Gravity-wave motions in the mesosphere and lower thermosphere observed at Mawson,Antarctica

We present the results of MF radar observations of mean winds and waves in the height range 78–108 km at Mawson (67°S, 63°E), Antarctica. The measurements were made in the period from 1984 to 1990. Climatologies of the prevailing zonal and meridional circulations made with a 12-day time resolution show that the mean circulation remained relatively stable over the 6 yr of observation. Climatologies of gravity-wave motions in the 1–24 h period range were also generated. These reveal that the r.m.s. amplitudes of horizontal wave motions near the mesopause (~90 km) are about 30 m s⁻¹, and that there is some anisotropy in the motions, especially at heights below 90 km. Meridional amplitudes are larger than zonal amplitudes, which suggests a preference for wave propagation in the north-south direction. Comparisons with MST radar wind observations made near the summer solstice at Poker Flat, Alaska (65°N) and at Andøya, Norway (69°N) show similarities with the Mawson observations, but the wave amplitudes and mean motions are larger in magnitude at the northern sites. This suggests hemispheric differences in wave activity that require further study.     

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.     

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.     

Simultaneous observations of the quasi 2-day wave at Mawson,Antarctica, and Adelaide,South Australia

Observations of the Austral quasi 2-day wave at Mawson, Antarctica (67°S, 63°E) are presented and compared with those from Adelaide (35°S, 138°E). The data were obtained from partial-reflection radars which have been measuring winds continuously since mid-1984, and the results presented here are the first to record the 2-day wave in middle atmosphere winds from Mawson. They show that 2-day period oscillations of 10–15 m s⁻¹ are a regular feature of the high latitude southern hemisphere summer. The wide longitude and latitude separation of the radar stations permits estimates of propagation velocity and latitude phase structure, and results are consistent with the passage of a westward travelling Rossby-gravity (3, 3) wave.     

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 semidiurnal tides at the midlatitude lower thermosphere over East Siberia

Results from the study of semidiurnal tides in the horizontal wind field at 85–95 km over East Siberia are presented. The seasonal variation of tidal amplitudes and the effects of stratospheric warmings are discussed.     

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.     

Vertical winds and turbulence over Thumba

Results of the analysis of barium cloud released over Thumba at an altitude 93 km are presented. In the initial stage, the cloud was in the form of two rings coupled together by a knot and later on the rings were distorted to become elongated loop-like structures. A strong north-south wind shear of 30 m/s/km and a vertical wind gradient are observed which explain the distortion of the initial rings into the loops. The lower loop showed development of turbulence, 280s after the release. However, the upper loop did not show any evidence of turbulence throughout the observed period (10 min), but expanded by wind. The delay of 280 s in the onset of turbulence for the lower loop suggests co-existence of turbulent and non-turbulent regions in the 93–95 km altitude range. The top portion of the upper loop moved upward upto an altitude of about 100km where it recorded maximum upward velocity of 19 ±7 m/s. The values of vertical velocities in the 95–100 km height region coupled with the vertical velocities recorded from other barium clouds released by the same rocket at the lower thermospheric altitudes reveal a wavy profile that is suggestive of the presence of an internal atmospheric gravity wave. The altitude of the turbopause observed in the present experiment is 95 ±2 km, about 10 km lower than that observed earlier. It is suggested that gradients in the vertical wind could cause lowering of turbopause level.     

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.     

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.     

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.     

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.     

Multi-instrument observations of mesospheric motions over Arecibo: comparisons and interpretations

The MF/HF partial-reflection technique of observing the mesosphere and lower thermosphere has been employed for more than two decades to measure motions, but there has never been complete agreement as to what motions were being detected. This paper reports on observations made during a major international campaign—AIDA '89—that was initiated with the objective of resolving this question.The partial-reflection system employed was an Imaging Doppler Interferometer operating at 3.175 MHz, but it stands here as a prototype for all MF/HF partial-reflection radar systems: its raw data were analyzed both in its own basic mode, derived on the assumption that it sees wind-borne multiple scattering centers and in modes adopted by other interferometric and ‘spaced antenna’ systems. The motions thus revealed are compared here with those found by what we consider to be more certain measurers of winds: an incoherent-scatteer radar at heights of 65–95 km, a meteor-wind radar at heights of 80–100 km and a Fabry-Perot interferometer measuring 0(¹S) emissions near a height of 97 km.Comparisons of the different sets of observations oblige us to conclude that

• 1.(1) MF/HF partial-reflection systems may be expected to give a good representation of ambient winds up to a height of about 80 km;
• 2.(2) they fail to give a consistently reliable measurement of the ambient winds above a height of about 80 km
• 3.(3) they yield, at the greater heights, what appears in our data to be some convolution of the horizontal phase velocities of atmospheric gravity waves, with the wave spectrum having been modified by passage through the underlying wind system and containing, on occasion, locally generated Kelvin-Helmholtz waves; and
• 4.(4) when the underlying winds change, the local wave spectrum will change in response and, in MF/HF partial-reflection measurements, will give the appearance of a changing local wind: if the underlying winds undergo tidal changes, the wave spectrum will undergo tide-like changes that will masquerade as true tidal winds.

These results are, of course, limited to a single site over a limited period of observation. Nevertheless, taken at face value they suggest that current methods of data reduction are inappropriate for partial-reflection velocities at heights above 80 km and that new methods of data reduction, perhaps extending certain older methods that have been applied successfully in the past to total-reflection measurements, should be employed in their place if the full potential of the MF/HF partial-reflecton technique is to be realized.     

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.     

A numerical model for gravity wave dissipation in the thermosphere

Two simplified models of internal gravity wave dissipation due to viscosity, thermal conduction and ion-drag, in a multilayered, isothermal thermosphere are developed. Each of these models uses the WKB approximation, ray theory and the time-averaged equation of energy conservation, but whereas one of the models (A) employs all of the gravity wave equations appropriate to a dissipative atmosphere, the other (B) does not. Results derived from these models for one particular wave are compared to each other and also to some previously published results of Klostermeyer, which employed a full-wave, model. A breakdown of the WKB approximation in the lower, non-isothermal thermosphere leads to models A and B underestimating the total dissipation there. In the middle thermosphere model A estimates the dissipation reasonably well, while model B grossly overestimates the dissipation. In the upper thermosphere model A underestimates the total upward energy flux, probably as a result of the neglect of coupling into the dissipative waves at these levels, while no energy remains in model B. Results from model A show that when dissipation due to viscosity and thermal conduction are included correctly and simultaneously, the dissipation due to viscosity can exceed that due to thermal conduction by a factor of three. It is argued that ray theory may either overestimate or underestimate the energy flux reaching the upper boundary of a thermospheric model depending on both its height and the particular thermospheric model used.     

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.     

Equatorial penetration of magnetic disturbance effects in the thermosphere and ionosphere

The neutral dynamic and electrodynamic coupling between high and low latitudes, and the mutual interactions between these two processes, are investigated. For 22 March 1979, when a sudden increase in magnetic activity occurred, we have analyzed the following experimental data: (a) neutral densities and cross-track neutral winds as a function of latitude (0°–80°) near 200 km from a satellite-borne accelerometer; (b) hourly mean H-component magnetic data from the Huancayo Observatory (0.72°S, 4.78°E; dipole geomagnetic coordinates) magnetometer; and (c) hourly mean foF2 measurements from the ionosonde at Huancayo. Comparisons are also made with a self-consistent thermosphere-ionosphere general circulation model and with observationally-based empirical models of winds and density.In concert with the increase in magnetic activity to K_p levels of 5–7, a nighttime (2230 LT) westward intensification of the neutral wind approaching 400 ± 100 ms⁻¹ occurred near the magnetic equator on 22 March 1979, accompanied by a 35% increase in neutral mass density. About 2 h after each of two substorm commencements associated with periods of southward IMF, ∼100γ and ∼200γ reductions in the daytime Huancayo H-component (corrected for ring current effects) are interpreted in terms of ∼0.5 and ∼1.0 mVm⁻¹ westward perturbation electric fields, respectively. An intervening 2-hour period of northward IMF preceded a positive equatorial magnetic perturbation of about 200γ. Time scales for field variations are a few hours, suggesting that processes other than Alfven shielding are involved. Variations in f₀F2 (∼ ± 1.0 MHz) over Huancayo are consistent with the inferred electric fields and magnetic variations. Similar equatorial perturbations are found through examination of other magnetic disturbances during 1979.