Semi-empirical models of the neutral atmosphere based on turbopause height and exospheric temperature variations

Based on mass spectrometer observations of the satellites Ogo 6 and Esro 4 the long periodic density variations of He, O, N₂ and Ar are interpreted to be a result of combined variations of the exospheric temperature and the turbopause height. Only these two parameters are used in the models. Thus in comparison with previous models the number of parameters needed is significantly reduced. This simplification does not diminish the accuracy of the analytic representation of the observations and provides new physical insight into the processes which govern the dynamic behaviour of the neutral thermosphere. The annual variation of the turbopause height is found to cover the height range between 90 and 115 km and is approximately equal for both the Ogo 6 and the Esro 4 data.     

Sporadic neutral metal layers in the mesosphere and lower thermosphere

Although the existence of thin ionized layers at heights around 100 km has been known for many years, it is only much more recently that thin neutral metal layers have been observed. Such layers, initially sodium and more recently calcium and iron, have been detected by lidar. The layers, with thicknesses between about 100m and several kilometres, and concentrations between about 10² and 10⁵ cm⁻³, occur most frequently between 90 and 100 km, and are normally superimposed on a background layer about 10 km thick. The occurrence of thin neutral layers appears to be latitude dependent, and is strongly linked to the appearance of Es on ionograms. Several causative mechanisms have been suggested, none of which appears to be capable of providing an altogether satisfactory explanation for the formation of the layers.     

Seasonal and tidal variations of neutral temperatures and densities in the high latitude lower thermosphere measured by EISCAT

Measurements of ion temperature, ion-neutral collision frequency and ion drift in the E-region from the period December 1984 to November 1985 are used to derive neutral temperatures, densities and meridional winds in the altitude intervals 92–120 km, 92–105 km and 92–120 km, respectively. Altitude profiles of temperature and density and their seasonal variations are compared with the CIRA 1972 and MSIS 1983 models and the effects of geomagnetic activity are demonstrated. Semi-diurnal tidal variations in all three parameters are derived and the comparison with lower latitude measurements is discussed.     

Non-homogeneous structure of neutral winds in the lower thermosphere during the MAC/EPSILON campaign

The paper presents the results of an investigation of the height variations of dynamic processes in the 80–110 km height region, carried out in Kazan, U.S.S.R. (56°N, 49°E) by the radiometeor method during the MAC/EPSILON campaign. Experimental results show that the largest values of vertical wind gradients, as well as zonal and meridional temperature gradients can be found at heights of ~ 83 km. At heights of 80 ⩽ h ⩽ 100 km, we can observe energy absorption of IGW and tides which are the major sources of turbulent energy in the above-mentioned height interval. Using the effects of IGW energy absorption, values of the turbulent eddy diffusion coefficient K_l ranging from 1600 to 4400 m²/s were calculated for October 1987. The energy dissipation rate ϵ was estimated to be from 0.1 to 0.4 W/kg.     

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.     

Eddy diffusion models for the mesosphere and lower thermosphere

Two-dimensional eddy diffusion around mesopause heights is modelled by means of a set of parameters relating eddy transports to average fields of meteorological quantities like wind variance and potential temperature. The so-called K-theory approach is used. The parameters chosen in this study allow easy modification of the K-models. This is thought to enable better adjustment and further development of the transport parameterization in accordance with future exploration of the transport phenomena and their physical background in the middle atmosphere. The models in their present state have been used to study a few problems associated with the climatological behaviour of the mesosphere and lower thermosphere: the relative importance of convective and eddy transports, implications of the mean winter anomaly of ionospheric absorption, the problem of strong mean vertical winds near the mesopause and the formation of the NO density minimum around 85 km altitude.     

EISCAT observations of tidal modes in the lower thermosphere

EISCAT has made regular measurements of plasma velocity at heights between 101 and 133 km in the E-region and at 279 km in the F-region as part of the Common Programme CP1. Correcting for the effect of the electric field as determined in the E-region, it is possible to estimate the neutral wind velocity in the E-region for a number of days in the period 1985–1987 when magnetic conditions were relatively quiet. These velocities display diurnal and semi-diurnal tidal oscillations. The diurnal tide varies considerably from day to day in both amplitude and phase. The semi-diurnal tide also varies in amplitude but displays a fairly consistent phase at each height and the variation of phase with height below 110 km indicates a dominant (2,4) mode. Above 120 km the variation of phase with height is slower which suggests that at these heights the (2, 4) mode is attenuated and the (2, 2) mode is more important. The results agree well with previous measurements at high latitude.     

High-latitude tidal behavior in the mesosphere and lower thermosphere

The high-latitude structure of the mean winds and tides is described in this paper using climatologies prepared from radar data during the Atmospheric Tides Middle Atmosphere Program. The monthly evolution of the amplitude and phase of the tides is discussed. Comparison between the southern and northern hemispheres indicate that the diurnal tide is stronger in the southern hemisphere and that the antisymmetric diurnal tidal modes are dominant. The semidiurnal tide is larger than the diurnal tide. The vertical wavelength structure is significantly different between the southern and northern hemisphere. Comparisons with recent tidal models show several discrepancies.     

Infrared radiative cooling in the mesosphere and lower thermosphere

This paper discusses the current status of calculating infrared cooling by CO₂ in the mesosphere and lower thermosphere. It is desirable to have fast but accurate procedures for use in dynamic models. The most difficult region is from 70 to 90 km, where cooling rates are strongly influenced or, in the case of the summer mesopause region, dominated by the absorption of radiation emitted by underlying layers, with the hot bands and isotopic bands playing a significant role. A three-energy-level model is derived for the excited population levels of a CO₂ molecule. Vibrational-vibrational coupling between isotopes is also included as significant. Results from model calculations for cooling rates and NLTE source functions are presented. Global average infrared cooling rates appear to be in reasonable balance with solar heating rates, considering the uncertainties in calculating both these terms. Radiative cooling rates by CO₂ above 100 km are strongly dependent on atomic oxygen concentrations and on the rate of energy exchange between atomic oxygen and CO₂. Likewise, NO cooling, which is important above 120 km, is proportional to atomic oxygen concentrations. Since CO₂, NO and O concentrations can all vary with motions, these dependencies suggest interesting feedbacks to atmospheric dynamics.     

Variability of winds and temperatures in the lower thermosphere

High-resolution daytime incoherent scatter radar measurements of plasma temperatures and drifts in the ionospheric E-region above Millstone Hill (42.6°N, 71.5°W) have been used to derive horizontal neutral winds and temperatures in the lower thermosphere (105–130 km) during five multi-day campaigns in 1987–1991. The underlying semi-diurnal tidal component has been determined from the observations, with characteristic average amplitudes of 50 ± 15 m/s and 30 ± 10 K. Phase propagation with altitude follows the expected structure of semi-diurnal tidal modes, but reveals complex coupling of tidal modes, particularly above 115 km. Day-to-day variability in the winds and temperatures is large, and the deviations from the semi-diurnal harmonic can exceed 40 m/s and 50 K. No strong correlations have so far been found with geophysical parameters to explain the observed variability.     

Mean geopotential fields for the lower thermosphere of the southern hemisphere

Satellite radiance data are used to compile monthly mean contour maps for the 0.005 mb and 0.001 mb constant pressure surfaces. Aspects of the seasonal and spatial distribution of the geopotential height fields are discussed. Zonal geostrophic wind speeds are calculated from the zonal mean heights.     

The impact of gravity waves on nitric oxide in the lower thermosphere

Observations of nitric oxide (NO) by the Solar Mesosphere Explorer (SME) during equinox indicate a lower-thermosphere equatorial minimum which is at variance with theoretical predictions. To address this discrepancy a zonally averaged model of the thermosphere and upper mesosphere is used to evaluate the influence of a latitude variation in turbulence. Five numerical simulations were performed with different latitude structures of eddy diffusion (K_T), ranging from uniform in latitude, peaks at low, mid-, or high-latitude, to a hemispherically asymmetric distribution. A local increase in eddy diffusion causes the lower thermosphere to cool and induces a latitude pressure gradient that drives horizontal and vertical winds. The circulation, turbulent transport and temperature dependent chemistry act to change the distribution of species. Comparison of the model predictions of NO with SME data, and simulated wind and temperature structure with empirical climatology, indicates a preference for a midlatitude peak in K_T.     

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.     

Possible radioindications of anthropogenic influences on the mesosphere and lower thermosphere

From an analysis of the variations of various ionospheric characteristics influenced by global anthropogenic effects, it is shown that the collision frequency v_en is the parameter that changes most when the ratio between the carbon dioxide plus methane and the other components is changed. Since this collision frequency is directly involved in the formation of ionospheric absorption, the latter is recommended to be regarded as the most sensitive ground-based indicator of the global ‘cooling’ of the near-Earth space. Expressions are obtained for the estimation of man-made influences by absorption measurements. For medium latitudes, we recommend the frequency range from 400 to 800 kHz as a sensitive range for these measurements.     

Interactions between diurnal tides and gravity waves in the lower thermosphere

Recent progress on interactions between breaking gravity waves and the diurnal tide in the upper mesosphere and lower thermosphere is reviewed, mainly based on the recent results of our numerical models.     

Diurnal tide in the Antarctic and Arctic mesosphere/lower thermosphere regions

The behaviour of the diurnal tide at 95 km over various years between 1965 and 1986 is studied using radar data from Heiss Island (81°N), Mawson (67°S), Molodezhnaya (68°S) and Scott Base (78°S). The observations are also compared with the model results of FORBES and HAGAN [(1988) Planet. Space Sci. 36, 579] for the same latitudes. There are substantial fluctuations in amplitude and phase at all stations, particularly in winter. Phase fluctuations can be as large as a uniform random distribution over the 24-h cycle. In summmer the phases of the meridional components are well defined and suggest the presence of a dominant symmetric mode. The meridional amplitudes are larger in summer whereas the zonal components have a greater variation and show no significant variation with season.     

Simultaneous measurements of large vertical winds in the upper and lower thermosphere

High resolution vertical wind measurements of the upper and lower thermosphere were made at Poker Flat, Alaska, using a scanning Fabry-Perot spectrometer (FPS). Observations of the λ558 and λ630 nm emissions of atomic oxygen were made on 21 nights and allowed for the simultaneous determination of wind and temperature at altitudes of about 130 and 240 km, respectively. On two occasions, significant upwelling events were measured which lasted between 15 and 25 min. Peak velocities were up to 42 m/s at 130 km and 138 m/s at 240 km. Auroral activity was monitored using a meridian scanning photometer (MSP). On both occasions, the region of upwelling was located on the poleward side of the auroral oval during geomagnetically active conditions. A schematic model is used to describe an event from which the horizontal scale of the upwelling region is estimated to be less than 320 km in the lower thermosphere and less than 800 km in the upper thermosphere.