On tidal variability and the existence of planetary wave-like oscillations in the upper thermosphere—II. Non-linear interactions and global scale oscillations

In Part 1 of this paper the variability of diurnal and semidiurnal components of the meridional thermospheric wind was discussed. The observed variability is discussed in the light of the non-linear theory of wave-wave interactions. It is shown that it is possible to explain, at least partially, the variability through the non-linear interactions of tides with long period oscillations having periods between 2 and 15 days. Given that the periods obtained both from the quarterly spectra of the tidal amplitudes and the non-linear interaction analysis coincide with periods reported in the literature for planetary waves in the lower and middle atmosphere, the existence of global scale oscillations are sought in the upper atmosphere. Wave events are observed, coincident with the above-mentioned periods, for a set of five longitudinally distributed locations between 30 and 35 S throughout the yearly samples. The wave events have basically westward phase displacements though eastward travelling phases are observed at times.     

Vertical winds in the midlatitude thermosphere from Fabry-Perot interferometer measurements

Monostatic and bistatic measurements of thermospheric winds have been made with Fabry-Perot Interferometers at the Millstone Hill and Laurel Ridge Observatories. Synchronized observing sequences have been chosen to enable the determination of vertical winds from the measurements. The vertical winds are found to be significant on some nights with velocities of tens of m/s. For geomagnetically quiet nights, the averaged values for eight samples in the July–October 1992 period show little variation during the night, averaging ∼10 m/s downward. The average for three geomagnetically disturbed nights oscillates from a downward maximum of ∼50 m/s at ∼02 UT to an upward maximum of ∼20 m/s at ∼07 UT. Temporal variations in the vertical motions suggest oscillatory behavior with periods of ∼0.7, ∼1.8 and ∼3.4 h, respectively, on three quiet nights, possibly associated with gravity wave or tidal-harmonic effects.     

The effects of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes

The influence of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes is investigated. A 3-dimensional neutral wind model is developed and used together with an atmospheric model in a gravity-wave ray-tracing analysis. It is demonstrated that the thermospheric wind can act as a filter for waves travelling at unfavorable angles to the mean flow, via the mechanisms of reflection and critical coupling. This wind filtering action rotates clockwise diurnally through 360° in the northern hemisphere. Observational evidence is presented which supports these predictions. Extensive modelling indicates that (a) faster and longer period waves are least affected by the neutral winds and (b) fixed-height (e.g. HF Doppler) observations of medium scale gravity waves is only likely to be possible for waves generated locally (within 500–1000 km). Waves generated at greater distances are probably dissipated before reaching the observation region.     

Heights of MF radar scatter (1986/87) and the wind field (55–95 km): Saskatoon,Canada

The Saskatoon MF radar (2.2 MHz) at 52°N, 107°W, has been used to measure the heights of occurrence of radar scatter during four seasons, and twelve months of 1986/87. Mean winds, and gravity waves are also available, by the spaced antenna method and from the same radar echoes. Certain heights, called elsewhere ‘preferred heights’, are identified near 60km, 70km, 75km in summer, and 80–86 km. Several layers have seasonal and diurnal variations. Associations with electron density gradients (rocket data), mean wind shear in summer, and gravity wave amplitude-minima in the equinoxes are effectively demonstrated. Case studies, involving 3 h data sets of radar scatter and wind elaborate the comparison: gravity waves of long period (τ > 6 h) are shown to modulate the scattering process.     

Comparison of short period TID morphologies in Antarctica during geomagnetically quiet and active intervals

Travelling ionospheric disturbances with periods in the range 10 < τ < 30 min were observed by an HF Doppier network on the Antarctic Peninsula. A distinction was made between TIDs associated with geomagnetically quiet and active intervals, in the expectation that their morphology might depend on the degree of magnetic activity. During quiet times the short period TIDs have speeds less than 300 m s⁻¹ and may be classified as Medium Scale TIDs. An anticlockwise diurnal azimuth rotation is established, with waves tending to propagate in the (modelled) antiwindward direction. Waves associated with magnetically active intervals often have high speeds and do not generally conform to the simple azimuth variation described above. These differences are explained in terms of perturbed neutral wind patterns and the existence of different wave sources during active times. These observations are presented in the context of previous morphological wave studies. The geomagnetic dependence observed in Antarctica may explain some of the conflicting or ambiguous conclusions resulting from investigations at other locations.     

Vertical motions in the thermosphere over Mawson,Antarctica

High time resolution measurements of Doppler shift and broadening of the (OI) >1630 nm emission in the night airglow and aurora have provided determinations of vertical velocities and temperatures in the neutral thermosphere over Mawson, Antarctica. The vertical wind exhibits a large, rapid and complex response to geomagnetic energy input. Upward winds greater than 50 m s⁻¹ are frequently associated with the expansion phase of auroral substorms. Following the disturbance, prolonged periods of downward winds produce temperature enhancements of 200K outside the source region, thus providing a mechanism for the redistribution of geomagnetic energy. Oscillatory behaviour consistent with thermospheric gravity waves is observed during both quiet and disturbed conditions.     

Determination of velocity vectors of thermospheric wind from dispersion relations of TID's observed by an HF Doppler array

A method of determining horizontal velocity vectors and temperature of thermospheric winds from azimuthally different ω-κ diagrams derived from the data of traveling ionospheric disturbances (TID's) observed by an HF Doppler array has been developed. In this method, an analogous property between the Brunt-Vaisala frequency in the atmospheric oscillations and the plasma frequency in the plasma oscillations is applied to the analysis of the observed data. A declination of the resonance branches of the Brunt-Vaisala frequency in the ω-κ diagrams due to a Lorentz transformation is used for the estimation of the velocity vector of the thermospheric wind. This method makes it possible to separate the velocity vector of the thermospheric wind and the propagating directions of the TID's from the azimuthally different ω-κ diagrams. Several results of the velocity vectors during the daytime data are consistent with the results obtained by incoherent scatter radars and theoretical results. Furthermore, a typical dispersion curve which agrees well with those of the acoustic gravity waves derived from the linear perturbation theory under the isothermal atmospheric condition has been also obtained.     

Vertical propagation characteristics of internal gravity waves around the mesopause observed by the Arecibo UHF radar

A high resolution wind observation of the mesosphere and lower thermosphere (73–95 km) was conducted with the aid of the high power UHF Doppler radar at Arecibo (18.4°N, 66.8°W). Zonal wind velocities were continuously observed during day-time hours on 1–15 August 1980. We discuss here the observed wind fluctuations with periods of 1–4 h in the light of internal gravity waves. The phase propagation associated with these fluctuations is, on average, shown to be downward, indicating an upward energy flux. A space-time spectral analysis shows that waves with vertical wavelengths shorter than 10 km disappear around the mesopause (about 85km), while those with longer vertical wavelengths exist throughout the observational height. This result is explained in terms of wave absorption at a critical layer where the mean zonal wind has a westerly shear with height. This feature is consistent with the behavior expected for internal gravity waves around the summer mesopause in order to explain general circulation models.     

HF Doppler observations of medium-scale travelling ionospheric disturbances at mid-latitudes

From 1972 to 1975 F-region medium-scale travelling ionospheric disturbances (MSTIDs) were observed at Leicester, U.K. (52°32′N 1°8′W) by means of the HF Doppler technique. Most of the features of the disturbances previously reported in the literature are confirmed, with the exception of the apparent seasonal variation in the propagation direction. The measured wave azimuth rotates clockwise through 360° in 24 h, supporting theoretical predictions concerning the filtering effect of the neutral wind in the northern hemisphere. The most commonly observed direction of wave propagation, however, is displaced from the antiwind direction and is located at an azimuth of 130–140° relative to the wind. A periodic variation of the direction of wave propagation with respect to the anti-wind direction is evident, which may indicate that lower atmospheric winds can have a greater influence on waves at thermospheric heights than previously supposed.A synoptic survey of the data set reveals little correlation between wave occurrence and auroral processes, and it is unlikely that high-latitude sources are responsible for many of the MSTIDs observed at mid-latitudes.     

Variations of the gravity wave characteristics with height,season and latitude revealed by comparative observations

This paper reviews some recent observations of gravity wave characteristics in the middle atmosphere, revealed by co-ordinated observations with the MU radar in Shigaraki (35°N, 136°E) and nearby rocketsonde experiments at Uchinoura (31°N, 131°E). We further summarize the results of comparative studies on the latitudinal variations of the gravity wave activity, which were detected by additionally employing data obtained with MF radars at Adelaide (35°S, 139°E) and Saskatoon (52, 107W) and lidar observations at Haute Provence (44, 6E).The seasonal variation of gravity wave activity detected with the MU radar in the lower stratosphere showed a clear annual variation with a maximum in winter, and coincided with that for the jet-stream intensity, indicating a close relation between the excitation of gravity waves and jet-stream activity at middle latitudes. The long-period (2–21 h) gravity waves seemed to be excited near the ground, presumably due to the interaction of flow with topography, and the short-period (5 min 2 h) components had the largest kinetic energy around the peak of jet-stream.We found an increase with height in the vertical scales of dominant gravity waves, which can be explained in terms of a saturation of upward propagating gravity waves. The values of the horizontal wind velocity variance generally increased in the stratosphere and lower mesosphere, but they became fairly constant above about 65 km due to the wave saturation, resulting in the active production of turbulent layers.Although the gravity wave energy showed an annual variation in the lower atmosphere, it exhibited a semiannual variation in the mesosphere, with a large peak in summer and a minor enhancement in winter. Lidar observations reasonably interpolated the seasonal variations in the intermediate height regions.The gravity wave energy in the mesosphere, with periods less than about 2 h, was consistently larger in summer than in winter at all the stations, i.e. at 35N, 44N,52 N and 35 S. However, the values were generally larger at 35 N than at 52 N. which was found from a comparison of l-yr observations at Shigaraki and Saskatoon. Furthermore, a comparison between Shigaraki and Adelaide, located at the conjugate points relative to the equator, revealed that the gravity-wave energy in the mesosphere was found to be fairly similar, when we compared the values in summer/winter in each hemisphere.     

Large-scale TIDs and upper atmospheric gravity waves

Excitation of the guided acoustic-gravity waves in the upper thermosphere in response to enhanced auroral electrojets is calculated in the absence of dissipation under a fully ducted condition. It is shown that a model atmosphere terminated with an isothermal half-space supports a long-period, high-speed mode, which is the interface mode guided along the half-space termination of the atmosphere. The dispersion properties and the vertical distributions of the kinetic energy density of this mode are similar to those of the so called ‘gravity pseudomode’. The excitation of this mode is computed to show how the wave generation depends on the source mechanism (the Lorentz force and joule heating) and on the source altitude. Joule heating can generate the waves with appreciable amplitudes. On the other hand, the Lorentz force prevailing in the lower region cannot excite the waves with any observable amplitudes. The waves are intensified with increasing the heat source altitude. The gross features of the calculated waves indicate that the ducted thermospheric gravity waves are capable of producing observable thermospheric waves. It is therefore suggested that further examination of the excitation of the ducted acoustic-gravity waves undergoing partial reflections due to viscosity and thermal conduction should be useful for the theory of large-scale travelling ionospheric disturbances.     

The analysis of hydroxyl rotational temperatures to characterize moving thermal structures near the mesopause

Hydroxyl (OH) rotational temperatures near 85 km altitude have been monitored at Calgary, Alberta, Canada (51°N, 114°W) since 1981 with the objective of determining velocities, wavelengths and periods associated with moving temperature structures. A technique is described whereby the velocity of moving patterns in two dimensional data sets can be accurately determined and used as a parameter for a global smoothing algorithm. Velocities of the structures in the meridional direction were found to be directed poleward. Corresponding Doppler bulk wind velocities measured near the 95 km height region were directed equatorward indicating the presence of filtering of internal gravity waves by the background wind. Two coherent wave structures were often observed simultaneously during a night. The smaller of the two structures had true wavelengths less than 15–30 km and may be related to billow clouds often reported in noctilucent cloud observations. The second wave has a period on the order of an hour and meridional wavelengths ranging from 100 to 2000 km.     

On tidal variability and the existence of planetary wave-like oscillations in the upper thermosphere—I. Observations of tidal variability

The evidence for the existence of tidal variability as observed in the meridional thermospheric wind (approx. 300 km height) is presented for a set of eight ionosonde locations (three in the northern hemisphere and five in the southern hemisphere). The data set corresponds to a full year (1984) of hourly values. The detected variability can be seen in the tidal components of the meridional wind. The diurnal and semidiurnal components are spectrally analysed. The quarterly spectra show that the tidal amplitudes oscillate with periods between 2 and 60 days. The more important oscillations have periods from 15 to 3 days. No direct link between solar and magnetic activity indices was detected. Possible reasons for the observed tidal variability are discussed in the light of the current theory developed for the mesosphere and lower thermosphere.     

Simulation of gravity wave effects under solstice conditions using a 3-D circulation model of the middle atmosphere

Vertically propagating gravity waves can transport momentum and energy from the troposphere up to the mesosphere and thus modify the circulation of the middle atmosphere. The effects of regional gravity wave sources, together with temporal changes of gravity wave activity, are studied under solstice conditions in a 3-D circulation model using a simplified parameterization scheme for the gravity momentum deposition. In this way we can reproduce the reversal of the mean zonal wind with height and very low temperatures at the summer mesopause region. Using a stochastic forcing by taking the gravity wave parameters at random, characteristic oscillations are found with periods in the planetary scale range (2, 4 and 5 days) and in the tidal range (1 day, 16 h and 12 h).     

Diurnal propagating tides in the low-latitude middle atmosphere

Using an equivalent gravity wave f-plane model it is shown that longitude variations in diurnal insolation absorption by tropospheric H₂O can account for longitudinal variations of at least ± 12–15% about zonal mean values in the diurnal wind amplitude at low latitudes (0–20°) between 80 and 100 km, by virtue of the non-migrating propagating tidal modes which are excited. Phase variations of about ± 0.75 h also occur. These percentage variations are conservative estimates, since the background migrating (1,1,1) mode appears to be slightly (20–25%) overestimated in amplitude. In addition, the assumed eddy dissipation values, which appear necessary to model the breaking (1,1,1) mode, are larger than generally considered ‘reasonable’ by photochemical modellers. For a photochemically more reasonable eddy diffusion profile, estimates of longitude differences in diurnal wind amplitude are quite similar to the above values below 87 km, but increase to ± 17–25% near 100 km, with accompanying phase variations of ± 1–2 h about zonal mean values. In addition, it is shown that radiative damping by CO₂ parameterized by a scale-dependent Newtonian cooling coefficient accounts for no more than a 20% reduction in the amplitudes of diurnal propagating tides above 80 km.     

Dynamic regime of the mesopause—lower thermosphere at mid-latitudes of the northern hemisphere by radio meteor observations

The coherent pulse Meteor Automatic Radar System (MARS) based at Kharkov (49°30′N, 36°51′E) was used to measure zonal winds in the altitude range 80–105 km in the period from November 1986 to December 1990. It was found that, for the greater part of the year, the zonal prevailing wind component was in the eastward direction. The change from eastward to westward direction begins in the lower thermosphere in February–March, propagating downwards to the mesosphere, and it remains there until June–July. The structure of semidiurnal tides has general regularities at different sites. Annual variations in the monthly mean values of semidiurnal vertical wavelengths are practically the same, both in the northern and southern hemispheres. Wavelengths are more than 100 km in summer months, whereas they are less than 60 km in winter months.Studies of internal gravity wave (IGW) parameters in the height range of 80–105 km have shown that the internal gravity wave amplitude does not exceed 30 m/s, the vertical wavelength is in the range of 10–30 km, the horizontal wavelengths are 100–800 km and the horizontal phase velocities are in the range 20–160 m/s. The propagation and breaking of upward and downward IGW at heights of 80–100 km have been recorded.     

Permanent monitoring of the upper mesosphere and lower thermosphere wind fields (prevailing and semidiurnal tidal components) obtained from LF D1 measurements in 1991 at the Collm Geophysical Observatory

The wind field of the upper mesosphere and lower thermosphere region (85–105 km) over Central Europe (52°N, 15°E) has been continually and reliably recorded by regular daily D1 radio wind measurements in the LF range (177, 225 and 270 kHz) using commercial radio transmitters. These measurements show the prevailing winds, the tidal wind components and the effects of internal gravity waves, as well as the seasonal and irregular variations of these parameters. The height of the wind measurements is determined by measuring the travel time differences between corresponding modulation bursts in the sky wave and in the ground wave. Using a quasi-online calculation procedure, the results are available immediately. Therefore they are useful for monitoring the upper atmospheric circulation with regard to upper atmosphere meteorology in the future. Vertical profiles of the wind field parameters can be derived with the aid of the combined wind and height measurements. Height-time cross-sections of the monthly mean prevailing winds and semidiurnal wind components have been calculated almost continuously for the last 10 years. The present paper deals with recent results for the year 1991.     

On the structure and circulation of the polar thermosphere under magnetically quiet conditions

A theoretical study is presented bearing on the thermospheric circulation and composition at polar latitudes. The observed motions and density perturbations in N₂, O and He have signatures which may be understood in terms of two different source mechanisms. We consider electric field momentum coupling and Joule heating as well as interactions between both processes. A spectral model in terms of vector spherical harmonics (with magnetic coordinates) is used, delineating the diurnal and mean (time independent) components. The important non-linearities are evaluated in configuration space. The electric field model of Volland and the global average density and temperature variations of Hedin (MSIS) are adopted as input. Our analysis leads to the following conclusions. (1) The vortex type double cell polar circulation (zonal wave number m = 1) is primarily driven by collisional momentum transfer from electric field induced ion convection. (2) Because of the thermospheric low pass filter, a large time independent component (zonal wave number m = 0) is produced by Joule heating; the heavier species (N₂) being concentrated where the lighter ones (O, He) are depleted, and vice versa due to wind induced diffusion (3) The electric field driven vortex circulation redistributes the mass and energy in the time independent density and temperature variations (from Joule heating), producing primarily diurnal variations (m = 1) in the temperature and composition near the pole and at auroral latitudes, again the heavier and lighter species varying out of phase. The above results are in substantial agreement with observations. It is worth noting that momentum rectification associated with the diurnally varying electric field and conductivity induces a weak zonally symmetric (single cell) prograde polar vortex. However, this motion is partially compensated by a retrograde vortex from geostrophic balance due to Joule heating, which dominates near the pole. These motions are small compared with the diurnally varying component in the polar circulation.     

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.     

Some further results on long term mesospheric and lower thermospheric wind observations by the Arecibo UHF radar

A second series of long term mesospheric and lower thermospheric wind observations was conducted at Arecibo (18.4°N, 66.8°W) between 6 and 20 March 1981 using the UHF Doppler radar, following the first observations in August 1980 (Hirota et al., 1983). Zonal and meridional wind velocities were measured during the morning (8–10 LT) and afternoon (13–15 LT) periods. The mean wind profile averaged over the entire observational period shows the predominance of the diurnal tide. The fluctuating wind vector rotates clockwise relative to height with a characteristic vertical scale of about 10 km. The phase difference inferred by a cross correlation analysis between morning and afternoon profiles indicates that the dominant period is about 20–30 h. This oscillation is discussed in relation to internal inertia-gravity waves observed by the same radar in the lower stratosphere. On the other hand, wind fluctuation with a vertical scale larger than 20 km shows a substantial day-to-day variation with a period of 5–8 days. This long period oscillation shows a good correlation with the global scale geopotential height anomalies at 1 mb (46–48 km) observed by the Tiros-N satellite at 20°N. Our evidence suggests that westward travelling planetary-scale waves with zonal wavenumber one may propagate up to the lower thermosphere.