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.     

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

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

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.     

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.     

Co-ordinated EISCAT-MICADO interferometer measurements of neutral winds and temperatures in E- and F-regions

The MICADO instrument has been built to measure temperature and wind in the E- and F-regions. It employs a thermally stable field-compensated Michelson interferometer to allow wind measurements. During the winter of 1988–1989, the MICADO instrument was operated at Sodankylä (67°22′N, Finland). Measurements were made by observing the O¹S (low thermosphere) and the O₁D lines (high thermosphere) emission. Two co-ordinated campaigns were organized with the EISCAT radar, which operated in special modes. Neutral wind and temperature are derived from EISCAT data. Results of the two instruments are shown. The differences between the two sets of results are discussed and show that most of the discrepancy is due to the presence of vertical winds during the observations where the magnetic activity was high.     

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 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.     

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

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

The influence of geomagnetic activity on the upper mesosphere lower thermosphere in the auroral zone. II. Horizontal winds

A spaced antenna partial reflection radar located at Mawson, Antarctica (67°S, 63°E, invariant latitude 70°S), has been used to measure the horizontal wind field in the height range 70–110 km. Three years of data (1985–1987) from the radar have been analysed in order to investigate correlations between geomagnetic activity (determined from the local K-index) and the horizontal wind. Results are analysed using a randomization technique and show that larger winds are measured during geomagnetically active periods in both the raw (or unfiltered) wind values and in the medium-frequency (2–6 h period) and high-frequency (1–3 h period) components. The raw winds tend to be shifted towards the geographic NW to NE quadrant in the early morning hours during high K-times. The observed correlation is seen down to 86 km and shows a seasonal dependence. The mean r.m.s. velocity of the radar scatterers and the angular spread of the return echoes are also found to be correlated with geomagnetic activity. The medium- and high-frequency components of the wind are polarized in the magnetic zonal direction during all seasons of the year.     

F-region neutral winds from ionosonde measurements of hmF2 at low latitude magnetic conjugate regions

The behavior of the F2-peak height difference Δh_mF2, between low latitude magnetic conjugate points, is known to be governed by thermospheric winds blowing along the magnetic meridian. Ground based ionosonde measurements of h_mF2, at two pairs of magnetic conjugate stations, have been analysed in conjunction with the results of a realistic dynamic computer model of the tropical ionospheric F-region, to determine thermospheric wind velocities. The behavior of monthly average values of the sum, at conjugate points, of the thermospheric horizontal wind velocity component in the magnetic meridian, at low latitudes, has been inferred for months of solstice and equinox, as well as for periods of low and high solar activity.     

Vertical neutral wind in the equatorial F-region deduced from electric field and ion density measurements

DC electric field and ion density measurements near density depletion regions (that is, equatorial plasma bubbles) are used to estimate the vertical neutral wind speed. The measured zonal electric field in a series of density depletions crossed by the San Marco D satellite at 01.47-01.52 UT on 25 October 1988, can be explained if a downward neutral wind of 15–30 m s⁻¹ exists. Simultaneously, the F-region plasma was moving downward at a speed of 30–50 m s⁻¹ These events appear in the local time sector of 23.002̄23.15 in which strong downward neutral winds may occur. Indeed, airglow measurements suggest that downward neutral velocities of 25–50 m s⁻¹ are possible at times near midnight in the equatorial F-region.     

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.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.     

Seasonal and latitudinal variations of the lower thermospheric tidal winds from meteor radar measurements in the U.S.S.R.

Meteor wind results obtained at 93–95 km altitude at seven sites, six in the Soviet Union and one in Antarctica, between 1965 and 1985 are reported. Attention is focussed on the amplitudes and phases of the semi-diurnal tide, showing a 22-yr oscillation, and of the generally weaker diurnal tide. The measurement results are compared with the results of theoretical models.     

Atomic oxygen determination from a nitric oxide point release in the equatorial lower thermosphere

Atomic oxygen density values in the 80–105 km altitude equatorial region have been obtained by analyzing the chemiluminescence of nitric oxide point releases from three CENTAURE II-C rockets. The light emission produced by the NO—O chemiluminous recombination was sufficiently high to render the artificial clouds observable only by ground-based instruments. The difficulties associated with these kind of experiments have been greatly avoided by a new technique ejecting the NO gas into the backward direction of the flight. It has been found that below 90 km the derived atomic oxygen densities are in relatively good agreement with those reported by other workers. At approximately 105 km the measured value is about two times higher than the n(O) density obtained by averaging a set of data from a great number of other nights but coincides rather well with the measurements of Dickinsonal. (1980).     

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.     

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.     

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.     

A study of meteor radar winds from two locations in the British Isles

Winds and tides have been measured by a two-station meteor radar system which has increased spatial resolution compared with single station radars used in the past. Narrow radar beams, pointing SW from Sheffield (53.5°N, 1.6°W) and 30°N of W from Shrivenham (51.5°N, 1.6°W), are arranged to converge over the U.K. MST radar site near Aberystwyth, thus defining a unique atmospheric volume in which meteor wind components are simultaneously measured from the two radar sites. The resultant ‘true’, or local, wind vector is compared with the spatially averaged vector obtained with the aid of beams pointing SW and NW from Sheffield only. It is found that the ‘true’ and averaged tidal winds are in good agreement, as expected from their large scale sizes, and that the main advantages of the dual station technique lie in the resolution of a small scale structure such as that related to internal atmospheric gravity waves. By the simultaneous deployment of two-station meteor radar, MST radar and LIDAR, such waves may now be studied through a large vertical section of the atmosphere in a geographically localized area.     

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.