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.     

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.     

EISCAT observations of ion composition and temperature anisotropy in the high-latitude F-region

The papers by Winseret al. [(1990) J. atmos. terr. Phys.52, 501] and Häggström and Collis [(1990) J. atmos. terr. Phys.52, 519] used plasma flows and ion temperatures, as measured by the EISCAT tristatic incoherent scatter radar, to investigate changes in the ion composition of the ionospheric F-layer at high latitudes, in response to increases in the speed of plasma convection. These studies reported that the ion composition rapidly changed from mainly O⁺ to almost completely (>90%) molecular ions, following rapid increases in ion drift speed by >1 km s⁻¹. These changes appeared inconsisent with theoretical considerations of the ion chemistry, which could not account for the large fractions of molecular ions inferred from the obsevations. In this paper, we discuss two causes of this discrepancy. First, we reevaluate the theoretical calculations for chemical equilibrium and show that, if we correct the derived temperatures for the effect of the molecular ions, and if we employ more realistic dependences of the reaction rates on the ion temperature, the composition changes derived for the faster convection speeds can be explained. For the Winser et al. observations with the radar beam at an aspect angle of ϕ = 54.7° to the geomagnetic field, we now compute a change to 89% molecular ions in < 2 min, in response to the 3 km s⁻¹ drift. This is broadly consistent with the observations. But for the two cases considered by Häggström and Collis, looking along the field line (ϕ = 0°), we compute the proportion of molecular ions to be only 4 and 16% for the observed plasma drifts of 1.2 and 1.6 km s⁻¹, respectively. These computed proportions are much smaller than those derived experimentally (70 and 90%). We attribute the differences to the effects of non-Maxwellian, anisotropic ion velocity distribution functions. We also discuss the effect of ion composition changes on the various radar observations that report anisotropies of ion temperature.     

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.     

Further evidence for a 6-h tide above Arecibo

Since the publication of results suggesting the existence of a 6-h tide in the E region above Arecibo [(Tonget al., 1988) J. geophys. Res.93, 10047–10051], much more data has been collected and analyzed. In particular, the time-height trajectories of middle and upper E region Tidal Ion Layers (TILs) for early January 1989 closely resemble those from early January 1981, which first revealed the presence of a 6-h quasi-periodic intermediate layer structure. Further, the January 1989 observations form an ‘overture’ to the March–May 1989 AIDA (Arecibo Initiative in Dynamics of the Atmosphere) campaign, which yielded a total of 28 days of additional data regarding TIL motion. Interestingly, the AIDA data set is dominated, above about 120 km altitude, by sporadic intermediate layers [(mathewset al., 1993) J. atmos. terr. Phys.55, 447–457] and certainly does not show the consistent 6-h period TIL feature seen in the two January data sets. In reviewing all data collected over the past 10 yr and the extensive 1989 observations in particular, we conclude that the basic TIL structure is controlled by two separate tidal wind patterns. We refer to these as the normal pattern and the ‘deep-winter’ pattern. The normal pattern includes the combination of diurnal and semidiurnal tides, while the deep-winter pattern has an additional 6-h tidal component. The deep winter pattern remains unexplained, but we suggest that the 6-h periodicity, which appears to be phase locked with the semidiurnal tide, is generated via in situ non-linear frequency doubling of the semidiurnal tide. The January 1989 results also manifest a TIL structure, below 100 km altitude, which has not been previously reported.     

Monthly simulations of the solar semidiurnal tide in the mesosphere and lower thermosphere

Monthly simulations of the solar semidiurnal tide in the 80–100 km height regime are presented. These calculations benefit from the recent heating rates provided by Groves G. V. (1982a,b) (J. atmos. terr. Phys. 44, 111; 44, 281), the zonally-averaged wind, temperature and pressure fields developed for the new COSPAR international reference atmosphere [Labitzke K., Barnett J. J. and Edwards B. (1985) Handbook for MAP 16, 318], and eddy diffusivities determined from gravity wave saturation climatologies and used by Garcia R. R. and Solomon S. (1985) (J. geophys. Res. 90, 3850) to simulate oxygen photochemistry and transport in the mesosphere and lower thermosphere. Some of the main characteristics of the observed semidiurnal tide at middle and high latitudes are reproduced in our simulations: larger amplitudes in winter months than in summer months, and the bi-modal behavior of the phase with summer-like and winter-like months separated by a quick transition around the two equinoxes. The phase transition is also more rapid in the spring, consistent with observations. The wavelengths are also longer in summer than in winter, at least below 95 km (whereas in July and August the simulations exhibit some discrepancies above this altitude), similar to the observational data. Semidiurnal amplitudes are generally smaller and the phases more seasonally symmetric at middle and low latitudes, as compared with the tidal structures above about 50° latitude. In addition, hemispheric differences in the mean zonal wind result in marked asymmetries in tidal behavior between the Arctic and Antarctic regions, and suggest that a comparative study of tide, gravity wave and mean flow interactions in the Arctic and Antarctic mesosphere and lower thermosphere would be fruitful.     

Tidal wind observations with incoherent scatter radar and meteorological rockets during MAP/WINE

Measurements of winds in the mesosphere and lower thermosphere were carried out during the main phase of the MAP/WINE project in January and February 1984 with the EISCAT UHF incoherent scatter radar near Tromsö, Norway, and with meteorological rockets launched from the Andøya Rocket Range, Norway. The radar measurements yield wind profiles between the altitudes of about 80 km and 105 km and the rockets between about 60 km and 90 km. Results from both techniques are combined to yield mean profiles which are particularly evaluated in terms of tidal variations. It is found that the semidiurnal tide constitutes an essential wind contribution between 85 km and 105 km. Whereas the tidal amplitudes are below 5 m s⁻¹ at about 80 km, they increase to 20–30 m s⁻¹ at 100 km. The average vertical wavelength of 35 km points to the S₄² mode, but coupling and superposition of different modes cannot be excluded.     

Computer simulations of the seasonal variations of the ionospheric equatorial anomaly in East Asia under solar minimum conditions

The previous dynamical, computer simulation model of the ionosphere at low latitudes of Chan H. F. and Walker G. O. (1984a, J. atmos. terr. Phys. 46, 1103; 1984b, J. atmos. terr. Phys. 46, 1113) has been modified to (1) include photoionization of molecular species NO⁺, N₂⁺ and O₂⁺ below 300km, (2) decouple the ionization and wind calculations below 180 km and (3) expand the geographical coverage to 46°N-30°S latitude. The first two modifications improved the model stability and the latter reduced the effect of the lateral boundaries on the equatorial anomaly. Results are presented for the representative seasonal months of January, April and July for East Asia, during solar minimum, comprising latitudinal-local standard time (120°E) contour plots of (1) the atmospheric pressure, (2) the computed meridional wind at 300 km, (3) the foF2 and (4) hmF2, together with latitudinal profiles of foF2 and N_T (electron content) showing the daytime development and nighttime decay of the equatorial anomaly.Comparisons have been made between the computer simulations and various experimental measurements of foF2, M(3000) F2 and N_T obtained in East Asia during periods of low solar activity. Most of the gross features of the development and decay of the equatorial anomaly at the various seasons were reproducible by the model simulations, the best agreement occurring for the equinoctial month of April.     

Characteristics of ionospheric irregularities capable of producing quasi-horizontal traces on ionograms

This paper presents simulated ionograms calculated for a parabolic ionospheric layer containing irregularities in the form of small amplitude waves. With small amplitudes, perturbation techniques can be used enabling results for the irregular ionospheres to be calculated from the results for smooth ionospheres. This approach is relatively straightforward and avoids having to ray trace new paths each time the irregularity parameters are changed. It is, however, restricted to irregularities which do not cause multiple echoes. Irregularities with vertical wavelengths of up to a few kilometres can produce significant changes in the ionosphere over height intervals smaller than those involved in reflecting a single pulse. Consequently, in the simulation procedure, it is essential to consider not just the carrier frequency but the complete frequency spectrum of the pulse. Irregularities with vertical wavelengths of the order of 10 km or more can produce ripples in an ionogram trace. These will, of course, be more evident on ionograms with high frequency resolution. Irregularities with vertical wavelengths of up to several kilometres and amplitudes up to a few per cent can produce significant pulse spreading and splitting. The actual effects depend not just on the irregularity properties but also on the ionosonde pulse width, gain and frequency and height resolutions. Some simulations show trace splitting and quasi-horizontal traces similar in many respects to effects observed by Bowman (1987, J. atmos. terr. Phys. 49, 1007) and Bowmanet al. (1988, J. atmos. terr. Phys. 50, 797). Consequently it is suggested that, at least in some cases, small amplitude (≤3%) and small scale (≤4 km) irregularities produce the spread-ifF reported by these authors.     

Tidal winds from the MLT global radar network during the first LTCS campaign—September 1987

Winds and tides were measured by a number of MLT (Mesosphere, Lower Thermosphere) radars with locations varying from 43–70°N, 35–68°S, during the first LTCS (Lower Thermosphere Coupling Study) Campaign, 21–25 September 1987. The mean winds were globally westerly, consistent with early winter-like (NH) and late winter (SH) circulations.The semi-diurnal tide had vertical wavelengths near or less than 100 km at most locations, with some latitudinal variation (longer/shorter at lower latitudes in the NH/SH)—amplitudes decreased at high latitudes. The global tide was closer to anti-symmetric, with northward components being in phase at 90 km. Numerical model calculations [Forbes and Vial (1989), J. atmos. lerr. Phys. 51, 649] for September have rather similar wavelengths and amplitudes; however, the global tide was closer to symmetric, and detailed latitudinal trends differed from observed.The diurnal tide had similar wavelengths in each hemisphere, with short values (~30 km) at 35°, long (evanescence) at 68–70°, and irregular phase structures at mid-latitudes. The tide was neither symmetric nor anti-symmetric. Model calculations for the equinox [Forbes. S and Hagan (1988), Planet. Space Sci. 36, 579] were by nature symmetric, and showed the short wavelengths extending to mid-latitudes (43–52°). Southern hemisphere phases were significantly (6–8 h) different from observations. Amplitudes decreased at high latitudes in model and observation profiles.     

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.     

Recent work on mid-latitude and equatorial sporadic-E

Theoretical and experimental work since 1970 is summarized. Mid-latitude sporadic-E is most likely due to a vertical shear in the horizontal east-west wind and this theory accounts for the detailed observations of the wind and electron density profiles. Preferred heights of sporadic-E are separated by about 6km and descending layers are often seen moving down with velocities in the range 0.6–4 ms⁻. Sometimes sporadic-E layers are very flat and uniform, and at other times form clouds of electrons 2–100km in size moving horizontally at 20–130 ms⁻¹. Sporadic-E is probably not correlated with meteor showers; this is a rather surprising result since the ions are meteor debris.The major problems with windshear theory are to account for the dramatic seasonal variation and, to a lesser extent, for the geographical and diurnal distributions.The Q-type equatorial sporadic-E appears to be due to the gradient instability. There is a very much smaller amount of new experimental data available in this area.     

Dynamics of the Antarctic and Arctic mesosphere and lower thermosphere regions—II. The semidiurnal tide

The semidiurnal tidal dynamics of the Antarctic and Arctic mesopause regions (95 ± 15 km) are investigated through comparative analyses of monthly mean tidal wind fields determined from radar measurements at the Scott Base (78°S), Molodezhnaya (68°S), and Mawson (67°S) stations in the Antarctic, and the near-conjugate stations of Heiss I. (81°N) and Poker Flat (65°) in the Arctic region. The main feature common to all stations is the fall equinoctial maximum in amplitude (10–20 m s⁻¹), which is also reproduced by the most recent numerical tidal model. However, the wintertime amplitude growth with height and the shorter vertical wavelengths characterizing the model are features not reflected in the data. There is also a spring equinoctial maximum in the Antarctic data which the model does not reproduce.Examination of interannual variability reveals characteristics similar to those noted in Part I for the mean zonal wind; namely, some degree of year-to-year variability superimposed on apparent long-term decreases of order 0.3–0.5 m s⁻¹ yr⁻¹ (depending on month) in the Southern Hemisphere semidiurnal tidal amplitudes. Numerical simulations presented herein indicate that changes of this magnitude cannot even be induced (via mode coupling) by a change in the mean zonal wind field of order 30%, and are more plausibly explained by a secular change in the tidal forcing by ozone insolation absorption. However, contrary to Part I, the annual mean tidal amplitude is not characterized by any significant secular trend, remaining within the 10.0 ± 2.5 m s⁻¹ range throughout the 1970–1986 period. Analyses of other data sets are required to ascertain confidence in the apparent trend reported here.     

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.     

On significant quantities of negative ions observed around the mesopause

Significant amounts of negative ions are sometimes observed at altitudes as high as the mesopause region (Kopp E. and Hermann L., 1984, Annales Geophysical2, 83; Ganguly S., 1984, J. atmos. terr. Phys. 46, 633). Using a suitable ion-chemical scheme in which water clustering to negative ions is also considered, the effect of change in temperature, T, and concentrations of NO, O, O₃ and H₂O on the abundance of negative ions in the mesopause region has been examined. It is shown that a drastic increase in the concentration of O₃ or a decrease in neutral temperature by about 20 K. around 85km could make the concentration of total negative ion comparable to that of electron.     

Observations of lunar tides in upper atmosphere winds at Poker Flat,Alaska

The lunar semidiurnal tide is extracted from hourly values of winds in the 75–105 km region measured by the Poker Flat Alaska MST radar used in the meteor mode. Since year-to-year variations are apparent, detailed results for 1983 and 1984 are presented. Inferred vertical wavelengths range from 17 km in March 1983 to 46–55 km in September of 1983 and 1984. The height progression of the phase is frequently too irregular to derive a vertical wavelength. Amplitudes of 3 m s⁻¹ are common and range up to 8 m s⁻¹. Amplitudes generally are largest at the equinoxes, especially in September, with another maximum in winter sometimes occurring. Reasonable agreement is found with lunar tidal measurements at Saskatoon, and some points of similarity are found with the solar semidiurnal tide at Poker Flat.     

Radar measurement of the seasonal variation in the velocity of the sunrise terminator

The HF phased-array pencil beam radar at Bribie Island, Australia, used to measure horizontal movements of the ionosphere, has been calibrated using the known velocity of the sunrise terminator. The seasonal variation in the velocity of the terminator has been resolved, both in magnitude and direction.The technique uses single-station ionospheric sounding, and requires the angle of arrival and Doppler shift of ionospheric echoes to be measured as the terminator passes overhead. Pfister's theorem [(1971), J. atmos. terr. Phys. 33, 999] then allows calculation of the velocity of the reflecting surface. The difference between theory and experiment is less than 3% in speed and 2° in direction on average.     

Incoherent scatter observations of thin ionization layers at Sondrestrom

High resolution incoherent-scatter observations of E-region thin (1–3 km) metallic ion layers are presented. Data were collected during three different periods from August 1990 to August 1991, in three different experimental modes. First, the antenna was directed vertically and the entire duty cycle was devoted to Barker coded multi-pulse [Zamlutti (1980) J. atmos. terr. Phys.42, 975–982] measurements to determine the densities and temperatures in the E-region with 300 m resolution. The second experiment measured the F-region electric field as well as the high resolution E-region densities. For the third experiment the antenna was scanned magnetic north-south while only the E-region densities were measured. The experiments were carried out on 16 different nights for a period of 4 h each night at a time near magnetic midnight. Thin ionization layers were observed on 12 of the 16 nights. The first experiment demonstrated that the thin layers are composed of a significant fraction of heavy metallic ions; assuming the layers are composed of a mixture of Fe⁺ and Mg⁺ a composition estimate of 63% Fe⁺ was obtained in one example. The second experiment investigated the relationship between the direction of the electric field and the presence of the thin layers. In these observations thin layers were only present when the electric field was pointed in the magnetic north-west or south-west quadrants, most frequently when the field was near magnetic west. Correlation between layer altitude and field direction was also observed, layers occurring at higher altitudes for fields directed in the north-west, and lower altitudes for fields directed to the south-west. The observations are compatible with the electric field mechanism for thin ionization layer formation. The scanning experiment showed that the layers were of a limited latitudinal extent, typically about 100 km up to a maximum of about 200 km.     

On Pfister's theorem and its generalization

The relation between angle of arrival and Doppler shift that has been called Pfister's theorem is shown to be a special case of a result that applies to ionospherically transmitted signals between fixed ground sites, whenever the ionosphere moves without change of shape. This general result does not require the existence of an effective mirror reflector or any special symmetry of the ionosphere, but depends only on the validity of ray theory. The formula obtained by Pfister [(1971) J. atmos. terr. Phys.33, 999] is shown to apply, provided the ray returning from the ionosphere follows the same path as the incident ray.