Seasonal variations of equatorial night-time thermospheric meridional winds

Using h'F data at two equatorial stations, night-time equatorial thermospheric meridional winds have been deduced for a period of two years to study their seasonal characteristics. It has been found that the thermospheric wind shows trans-equatorial flow from summer to winter hemisphere. During equinoxes the flow is mainly equatorward with a reversal to poleward direction around midnight hours. The abatement and reversal of equatorward wind which is weaker in summer compared to equinoxes is attributed to Midnight Temperature Maximum (MTM). The results of the present investigation are compared with those at other equatorial stations and also with the empirical model of Hedin et al. (1991).     

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.     

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.     

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.     

A tidal wind model for 85–120 km based on CIRA-86: a feasibility study

This paper examines the feasibility of deriving a climatology of the diurnal variations of the wind in the 85–120 km region from the tidal components of temperature, density, and composition contained in the new COSPAR International Reference Atmosphere, CIRA-1986, Part I: Thermosphere Models [(1988), Adv. Space Res.8, 9]. To derive the wind field, we used the zonal and meridional momentum equations which have been modified from the characteristic scales of the tidal components observed in the 85–120 km region. The CIRA temperature and density model was used to derive the eastward (westerly) and northward (southerly) pressure gradient forces which serve as the forcing functions in the coupled momentum equations. Ground-based wind data from the Mesosphere-Lower Thermosphere (MLT) radar network is used as an independent data set to check the accuracy of the derived tidal wind model. At midlatitudes, the model reproduces some of the general features observed in the radar tidal data, such as the dominant semidiurnal tide with increasing amplitude with height and clockwise (counterclockwise) rotation of the velocity vector observed in the northern (southern) hemisphere. The model overestimates the semidiurnal amplitudes observed by radar by 50–75% during most seasons with the best agreement found during the equinoctial months. The model exhibits little phase variation with height or season, whereas the radar data exhibit a downward phase progression during most seasons (other than summer) characteristic of upward propagating tidal waves, and large seasonal phase variations associated with seasonal changes in vertical wavelengths. The diurnal tidal amplitudes, which are generally 5–20 m s⁻¹ at mid-latitude radar stations and are dominant over the semidiurnal amplitudes at lower latitudes, are less than 5 m s⁻¹ at all latitudes in the model.     

Long-period motions in the equatorial mesosphere

The seasonal variations in winds measured in the equatorial mesosphere and lower thermosphere are discussed, and oscillations in zonal winds in the 3–10 day period range are examined. The observations were made between January 1990 and June 1991 with a spaced-antenna MF radar located on Christmas Island (2°N, 157°W). The seasonal variations are analyzed in terms of the mean, annual, and semiannual (SAO) harmonic components. The SAO is the dominant component in the zonal winds, with the amplitude and phase characteristics being in good agreement with earlier rocketsonde measurements at Kwajalien (9°N) and Ascension Island (8°S). The annual and semiannual oscillations combine to produce a stronger change in zonal wind strength in the first half-year (January–June) than in the second half-year (July–December). An annual cycle dominates the meridional winds with maximum velocities (5–10m s⁻¹) attained at about 90km. The meridional circulation at the solstices is consistent with a flow from the summer to the winter pole. Power spectral analyses indicate that motions in the 3–10 day period range occur mainly in the zonal winds, behavior which is interpreted as being due to eastward propagating Kelvin waves. Despite the intermittent nature there is an overall semiannual variation in Kelvin-wave activity. Maximum amplitudes are achieved at the mesopause in January/February and August/September which are times when the zonal winds are westward.     

Wind corners in the 70–100 km altitude range as observed at andenes (69° latitude)

In the altitude range 70–100 km, high-resolution wind profiles have been measured during the summers of 1987 and 1988 at Andenes (69°N). We report on the wind corners observed in these profiles and compare their properties with those of wind corners seen in the winter of 1983–1984 and autumn 1987. Five of the main results are as follows. (1) The occurrence rates for wind corners in general are similar in summer and in winter. The database for autumn (only 7 flights) was too small to draw any firm conclusions. A strong wind corner was seen, roughly, on every third experiment, both in summer and in winter. (2) The results obtained on the temporal occurrence of wind corners suggest that wind corners seem to have no preference to appear at certain hours of the day. (3) Wind corners tend to appear at preferred heights which are higher in summer than in winter. The spacing between these preferred heights is about 5 km in summer and about 3-3.5 km in winter. (4) In strong wind corners the sense of rotation of the wind direction is positive in summer and negative in winter (with positive being defined as a rotation of the wind direction from northward towards eastward with increasing altitude). (5) At altitudes below 90km wind corners tend to occur at or close to atmospheric layers having Ri ≈ 0.25.     

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.     

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.     

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.     

Climatologies of semi-diurnal and diurnal tides in the middle atmosphere (70–110 km) at middle latitudes (40–55°)

The radars utilized are meteor (2), medium-frequency (2) and the new low-frequency (1) systems: analysis techniques have been exhaustively studied internally and comparatively and are not thought to affect the results. Emphasis is placed upon the new height-time contours of 24, 12 h tidal amplitudes and phases which best display height and seasonal structures; where possible high resolution (10 d) is used (Saskatoon) but all stations provide monthly mean resolution. At these latitudes the semi-diurnal tide is generally larger than the diurnal (10–30 m s⁻¹ vs. < 10 ms⁻¹), and displays less month to month variability. The semi-diurnal tide does show significant regular seasonal structure; wavelengths are generally small (⩽50 km) in winter, large in summer (≲ 100 km), and these states are separated by rapid equinoctial transitions. There is some evidence for less regularity toward 40°C. Coupling with mean winds is apparent. The diurnal tide has weaker seasonal variations; however there is a tendency for vertical wavelengths and amplitudes to be larger during summer months. On occasions in winter and fall wavelengths may be less than 50 km. Again the seasonal transitions are in phase with reversals of the zonal wind. Agreement with new numerical models is to be shown encouraging.     

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.     

Electron density near the plasmapause measured over one year by GEOS-2: a statistical analysis

This paper presents the results of a statistical analysis made under the 1 h average regime from the electron density data obtained over one year by the relaxation sounder on board the satellite GEOS-2 of ESA. The electron density diurnal variations, monthly and annually averaged, are sorted out. A comparison between monthly averaged, daily electron density profiles obtained over all the year has revealed the existence of a seasonal variation of plasma density in the equatorial region, the electron density being larger, on average, in summer than in winter. This seasonal variation is superimposed on the variation related to geomagnetic activity. The asymmetry of the Earth's internal magnetic field is mentioned as being a potential candidate for explaining this seasonal modulation. The annually averaged, daily profile is given. It is found to reach a maximum at 1600 LT, which indicates a plasmaspheric bulge in the predusk, rather than dusk, sector. This is found to be significantly earlier than the average LT location of the stagnation point (≳ 1800 LT) inferred from previous empirical studies for comparable geomagnetic activity levels. This latter feature is interpreted as resulting from the asymmetry of the individual daily density profiles with respect to their maximum, which has been previously reported, and whose effect, after averaging individual daily profiles having their maxima distributed around dusk, is to shift the maximum of the average profile toward the predusk sector.     

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.     

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.     

Climatology of the middle atmosphere of the Southern hemisphere

Main features of spatial distribution and time variations of meteorological parameters in the Southern hemisphere at altitudes 25–80 km are reviewed on the basis of zonal empirical models revised in 1982. Meridional distribution and seasonal variations are analysed. For comparison purposes with the Northern hemisphere, a model developed by Cole and Kantor in 1978 is used. It is revealed that the compilation of new models of the Southern hemisphere atmosphere has not resulted in substantial revision of hemispheric-structure outlined earlier in studies conducted in the Central Aerological Observatory. Meridional distribution of the parameters in summer is characterized by higher values of temperature, pressure and density gradients in the stratosphere of the Southern hemisphere than in that of the Northern hemisphere. This resulted in greater advancement of the core of the summer-time easterly (low towards the equator in the Southern hemisphere than in its northern counterpart. In winter, meridional temperature gradients in the middle stratosphere are greater in the Southern hemisphere than those in the Northern hemisphere, however, rapid attenuation of the gradients with height is observed in the Southern hemisphere, and above 35–40 km they become negative near 50–60°S, in contrast to positive values typical for the Northern hemisphere stratosphere. In the wind field, specific features of the Southern hemisphere westerly flow are high intensity and relatively low altitude of the maximum speed (as compared to the Northern hemisphere).The phases of the annual temperature wave at low latitudes are similar south and north of the equator; south of 30°S a reversal of the phase is observed. The semi-annual oscillation of temperature and wind is less pronounced in middle and high latitudes of the Southern hemisphere than in the Northern hemisphere.The origin of the primary differences between the hemispheres is related mainly to lower intensity of large-scale stratospheric processes in the Southern hemisphere as compared to those in the Northern hemisphere, which is confirmed by values of the standard deviation of the parameters in the two hemispheres.In summer, temperature and pressure fields based on satellite data are symmetric relative to the poles in both hemispheres. In winter, the distortion of the mean pressure field in the mesosphere may be as great in the Southern as in the Northern hemisphere.     

The analysis of hydroxyl rotational temperatures to characterize moving thermal structures near the mesopause

Hydroxyl (OH) rotational temperatures near 85 km altitude have been monitored at Calgary, Alberta, Canada (51°N, 114°W) since 1981 with the objective of determining velocities, wavelengths and periods associated with moving temperature structures. A technique is described whereby the velocity of moving patterns in two dimensional data sets can be accurately determined and used as a parameter for a global smoothing algorithm. Velocities of the structures in the meridional direction were found to be directed poleward. Corresponding Doppler bulk wind velocities measured near the 95 km height region were directed equatorward indicating the presence of filtering of internal gravity waves by the background wind. Two coherent wave structures were often observed simultaneously during a night. The smaller of the two structures had true wavelengths less than 15–30 km and may be related to billow clouds often reported in noctilucent cloud observations. The second wave has a period on the order of an hour and meridional wavelengths ranging from 100 to 2000 km.     

A comparison of foF2 obtained from a time-dependent ionospheric model with Argentine Islands' data for quiet conditions

In this study a comparison is made of the Utah State University Time-Dependent Ionospheric Model (TDIM) and an ionosonde data set from Argentine Islands. This study is unique in that the Argentine Islands data set of foF2 spans complete diurnal, seasonal and solar cycle conditions for low geomagnetic activity. The TDIM reproduces these foF2 variations extremely well. Although the observed winter and summer solstice foF2 diurnal curves have opposite phases, they are readily modelled. At equinox where a sharp transition occurs from winter to summer, or vice versa, the monthly average is complicated by this feature and hence the TDIM does not reproduce the diurnal fine structure.The neutral wind induced vertical plasma drift is the only free parameter in this study. All the other inputs are fixed for the specific solar, seasonal and diurnal conditions. A vertical plasma drift variation is presented; although simplistic, it couples the geographic and geomagnetic frames. With additional information such as hmF2, it would be possible to deduce a unique vertically induced drift pattern.     

Some further results on long term mesospheric and lower thermospheric wind observations by the Arecibo UHF radar

A second series of long term mesospheric and lower thermospheric wind observations was conducted at Arecibo (18.4°N, 66.8°W) between 6 and 20 March 1981 using the UHF Doppler radar, following the first observations in August 1980 (Hirota et al., 1983). Zonal and meridional wind velocities were measured during the morning (8–10 LT) and afternoon (13–15 LT) periods. The mean wind profile averaged over the entire observational period shows the predominance of the diurnal tide. The fluctuating wind vector rotates clockwise relative to height with a characteristic vertical scale of about 10 km. The phase difference inferred by a cross correlation analysis between morning and afternoon profiles indicates that the dominant period is about 20–30 h. This oscillation is discussed in relation to internal inertia-gravity waves observed by the same radar in the lower stratosphere. On the other hand, wind fluctuation with a vertical scale larger than 20 km shows a substantial day-to-day variation with a period of 5–8 days. This long period oscillation shows a good correlation with the global scale geopotential height anomalies at 1 mb (46–48 km) observed by the Tiros-N satellite at 20°N. Our evidence suggests that westward travelling planetary-scale waves with zonal wavenumber one may propagate up to the lower thermosphere.     

Investigations of the winter anomaly in ionospheric absorption in January 1981

Ground-based and rocket-borne investigations were carried out in January 1981 in the Volgograd region to study space-time peculiarities of the winter anomaly in ionospheric radio wave absorption (WA). Electron-density altitude profiles N_e(h) were measured with rockets, by the coherent frequency method and by using electrostatic probes; temperature profiles T(h) were measured by a resistance thermometer: wind velocity and direction were measured by radio-observations of a chaff cloud and of the payload parachute drift. At the same time, ionospheric radio wave absorption was measured in Volgograd at two frequencies, 2.2 and 2.7 MHz, by the A1 method. The condition of the lower ionosphere could be determined from absorption data and from f min parameter data obtained from vertical sounding ionograms. “Salvo” launchings of the rockets were performed on 14 January, when absorption was anomalously large, and on 21 and 28 January, which were days of normal winter absorption.Data analysis has shown that N_e values on the day with excessive absorption exceeded the same values on a normal day at altitudes from 72 to 95 km; on 21 January N_c values exceeded those of 29 February 1980 (without WA) at all altitudes below ~ 90 km. The absorption at Volgograd on 28 January was somewhat higher than on 21 January and than at stations at higher latitude, which may be due to a stable local increase of N_e values in the altitude range 80–90 km. The temperature in the region of the N_e-enhanced values (up to the limit altitude of measurements, about 80 km) was below the standard temperature (COSPAR, 72), both on 14 January and on the normal days. Measurements carried out at night have shown that winter N_c values considerably exceeded those during the autumn. The zonal and meridional wind profiles (up to about 80 km) at Volgograd exhibit a stable eastward flux, both in the stratsophere and in the mesosphere. The value of the wind velocity meridional component on 21 January is close to zero at all altitudes. On 14 and 28 January the wind profiles show an irregular structure with large velocity gradients at all altitudes above about 50–60 km.The absorption data and f min data from a number of stations, viz. from Juliusruh to Yakutsk (in longitude) and from Arkhangel'sk to Rostov-on-Don (in latitude), show that anomalously excessive absorption occurred over a vast distance exceeding 100° of longitude at ~ 55° latitude and that, based on the dates of absorption maxima (f min), one may conclude that the source of the disturbance was moving from west to east. Data on the motion of the air as shown by rocket and radiometeoric observations, indicate the same wind direction in the stratosphere as in the mesosphere. These data and the constant pressure charts point to the conclusion that the enhanced radio absorption values at mid-latitudes may be explained by a transport of dry air rich in nitric oxide from the auroral zone towards lower latitudes. The transport is provided by a stable circumpolar vortex existing in winter time. This mechanism may explain both the normal and anomalous winter absorption, as well as the post-storm effect.