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.     

Effects of solar tides on the zonal mean circulation in the lower thermosphere: solstice condition

Effects of momentum deposition due to solar diurnal and semi-diurnal tidal waves on the zonal mean circulation in the mesosphere and lower thermosphere for a solstice condition are discussed. In the present model, the system of zonally averaged equations and the system of perturbation equations are integrated simultaneously, so that the propagation of tidal waves is affected not only by the basic mean fields but also by the induced zonal mean fields due to the momentum deposition. Results for two different vertical eddy diffusion profiles are presented. It is shown that the solar tides make a significant contribution to the generation of the mean zonal winds in the upper mesosphere and the lower thermosphere. Below 120 km the main contribution is due to propagating diurnal tides, while above 120 km it is due to semidiurnal tides.     

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.     

The quasi 2-day wave at Durham (43°N): solar and magnetic effects

A quasi 2-day oscillation has been observed in the meteor winds at Durham since 1970. On a four-day basis the oscillation occurs throughout the year with amplitudes of 10 m s⁻¹ and standing wave or evanescent (λ_z > 150 km) behavior with height. During late summer the oscillation increases in amplitude to ~30 m s⁻¹ with increased phase coherence. When analyzed as a 48 h component the time of maximum of the North-South oscillation prefers the value 15 h LST implying some interaction with the solar tides. The amplitude of the 2-day component is correlated with the daily magnetic index A_p indicating magnetic activity as a possible forcing for this oscillation.     

Mesospheric wind studies during AIDA Act '89: morphology and comparison of various techniques

The Arecibo Initiative in Dynamics of the Atmosphere (AIDA) '89 was a multi-instrument campaign designed to compare various mesospheric wind measurement techniques. Our emphasis here is the comparison of the incoherent scatter radar (ISR) measurements with those of a 3.175 MHz radar operating a s an imaging Doppler interferometer (1131). We have performed further analyses in order to justify the interpretation of the long term IDI measurements in terms of prevailing winds and tides. Initial comparison of 14 profiles by Hines et al., 1993, J. atmos. terr. Phys. 55, 241–288, showed good agreement between the ISR and IDI measurements up to about 80 km, with fair to poor agreement above that altitude. We have compiled statistics from 208 profiles which show that the prevailing wind and diurnal and semidiurnal tides deduced from the IDI data provide a background wind about which both the IDI and ISR winds are normally distributed over the height range from 70 to 97 km. The 3.175 MHz radar data have also been processed using an interferometry (INT) technique [Van Baelen and Richmond 1991, Radio Sts. 26, 1209–1218] and two spaced antenna (SA) techniques [Meek, 1980, J. atmos. terr. Phys. 42, 837–839; Briggs. 1984, MAP Handbook, Vol. 13, pp. 166–186] to determine the three dimensional wind vector. These are then compared with the IDI results. Tidal amplitudes and phases were calculated using the generalized analysis of Groves, 1959, S. atmos. terr. Phys. 16, 344–356, historically used on meteor wind radar data. Results show a predominance of the diurnal S₁¹ tidal mode in the altitude range 70–110 km, reaching a maximum amplitude 45 ms⁻¹ at 95 km, with semidiurnal amplitudes being about 10–15 ms⁻¹ throughout the height range considered. There is evidence of the two day wave in data from 86–120 km, with amplitudes on the order of 20 ms⁻¹.     

Net radiative heating in the middle atmosphere

The meridional distributions of both total solar and net radiative heating rates have been obtained between 30 and 110 km at both the solstice and equinox using Fomichev et al.'s total radiative long wave cooling data in the calculations of the net radiative heating. The contributions to the solar heating of O₃, O₂, CO₂ and H₂O have been investigated. For the ozone heating, the absorption of diffusive solar radiation from the ground and troposphere has been estimated. The 50–90 km layer is close to radiative equilibrium on a globally averaged basis. The importance of radiative cooling as an energy sink in the 90–110 km layer is apparently not less than that of the vertical eddy heat conduction. The ordered meridional circulation has been obtained under the assumption that the temperature variation, due to net radiative heating, is balanced by the adiabatic and temperature variations due to vertical air motion. The circulation model obtained is compared with other empirical models, which are reviewed. For the hemisphere and the 60–80 km layer, the two-cell circulation with the rising motion near the equator and pole from spring to autumn and above 80 km, the one-cell circulation with the sinking motion near the equator and equinox, seem to be most realistic. Also quite realistic for the period near the solstice is the same type of two-cell circulation in the 40–50 km layer and the sinking motion at low latitudes in the 50–60 km layer.     

The EISCAT mesospheric measurements during the CAMP campaign

A brief outline is given of the experimental technique used during the Cold Arctic Mesopause Project to record the first D-region ion line spectra with the EISCAT incoherent scatter radar. The data analysis shows that echoes from mesospheric heights between about 70 km and 90 km can be detected during disturbed periods of enhanced electron density during particle precipitation events. Electron density profiles were determined which show a fairly high density, up to 5 × 10¹⁰ m⁻³ in the upper D-region. The measured meridional winds were lower than 10 m s⁻¹. A fit of the measured height profile of spectral width to temperature and neutral density models yielded a measured temperature profile in good agreement with simultaneous rocket data. The mesopause temperature was determined to be as low as 130 K. This detailed analysis of the spectral width profile indicates that below about 77–80 km the ratio of negative ions to electrons exceeded unity. Finally, some discussions are added on the limitations and significance of these first mesosphere observations.     

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.     

Meteor radar observation at Kyoto in two C.T.O.P. periods (July 20–August 7, 1978, and March 13–29, 1979)

Meteor radar data taken in 1978–1979 at Kyoto station (35°N, 136°E) are processed statistically by Groves' algorithm (1959). Analysis is done especially for data in two C.T.O.P. campaigns carried out in summer and in spring to study a seasonal variation of tides. The reliability of the algorithm is clarified by a simulation with a realistic model. Vertical wavelengths of the diurnal tide and the semidiurnal tide are estimated from height profiles. The diurnal tide becomes evanescent in summer, and the semidiurnal tide appears to be the S_2,2 mode in summer and the S_2,4 mode in spring. This suggestion regarding the semidiurnal tidal modes agrees with the latitudinal variation deduced from many other meteor radar data. Tidal wave energy in the meteor region is estimated. A sudden enhancement of energy is observed in both the diurnal and the semidiurnal tides. There is a clear correlation between an energy decrease of the diurnal tide and an energy increase of the semidiurnal tide.     

First summer results on winds in the upper mesosphere derived from the 843 nm hydroxyl emissions measured from the Bear Lake Observatory,Utah

The Imaging Fabry-Perot Interferometer (IFPI) at the Bear Lake Observatory (BLO), Utah (41.9°N, 111.4°W) is used for studies of the aeronomy of the middle and upper atmosphere. Wind and temperature structure can be determined from observations of the Doppler shift and Doppler broadening of the airglow and auroral emissions from the mesosphere and thermosphere. The mesospheric winds recorded at the end of August, September and early October 1992 are consistent with a semi-diurnal tidal variation. The amplitude of this variation is approximately 30 ms⁻¹ at the end of August and early September and approximately 20 ms⁻¹ at the end of September and early October. However, during June and July, the semi-diurnal tidal variation, if present, is weak, with amplitude < 5 ms⁻¹. No consistent semi-diurnal tidal variation is observed during late October 1992. During the solstice period, antisymmetric tidal components may be preferentially generated in such a way that they can result in destructive interference with the normally dominant symmetric modes, resulting in a decrease of tidal variation. This is consistent with the observed decrease in tides during the June, July and late October periods. Near the equinoxes, however, the excitation of these antisymmetric modes is expected to be weaker, possibly explaining why a pronounced and consistent semi-diurnal tidal variation has been observed during the August, September and early October periods. In contrast, the mesospheric winds derived from the Sheffield Meteor Wind Radar (53.4°N, 1.5°W) reveal a clear semi-diurnal tidal variation throughout the year, with an amplitude that may vary between 15 ms⁻¹ and 50 ms⁻¹, being about 25 ms⁻¹ on average. The IFPI records winds from a region of the atmosphere centred at 87 km, whereas the Sheffield Meteor Wind Radar measures winds centred at 95 km. Therefore, the two regions may experience different tidal modes due to the different latitude, longitude and altitude of the observed regions and/or the different topography of the observing sites. Some proposed reasons for these differences are presented.     

Thermospheric and mesospheric temperatures during geomagnetic storms at 23°S

Night-time thermospheric temperatures, T63o, and mesospheric rotational temperatures, T(OH) and T(O₂), have been measured at Cachoeira Paulista (23°S, 45°W, 16°S dip latitude), located in both the equatorial ionospheric anomaly and the South Atlantic Geomagnetic Anomaly, with a Fabry-Perot interferometer and a multi-channel tilting filter-type photometer, respectively. The thermospheric temperatures are obtained from the Doppler line broadening of the OI 630.0 nm emission and the mesospheric rotational temperatures from the OH(9,4) and O₂A(0,1) band emissions. Measurements made during three geomagnetic storms showed that the nocturnal mean values of T₆₃₀ during the recovery phase of the storms were lower than those observed during quiet time and from model predictions. Also, the nocturnal mean value of the T₆₃₀ soon after the SSC event on 27 June 1992 was higher than the quiet time and model predictions. The observed mesospheric nocturnal mean rotational temperatures, T(O₂) and T(O₂), were unaffected by the storms. A comparison of the night-time observed temperatures T₆₃₀, T(OH) and T(O₂) with those calculated using the MSIS-86 model is also presented.     

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.     

Tidal winds at mesopause altitudes over Arecibo (18°N, 67°W), 5–11 April 1989 (AIDA '89)

An imaging Doppler interferometer (IDI) radar was operated during the three AIDA '89 campaigns in Puerto Rico over the period March–May of 1989. The output of the IDI analysis characterizes radar scattering in terms of a number of discrete ‘scattering points,’ also referred to as ‘multiple scattering centers,’ IDI/MSC for short. For each of these points the three-dimensional location, radial velocity and amplitude and phase are determined, similar to the output of meteor radars. We have applied the conventional Groves [(1959) J. atmos. terr. Phys. 16, 344–356] meteor wind radar analysis to the scattering points to produce the mean apparent motions over the height range from 70 to 110 km which are presented here. The mean apparent motion of the scattering centers is the quantity that would correspond to the neutral atmosphere wind or bulk motion if the scattering points are physical entities (such as turbulent eddies) whose motions are determined solely by advection. This is the quantity which is treated as the ‘wind’ in the analysis which follows and which should be compared to the wind measurements as deduced from the other methods employed during this campaign. There is, however, a caveat which supports the contention of Hineset al. [(1993) J. atmos. terr. Phys. 55, 241–287] that extreme care must be used in interpreting the velocities measured by partial reflection radars as winds. The current application of the Groves method of analysis has revealed motions from which one would infer a typical equatorial easterly circulation, with mean meridional circulation becoming significant only above 96 km. A periodogram analysis of the complete data interval (5–11 April) has shown the diurnal tide to be the most significant feature of the wind field at these altitudes, with zonal amplitudes up to some 50 m/s and meridional amplitudes approximately half this value. The 12 and 6 h tides become as significant as the diurnal above 100 km. The two day (48 ± 5 h) wave is the next most significant feature, with zonal amplitude increasing with height up to 30 m/s at 110km. The semidiurnal tide is not at all well developed below 100 km. However, analysis on a day by day basis reveals a significant semidiurnal component which is not phase coherent over the total interval. Mean vertical velocities are of the order of tens of centimeters per second and are considered to be more realistic than the meters per second velocities usually inferred from analyses of meteor trail drifts.     

Tidal winds in the radio-meteor region over Trivandrum (8.5°N, 77°E)

Wind measurements using a meteor trail radar which performed during two June solstice and three December solstice months were analysed to study the tidal winds over Trivandrum (8.5°N). The ter-diurnal (8 h) component of wind was found to be as significant as the diurnal and semi-diurnal components. The modulations in the altitude profiles of amplitude and phase of the tidal wind components are interpreted to be due to higher order tidal modes. The amplitudes and vertical wavelengths of the tidal modes observed in the meteor zone are discussed.     

The effects of non-linearity on thermal tides in a 3-D numerical model

Absorption of solar radiation is taken as the cause of atmospheric tides, which are simulated by a global, 3-D primitive equation model for the altitude region 0 km–120 km. To investigate non-linear effects, two model versions are used, one with the complete non-linear equation set, the other with the linear equation set. Tide simulations are then performed under the same conditions (background atmosphere and radiation for solstice) with both model versions and directly compared. The diurnal tide can be regarded as a linear phenomenon, whereas the semi-diurnal tide is modified in lower latitudes above 65 km by non-linear effects. Due to the interaction of the tidal components with the background wind, a strong westward zonal flow is generated in the non-linear model above 70 km.     

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.     

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).     

Dynamic coupling between the lower and upper atmosphere by tides and gravity waves

Results of a General Circulation Model simulation of the dynamics of the middle atmosphere are shown focusing our attention to the tidal wave mean flow interaction and propagation of migrating diurnal and semidiurnal tides in the model. It is shown that migrating tidal waves are well simulated and the amplitude growth with height is effectively suppressed by the convective adjustment in the model. It is also shown that the dissipating solar diurnal tide plays an important role in inducing mean zonal winds in the low latitude region of the lower thermosphere. The behavior of non-migrating diurnal tides is also analyzed to show that non-migrating diurnal tides have significant amplitudes in the lower thermosphere. It is suggested that the non-migrating diurnal tide, which propagates against background mean zonal winds, has the possibility to propagate into the middle to high latitude region due to the Doppler effect.     

Plasmaspheric zonal electric fields and coupling fluxes from the Dunedin VLF Doppler experiment,for 180°E,L ≈ 2.3, at solstice and equinox

Group delays and Doppler shifts from ducted whistler-mode signals are measured using the VLF Doppler experiment at Dunedin, New Zealand (45.8°S, 170.5°E). Equatorial zonal electric field and plasmasphere-ionosphere coupling fluxes are determined for L ≈ 2.3 at June solstice and equinox during magnetically quiet periods. The general features of the electric field measured at Dunedin agree with those predicted from ionospheric dynamo theory with a (1,−2) tidal component. Some seasonal variations are observed, with the electric field measured during equinox being smaller and predominantly westward during the night. The electric field at June solstice is also westward during the evening and for part of the night, but turns sharply eastward during the pre-dawn and dawn period at the duct entry site. The June electric field appears to follow a diurnal variation whereas the equinox electric field shows a possible 4-hourly periodic variation. Seasonal variations in the neutral wind pattern, altering the configuration of the ionospheric dynamo field, are the probable cause of the seasonal differences in the electric field. The seasonal variation of the coupling fluxes can be explained by the alteration of the E x B drift pattern, caused by the changes in the electric field.     

Some results on mean tidal structure and day-to-day variability over Arecibo

A long series of monthly 36 h observations, plus a series of observations at the same local time each day over two months at Arecibo, have been analyzed for tidal structure and variability. Some of the results are as follows. (1) A diurnal tide with vertical structure similar to that of the S_1,1 mode dominates the wind field up to heights of the order of 110 km over Arecibo. A semidiurnal oscillation dominates above that height. For non-winter conditions the semidiurnal oscillation in the 80–200 km region closely resembles the S_2,2 mode, though there is the possibility of contributions from higher order modes in the 100–120 km region. (2) Larger semidiurnal amplitudes are observed in the lower thermosphere for winter conditions. The data appears roughly consistent with Bernard's (1979) hypothesis that S_2,2, S_2,4 and S_2,5 modes are thermally excited, with the S_2,4 and S_2,5 modes out of phase in the meteor region in summer and in phase in winter. (3) The day-to-day variability of the tides is at least half the amplitude of the mean oscillations. The maximum wind variability was observed to occur in the 100–110 km region where the diurnal tide is strongly dissipated. (4) The day-to-day deviations in the wind and temperature oscillations from a long term mean at one local time tend to be wave-like structures which are generally correlated from day to day. The structures tend to move upwards, i.e., appear at a later local time, from day to day.