Summer circulation in the Antarctic middle atmosphere

In November 1982 a partial reflection drifts system for the measurement of winds in the mesosphere and lower thermosphere was installed as part of the New Zealand Antarctic Research Programme at Scott Base (77.8 S, 166.7 E). Ross Island, Antarctica. The wind speed and direction are measured once an hour from echoes available at the time within a height range of 67–97 km. Initial observations made during December 1982, show westward winds between 70 and 90 km, reaching a broad maximum of about 25 m s⁻¹ around 85 km. There is a strong (10 m s⁻¹) meridional component away from the pole at heights of 85–95 km.     

The influence of geomagnetic activity on the upper mesosphere/ lower thermosphere in the auroral zone. I. Vertical winds

A scanning Fabry-Perot spectrometer (FPS), located at Mawson station, Antarctica (672S, 63°E, invariant latitude 70°S) was used to obtain vertical wind, temperature, and emission intensity measurements from the λ558 nm emission of atomic oxygen. The measured temperature is used to assign an approximate emission height to the observations. A spaced-antenna partial-reflection radar was run concurrently with the FPS from which the presence of enhanced ionization in the D-region could be inferred from the return heights and strengths of the echoes. Large upwards winds of approximately 30 m s⁻¹, at altitudes less than 110 km, appear to be a direct response of the neutral atmosphere to intense auroral events. It is suggested that the observed upwelling is a result of particle heating at heights below the principal emission height. At higher altitudes, vertical winds of a similar magnitude are also measured during geomagnetically disturbed conditions, although here they do not appear to be associated with particular auroral events. In this case it is suggested that upwelling is produced by a combination of Joule and particle heating.     

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.     

Comparison of lower thermospheric winds measured by a Polarizing Michelson Interferometer and a Fabry-Perot spectrometer during the AIDA campaign

The Polarizing Atmospheric Michelson Interferometer, PAMI, a new version of the Wide Angle Michelson Interferometer, was used to measure winds in the lower thermosphere during the AIDA campaign. In the polarizing instrument, the optical path difference is changed simply by rotating a polarizing filter external to the interferometer. This allows a very simple scanning mechanism as described by Birdet al. [(1992) J. Phys.]Results of measurement of the 557.7 nm emission were obtained from the AIDA observation campaign in Puerto Rico (with PAMI located at 17°57′0″N, 66°52′42″W). Co-ordinated observations of atmospheric motions were made by PAMI along with other optical and radio measurements during April and May 1989. By comparing with the Arecibo Fabry-Perot instrument, the first wind comparisons of a FabryPerot spectrometer and a Michelson interferometer are presented. The agreement is very good in most cases, but there are times when there is a constant wind offset for several hours and a few occasions of major disagreement. It is concluded that the constant offset results from the 50 km difference in the locations of the two stations; the major disagreement may result from contamination, for the MI, of winds in the F-region during ionospheric disturbances.     

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.     

Tidal wind observations with incoherent scatter radar and meteorological rockets during MAP/WINE

Measurements of winds in the mesosphere and lower thermosphere were carried out during the main phase of the MAP/WINE project in January and February 1984 with the EISCAT UHF incoherent scatter radar near Tromsö, Norway, and with meteorological rockets launched from the Andøya Rocket Range, Norway. The radar measurements yield wind profiles between the altitudes of about 80 km and 105 km and the rockets between about 60 km and 90 km. Results from both techniques are combined to yield mean profiles which are particularly evaluated in terms of tidal variations. It is found that the semidiurnal tide constitutes an essential wind contribution between 85 km and 105 km. Whereas the tidal amplitudes are below 5 m s⁻¹ at about 80 km, they increase to 20–30 m s⁻¹ at 100 km. The average vertical wavelength of 35 km points to the S₄² mode, but coupling and superposition of different modes cannot be excluded.     

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.     

Fabry-Perot spectrometer observations of the auroral oval/polar cap boundary above Mawson,Antarctica

Zenith observations of the oxygen λ1630 nm auroral/airglow emission (produced at an altitude of ∼220 to ∼250 km) were obtained with the Mawson Fabry-Perot Spectrometer (FPS) during three ‘zenith direction only’ observing campaigns in 1993. The data show many instances of strong (50 to 100 m s⁻¹) upwellings in the vertical wind, when the auroral oval is located equatorward of the zenith. Our data appear consistent with the existence of a region of upwelling up to ∼ 4° poleward of the poleward boundary of the visible auroral oval, rather than short duration, explosive heating events. The upwellings are probably the vertical component of wind shear produced by reversal of the zonal thermospheric winds, which occurs near the poleward boundary of the visible auroral oval. Zenith temperature was also seen to increase when the oval was equatorward of Mawson, showing rises of up to 300 K or more. However, this increase is at times unrelated to the upwellings, and seems to be caused by the expansion of the warm polar cap over the observing site.On a number of nights the boundary between the polar cap and the auroral oval was observed to pass over our site several times, occasionally showing a quasi-periodic expansion and contraction. We speculate that this quasi-periodic movement may be related to periodic auroral activity that is known to generate large-scale gravity waves.     

The 2-day wave in the middle atmosphere as simulated in a general circulation model extending from the surface to 100 km

The 2-day wave observed in the mesosphere and lower thermosphere has been reproduced in a general circulation model of the atmosphere run for fixed January conditions. The wave was confined to the summer hemisphere between 50 and 100 km, and was most strongly evident in the meridional velocity where it caused a reversal in the direction of this wind approximately every 24 h. Similar but smaller fluctuations could be detected in the zonal wind and temperature. The synoptic distributions from the model confirm that the 2-day wave is a zonal wave number 3 phenomenon and that it progresses westwards. These distributions have maximum amplitudes occurring at higher latitudes than observed, probably owing to the mean wind intensity in the model summer hemisphere being slightly underestimated. Quite marked interactions occurred between the high latitude and tropical features of the synoptic meridional velocity distribution as the wave progressed. The wave had a very small phase variation with altitude, and, except for a region near 70 km, exhibited hardly any sign of baroclinic activity. The formulation of the model eliminates atmospheric tides or orography as forcing agents responsible for the excitation of the 2-day wave.     

A study of equatorial waves in the Indian zone

An equatorial wave campaign was conducted at Trivandrum (8.5°N, 77°E), Minicoy (8.3°N, 73°E) and Port Blair (11.7°N, 92.7°E) during June-July 1988. The campaign provided balloon-measured daily wind profiles at all the three stations for 48 days in the 0–30 km altitude range and rocket-measured daily wind profiles at Trivandrum for 42 days in the 31–60 km altitude range. Using these daily wind data a study was made on different equatorial wave modes present in this region. The study revealed evidence of Kelvin waves with period 12–16 days and vertical wavelength ∼ 10 km in the lower stratosphere, with period 6–9.6 days and vertical wavelength of ∼ 10–15 km in the stratospheric-lower mesospheric region and MRG waves with periods 4–4.4 days and vertical wavelength of 10 km in the upper troposphere and lower stratosphere.     

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.     

Observations of thermospheric neutral winds by the UCL Doppler imaging system at Kiruna in northern Scandinavia

Since the 1982/1983 winter, the UCL group, in collaboration with the Swedish Institute for Space Physics (previously Kiruna Geophysical Institute), has operated a Doppler imaging system at the high latitude station of Kiruna (67°N, 22°E). The Doppler imaging system is an imaging Fabry-Perot interferometer of 13.2 cm aperture. This instrument has been operated on a ‘campaign’ basis for mapping thermospheric winds using the OI emission at 630 nm (240 km altitude) from a region up to about 400 km radius about Kiruna. In November 1986, the performance of this wide-field Doppler imaging system was augmented by improvements to the detector and all-sky optics. We present data from December 1986, obtained during periods with both clear skies and active auroral and geomagnetic conditions. Maps of the neutral wind flow within the auororal oval during disturbed conditions and near magnetic midnight show continuous and rapid changes of thermospheric winds. The typical scale sizes of eddies observed within the mean flow around magnetic midnight are 100–300 km, with fluctuations at all time scales resolved by the 10 min between successive Doppler images. The local and short period fluctuations appear to be a filtered response of the thermosphere to rapid local variations of the convection and precipitation patterns, within a background of global scale changes     

Vertical structure of AGW associated ionospheric fluctuations in the E- and lower F-region observed with EISCAT—a case study

The vertical structure of AGW (atmospheric gravity wave) associated fluctuations of ionospheric plasma parameters for the 100–240 km altitude range in the daytime of 7 September 1988 has been investigated by making use of the data provided by the Tromsø measurements in the EISCAT CP1 observation mode.The wave power profile vs height has been studied by integrating the power spectral density in each altitude. The essential feature of the power variation can be explained in terms of the energy conservation of AGWs propagating in a dissipative thermosphere. Intrinsic propagation parameters of the dominant AGW have been successfully estimated with a method based on the retrieval of the Doppler effect due to the horizontal prevailing wind. From the fluctuation structure analysis in a time-altitude frame, a downcoming AGW has been clearly identified. This downcoming wave might have been reflected from a wind shear at the altitude around 200 km, which is inferred from the meridional prevailing wind profile.     

Influence of latitudinal and longitudinal variations of ozone and water vapour on the solar semidiurnal tide

A new tidal source model, based on climatological global ozone and water vapour distributions, has been obtained for January, April, July and October. The source model is used for modelling the solar semidiurnal tide in the lower thermosphere within the framework of the classical tidal theory. The observed phase quasibimodality of the semidiurnal tide at middle latitudes is possibly formed, to a great extent, by two types of hemispheric asymmetry (changing sharply near the equinox) of the ozone distribution. Near 95 km at middle latitudes, the ozone and water vapour distribution nonzonality yields typical values ~2 m/s for maximum longitudinal variations of the zonal and meridional wind amplitudes, and the range ~0.2–0.5 h for maximum longitudinal phase variations in the Northern hemisphere, while they can reach ~10 m/s and ~1.5 h in the Southern hemisphere. The hemispheric asymmetry is mainly caused by the effect of the water vapour tidal source.     

Tides in the 80–100 km height region

The Institute of Experimental Meteorology (U.S.S.R.) has carried out long-term continuous wind velocity measurements by the meteor radar method for the 80–100 km height region. From this experimental data the seasonal and latitudinal variations of atmospheric tides, as well as the spatial and temporal scales of tidal variability, have been determined.Atmospheric variations with a period of about half-a-day are investigated on the basis of a numerical model. A dependence between the mean wind structure and the semi-diurnal oscillation in the lower thermosphere is established. The influence of stratospheric warmings on semi-diurnal oscillations is also discussed. Numerical experiments show that the mean wind variations cannot explain the observed seasonal variations of a semi-diurnal tide.     

Dominant vertical scales of gravity waves in the middle atmosphere observed with the MU radar and rocketsondes

We have simultaneously observed wind motions in the altitude range of 5–90 km by means of the MU radar, rocketsondes and radiosondes. Dominant vertical scales of wind fluctuations due to gravity waves were 2–5 km in the lower stratosphere, about 5–15 km in the upper stratosphere and longer than 15 km in the mesosphere. The increase in the vertical scale with altitude is interpreted in terms of the saturation of upward propagating gravity waves. In the stratosphere, the observed vertical wavenumber spectra showed smaller amplitudes and more gradual slopes than the model values. Furthermore, the wind velocity variance in the stratosphere increases exponentially with an e-folding height of about 9 km, implying that the gravity waves were not fully saturated. On the other hand, the spectra in the upper stratosphere and mesosphere agreed fairly well with the model spectra. The variance in the mesosphere seems to cease increase of the wave amplitudes and agrees reasonably well with the model value.     

Vertical winds in the thermosphere within the polar cap

Vertical winds measured in the upper and lower thermosphere above the South Pole station show a predominantly diurnal variation with an average amplitude of 40 m/s and 10 m/s, respectively. Downward motion was typical of the dayside polar cap in the vicinity of the cusp and cleft, and upward motion of about the same magnitude occurred in the midnight sector. Observations during the June 1991 storm period showed that the amplitude of the diurnal variation was well correlated with the daily sum of K_p or ΣK_p, and also that the downward wind was the most sensitive to K_p change. Vertical winds in excess of 150 m/s were observed on the most active day. These measurements bear strong similarities to vertical wind data from Longyearbyen, Svalbard, at a similar geomagnetic latitude in the northern hemisphere. It was found that the downward vertical wind was proportional to the calculated divergence of the horizontal wind with a constant of proportionality equal to about twice the typical scale height at the altitude of measurement. Following the arguments of Burnside et al. (1981) and Rees et al. (1984b), we show that there is good evidence that the observed vertical winds are driven by divergence in the horizontal wind.     

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.