Longitudinal differences and inter-annual variations of zonal wind in the tropical stratosphere and troposphere

A quantitative assessment has been made of the longitude-dependent differences and the interannual variations of the zonal wind components in the equatorial stratosphere and troposphere, from the analysis of rocket and balloon data for 1979 and 1980 for three stations near ±8.5° latitude (Ascension Island at 14.4°W, Thumba at 76.9°E and Kwajalein at 67.7°E) and two stations near 21.5° latitude (Barking Sands at 159.6°W and Balasore at 86.9°E). The longitude-dependent differences are found to be about 10–20 m s⁻¹ (amounting to 50–200% in some cases) for the semi-annual oscillation (SAO) and the annual oscillation (AO) amplitudes, depending upon the altitude and latitude. Inter-annual variations of about 10 m s⁻¹ also exist in both oscillations. The phase of the SAO exhibits an almost 180° shift at Kwajalein compared to that at the other two stations near 8.5°, while the phase of the AO is independent of longitude, in the stratosphere.The amplitude and phase of the quasi-biennial oscillation (QBO) are found to be almost independent of longitude in the 18–38 km range, but above 40 km height the QBO amplitude and phase have different values in different longitude sectors for the three stations near ±8.5° latitude. The mean zonal wind shows no change from 1979 to 1980, but in the troposphere at 8.5° latitude strong easterlies prevail in the Indian zone, in contrast to the westerlies at the Atlantic and Pacific stations.     

Long period wind oscillations in the meteor region over Trivandrum (8°N, 77°E)

The spectra of long period wind oscillations in the meteor zone over Trivandrum are presented. The spectral amplitudes were found to be much larger during June 1984 when the QBO in the stratospheric zonal wind was in a strong easterly phase compared with June 1987 when the zonal winds at the altitude of maximum QBO were weak westerlies. Zonal wind amplitudes for periods of 15 and 5 days were found to be most significant during these two June months. The amplitudes of these two oscillations in meridional wind were found to be as large as the amplitudes in the zonal wind. The vertical wavelength in both zonal wind and meridional winds of the 15-day oscillation is very large whereas for the 5-day oscillation the vertical wavelengths were 80 and 65 km during June 1984 and June 1987, respectively. The results are discussed.     

Mean winds at 60–90 km observed with the MU radar (35°N)

Mean winds at 60–90 km altitudes observed with the MU radar (35°N, 136°E) in 1985–1989 are presented in this paper. The zonal wind at 70 km became westward and eastward in summer and winter, respectively, with a maximum amplitude of 45 m s⁻¹ westward in early July and 80 m s⁻¹ eastward at the end of November. The meridional wind below 85 km was generally northward with the amplitudes less than 10 m s⁻¹. In September to November, the meridional wind at 75–80 km becomes as large as 20–30 m s⁻¹. Those zonal wind profiles below 90 km show good coincidence with the CIRA 1986 model, except for the latter half of winter, from January to March, when the observational result showed a much weaker eastward wind than the CIRA model. The height of the reversal of the summer wind from westward to eastward was determined as being 83–84 km, which is close to the CIRA 1986 model of 85 km. The difference between the previous meteor radar results at 35–40°N, which showed the reversal height below 80 km, could be due to interannual variations or the difference in wind measurement technique. In order to clarify that point, careful comparative observations would be necessary. These mean winds were compared with Adelaide MF radar observations, and showed good symmetry between the hemispheres, including the summer reversal height, except for the short period of eastward winds above Kyoto and the long period over Adelaide.     

Long period wind oscillations observed by the Kyoto meteor radar and comparison of the quasi-2-day wave with Adelaide HF radar observations

We have detected wind oscillations with periods ranging from 1.4 to 20 days at 80–110 km altitude using Kyoto meteor radar observations made in 1983–1985. Among these oscillations, the quasi-2-day wave is repeatedly enhanced in summer and autumn. We found that the period of the quasi-2-day wave ranges from 52 to 55 h in summer, and becomes as short as 46 to 48 h in autumn in 1983 and 1984. The change in the wave period seems to coincide with a decrease in the amplitude of the zonal mean wind. A quasi-2-day wave event was simultaneously observed in January 1984 at Kyoto (35° N, 136°E) and Adelaide (35° S, 138° E), which are located at conjugate points relative to the geographic equator. Amplitudes of the meridional component at Adelaide are approximately four times larger than those observed at Kyoto. Comparison observations clearly show that the meridional component is in phase and the zonal component is out of phase, respectively, implying antisymmetry of the quasi-2-day wave between the northern and southern hemispheres. Relative phase progressions with height are similar between the Kyoto and Adelaide results for both meridional and zonal components, and indicate the presence of an upward energy propagating wave with a vertical wavelength of about 100 km.     

Observations of winds in the Antarctic summer mesosphere using the spaced antenna technique

Observations of winds in the 60–100 km height range were made at Mawson (68°S, 63°E) during December 1981 and January 1982 with the MF spaced antenna technique. The prevailing winds are in accord with other recent observations made at high latitudes and show a peak in the zonal wind near 80 km with westward winds of 30 m s ⁻¹. The meridional winds maximize near 90 km with an equatorward flow of 10 m s⁻¹. The diurnal tidal components are in reasonable agreement with recent model predictions, especially in phase. The amplitudes tend to be larger than the model values. The semidiurnal tide is not as stable as the diurnal tide and shows evidence for interference effects between different modes.     

Variations of mean winds and tides in the upper middle atmosphere over a solar cycle,Saskatoon, Canada, 52°N, 107°W

Saskatoon (52 N, 107 W) medium frequency (MF) radar data from 1979 to 1990 have been analyzed to investigate the solar activity effects on upper middle atmospheric winds and tidal amplitudes. The period of study covers two solar maxima and a solar minimum; the continuous data allow a systematic analysis of solar cycle dependence on mean winds and tides. The height region of 79–97 km sampled in the study shows an apparent but very weak dependence of mean winds and tidal amplitudes on solar activity variation. The observed features are fairly consistent with the early results reported by Sprenger and Schmindkr [(1969) J. atmos. terr. Phy. 31, 217). The mean zonal wind and the semidiurnal tidal amplitudes appear to exhibit positive and negative correlations with the solar activity, respectively; the statistical significances of these correlations are generally low. There is a biennial periodicity evident in the zonal wind oscillations but this docs not have a consistent phase relationship with the equatorial stratospheric wind oscillations (QBO). The meridional winds and the tidal amplitudes are characterized with different and quite irregular periods of oscillations (2–5 yr). The diurnal tidal variations over the solar cycle are small and irregular, although amplitudes are slightly larger during the solar minimum years.     

Quasi-biennial oscillation in ionospheric parameters measured at Juliusruh (55°N, 13°E)

When seasonal variations were eliminated by evaluating 12-month running means, the ionospheric parameters foE, foF2 and hmF2 at Juliusruh (54.6°N, 13.4°E) showed large solar cycle variations. However, when further 3-yr running averages were evaluated and subtracted, QBO (Quasi-biennial oscillations) were noticed in all these parameters. Sunspot series did not reveal a QBO, but geomagnetic Ap did show a QBO. The peaks of the ionospheric QBO and QBO of Ap could be roughly compared, with lags or leads of a few months. Also, these compared roughly with the well-known QBO peaks of tropical stratospheric (50 mb) zonal winds. Similar analyses at other locations are warranted.     

Modelling studies of the conjugate-hemisphere differences in ionospheric ionization at equatorial anomaly latitudes

The relative importance of the equatorial plasma fountain (caused by vertical E x B drift at the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis on conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospherec electron content (IEC) and peak electron density (Nmax) computed at conjugate points in the magnetic latitude range 10–30° at longitude 158°W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and Nmax, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes.At equinox., the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements. in IEC and Nmax (with delayed peaks) in the southern hemisphere at all latitudes (10–30°). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F-region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and Nmax and stronger daytime Nmax in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.     

Predominant semi-annual oscillation of the upper mesospheric airglow intensities and temperatures in the equatorial region

The upper mesospheric and lower thermospheric airglow emissions, OI 557.7 nm, NaD 589.3 mn and the OH (9,4) band and its rotational temperature have been measured using a ground-based multichannel airglow photometer located at Fortaleza (3.9°S, 38.4°W) since 1986. The observed emission intensities show predominantly semi-annual oscillations with maxima at the equinoxes and minima during the solstices. The amplitudes of the oscillations are larger than those observed from the low latitude station, Cachoeira Paulista (22.7°S, 45.0°W). The OH rotational temperature, which represents a gas kinetic atmospheric temperature at around 85–95 km, also shows a strong semi-annual oscillation, 18 K peak to peak, with an. average value around 10 K higher than that observed from Cachoeira Paulista. These results do not agree with model atmospheres presently available. It is suggested that the differences result from the effects of seasonal variations in vertical eddy transport and/or meridional circulation.     

Mesopause temperatures and integrated band brightnesses calculated from airglow OH emissions recorded at Maynooth (53.2°N, 6.4°W) during 1993

Spectra of the hydroxyl emissions in the wavelength range 1.0–1.6 μm, which originate at mesopause altitudes, have been obtained, using a Fourier transform spectrometer at Maynooth (53.2°N, 6.4°W), on all suitable nights during the period January–December 1993. Rotational temperatures and integrated band brightnesses have been calculated from the spectra of the OH(3, 1) and (4, 2) vibration-rotation bands. The mean annual temperatures calculated over all measurements were T(3, 1)=200±19 K and T(4, 2)=206±19 K, where the uncertainty represents the standard deviation on the measurements. Harmonic analysis of the nightly averaged temperature values revealed an amplitude of 27 ± 1 K and a phase of 95 ± 2 days in the annual variation of the (3, l) band at our latitude. The semiannual component was found to have an amplitude of 7 ± 1 K and a phase of −51 ± 9 days for this band. Results for the (4, 2) band were identical in both amplitude and phase for the annual component, while the semiannual component gave an amplitude of 8 ± 1 K and a phase of − 43 ± 7 days. These results are compared with data recorded by the SME satellite, and with the predictions of the MSISE-90 model for a station at 53° latitude. Temperatures predicted by the MSISE-90 model for Maynooth are consistently below the values obtained in this study by 15–20 K. Excellent agreement is observed between the absolute value of temperature, in the case of the SME satellite, and in the amplitude and phase of the annual variation predicted by MSISE-90. The phase of the semiannual component observed in our data deviates somewhat from the −99 ± 1 days predicted by MSISE-90.The annual mean brightness of the OH (3, 1) band was found to be 75 ± 18 kR, while that of the (4, 2) band was 106±26 kR. Diurnal variations generally showed a steady decrease from dusk to dawn, apart from a brief period in June and July. Monthly average values of band brightness have been calculated for each band and are compared with the predictions of a recent photochemical model (Le Texier et al., 1987). The model shows some elements of agreement with our observations, particularly a pair of maxima near the equinoxes, but it does not predict the broad winter maximum observed in both bands at this latitude.     

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.     

Winds oscillations (∼ 6h–6days) in the upper middle atmosphere at Monpazier (France, 45°N, 1°E) and Saskatoon (Canada, 52°N, 107°W) in 1979–1980

The medium frequency radar (∼ 2.2 MHz) at Saskatoon has been run continuously since 1978 and the Meteor Radar at Monpazier ran continuously for ∼ 10 day intervals in most months of 1979/1980. The radars are separated by ∼ 8000 km. Because of the excellent quality of the data, spectral and harmonic analyses have been completed from ∼ 70 to 100km and oscillations with periods of ∼ 6h–6days studied.There are substantial similarities in the mean zonal winds, both with regard to magnitudes and times of seasonal reversals; also in annual and semi-annual oscillations. In general, the semi-diurnal tide has similar amplitudes, phases and vertical wavelengths : there are consistent summer (long λ) and winter (short λ) characteristics, with rapid transitions between them. Differences between the timing of these transitions and in some of the mid-season tides are discussed. The diurnal tide is less regular and of smaller amplitude at both locations, often being too small to reliably characterize at Monpazier. However, seasonal variations between summer and winter months may be discerned.In addition to the 24 and 12 h tidal oscillations, which traditionally are studied in most detail, there is clear evidence for additional osculations near 6,8, ∼ 10 and ∼ 16 h and longer periods of ∼ 2 and ∼ 5 days. The amplitudes of these are often comparable or larger than the ‘dominant’ 24 and 12 h tides. The monthly and seasonal variations of these additional oscillations are studied, as a function of height, at the two locations. There is evidence for large scale (global) and small scale (local) disturbances in the wind field.     

Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data

The daily variations of the meridional wind at ±18° latitude have been obtained for summer and winter between 1977 and 1979 using the in situ measurements from the Atmosphere Explorer-E (AE-E) satellite. The AE-E altitude increased from about 250 to about 450 km during this period, with solar activity increasing simultaneously. Data are presented at three altitudes, around 270, 350 and 440 km. It was possible to average the data to obtain the 24 h variations of the meridional wind simultaneously at northern and southern latitudes and thereby study the seasonal variation of the meridional wind in the altitude range covered. Two features are found showing significant seasonal variation: (a) a late afternoon maximum of the poleward wind occurring only in winter at 1800 LT at all three altitudes; (b) a night-time maximum in the equatorward wind—the summer equatorward wind abating earlier (near 2130 LT) and more rapidly than the winter wind (after 2300 LT). Furthermore, in summer the night-time wind reaches higher amplitudes than in winter. The night-time feature is consistent with the observed seasonal variation of the equatorial midnight temperature maximum, which occurs at or before midnight in summer and after midnight in winter, showing a stronger maximum in summer. The observed night-time abatement and seasonal variations in the night-time winds are in harmony with ground based observations at 18° latitude (Arecibo). The time difference found between summer and winter abatements of the night-time equatorward wind are in large part due to a difference between the phases of the summer and winter diurnal (fundamental) components, and diurnal amplitudes are larger in summer than in winter at all threee altitudes. However, the higher harmonics play an important role, their amplitudes being roughly 50% of the diurnal and in some instances larger. The 24 h variation is mainly diurnal at all altitudes in both summer and winter, except in winter around 2700 km altitude where the semi- and ter-diurnal components are approximately equal to or larger than the diurnal.     

Diurnal tide in the Antarctic and Arctic mesosphere/lower thermosphere regions

The behaviour of the diurnal tide at 95 km over various years between 1965 and 1986 is studied using radar data from Heiss Island (81°N), Mawson (67°S), Molodezhnaya (68°S) and Scott Base (78°S). The observations are also compared with the model results of FORBES and HAGAN [(1988) Planet. Space Sci. 36, 579] for the same latitudes. There are substantial fluctuations in amplitude and phase at all stations, particularly in winter. Phase fluctuations can be as large as a uniform random distribution over the 24-h cycle. In summmer the phases of the meridional components are well defined and suggest the presence of a dominant symmetric mode. The meridional amplitudes are larger in summer whereas the zonal components have a greater variation and show no significant variation with season.     

Comparison of plasma flow and thermospheric circulation over northern Scandinavia using EISCAT and a Fabry-Perot interferometer

The EISCAT incoherent scatter radar, operating in a full tristatic mode, provided data on the ionospheric plasma drift above northern Scandinavia, during the 24 h period, 11 UT 25 November to 11 UT 26 November 1982. For the hours of darkness, 14 UT until 05 UT, observations of thermospheric winds were made by means of a ground-based Fabry-Perot interferometer (FPI) operated at Kiruna Geophysical Institute (21° E, 68° N). During this period, the radar observations describe well the ebbing and flowing of regions of strong convective ion flow associated with the auroral oval. As individual geomagnetic disturbances occur, the overall ion flow pattern intensifies and moves equatorward. The zonal thermospheric wind observed by the FPI responds rapidly to surges of the local ionospheric convection, while the meridional wind response is slower and apparently to much larger-scale features of the geomagnetic input to the high latitude thermosphere. From the data base, periods of strong heating of the ionospheric ions and of the thermospheric gas can be identified, which can be compared with Joule and particle heating rates deduced from the observations of ionospheric drifts, neutral winds, electron densities and auroral emission rates. A three-dimensional, time-dependent global thermospheric model is used to distinguish local and global features of the thermospheric wind field. Meridional and zonal wind components at 312 km may be theoretically derived from the EISCAT data using an appropriate model (MSIS) for neutral temperature. The EISCAT-derived meridional wind is within about 50 m s⁻¹ of the FPI observations throughout the period of joint observations. The EISCAT-derived zonal wind is systematically larger (by about 50%) than the FPI measurement, but the two independent measurements follow closely the same fluctuations in response to geophysical events until 03 UT, when the EISCAT solution is driven away from the FPI measurement by a sharp increase in both neutral and ion temperatures. Between 03 and 05 UT the EISCAT-derived zonal wind is 200–400 m s⁻¹ westward. Allowance for the neutral temperature rise would reduce the EISCAT values towards the very small zonal winds shown by the FPI during this period. We describe the relatively straightforward analysis required to derive the meridional wind from the radar data and the limitations inherent in the derivation of zonal wind, using the ion energy equation, due to the lack of precise knowledge of the background neutral temperature from the EISCAT data alone. For analysis of EISCAT ion drift observations at 312 km, the ground-based FPI temperature measurements do not improve the accuracy of the analysis, since the median altitude of the FPI measurement is probably in the range 180–240 km throughout the observation period. This median altitude and the temperature gradient both fluctuate in response to local geomagnetic events, while the temperature gradient may be considerably greater than that predicted by standard atmospheric models. When the neutral temperature is well known, or when there is a large enhancement of the ion temperature, the EISCAT-derived zonal wind exceeds the FPI measurement, but the consistency with which they correlate and follow ion-drag accelerations suggests that the differences are purely due to the considerable altitude gradients which are predicted by theoretical models.     

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.     

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.     

Mean winds observed by the Kyoto meteor radar in 1983–1985

Mean winds at 82–106 km altitude have been almost continuously monitored by the Kyoto meteor radar over the period from May 1983 to December 1985. The mean zonal wind becomes eastward with amplitudes as large as 30 m s⁻¹ in the summer months (May–August), maximizing early in July at 95 km altitude, while it is less than 10 m s⁻¹ at all the observed altitudes during the equinoxes. It is normally eastward in winter at low altitudes, although it sometimes becomes westward during sudden stratospheric warmings. The mean meridional wind is usually equatorward and is weaker than the zonal component. A southward wind exceeding 10 m s⁻¹ is detected in July and August. The observed mean winds are compared with the CIRA 1972 model and coincidences with sudden warmings of changes in zonal wind direction are pointed out.     

Plasma drifts inferred from thermospheric neutral winds and temperature gradients observed at low latitudes

Low-latitude plasma drifts (zonal and meridional) in the F-region are inferred from the observed night-time thermospheric neutral wind velocities and temperature gradients, together with models for the neutral density (MSIS-86 model) and the electron density (IRI model). The thermospheric neutral winds and temperatures are derived from measurements of Doppler shifts and widths of the Oi 630.0 nm airglow emission line, respectively, using a Fabry-Perot interferometer at Cachoeira Paulista (23°S, 45°W), Brazil. The equations considered are the ideal gas law and the momentum equation for the thermosphere, which includes the time variation of the neutral wind, the pressure gradient which is related to the temperature and density gradients and the ion drag force. The present method to infer the night-time plasma drift using observed neutral parameters (time variation of neutral wind velocities and temperature gradients) showed results that are in reasonable agreement with our calculated plasma drifts and those observed in other low-latitude locations. On the other hand, it is surprising that sometimes the winds flow from the observed coldest sector to the hottest part of the thermosphere during many hours, suggesting that plasma drift can drive the neutral winds at low latitudes for a period of time.     

Seasonal and tidal variations of neutral temperatures and densities in the high latitude lower thermosphere measured by EISCAT

Measurements of ion temperature, ion-neutral collision frequency and ion drift in the E-region from the period December 1984 to November 1985 are used to derive neutral temperatures, densities and meridional winds in the altitude intervals 92–120 km, 92–105 km and 92–120 km, respectively. Altitude profiles of temperature and density and their seasonal variations are compared with the CIRA 1972 and MSIS 1983 models and the effects of geomagnetic activity are demonstrated. Semi-diurnal tidal variations in all three parameters are derived and the comparison with lower latitude measurements is discussed.