14 as deduced from the SOUSY spectral width, an increase in the electron density which is consistent with riometer data, and a 33% decrease in the electron density gradient scale length which is hypothesized. We also estimate the radar reflectivity at 933 MHz and conclude that signals in excess of thermal scatter levels would have occurred at the peak of the event studied, provided that the electron fluctuation spectrum decreases as k⁻⁷ in the viscous subrange. If the spectrum has an exponential form, however, a turbulent source cannot explain the enhanced 933 MHz echoes reported by EISCAT.     

Variability of die semi-diurnal tide in the upper mesosphere

The observed variability of the semi-diurnal tide can be described as a composition of three effects:

• 1.(i) a variation of the main tide, over scales larger than 5 days, which can be explained as a variation of the relative amplitude or phase of the different tidal modes;
• 2.(ii) a short time-scale variation, which can be attributed to a combination of local effects;
• 3.(iii) a longitudinal variation, related to non-solar migrating modes.

Though we can account for the main tidal structure with classical thermal excitation and propagation, coupling mechanisms related to background atmospheric structure as computed by Lindzen and Hong (1974) are necessary to explain the large scale variations. In this paper, we develop simplified coupling mechanism to extend such coupling to non-solar migrating modes, through longitudinal variations of the background atmosphere.     

Inter-annual variability of tides in the mesosphere and lower thermosphere

The inter-annual variation in diurnal and semi-diurnal atmospheric tides between 85 and 95 km has been studied for various years between 1978 and 1988. Observations comprised wind measurements from the medium frequency SA mode wind radars at Adelaide (35°S), Christchurch (44°S) and Saskatoon (52°N) and the meteor wind radar at Durham (43°N). Although the observations include the interval between solar maximum and solar minimum, there is in general no correlation between tidal amplitudes and solar activity. In contrast with earlier studies there does appear to be a positive correlation between solar activity and the amplitude of the semi-diurnal tide, but only during the southern summer and simultaneous northern winter.     

VHF radar observations of nonlinear interactions in the summer polar mesosphere

VHF radar measurements of velocities and echo power in the summer polar mesosphere have been analysed using maximum entropy, bispectral and cross-spectral methods in order to study wave-wave interactions. The results show nonlinear interactions of second and even third order between the diurnal and semidiurnal tides and planetary waves with periods of 2 and 3 days in the velocity field. Similar analyses of time series of echo power suggest corresponding variations of the temperature field.     

Observations of winds in the Antarctic summer mesosphere using the spaced antenna technique

Observations of winds in the 60–100 km height range were made at Mawson (68°S, 63°E) during December 1981 and January 1982 with the MF spaced antenna technique. The prevailing winds are in accord with other recent observations made at high latitudes and show a peak in the zonal wind near 80 km with westward winds of 30 m s ⁻¹. The meridional winds maximize near 90 km with an equatorward flow of 10 m s⁻¹. The diurnal tidal components are in reasonable agreement with recent model predictions, especially in phase. The amplitudes tend to be larger than the model values. The semidiurnal tide is not as stable as the diurnal tide and shows evidence for interference effects between different modes.     

Monthly simulations of the solar semidiurnal tide in the mesosphere and lower thermosphere

Monthly simulations of the solar semidiurnal tide in the 80–100 km height regime are presented. These calculations benefit from the recent heating rates provided by Groves G. V. (1982a,b) (J. atmos. terr. Phys. 44, 111; 44, 281), the zonally-averaged wind, temperature and pressure fields developed for the new COSPAR international reference atmosphere [Labitzke K., Barnett J. J. and Edwards B. (1985) Handbook for MAP 16, 318], and eddy diffusivities determined from gravity wave saturation climatologies and used by Garcia R. R. and Solomon S. (1985) (J. geophys. Res. 90, 3850) to simulate oxygen photochemistry and transport in the mesosphere and lower thermosphere. Some of the main characteristics of the observed semidiurnal tide at middle and high latitudes are reproduced in our simulations: larger amplitudes in winter months than in summer months, and the bi-modal behavior of the phase with summer-like and winter-like months separated by a quick transition around the two equinoxes. The phase transition is also more rapid in the spring, consistent with observations. The wavelengths are also longer in summer than in winter, at least below 95 km (whereas in July and August the simulations exhibit some discrepancies above this altitude), similar to the observational data. Semidiurnal amplitudes are generally smaller and the phases more seasonally symmetric at middle and low latitudes, as compared with the tidal structures above about 50° latitude. In addition, hemispheric differences in the mean zonal wind result in marked asymmetries in tidal behavior between the Arctic and Antarctic regions, and suggest that a comparative study of tide, gravity wave and mean flow interactions in the Arctic and Antarctic mesosphere and lower thermosphere would be fruitful.     

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.     

Simulation of the global structure of stationary planetary waves in the mesosphere and lower thermosphere

When simulating the global structure of stationary planetary waves (SPW) the problem of obtaining the numerical solution in the equatorial region appears. It results from the presence of apparent singularities in the operator of the SPW latitudinal structure when the Coriolis parameter is small. The new method based on SPW latitudinal operator inversion is proposed. This method permits the difficulties arising from the simulation of stationary large scale disturbances at low latitudes to be avoided. The global structure of SPW with zonal wave number m = 1 at the mesosphere and lower thermosphere heights has been calculated for the background zonal wind distribution representing a climatic picture of the solstice conditions. In the region of the mean zonal westerlies the SPW penetration across the equator is obtained. The SPW at low latitudes are shown to appear most significantly in the zonal component of the wind velocity. The influence of planetary wave motions on the distribution of longlived species in the ionospheric D-region and at the heights of lower thermosphere are discussed.     

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.     

Polar thermospheric Joule heating,and redistribution of recombination energy in the upper mesosphere

Kellogg, W. W. (1961, J. Met. 18, 373) suggested that transport of atomic oxygen from the summer into the winter hemisphere and subsequent release of energy by three body recombination, O + O + N₂→O₂ + N₂ + E, may contribute significantly to the so-called mesopause temperature anomaly (increase in temperature from summer to winter). Earlier model calculations have shown that Kellogg's mechanism produces about a 10% increase in the temperature from summer to winter at 90 km. This process, however, is partly compensated by differential heating from absorption of UV radiation associated with dissociation of O₂. In the auroral region of the thermosphere, there is a steady (component of) energy dissipation by Joule heating (with a peak near 130 km) causing a redistribution and depletion of atomic oxygen due to wind-induced diffusion. With the removal of O. latent chemical energy normally released by three body recombination is also removed, and the result is that the temperature decreases by almost 2% near 90 km. Through dynamic feedback, this process reduces the depletion of atomic oxygen by about 25% and the temperature perturbation in the exosphere from 10% to 7% at polar latitudes. Under the influence of the internal dynamo interaction, the prevailing zonal circulation in the upper thermosphere (small in magnitude) changes direction when the redistribution of recombination energy is considered. The above described effects are very sensitive to the adopted rates of eddy diffusion. They are also strongly time dependent and are significantly reduced for disturbances associated with magnetic storms.     

Comparison of geostrophic and observed winds in the upper mesosphere over Saskatoon,Canada

Thicknesses derived from SAMS (Stratospheric and Mesospheric Sounder, on-board Nimbus 7) and 30 mbar height analyses based on radiosondes have been combined to give monthly mean height charts of the upper mesosphesre, (0.01 mbar level, approximately 78–80 km). Four years of data are available, 1979–1982.Based on this material, the geostrophic winds were derived for the grid point 50°N, 110°W and compared with winds observed over Canada (52°N, 107°W) using the Saskatoon medium frequency (MF) radar. Agreement, especially in the summer months, was very good, however, significant ageostrophy was evident in autumnal and winter months. Possible causes are confluence associated with large scale circulations and gravity wave drag.     

Observational evidence of local turbulence production due to gravity wave saturation in the summer mesosphere

Five foil chaff and two falling sphere rockets flown during the MAC/SINE Campaign on 15 July 1987 at Andenes, Northern Norway (69°17′N). From these rocket measurements, turbulent energy dissipation rates, vertical wind shears and Richardson numbers as functions of height were derived in the range from 82 to 92km. Turbulent energy dissipation rates generally range from 1.4 × 10⁻⁵ to 2.0 × 10⁻²W/kg and are consistent with other experiments performed at the same latitude. Strong wind shears of the order of 50–90 m/s/km are observed at various heights. Good correspondence between turbulence intensity peaks, regions of strong wind shear and low Richardson number is found. Vertical wavenumber spectra of the five scalar winds measured by the foil chaff rockets indicate that there is an excellent agreement with the saturation hypothesis, suggesting that the turbulence intensity peaks measured in this salvo are linked directly to the saturation of gravity wave motions via dynamical instabilities.     

Concentrations of H2O and NO in the mesosphere and the lower thermosphere at high latitudes

Water vapour and nitric oxide concentrations in the mesosphere and lower thermosphere were derived from infrared emission and positive ion composition measurements above northern Europe during the Energy Budget Campaign 1980. The experiments were performed at different levels of geomagnetic disturbance. Both water vapour and nitric oxide are highly variable. Water vapour mixing ratios between 0.2 ppm and 10 ppm were observed. The nitric oxide peak densities varied by more than a factor of ten. Maximum values of 2 × 10⁹cm⁻³ were obtained.     

Securing the heights: The vertical dimension of the Siachen conflict between India and Pakistan in the Eastern Karakoram

The Siachen conflict between India and Pakistan is often referred to as the coldest war, or, the endless war atop the roof of the world. The high altitude and extreme climate create a hostile environment that has caused by far the most casualties and imposed tremendous costs on both sides. This environmental setting is usually only cited to underline the absurdity of this more than 30 year old conflict. We, however, argue that rather than being a constraint upon the conflict, the terrain itself is central to the genesis and continuation of the conflict. Further, the vertical dimension is the focus of contestation and the site where mountaineering practices, cartographic imagination and military logic intersect. The inaccessibility imposed by the terrain also implies that far from being a frozen conflict there is a temporal dynamism, as improvements in technology and logistics alter the possibility of maintaining the status quo.     

Gravity wave activity in the upper mesosphere over Urbana,Illinois: lidar observations and analysis of gravity wave propagation models

We analyze 375 h of Na Wind/Temperature lidar measurements of the mesopause region (≈ 80–105 km) Na density and temperature profiles on 57 nights distributed over 2 yr at Urbana, Illinois. These observations yield a high-resolution seasonal data set of gravity wave activity in the upper mesosphere. From this data, we present measurements of the Brunt-Väisälä period, the relative atmospheric density perturbations and their spectra, and the parameters of 143 quasi-monochromatic gravity waves. The direct measurement of the Brunt-Väisälä period allows accurate calculation of the horizontal velocity perturbations and vertical displacement perturbations from the density measurements. The horizontal velocity and vertical displacement vertical wave number spectrum magnitudes and indices show considerable seasonal and nightly variability. The gravity wave amplitudes, wavelengths, and observed periods exhibit systematic relationships similar to those found in previous studies, and are consistent with the MU radar measurements of intrinsic gravity wave parameters. Here, we present a detailed analysis of the observations in terms of Diffusive-Filtering Theory models of gravity wave propagation. The magnitudes of the vertical wave number spectrum, the form of the joint vertical wave number and frequency spectrum, and the systematic relationships between the monochromatic gravity wave parameters are consistent with the Diffusive-Filtering model. We compare these results with a variety of radar, lidar, and airglow observations from other sites. This observational study suggests that the complex nonlinear interactions of the gravity wave field may be modeled successfully as a diffusive damping process, where the effective diffusivity is a function of the total wave variance.     

Estimates of gravity wave momentum fluxes in the winter and summer high mesosphere over northern Scandinavia

As part of the MAP/WINE campaign (winter 1983–1984) and the MAC/SINE campaign (summer 1987) high resolution wind profiles were obtained in the upper mesosphere using the foil cloud technique. Vertical winds were derived from the fall rate of the foil clouds and are used for estimating the momentum fluxes associated with vertical wavelengths shorter than about 10 km. From the ensemble average of 15 observations over an altitude range of 74–89 km we calculate a zonal net momentum flux of +12.6 ± 4.5 m²s⁻² in summer. The average of 14 measurements in winter between 73 and 85 km indicates a zonal net momentum flux of −3.7 ± 2.4 m²2 s⁻².     

Applicability of the two-layer model for simulation of non-linear evolution of a large scale plasma cloud in the ionospheric F-region

The adequacy of the two-layer model of Lloyd and Haerendel for describing the behaviour of an ionospheric irregularity is verified by numerical simulation of large plasma cloud dynamics. The background ionosphere is approximated by a set of conductive layers with ion mobilities and concentrations corresponding to the real ionospheric conditions. Polarization electric field produces positive and negative image clouds, i.e. plasma density enhancements and depletions in each layer. Their intensity, form and orientation turn out to change with height depending on the local conditions. However, the drift and deformation of the released cloud slightly differ from the case when the ionosphere is characterized by constant, height averaged parameters, at least if altitude dependent neutral wind and photochemical processes are ignored.     

The influence of geomagnetic activity on the upper mesosphere lower thermosphere in the auroral zone. II. Horizontal winds

A spaced antenna partial reflection radar located at Mawson, Antarctica (67°S, 63°E, invariant latitude 70°S), has been used to measure the horizontal wind field in the height range 70–110 km. Three years of data (1985–1987) from the radar have been analysed in order to investigate correlations between geomagnetic activity (determined from the local K-index) and the horizontal wind. Results are analysed using a randomization technique and show that larger winds are measured during geomagnetically active periods in both the raw (or unfiltered) wind values and in the medium-frequency (2–6 h period) and high-frequency (1–3 h period) components. The raw winds tend to be shifted towards the geographic NW to NE quadrant in the early morning hours during high K-times. The observed correlation is seen down to 86 km and shows a seasonal dependence. The mean r.m.s. velocity of the radar scatterers and the angular spread of the return echoes are also found to be correlated with geomagnetic activity. The medium- and high-frequency components of the wind are polarized in the magnetic zonal direction during all seasons of the year.