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The structure, variability, and mean-flow interactions of the quasi-2-day wave (Q2DW) in the mesosphere and lower thermosphere during January 2015 were studied employing meteor and medium-frequency radar winds at eight sites from 23°S to 76°S and Microwave Limb Sounder (MLS) temperature and geopotential height measurements from 30°S to 80°S. The event had a duration of ~20–25 days, dominant periods of ~44–52 hr, temperature amplitudes as large as ~16 K, and zonal and meridional wind amplitudes as high as ~40 and 80 m/s, respectively, at middle and lower latitudes. MLS measurements enabled definition of balance winds that agreed well with radar wind amplitudes and phases at middle latitudes where amplitudes were large and quantification of the various Q2DW modes contributing to the full wave field. The Q2DW event was composed primarily of the westward zonal wavenumber 3 (W3) mode but also had measurable amplitudes in other westward modes W1, W2, and W4; eastward modes E1 and E2; and stationary mode S0. Of the secondary modes, W1, W2, and E2 had the larger amplitudes. Inferred MLS balance winds enabled estimates of the Eliassen-Palm fluxes for each mode, and cumulative zonal accelerations that were found to be in reasonable agreement with radar estimates from ~35°S to 70°S at the lower altitudes at which radar winds were available.

The statistics of gravity wave momentum flux estimation are investigated using data from the MU radar at Shigariki, Japan (136°E, 35°N). The radar has been operating during campaign periods since 1986. The first part of the paper focuses on a multi-day campaign during October 13–31, 1986. The second part of the paper investigates data after 2006 when the radar was operated in a meteor scatter mode. Momentum fluxes are derived from both the turbulent scatter and the meteor scatter measurements, but the techniques are quite different. Probability Distribution Functions are formed using turbulent scatter data. These show that wave packets sometimes have momentum flux magnitudes in excess of 100 m 2 s −2 . The technique for meteor radars, introduced by Hocking (2005), has been widely adopted by the radar community in recent years. The momentum flux estimated using this technique is found to be anti-correlated with the background tidal winds. A validation investigation is carried out for periods with a high meteor echo data rate. The conclusion was that the method can be used to calculate the sign of momentum flux, but does not accurately specify the magnitude.

Two nearly identical meteor radars were operated at Koto Tabang (0.20° S, 100.32° E), West Sumatra, and Biak (1.17° S, 136.10° E), West Papua, in Indonesia, separated by approximately 4000 km in longitude on the Equator. The zonal and meridional momentum flux, u′w′ and v′w′, where u, v, and w are the eastward, northward, and vertical wind velocity components, respectively, were estimated at 86 to 94 km altitudes using the meteor radar data by applying a method proposed by Hocking (2005). The observed u′w′ at the two sites agreed reasonably well at 86, 90, and 94 km during the observation periods when the data acquisition rate was sufficiently large enough. Variations in v′w′ were consistent between 86, 90, and 94 km altitudes at both sites. The climatological variation in the monthly averaged u′w′ and v′w′ was investigated using the long-term radar data at Koto Tabang from November 2002 to November 2013. The seasonal variations in u′w′ and v′w′ showed a repeatable semiannual and annual cycles, respectively. u′w′ showed eastward values in February–April and July–September and v′w′ was northward in June to August at 90–94 km, both of which were generally anti-phase with the mean zonal and meridional winds, having the same periodicity. Our results suggest the usefulness of the Hocking method.

A minor stratospheric sudden warming (SSW) event was noticed in the southern hemisphere (SH) during September (day 259) 2010 along with two episodic warmings in early August (day 212) and late October (day 300) 2010. Among the three warming events, the signature of mesosphere response was detected only for the September event in the mesospheric wind dataset from both meteor radar and MF radar located at King Sejong Station (62°S, 59°W) and Rothera (68°S, 68°W), Antarctica, respectively. The zonal winds in the mesosphere reversed approximately a week before the September SSW event, as has been observed in the 2002 major SSW. Signatures of mesospheric cooling (MC) in association with stratospheric warmings are found in temperatures measured by the Microwave Limb Sounder (MLS). Simulations of specified dynamics version of Whole Atmosphere Community Climate Model (SD-WACCM) are able to reproduce these observed features. The mesospheric wind field was found to differ significantly from that of normal years probably due to enhanced planetary wave (PW) activity before the SSW. From the wavelet analysis of wind data of both stations, we find that strong 14–16 day PWs prevailed prior to the SSW and disappeared suddenly after the SSW in the mesosphere. Our study provides evidence that minor SSWs in SH can result in significant effects on the mesospheric dynamics as in the northern hemisphere.

[1] A mesospheric horizontal wind data set measured during 1991–2006 by the medium frequency (MF) radar at Kauai, Hawaii (22°N, 160°W) is analyzed to examine the long-term variability of the quasi two-day wave (QTDW). The QTDW over Hawaii is amplified twice a year, with the January and July events most likely being the representation of zonal wave numbers 3 and 4 modes, respectively. The amplitudes of the January monthly mean QTDW in both meridional and zonal winds and the July monthly mean QTDW in meridional component are nearly in phase with the solar cycle but with the solar maxima leading the QTDW maxima by 1 or 2 years. However, the July monthly mean QTDW in zonal wind is more antiphase with the solar cycle. Enhanced QTDW oscillations are evident in both wind components in January 1998, which is likely related to the strong El Nino event during the winter of 1997/1998. The enhanced gravity wave activity and the increased barotropic/baroclinic/inertial instability related to the strengthened stratosphere summer easterly jet might provide additional forcing to amplify the QTDW. Moreover, the enhanced migrating diurnal tide during warm El Nino-Southern Oscillation events could also contribute to the abnormally strong QTDW by increasing the refractive index and thus the growth rate of the QTDW. Additional enhancement of the QTDW with a short period of ~43 h is observed during the major sudden stratospheric warming in January 2006.

For many years the amplitude of the diurnal tide in the mesosphere, lower thermosphere (MLT) has been known to be deeply modulated over a wide range of time scales. These time scales range from a few days, to seasonal dependencies, and up to interannual time scales. The causes of the variability have remained elusive, but can fall into two main categories. The first is source variability, the variability arising from variations in water vapor heating, latent heating, and ozone heating. The second potential cause is interaction of the diurnal tide with the background, including the effects of the zonal mean structure, planetary waves, gravity waves and turbulence. In this paper we focus on the second category of tidal variability. Measurements at the equator allow us to focus on particular mechanisms of tidal variability. We are close to the main sources driving the tide, equatorial convection and water vapor heating. The equatorial mean zonal winds are relatively weak so we also avoid the filtering of the tide due to the midlatitude jets. Although we cannot observe wave–mean and wave–wave interactions directly, a decadal climatology allows to diagnose how much tidal–background interaction might contribute to tidal variability. We make use of data from the SABER instrument, providing continuous temperature measurements and augment these with some equatorial radar wind observations. Two-month averages of SABER temperatures are required to unambiguously define migrating tidal temperature fields which confine our study mainly for variability on a seasonal time scale or longer. The temperature tide has a repeatable seasonal variation that is shown to stem directly from the sources and which can be observed as low as tropopause heights. Refraction and reflection are found to play an important role in the vertical structure of the tide and found to influence temporal variability in both amplitude and phase when the tide propagates into the mesosphere. An anti-symmetric migrating tidal mode was found to be present during some seasons of the year above 80 km.

The Spread F Experiment, or SpreadFEx, was performed from September to November 2005 to define the potential role of neutral atmosphere dynamics, primarily gravity waves propagating upward from the lower atmosphere, in seeding equatorial spread F (ESF) and plasma bubbles extending to higher altitudes. A description of the SpreadFEx campaign motivations, goals, instrumentation, and structure, and an overview of the results presented in this special issue, are provided by Fritts et al. (2008a). The various analyses of neutral atmosphere and ionosphere dynamics and structure described in this special issue provide enticing evidence of gravity waves arising from deep convection in plasma bubble seeding at the bottomside F layer. Our purpose here is to employ these results to estimate gravity wave characteristics at the bottomside F layer, and to assess their possible contributions to optimal seeding conditions for ESF and plasma instability growth rates. We also assess expected tidal influences on the environment in which plasma bubble seeding occurs, given their apparent large wind and temperature amplitudes at these altitudes. We conclude 1) that gravity waves can achieve large amplitudes at the bottomside F layer, 2) that tidal winds likely control the orientations of the gravity waves that attain the highest altitudes and have the greatest effects, 3) that the favored gravity wave orientations enhance most or all of the parameters influencing plasma instability growth rates, and 4) that gravity wave and tidal structures acting together have an even greater potential impact on plasma instability growth rates and plasma bubble seeding.

The coupling of the dynamical regimes in the high- and low-latitude stratosphere and mesosphere during the major SSW in the Arctic winter of 2003/2004 has been studied. The UKMO zonal wind data were used to explore the latitudinal coupling in the stratosphere, while the coupling in the mesosphere was investigated by neutral wind measurements from eleven radars situated at high, high-middle and tropical latitudes. It was found that the inverse relationship between the variability of the zonal mean flows at high- and low-latitude stratosphere related to the SSW is produced by global-scale zonally symmetric waves. Their origin and other main features have been investigated in detail. Similar latitudinal dynamical coupling has been found for the mesosphere as well. Indirect evidence for the presence of zonally symmetric waves in the mesosphere has been found.

Vertical coupling in the low-latitude atmosphere–ionosphere system driven by the 5-day Rossby W1 and 6-day Kelvin E1 waves in the low-latitude MLT region has been investigated. Three different types of data were analysed in order to detect and extract the ∼6-day wave signals. The National Centres for Environmental Prediction (NCEP) geopotential height and zonal wind data at two pressure levels, 30 and 10 hPa, were used to explore the features of the ∼6-day waves present in the stratosphere during the period from 1 July to 31 December 2004. The ∼6-day wave activity was identified in the neutral MLT winds by radar measurements located at four equatorial and three tropical stations. The ∼6-day variations in the ionospheric electric currents (registered by perturbations in the geomagnetic field) were detected in the data from 26 magnetometer stations situated at low latitudes. The analysis shows that the global ∼6-day Kelvin E1 and ∼6-day Rossby W1 waves observed in the low-latitude MLT region are most probably vertically propagating from the stratosphere. The global ∼6-day W1 and E1 waves seen in the ionospheric electric currents are caused by the simultaneous ∼6-day wave activity in the MLT region. The main forcing agent in the equatorial MLT region seems to be the waves themselves, whereas in the tropical MLT region the modulated tides are also of importance.

As part of an ongoing effort to understand the migrating diurnal tide generated by the NCAR Whole Atmosphere Community Climate Model, version 3 (WACCM3), we compare the WACCM3 migrating diurnal tide in the horizontal wind and temperature fields to similar results from the Global Scale Wave Model (GSWM). The WACCM3 diurnal tidal wind fields are also compared to tropical radar measurements at Kauai (22°N, 200.2°E) and Rarotonga (21.3°S, 199.7°E). The large-scale features of the WACCM3 results, such as the global spatial structure and the semiannual amplitude variation are in general agreement with past tidal studies; however, several differences do exist. WACCM3 exhibits a much higher degree of hemispheric asymmetry, lower overall amplitudes around the equinoxes, and peaks which are more confined in latitude when compared with the GSWM. Factors which may contribute to such differences between WACCM3 and GSWM are the solar heating profiles from ozone and water vapor, dissipation, and the zonal mean zonal winds. We find that the internally generated heating in WACCM3 and eddy dissipation values are both smaller than the values specified in the GSWM; the eddy dissipation fields and zonal mean zonal winds of the two models also display measurable differences in spatial structure. Comparisons with radar data show several differences in spatial and seasonal structure. In particular, the diurnal tide zonal winds in WACCM3 above Kauai are considerably larger in amplitude than those observed in the radar data, due to contributions from nonmigrating tidal components including wave numbers eastward 1 through 3, westward 2, and stationary components, which interfere constructively with the migrating component around equinox in WACCM3.

Images of mesospheric airglow and radar-wind measurements have been combined to estimate the difference in the vertical flux of horizontal momentum carried by high-frequency gravity waves over two dissimilar Antarctic stations. Rothera (67° S, 68° W) is situated in the mountains of the Peninsula near the edge of the wintertime polar vortex. In contrast, Halley (76° S, 27° W), some 1658 km to the southeast, is located on an ice sheet at the edge of the Antarctic Plateau and deep within the polar vortex during winter. The cross-correlation coefficients between the vertical and horizontal wind perturbations were calculated from sodium (Na) airglow imager data collected during the austral winter seasons of 2002 and 2003 at Rothera for comparison with the 2000 and 2001 results from Halley reported previously (Espy et al., 2004). These cross-correlation coefficients were combined with wind-velocity variances from coincident radar measurements to estimate the daily averaged upper-limit of the vertical flux of horizontal momentum due to gravity waves near the peak emission altitude of the Na nightglow layer, 90km. The resulting momentum flux at both stations displayed a large day-to-day variability and showed a marked seasonal rotation from the northwest to the southwest throughout the winter. However, the magnitude of the flux at Rothera was about 4 times larger than that at Halley, suggesting that the differences in the gravity-wave source functions and filtering by the underlying winds at the two stations create significant regional differences in wave forcing on the scale of the station separation.

The 5-day wave is the gravest symmetric Hough mode of westward propagating zonal wavenumber 1. This wave is observed using the SABER instrument aboard the TIMED satellite during the first three years of the spacecraft mission (2002–2004). Supporting measurements were made with mesospheric radar systems. To better interpret the observations, the NCAR thermosphere–ionosphere–mesosphere–electrodynamics general circulation model (TIME-GCM) simulation of year 2003 is used for comparative analysis. For the simulation the lower boundary was specified using NCEP data. The climatology from SABER shows a May maximum in the amplitude of the 5-day wave, which is consistent with the seasonal dependence found in earlier studies. A particularly strong wave with a ∼ 6 day period was observed in May 2003 and is studied in some detail. There is considerable evidence from both data and model in our study that a major source for this wave was in the southern (winter) hemisphere. Cross-equatorial ducting allowed the wave to propagate into the northern (summer) hemisphere, where it was amplified by baroclinic instability.

At Rothera Research Station (67.3°S, 68.1°W), Antarctica, 296 h of day and nighttime Fe-Boltzmann temperature lidar data were accumulated in 2003. During this time, sporadic iron layers (Fe S ) were observed with an annual average occurrence probability of 14%. The peak altitude of the Fe S layers was highest during the summer, with a fitted value of 103 km while during the winter the layer decreased in height to 90 km with an annual average of 97 km. The atmospheric temperature perturbations and potential energy density profiles computed from the same lidar data exhibited increased temperature but constant-with-height potential energy density when sporadic Fe layer occurred. Once sporadic Fe layers disappeared, the potential energy density decreased with height, indicating an energy loss due to atmospheric gravity wave breaking. These results suggest a link between Fe S and atmospheric dynamics.

There have been reports in the literature that spaced-antenna MF radars may provide a source of data on Polar Mesospheric Summer Echoes (PMSE). Even though the expected scatter from PMSE at MF frequencies is very much weaker than at VHF, the wide distribution of sites and long duration of data sets for MF radar systems could provide valuable information about the occurrence of PMSE. This paper tests whether there is any evidence of PMSE in the profiles derived using the MF radar at Rothera, Antarctica, one of the few such radars at high southern latitudes. Over a year of data during 1997/1998 has been analysed for the occurrence of persistent features around midday in the altitude range 60-95km. Criteria were chosen to test the likelihood that some of the narrow peaks in the power profiles were manifestations of electron density structures associated with PMSE. Although a small number of persistent features were seen at altitudes of 80-85km that are typically associated with PMSE, there was no seasonality in their occurrence. A detailed analysis of specific days showed that two peaks were often seen with altitude separations consistent with the vertical wavelength of the diurnal tide. Persistent features were also detected at altitudes of 70km and 90km during the winter months, thus showing a quite different seasonality to that of PMSE. An estimate of the turbulence caused by the breaking of gravity waves that have propagated up from the lower atmosphere shows that at Rothera significant energy is deposited near 80km during summer, and near 70 and 90km during winter. This seasonal variability is driven by the screening effect of stratospheric winds, and it appears that breaking gravity wave dynamics, rather than PMSE phenomena, can explain many of the localised altitude features in the MF radar data.

Using simple numerical filters, wind variances with time scales of 0.1 – 1 h and 1 – 5 h are estimated from medium frequency radar wind observations at altitudes 70 – 94 km over Hawaii (22°N, 160°W) during the years 1990–2000. The results show interannual variations of the mean wind and variances. The variances can be attributed to the presence of atmospheric gravity waves (GWs). At altitudes below 80 km a maximum of GW intensity was observed between the years 1992 and 1996. An average seasonal cycle in wind variances is calculated by compositing over the 11 years of data. The monthly residual values, obtained after subtraction of the average seasonal variations show quasi-biennial (∼27 month) and longer period oscillations. Maximum variances are observed in 1993 and 1998–1999 at altitudes above ∼84 km . A correlation is found between GW wind variances and the Southern Oscillation Index, which in turn reflects El Nino activity in the tropical Pacific. However, more analysis with several El Nino events is required to establish the correlation with greater confidence.

Tropospheric and stratospheric radar wind data obtained at Jicamarca, Peru (11.95°S, 76.87°W) during August 17–26, 1998 have been used to estimate the momentum flux of short-period (T h ) wind fluctuations together with the estimation uncertainties. As a result of our study we conclude that momentum flux estimates exceeding the measurement uncertainties can be obtained after about one day of integration.

Data obtained with multiple instruments at the main site of the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99) are employed to examine the character and variability of wave motions occurring in the stable nocturnal boundary layer during the night of 14 October 1999. The predominant motions are surprisingly similar in character throughout the night, exhibiting largely westward propagation, horizontal wavelengths of ∼1 to 10 km, phase speeds slightly greater than the mean wind in the direction of propagation, and highly coherent vertical motions with no apparent phase progression with altitude. Additionally, vertical and horizontal velocities are in approximate quadrature and the largest amplitudes occur at elevated altitudes of maximum stratification. These motions are interpreted as ducted gravity waves that propagate along maxima of stratification and mean wind and that are evanescent above, and occasionally below, the altitudes at which they are ducted. Modal structures for ducte...

The semidiurnal tide (SDT) is investigated through comparative analysis of horizontal winds measured at Poker Flat (65°N, 147°W), Andenes (69°N, 16°E), Davis (69°S, 78°E), and Rothera (68°S, 69°W). At the northern hemisphere sites the SDT maximizes around the autumn equinox. Poker Flat and Andenes results from 1999–2001 are used to demonstrate that there is a clear repeatable enhancement in SDT amplitudes around the autumn equinox, and that the maximum is localized in height around 86 km . In the southern hemisphere seasonal dependence of the SDT during 1997–1998 is more complicated, and the autumn enhancement is less pronounced. Many competing mechanisms might contribute to the observed seasonal dependence of the SDT, but this study focuses on the refractive effects of shears in the mean zonal wind and gradients in temperature. The main evidence for a refractive influence is that the seasonal enhancement in the SDT amplitude is accompanied by a dramatic shortening in the wave's vertical scale. This shortening of the vertical scale is consistent with refraction of the SDT energy into the horizontal wind component. Simplified linear tidal theory equations are used to estimate the expected magnitude of the refractive effects using wind and temperature fields observed over Andenes, Norway. The predicted refractive effects are shown to be potentially significant and qualitatively consistent with the observations. In addition to a seasonal dependence, the SDT amplitudes obtained at all the radar sites exhibit a deep amplitude modulation on a time scale characteristic of planetary waves. This sort of modulation has most often been attributed to nonlinear interactions between the tides and planetary waves. We suggest that refraction might instead produce, or at least contribute to, the observed modulation. Although the planetary waves are of weak ( m s −1 ) amplitude, the SDT (particularly the gravest S(2,2) mode) is only marginally propagating at high latitudes. Thus, small perturbations to the background are enough to periodically inhibit propagation of the SDT to higher levels.

Observations of mean winds and semidiurnal and diurnal tides in the mesosphere/lower-thermosphere (MLT) region were made during the 3-month Planetary-Scale Mesopause Observing System Summer 1999 campaign. Data from 22 ground-based radars (and from two other instruments with measurements for the same period but in 1998) allow us to investigate the ability of the GSWM-00 to simulate the solar tides in the mesopause region (90-95 km). Here we have found that the GSWM-00 provides an increasingly reasonable estimate of most of the tidal characteristics in the MLT region. However, the representation of the 24 h tide appears superior to that of the 12 h tide. Some of these discrepancies are studied in detail. In particular, the observations reveal significant 12 h tidal amplitudes at high latitudes in the Northern Hemisphere summer. There is evidence for relation between the longitudinal variability of the mean zonal wind and the tidal characteristics seen from the radar wind measurements in the summer middle latitudes and a quasi-stationary planetary wave with zonal wave number one.

It has become increasingly clear that Gravity Waves (GW) have an essential and often dominant role in the dynamics of the Middle Atmosphere. This leads to them having strong impacts upon the thermal structure and the distribution of atmospheric constituents. However, the radar observations of GW have been limited in their latitudinal extent during the past decade, and although satellite observations are now significantly contributing, global-seasonal climatologies of important characteristics are still inadequate. With regard to models, the inclusion of GW-drag effects has been problematic. Usually no seasonal or latitudinal variation in the subgrid-scale GW-drag parameterization scheme is included, and varieties of parameterization schemes have been used. Although these often make conflicting assumptions, they generally produce similarly acceptable end-products, e.g. zonal-mean zonal wind fields. In this paper, we report upon the beginnings of a substantial program, using observations from a network of MF radars (North America, Pacific and Europe), and data from the Canadian Middle Atmosphere Model (CMAM). This model allows the tidal and planetary wave fields to be assessed, characteristics and climatologies of which are well known from the MF Radars. Here we focus upon the tides. There are useful similarities in the observed and modeled background wind and wave fields, and strong indications that the two non-orographic GW-drag parameterization schemes (Hines; Medvedev–Klaassen) have significant and differing effects upon the dynamics of the modeled atmosphere. It is shown that this comparison process is valuable in the evaluation, and potentially the optimization, of parameterization schemes.

Continuous observations of the wind field have been made by six Medium Frequency Radars (MFRs), located between the equator and high northern latitudes: Christmas Islands (2°N), Hawaii (22°N), Urbana (40°N), London (43°N), Saskatoon (52°N) and Tromso (70°N). Data have been sought for the time interval 1990–1997, and typically 5 years of data have become available from each station, to demonstrate the level of annual consistency and variability. Common harmonic analysis is applied so that the monthly amplitudes and phases of the semi-diurnal (SD) and diurnal (D) wind oscillations are available in the height range of (typically) 75–95 km in the upper Middle Atmosphere. Comparisons are made with tides from the Global Scale Wave Model (GSWM), which are available for 3-month seasons. The emphasis is upon the monthly climatologies at each location based upon comparisons of profiles, and also latitudinal plots of amplitudes and phases at particular heights. For the diurnal tide, the agreement between observations and model is now quite excellent with modelled values frequently lying within the range of yearly values. Both observations and model demonstrate strong seasonal changes. This result is a striking improvement over the comparisons of 1989 (JATP, Special issue). In particular, the phases and phase-gradients for the non-winter months at mid- to high-latitudes are now in excellent agreement. Some of the low latitude discrepancies are attributed to the existence of non-migrating tidal components associated with tropospheric latent heat release. For the semi-diurnal tide, the observed strong transitions between clear solstitial states are less well captured by the model. There is little evidence for improvement over the promising comparisons of 1989. In particular, the late-summer/autumnal tidal maximum of mid-latitudes is observed to be larger, and with strong monthly variability. Also the summer modelled tide has unobserved short (20 km) wavelengths at high latitudes, and much smaller amplitudes than observed at all extratropical locations. Possible improvements for the GSWM’s simulations of the SD tide are discussed, which involve migrating tidal modes due to tropospheric latent heating.

Observations of winds and gravity waves (GW) by MF radars from the Arctic to the Equator are used to provide frequency spectra and spectral variances of horizontal motions, and information on the predominant azimuthal directions of propagation for the waves. The years used are mainly 1993/4; the height layer 76–88 km; and the GW bands 10 100 min. and 1–6 hrs. The high/mid-latitude locations of Tromso, Saskatoon, London/Urbana, Yamagawa, generally demonstrate similar behaviour: the monthly spectra have slopes near −5/3 in winter months, but smaller (absolute) slopes at higher frequencies (

The behavior of Kelvin waves in the mesosphere and lower thermosphere is examined in three 10-day sequences of high resolution Doppler imager mesospheric and lower thermospheric zonal winds at solstice. Eastward propagating signatures are observed in wavenumbers 1, 2, and 3 of the zonal wind field with periods near 3 and 5 days. These structures are coherent in latitude and altitude, and maximize near or on the equator. In the mesosphere, lines of constant phase move downward in time, implying that these waves are forced from below. Above 90 km the phase lines of Kelvin zonal wavenumbers 2 and 3 traverse upward in time, suggesting in situ or higher-level sources. Estimates of the Eliassen-Palm fluxes indicate that wavenumber 1 may significantly contribute to the eastward momentum budget of the equatorial lower thermosphere.

Radar observations of winds and momentum fluxes in the stratosphere and mesosphere at the Jicamarca Radio Observatory in Peru ( JRO; 12°S, 77°W) were taken during three 10 day campaigns in January 1993, March 1994, and August 1994. In order to interpret features in the campaign mean JRO wind profiles, we examined global circulation patterns as depicted by long time series of radiosonde profiles, analyses from the European Center for Medium Range Weather Forecasting (ECMWF), and winds from the High Resolution Doppler Imager (HRDI) aboard the Upper Atmosphere Research Satellite. In the tropical stratosphere, large-scale analyses show that a geographically varying annual cycle significantly affects winds over JRO, as does the quasi-biennial oscillation ( QBO). The spatial structure of the annual cycle and QBO is shown for the three campaigns, emphasizing the upward influence of subtropical tropospheric monsoon anticyclones. These anticyclones tilt poleward and merge zonally, underlying the zonal summer easterlies, which also merge zonally and tilt poleward with altitude. The annual cycle at Singapore includes a substantial easterly acceleration during March-August, which causes an apparent stalling of descending QBO westerlies or a more rapid descent of QBO easterlies. In the mesosphere, JRO and HRDI winds agree reasonably well, with zonal winds over JRO varying on a semiannual basis and meridional winds exhibiting structures expected from the diurnal tide. For vertical motion, separate north-south and east-west beam pair estimates agree, yet campaign-averaged vertical motions are large: ∼1–5 cm/s in the stratosphere and ∼ 10–50 cm/s in the mesosphere. In both the stratosphere and mesosphere, vertical winds are anticorrelated with horizontal wind. Possible explanations for the large vertical motions include aspect sensitivity and the diurnal tide. Uncertainties in the meaning of radar vertical motions create a challenge for interpreting momentum fluxes.

[1] Wind measurements in the mesosphere and lower thermosphere obtained with a medium frequency (MF) radar in Hawaii (22°N, 160°W) spanning ∼16 years are employed to examine the intra-annual and interannual variability of the mean and tidal motions at altitudes between 84 and 94 km. Intra-annual periodicities range from ∼3 to 12 months, with significant coherence in altitude and between the zonal and meridional components of each motion field. Interannual variations confirm the dominant periodicities identified previously at this site and elsewhere, in particular, the significant diurnal, and less significant semidiurnal, tidal responses at periods of ∼28 and ∼48 months. Amplitudes of these long-period oscillations of the diurnal tide increase with altitude below 92 km and are larger than the amplitudes of the 12 month oscillations above ∼90 km. Phases of the ∼28 and ∼48 month oscillations show a downward progression with a slightly larger altitude variation in the meridional diurnal tide for the ∼28 month oscillation and a significantly larger altitude variation in the zonal diurnal tide for the ∼48 month oscillation. The long and nearly continuous Hawaii data set also enables characterization of the responses of the wind fields to the 11 year solar cycle. Both wind and tidal fields exhibit this periodicity, but these responses display interesting and different relations to the phase of the solar cycle. The 11 year oscillation of the meridional wind is nearly in phase with the solar cycle, while the 11 year oscillation of the zonal wind is in approximate quadrature with the solar cycle. The 11 year oscillations of the semidiurnal tidal amplitudes and the meridional diurnal amplitude are all in approximate quadrature with the solar cycle (with the tidal amplitudes leading by ∼29 to 37 months), while the 11 year oscillation of the zonal diurnal amplitude is somewhat nearer an antiphase than a quadrature relation to the solar cycle (leading by ∼54 months).

The coupling processes in the middle atmosphere have been a subject of intense research activity because of their effects on atmospheric circulation, structure, variability, and the distribution of chemical constituents. In this study, the day-to-day variability of Aura-MLS (Microwave Limb Sounder) temperature data are used to reveal the vertical and interhemispheric coupling processes in the stratosphere-mesosphere during four Northern Hemisphere winters (2004/2005–2007/2008). The UKMO (United Kingdom Meteorological Office) assimilated data and mesospheric winds from MF (medium frequency) radars are also applied to help highlight the coupling processes. In this study, a clear vertical link can be seen between the stratosphere and mesosphere during winter months. The coolings and reversals of northward meridional winds in the polar winter mesosphere are often observed in relation to warming events (Sudden Stratospheric Warming, SSW for short) and the associated changes in zonal winds in the polar winter stratosphere. An upper-mesospheric cooling usually precedes the beginning of the warming in the stratosphere by 1–2 days. Inter-hemispheric coupling has been identified initially by a correlation analysis using the year-to-year monthly zonal mean temperature. Then the correlation analyses are performed based upon the daily zonal mean temperature. From the original time sequences, significant positive (negative) correlations are generally found between zonal mean temperatures at the Antarctic summer mesopause and in the Arctic winter stratosphere (mesosphere) during northern mid-winters, although these correlations are dominated by the low frequency variability (i.e. the seasonal trend). Using the short-term oscillations (less than 15 days), the statistical result, by looking for the largest magnitude of correlation within a range of time-lags (0 to 10 days; positive lags mean that the Antarctic summer mesopause is lagging), indicates that the temporal variability of zonal mean temperature at the Antarctic summer mesopause is also positively (negatively) correlated with the polar winter stratosphere (mesosphere) during three (2004/2005, 2005/2006, and 2007/2008) out of the four winters. The highest value of the correlation coefficient is over 0.7 in the winter-stratosphere for the three winters. The remaining winter (2006/2007) has more complex correlations structures; correspondingly the polar vortex was distinguished this winter. The time-lags obtained for 2004/2005 and 2006/2007 are distinct from 2005/2006 and 2007/2008 where a 6-day lag dominates for the coupling between the winter stratosphere and the summer mesopause. The correlations are also provided using temperatures in northern longitudinal sectors in a comparison with the Antarctic-mesopause zonal mean temperature. For northern mid-high latitudes (~50–70° N), temperatures in Scandinavia-Eastern Europe and in the Pacific-Western Canada longitudinal sectors often have opposite signs of correlations with zonal mean temperatures near the Antarctic summer mesopause during northern mid-winters. The statistical results are shown to be associated with the Northern Hemisphere's polar vortex characteristics.

[1] A function that approximates atmospheric tidal behavior in the polar regions is described. This function is fitted to multistation radar measurements of wind in the mesosphere and lower thermosphere with the aim of obtaining a latitude-longitude-height description of the variation of tides over the whole Antarctic continent. Archival wind data sets are combined with present-day ones to fill the spatial distribution of the observations and to reduce the potential effects of spatial aliasing. Multiple years are combined through the compilation of monthly station composite days, yielding results for each month of the year. Despite potential problems associated with year-to-year variations in the tidal phase, a useful climatology of Antarctic zonal and meridional tidal wind components is compiled. The results of the fits reproduce the major features of the high-latitude tidal wind field: the dominance of the semidiurnal migrating mode in the winter months and the presence of a semidiurnal zonal wave number one component in the summer months. It is also found that the summer semidiurnal tide contains a zonal wave number zero component.

[1] We report here and in a companion paper by Fritts et al. (2006a) on a new use of the UHF radar at the Arecibo Observatory in Puerto Rico. We have employed the 430 MHz radar for incoherent scatter measurements of radial wind spectra at altitudes from ∼71 to 95 km using the Gregorian and line-feed antennas to define beam angles inclined 15° to the east and west of zenith. We find that the two beams define radial velocities with sufficient accuracy to characterize both the gravity waves and the momentum fluxes due to these waves over the majority of the observed altitude range during daylight hours. The characteristics of the gravity waves inferred from these measurements include (1) vertical scales ranging from ∼2 to 20 km, (2) downward phase progression of the dominant gravity waves up to ∼5 ms−1, and (3) vertical wave number spectra having slopes near a value (−3) expected for saturated gravity waves. Gravity wave frequency spectra and momentum fluxes are addressed in the companion paper.

[1] Janches et al. (2006) described a new dual-beam use of the 430 MHz incoherent scatter radar at the Arecibo Observatory in Puerto Rico. We found the technique to define the radial wind field in the mesosphere and lower thermosphere with sufficient accuracy to characterize gravity waves occurring at high frequencies and small spatial scales over an extended altitude range. The coplanar, dual-beam experiment was also designed to test the ability of the system to measure gravity wave momentum fluxes and their frequency distributions, and we report here on those results. Initial measurements were of limited duration and necessarily represent a case study, but they demonstrate the value of such measurements for studies of GW variability and large-scale interactions. Radial velocity variances reveal preferential eastward propagation for most intervals and altitudes, with the greatest propagation bias at lower altitudes and later times on 11 September when strong westward mean winds favor strong gravity filtering. The momentum fluxes observed during this experiment had � 50-min averages that were often near zero, occasionally achieved amplitudes of � 20 to 50 m 2 s � 2 , displayed significant consistency in altitude, and exhibited an approximate anticorrelation with the zonal wind field in cases with significant momentum fluxes. Frequency spectra defined the major contributions to the momentum fluxes, while S transforms were employed to examine the temporal variability of the GWs and momentum fluxes in greater detail.

The purpose of this study is to seek observational evidence for diurnal, vertically propagating inertia-gravity waves (IGWs) in the mesosphere and lower thermosphere (MLT). Numerical modeling studies indicate that vertically propagating IGWs excited by tropical heating provide a significant source of momentum for the semiannual oscillation (SAO) in the equatorial zonal mean winds. The power spectrum of these waves has a strong diurnal component. We analyze global patterns of ascending and descending node differences in MLT satellite temperatures, which are assumed in this study to be proxies for waves of diurnal period. Juxtaposed upon the planetary-scale features are localized variations with longitudinal wavelengths ranging from 25 deg to 50 deg, and with vertical wavelengths between 13 and 25 km. These intermediate-scale variations are spatially coherent, and persist for several weeks. Their amplitudes generally increase with altitude, while their phase structures suggest both eastward and westward propagation. The variance of wave numbers 9-17 in the MLT is examined in relation to the underlying stratospheric zonal mean zonal gradient winds. Stronger variances generally coincide with periods where the underlying zonal mean winds are relatively light, or unidirectional. The weak inverse association between variance strength and zonal wind magnitude is suggestive of a wave filtering mechanism.

[1] Vertical coupling in the low-latitude atmosphere-ionosphere system driven by the 2-day wave in the tropical MLT region has been investigated. The problem is studied from an observational point of view. Three different types of data were analyzed in order to detect and extract the 2-day wave signals. The 2-day wave event during the period from 1 December 2002 to 28 February 2003 was identified in the neutral winds by radar measurements located at four tropical stations. The 2-day variations in the ionospheric electric currents (registered by perturbations in the geomagnetic field) and in the F-region electron densities were detected in the data from 23 magnetometer and seven ionosonde stations situated at low latitudes. Two features for each kind of wave were investigated in detail: the variation with time of the wave amplitude and the zonal wave number. The results show that the westward propagating global 2-day wave with zonal wave number 2 seen in the ionospheric currents and in F-region plasma is forced by the simultaneous 2-day wave activity in the MLT region. The main forcing agent in this atmosphere-ionosphere coupling seems to be the modulated tides, particularly the semidiurnal tide. This tide has a larger vertical wavelength than the diurnal tide and propagates well into the thermosphere. The parameter that appears to be affected, and thus drives the observed 2-day wave response of the ionosphere, is the dynamo electric field.

[1] Simultaneous iron resonance lidar density profiles, OH intensity images and MF-radar wind measurements have been used to determine the horizontal and vertical components of high-frequency (

International audience; A medium frequency spaced-antenna radar has been operating at Rothera station, Antarctica (67° S, 68° W) for two periods, between 1997-1998 and since 2002, measuring winds in the mesosphere and lower thermosphere. In this paper monthly mean winds are derived and presented along with three years of radiosonde balloon data for comparison with the HWM-93 model atmosphere and other high latitude southern hemisphere sites. The observed meridional winds are slightly more northwards than those predicted by the model above 80 km in the winter months and below 80 km in summer. In addition, the altitude of the summer time zero crossing of the zonal winds above the westward jet is overestimated by the model by up to 8 km. These data are then merged with the wind climatology obtained from falling sphere measurements made during the PORTA campaign at Rothera in early 1998 and the HWM-93 model atmosphere to generate a complete zonal wind climatology between 0 and 100 km as a benchmark for future studies at Rothera. A westwards (eastwards) maximum of 44 ms-1 at 67 km altitude occurs in mid December (62 ms-1 at 37 km in mid July). The 0 ms-1 wind contour reaches a maximum altitude of 90 km in mid November and a minimum altitude of 18 km in January extending into mid March at 75 km and early October at 76 km.

[1] Wind measurements from a series of atmospheric radars located around Antarctica have been used to characterize the mesospheric planetary-wave field during the winter of 2002. Combining winds from the medium-frequency (MF) radar at Rothera (68°S, 68°W) and the SuperDARN high-frequency meteor-wind radars at Halley (76°S, 27°W), Sanae (72°S, 3°W) and Syowa (69°S, 40°E) stations, we have been able to measure the period, wavenumber and propagation direction of the most prominent planetary waves. The results show that the planetary-wave field before the unusual stratospheric warming in 2002 was dominated by a very long-period (τ ≈ 43 days), westward and upward-propagating zonal planetary wavenumber 1. However, after the stratospheric warming events began in late winter, the character of the wave field changed and a shorter period (τ ≈ 14 days), westward, zonal wavenumber 1 became established. It would appear that the previously reported oscillations of the mesospheric hydroxyl airglow temperatures at Rothera and Halley, which were strongly anti-correlated to the meridional wind, were the result of these planetary waves.

[1] The zonal wave number 1 planetary wave of period near 6.5 days is a robust feature in the mesosphere and lower thermosphere (MLT) region with prominent seasonal variability as revealed by ground based and satellite observations. This wave and its seasonal variability are well reproduced in a recent one model year run of the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) with its lower boundary specified according to the National Centers for Environmental Prediction analysis (year 1993). Wavelet analysis of the model output shows that in the MLT region the wave maximizes before and after the equinoxes and minimizes at solstices. The wave amplitudes at the equinoxes are smaller than the peaks before and after but are still larger than the wave amplitudes at solstices. However, at the lower boundary near 30 km the wave peaks are predominantly between fall and the following spring. By examining the episodes of maximum and minimum wave amplitude and by conducting additional control experiments using the TIME-GCM, the structure of this planetary wave and the factors determining the wave characteristics and seasonal variability are studied in detail. It is found that the wave source, mean wind structure, instability, and the critical layers of the wave can all affect the wave response in the MLT region and can have a strong seasonal dependence. Before and after equinox, the wave follows the waveguide and propagates from the stratosphere to the summer mesosphere/mesopause, where it may amplify due to baroclinic/barotropic instability. Such instability is usually absent from the equinoctial atmosphere, so that there is no wave amplification at equinox. At solstice the wave decays significantly when propagating away from its winter source due to the strong eastward winter stratospheric jet. In the summer side the westward jet is also strong, and the meridional and vertical extension of the critical layer of the wave is large enough to enclose the instability in the summer mesosphere/mesopause at middle to high latitudes. The wave is thus reflected away and prevented from reaching and amplifying at the unstable region. The seasonal variation of the quasi-two-day wave, which has zonal phase speed similar to the 6.5-day wave, is also studied using similar diagnostics. It is further shown that within certain seasonal “window” periods, the variability in the MLT, especially the summer MLT, may closely track the lower atmospheric variability associated with these waves.

[1] The 2-day wave during the boreal summer of 1994 was observed using stratospheric analyses from the British Met Office and at mesospheric heights using medium-frequency (MF) radars and the microwave limb sounder (MLS) and high-resolution Doppler imager (HRDI) satellite instruments. Most of the evidence from our study points to a high latitude source for the boreal 2-day wave event we observed. We found little evidence for a connection between the 2-day wave event in the mesosphere and activity at lower altitudes. Instead we contend that the 2-day wave observed at upper mesospheric heights was excited in-situ. This event was predominantly zonal wave number 2, was highly localized in time, and the disturbance propagated equatorially from high northern latitudes. At stratopause and lower mesospheric heights the largest 2-day wave amplitudes were seen at high Southern Hemisphere latitudes (i.e., in the winter hemisphere). However, the austral winter 2-day wave was trapped and did not penetrate to upper mesospheric heights.

An unprecedented major stratospheric warming occurred in the Antarctic winter of 2002. We present measurements of winds in the mesosphere-lower thermosphere (MLT) made with MF radars located at Davis (69degreesS, 78degreesE), Syowa (69degreesS, 40degreesE) and Rothera (68degreesS, 68degreesW). The mesospheric wind field in 2002 was found to be considerably different to other years due to increased planetary wave activity throughout the winter. Zonal winds were weaker than usual during the 2002 winter and also during the transition to the summer circulation. The MLT zonal winds showed a reversal about one week earlier than the stratospheric reversal associated with the warming. Meridional winds showed oscillations consistent with the presence of traveling wave-1 planetary waves with periods similar to14 days. The results are compared with similar mesospheric observations made during northern hemisphere stratospheric warmings. Some similarities between hemispheres were found, notably that the reversal in the mesospheric winds precedes the warming events.

[1] Mesospheric temperatures derived from spectroscopic measurements of the hydroxyl (OH) nightglow have been observed from Rothera (67.6°S, 68.1°W) and Halley (75.6°S, 26.6°W) stations in Antarctica during the 2002 austral winter season. In addition to the normal seasonal changes, the temperatures at both sites, separated by 1658 km, showed several simultaneous shifts in temperature of between 10 and 20 K. These changes abruptly occurred in the space of 2–3 days and lasted for several days. These rapid variations in temperature were associated with large swings observed in the meridional component of the mesospheric wind measured by the Rothera MF radar. As there appeared to be no phase shift between the temperature variations at the two longitudinally separated stations indicating that planetary-waves caused the changes, these large-scale changes have been interpreted as variations in the inter-hemispheric meridional jet, and a corresponding modulation of the mesospheric descent and adiabatic heating rates over the polar region.

[1] A westward propagating zonal wave number 1 wave with a period near 6.5 days was a prominent feature in the mesosphere and lower thermosphere (MLT) during the 1994 equinoxes. The meridional structure of the wave in the upper stratosphere and the MLT is consistent with the 5-day wave structure predicted by normal mode theory. However, the amplitude increases sharply above 80 km, where the wave exhibits a highly organized baroclinic circulation. The eddy fluxes and the background state suggest that the wave is amplified by instability of the mesospheric winds.

[1] Using simple numerical filters, estimates of wind variances with period bands of 0.1 to 1 hour and 1 to 5 hours have been derived from data taken with the medium-frequency radar on the island of Kauai, Hawaii (22°N, 160°W). The observations cover altitudes of 70 to 90 km and extend over the years 1990–2000. The results show seasonal and interannual variations of the wind variances, which can be attributed to atmospheric gravity waves. The mean zonal wind has mainly an annual variation below an altitude of ∼83 km and a semiannual variation above. The gravity waves have maximum intensity at the solstices below ∼83 km, but at higher altitudes the times of the maxima shift to the equinoxes. Numerical simulations suggest this behavior results from the dependence of gravity-wave generation and propagation on the background wind and temperature.

[1] The mesosphere-stratosphere-troposphere (MST) radar at Jicamarca, Peru (12°S, 77°W), made extended (15 day or longer) observations of the horizontal and vertical winds that were used to infer the diurnal and semidiurnal tides. The measurements were made during several months from mid-1997 through mid-1998 and using a higher-power transmitter and finer range resolution during 10 days of August 1998. The three-component winds are used to estimate amplitudes, phases, and momentum fluxes associated with the tides. Thermal forcing of the diurnal tide is also examined with diurnal water vapor heating rates calculated using data from the NASA Water Vapor Project (NVAP). For the region near Jicamarca the calculations from NVAP showed the temporal variability of the diurnal heating to be dominated by an annual cycle with maximum around the summer solstice. When projected into tidal modes, about 25% of the total water vapor heating rate amplitude near Jicamarca is found to be nonmigrating. The meridional amplitude of the semidiurnal tide was found to be generally greater than the zonal amplitude, although tidal theory predicts that the zonal amplitude should be much greater at the latitude of Jicamarca (assuming the tide was migrating). The phase of the semidiurnal tide lagged (by about 3 hours) the phase expected from surface pressure climatologies. According to tidal theory the migrating semidiurnal tide should transport little meridional momentum flux. However, substantial southward fluxes (〈v′w′〉 ∼ −1 × 10−3 m2 s−2) were observed at Jicamarca, and the meridional component of momentum flux was typically larger in magnitude than the zonal component was. The diurnal tide was somewhat weaker, was less coherent, and transported less momentum. The semidiurnal tide had a very long vertical wavelength throughout the troposphere and into the lower stratosphere, while the diurnal tide was only observed to propagate at heights above the tropopause with a much shorter (∼10 km) vertical wavelength. Below the tropopause the dominant diurnal motions were not traveling waves, but rather convective motions that exhibited little phase progression with altitude. These motions were broadly peaked in frequency around 24 hours and were presumably standing oscillations with no horizontal propagation and probably with small horizontal scale. Despite the lack of coherence of these quasi-diurnal motions, the associated vertical wind amplitudes were sizable (∼0.02 m s−1), and thus the fluctuations can presumably transport significant horizontal momentum.

A description is given of E region auroral plasma irregularities associated with an intense auroral morning arc observed over Fort Churchill by radar. The observations are compared with data from an all-sky camera (ASC) operated at Fort Churchill by the National Research Council of Canada. The particular event described was chosen because of the rapid variation in structure and motion of the arc as it traveled through the radar beam. The horizontal vector electron drift velocity and electric field along the poleward boundary of the morning discrete auroral arc was successfully measured with a radar interferometer. This instrument provided information concerning the temporal and spatial structure of the electrostatic plasma turbulence in the arc. The observations are described.

We provide here an overview of, and a summary of results arising from, an extensive experimental campaign (the Spread F Experiment, or SpreadFEx) performed from September to November 2005, with primary measurements in Brazil. The motivation was to define the potential role of neutral atmosphere dynamics, specifically gravity wave motions propagating upward from the lower atmosphere, in seeding Rayleigh-Taylor instability (RTI) and plasma bubbles extending to higher altitudes. Campaign measurements focused on the Brazilian sector and included ground-based optical, radar, digisonde, and GPS measurements at a number of fixed and temporary sites. Related data on convection and plasma bubble structures were also collected by GOES 12, and the GUVI instrument aboard the TIMED satellite. Initial results of our SpreadFEx analyses are described separately by Fritts et al. (2009). Further analyses of these data provide additional evidence of 1) gravity wave (GW) activity near the mesopause apparently linked to deep convection predominantly to the west of our measurement sites, 2) small-scale GWs largely confined to lower altitudes, 3) larger-scale GWs apparently penetrating to much higher altitudes, 4) substantial GW amplitudes implied by digisonde electron densities, and 5) apparent influences of these perturbations in the lower F-region on the formation of equatorial spread F, RTI, and plasma bubbles extending to much higher altitudes. Other efforts with SpreadFEx data have also yielded 6) the occurrence, locations, and scales of deep convection, 7) the spatial and temporal evolutions of plasma bubbles, 8) 2-D (height-resolved) structures in electron density fluctuations and equatorial spread F at lower altitudes and plasma bubbles above, and 9) the occurrence of substantial tidal perturbations to the large-scale wind and temperature fields extending to bottomside F-layer and higher altitudes. Collectively, our various SpreadFEx analyses suggest direct links between deep tropical convection and large GW perturbations at large spatial scales at the bottomside F-layer and their likely contributions to the excitation of RTI and plasma bubbles extending to much higher altitudes.

The Cornell University Portable Radar Interferometer (CUPRI), a 50-MHz Doppler radar system, was operated during May and August/September 1983 on the island of St. Croix (17.7°N, 64.8°W) to study the plasma instabilities associated with nighttime sporadic E layers. Two events, on May 7 and August 22, show evidence of large-amplitude waves, with apparent horizontal wavelengths of 10–12 km and periods of 2–6 min. These apparent wavelengths are upper limits for the true wavelengths. The CUPRI beam was directed over Arecibo, Puerto Rico, and on May 7, concurrent electron density profiles within the CUPRI scattering volume were measured by the Arecibo Observatory's 430-MHz radar. During the stronger event of August 22, radar echoes were received from several altitudes up to 130 km. Large mean Doppler velocities (at times exceeding 250 m/s) were observed during this event, and the power spectra closely resemble those obtained at the magnetic equator during type 1 conditions. We believe (1) that the mid-latitude large-scale waves are generated by the same gradient drift instability mechanism responsible for equatorial large-scale waves and (2) that the type 1 3-m waves can be generated at mid-latitudes with drift velocities well below the sound speed because of the very sharp plasma density gradients associated with metallic ion sporadic E layers.