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Secure antenna polarization modulation, or SAPM, is a method for increasing security at the physical layer via spatial signal distortion. We derive its expected performance in additive white Gaussian noise (AWGN) channels with the goals of establishing theoretical limits and comparing them to other techniques in a controlled environment. The theory is verified with measurements from a proof-of-concept system, and security is evaluated using information beamwidths calculated from error probability overlook angle. Results indicate that, when compared to conventional methods and keeping the hardware consistent, SAPM has a much narrower angular range of demodulation around the intended receiver—often by over 20°, depending on the receiver’s error rate threshold. Different system configurations are explored in this work, including a variety of power allocations and orders of modulation, as well as the effect of antenna design. In all cases, SAPM shows the potential to significantly improve wireless security over other techniques. These findings provide the foundation and motivation for future characterization in practical channels.

We investigate the impact of additive white Gaussian noise on the performance of secure antenna polarization modulation (SAPM), a technique for wireless physical layer security. The secrecy rate, calculated from the mutual information of intended versus eavesdropper channels, serves as a metric for evaluation. With increasing signal-to-noise ratio (SNR), system designers can also choose to increase spectral efficiency by using a higher order of modulation. This significantly improves the level of security provided by SAPM by narrowing the range of spatial angles for which information may be received. We use simulation data to calculate the mutual information and symbol error probability over varying SNR levels to illustrate these effects.

Climatological structure of the quasi-2-day wave (Q2DW) at middle latitudes in temperature and horizontal winds in the mesosphere and lower thermosphere (MLT) was compared between the northern and southern hemispheres. Determination of the Q2DW in temperature was based on observation data by the Microwave Limb Sounder (MLS) onboard NASA's Earth Observing System (EOS) Aura satellite over 17 years from August 2004 to May 2021 and the Q2DW in horizontal winds was derived from Aura/MLS geopotential height data using balance equations. Amplitudes were maximized in summer in the southern hemisphere and in the meridional wind in the northern hemisphere, but in winter in the zonal wind in the northern hemisphere. Summer amplitudes were larger in the meridional wind than the zonal wind in the southern hemisphere, but zonal amplitudes in winter were larger than meridional amplitudes in summer in the northern hemisphere. Westward propagating zonal wavenumber 3 (W3) was largest in both hemispheres, but in addition to well-known W4, W3, W2 and eastward propagating zonal wavenumber 2 (E2), we also found W1, zonally symmetric standing (S0), and E1. Eliassen-Palm fluxes were derived for each mode. W3, W2, W1, and E2 fluxes were exhibited upward and poleward in January in the southern hemisphere while only W3 fluxes were exhibited clearly upward and poleward in July in the northern hemisphere. The balance winds and radar winds agreed in both amplitude and phase in the southern hemisphere and at lower latitudes in the northern hemisphere in January, and at lower latitudes in both hemispheres in July. Furthermore, the Q2DW is modulation in amplitude and phase from the W3 by accumulating other modes.

The Southern Hemisphere summer 2 day wave (TDW) is the most dramatic large-scale event of the upper mesosphere. The winds accelerate over ~1 week, may attain > 70 m/s, and are often accompanied by a near disappearance of the diurnal tide and stabilization of the period close to 48 h. We denote this as the phase-locked 2 day wave (PL/TDW). We have examined airglow and meteor radar (MR) wind data from the Andes Lidar Observatory (Cerro Pachon, Chile:30°S, 289.3°E), MR data from Darwin (12.5°S, 131°E) and airglow and medium frequency radar data from the University of Adelaide (34.7°S, 138.6°E) for the behavior of the TDW during the austral summers of 2010, 2012, and 2013. The Cerro Pachon and Adelaide sites are located at similar latitudes separated in longitude by about 120°. We find a remarkable coincidence between the TDW oscillations at Chile and Adelaide for the period January–February 2010. The oscillations are nearly in phase in terms of local time and the minima and maxima repeat at nearly the same local time from cycle to cycle consistent with a phase-locked wave number 3 TDW. Data for this and other years (including Darwin) show that the amplitude of the diurnal tide decreases when the TDW is largest and that this occurs when the period is close to 48 h. These observations support the proposal that the PL/TDW is a subharmonic parametric instability wherein the diurnal tide transfers energy to a TDW that is resonant at nearly 48 h.

The Aerospace Corporation's Nightglow Imager (ANI) observes nighttime OH emission (near 1.6 μm) every 2 s over an approximate 73° field of view. ANI had previously been used to study instability features seen over Maui. Here we describe observations of instabilities seen from 5 to 8 UT on 24 March 2012 over Cerro Pachon, Chile, and compare them with previous results from Maui, with theory, and with Direct Numerical Simulations (DNS). The atmosphere had reduced stability because of the large negative temperature gradients measured by a Na lidar. Thus, regions of dynamical and convective instabilities are expected to form, depending on the value of the Richardson number. Bright primary instabilities are formed with a horizontal wavelength near 9 km and showed the subsequent formation of secondary instabilities, rarely seen over Maui, consistent with the primaries being dynamical instabilities. The ratio of the primary to secondary horizontal wavelength was greater over Chile than over Maui. After dissipation of the instabilities, smaller-scale features appeared with sizes in the buoyancy subrange between 1.5 and 6 km. Their size spectra were consistent with the model of Weinstock (1978) if the turbulence is considered to be increasing. The DNS results produce secondary instabilities with sizes comparable to what is seen in the images although their spectra are somewhat steeper than is observed. However, the DNS results also show that after the complete decay of the primary features, scale sizes considerably smaller than 1 km are produced and these cannot be seen by the ANI instrument.

[1] The diurnal variation of gravity wave (GW) momentum flux is derived from the 5 years of meteor radar wind measurements at Maui, Hawaii. The amplitude and phase relationships between the GW forcing and the diurnal tide are analyzed by calculating their equivalent Rayleigh frictions. The results show that the GW momentum flux is clearly modulated by the diurnal tide. The forcing from the momentum flux convergence has strong effects on both the amplitude and phase of the diurnal tide. They can reach 80 ms − 1 day − 1 for the amplitude and 15 h day − 1 for the phase. The GW forcing tends to increase the diurnal tide amplitude above 90 km but has a small damping effect below 90 km. It tends to increase the phase of the diurnal tide throughout all altitudes. Seasonal variations of the GW forcing exist, which result in differences in their effects on the diurnal tide. The magnitudes of the forcing are in agreement with recent results from satellite observations but are much larger than values used in general circulation models. This work also demonstrates that meteor radar measurements can provide a valuable data set for the study of GW-tide interactions.

Semidiurnal tidal features have been examined in the Mesosphere and Lower Thermosphere (MLT) from the long-term (2002–2007) meteor wind data over Maui (20.75°N, 156.43°W). Amplitude and phase obtained from the harmonic analysis exhibit large day to day variability. Mean amplitude obtained from the monthly mean data over the observation period is found to vary within ∼8–28 m/s and 10–32 m/s for the zonal and meridional winds, respectively. The amplitude has revealed clear semiannual oscillation (SAO) pattern with maxima during solstices and altitudinal growth in both wind components. Significant resemblance in its variability with other observations carried out from the low latitude sites all over the globe is obtained. Vertical wavelength estimated from the phase gradients exposes large values (>90 km) in all seasons. Contribution of the semidiurnal tide to the total tidal variability in the MLT is found to vary over wide range throughout the year with generally higher influence during winter season over diurnal and terdiurnal components.

Short-period gravity waves of especially-small horizontal scale have been observed in the Maui, Hawaii airglow. Typical small-scale gravity wave events have been investigated, and intrinsic wave propagation characteristics have been calculated from simultaneous meteor radar wind measurements. Here we report specific cases where wave structure is significantly determined by the local wind structure, and where wave characteristics are consistent with ducted or evanescent waves throughout the mesopause region. Two of the documented events, exhibiting similar airglow signatures but dramatically different propagation conditions, are selected for simple numerical modeling case studies. First, a Doppler-ducted wave trapped within relatively weak wind flow is examined. Model results confirm that the wave is propagating in the 85–95 km region, trapped weakly by evanescence above and below. Second, an evanescent wave in strong wind flow is examined. Model results suggest an opposite case from the first case study, where the wave is instead trapped above or below the mesopause region, with strong evanescence arising in the 85–95 km airglow region. Distinct differences between the characteristics of these visibly-similar wave events demonstrate the need for simultaneous observations of mesopause winds to properly assess local propagation conditions.

Wind data collected by the Maui meteor radar (20.75° N, 156.43° W) are used to study the 8-h tide in the mesosphere and lower thermosphere (MLT) region at a low-latitude station. The data set spans the time interval from 19 May 2002 to 24 May 2007. Our results show that the 8-h tide is a regular and distinct feature over Maui. The meridional component of this wave is significantly larger than the zonal component. The meridional component exhibits a semiannual variation in amplitude, with peaks near the equinoxes, whereas the variation of the zonal component does not show this seasonal characteristic. The strongest wave motions mostly occur in the height range of 92–96 km near spring equinox (March) and at higher altitudes near autumn equinox (October). The vertical variations of 8-h tidal phase at Maui indicate an upward wave energy flux. The vertical wavelengths are ≥54 km in equinox months.

Based on zonally averaged TIDI meridional wind data from one yaw period (March 19–May 19, 2004) near equinox, we examine the latitudinal and altitudinal distribution of the migrating diurnal and semidiurnal tides using least-squares fitting method to provide a global view of these tidal waves. The TIDI results are compared with Global Scale Wave Model 00 output for the month of April. The diurnal tide amplitude distribution are in a good agreement in the northern hemisphere and differ in amplitude in the southern hemisphere. The TIDI results show a lower peak altitude (97 km), while GSWM00 peaks at 102 km.The peak latitudes for the diurnal tide are at 20°N and 20°S for both the TIDI data and GSWM00 model. The TIDI data seem to show that the amplitude of the diurnal tide stronger in the northern hemisphere than in the southern hemisphere based on least-squares fitting results over the yaw period. The visually estimated vertical wavelength from the TIDI is about 20 km, while least-square fit results over the yaw period provided longer vertical wavelengths in northern (∼40 km) and southern hemisphere (∼30 km). We suspect that the seasonal change in the tide may cause the larger vertical wavelength. The GSWM00 provides a vertical wavelength of ∼26 km for the two hemispheres. Both model and TIDI data show semidiurnal tide peak amplitudes at 45° latitude. The measured semidiurnal tide peaked at lower altitudes (102 km in the southern hemisphere and 105 km in the northern hemisphere), whereas the GSWM00 shows peak altitudes at 110 km or above. The phases for the semidiurnal tide show a good agreement between the data and the GSWM00 model. The TIDI meridional winds are compared with ground based meteor radar measurements at Maui. The meteor data show smaller diurnal tide amplitude than that shown in the TIDI data. The phases of the diurnal tide appear to be consistent between the two data sets.

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.

Atmospheric gravity waves play a significant role in the dynamics and thermal balance of the upper atmosphere. In this paper, we present a novel technique for automated and robust calculation of momentum flux of high-frequency quasi-monochromatic wave components from spectroscopic imaging and horizontal radar wind measurements. Our approach uses the two-dimensional (2-D) cross periodogram of two consecutive Doppler-shifted time-differenced (TD) images to identify wave components and estimate intrinsic wave parameters. Besides estimating the average perturbation of dominant waves in the whole field of view, this technique applies 2-D short-space Fourier transform to the TD images to identify localized wave events. With the wave parameters acquired, the momentum flux carried by all vertically propagating wave components is calculated using an analytical model relating the measured intensity perturbation to the wave amplitude. This model is tested by comparing wave perturbation amplitudes inferred from spectroscopic images with those from sodium lidar temperature measurements. The proposed technique enables characterization of the variations in the direction and strength of gravity waves with high temporal resolution for each clear data-taking night. The nightly results provide statistical information for investigating seasonal and geographical variations in momentum flux of gravity waves.

A network of 15 northern hemisphere radars has been used to measure horizontal winds in the mesosphere and lower thermosphere during the PSMOS campaign of Summer 1999. The radars are sited at latitudes ranging from 21°N to 75°N and longitudes from 142°E to 157°W. The data were examined to investigate the Northern Hemisphere structure of the quasi-2-day planetary wave during the interval June–August. The amplitude of the 2-day wave was found to exhibit great day-to-day variability. In particular, significant periodic fluctuations in amplitude occurred with periods of 8–10 and 14–17 days. These modulations were strongest in July and largely absent in June and August. In July, the wave activity can be resolved into three westward-propagating waves with zonal wave numbers of 2, 3 and 4. The periods associated with these wave numbers were 53–56, 48–50 and 42– 43 h , respectively. The simultaneous presence of at least two spectral components with periods close to each other may serve to explain the observed amplitude modulations as a result of a beating between different spectral components. An earlier analysis of the planetary-wave field during this interval has revealed a westward propagating ∼16-day wave with zonal wave number 1 (Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002b) 1865–1896). A non-linear interaction between this ∼16-day planetary wave and the (3,0) Rossby-gravity mode (the 2-day-wave) provides a possible mechanism to generate the above ∼42 h /wavenumber 4 wave and the ∼55 h /wavenumber 2 waves as sum and difference secondary waves. A bispectral analysis was used to further investigate non-linear interactions between members of the planetary-wave field and suggested a number of interactions occur within the planetary-wave field, but that some of the interactions also involve the non-migrating diurnal tide with zonal wavenumber 6.

Long period variations in the mesosphere wind have been observed for some time by ground-based radars. These planetary scale disturbances have reoccurring periods at or near 5–7, 10, and 16 days and at times dominate the wind field at mesospheric heights. Recently, due to the continuous operation of several of the MLT radars and the availability of measurements from the UARS satellite, it has been possible to compare observations during periods of large planetary wave activity. Wind measurements from four MLT radars; the meteor radars at Durham, NH (43°N,71°W) and Sheffield, UK (53°N,2°W) and MF radars at Urbana, IL (40°N,88°W) and Saskatoon, Canada (52°N,107°W) were compared with the HRDI measurements during intervals when 7-d planetary waves were present. Wind data from the HRDI instrument on UARS has been processed to show the latitudinal structure and the seasonal variation of the planetary scale wind variation. The phases and amplitudes of the waves as determined by both the satellite and the radars are in good agreement. The ground-based measurements show large modulation of tides by these long period components, and also show comparable responses of these low frequency components over thousands of kilometers. The satellite and the ground-based results both indicate a preponderance of wave occurrence during the equinoxes and at preferred latitudes.

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.

Simultaneous wind measurements in the mesopause region at Starfire Optical Range near Albuquerque, NM with Na wind/temperature lidar and meteor radar have been performed and compared. 20 nights of hourly data recorded with these two instruments at two layers around 86 and 93 km altitude are compared for both zonal and meridional wind components. The mean values are found to have no statistically significant differences. The correlation coefficients vary between 0.63 and 0.70, indicating that the two sets of measurements are broadly consistent. When comparing the averaged variations over the night, the two measurements are highly correlated, with correlation coefficients varying from 0.84 to 0.95. It indicates that the strong tidal variation is well captured by both instruments. Differences are however significant at individual hours, which are believed to be mainly due to the fact that the meteor radar wind is an average over the entire sky while the lidar measures wind within a volume about 100 m in diameter.

Using more than 350 ionospheric images reconstructed tomographically, studies on the motion of the anomaly crest and its geophysical implications are carried out. On an average day, the crest forms at 09:00 LT and, in the next two hours, moves poleward with a speed of about 1° per hour as it intensifies. This poleward motion is slowed as the crest reaches its highest latitude where it stays for several hours until early afternoon. Thereafter, the crest starts to weaken as it recedes with a speed of about 0.5° per hour equatorward. During 12:00 - 14:00 LT, the crest latitude is found to correlate with the fountain strength and the total number of electrons in a cross sectional plane at the observational longitude of the whole equatorial ionosphere.

Simultaneous medium frequency (MF) radar and Na Doppler lidar measurements of meridional winds in the mesosphere (84– 96 km ) were collected at Urbana, IL in September and October, 1996. Comparisons on time scales of 1 h and longer and with height resolution of 3 km show that the two instruments measure winds that agree to within tolerances established by statistical estimation errors at night, but reveal large differences that cannot be explained by estimation errors in the daytime. After removing the altitude-independent component of both data sets, the wind estimates are shown to agree to within expectations based on measurement uncertainty at all times of the day. The most likely explanation for differences between the two data sets is determined to be undetected/uncorrected wavelength shifts in the lidar, which would introduce offsets into the measured winds at all altitudes.

Resonance Lidar observations of neutral sodium and VHF coherent scatter radar observations of field-aligned 3-meter irregularities were obtained during the Coqui II rocket campaign in Puerto Rico. The Lidar, a facility instrument at the Arecibo Observatory (18.3°N, 66.8°W), and the University of Illinois Radar, located near Salinas on the south of the island, both monitored volumes near where the uplegs of the nominal rocket trajectories intersected the E-region. The Observatory's Incoherent Scatter Radar was also used to characterize the plasma layers. Preliminary investigation of the data sets has shown a potential correspondence between VHF backscatter from plasma layers and a new class of characteristic enhancements in the neutral sodium.

Radio tomography experiments have demonstrated the promising potential of applying tomographic methods in imaging various ionospheric structures. In actual implementation of image reconstructions one is faced with many choices, which include the following: whether to use the total phase, relative phase, or Doppler as the projection data, how to approximate the projection operator, what inversion algorithm to employ, and the choice of how to include the ancillary data and constraints on the constructed image. Each choice results in an image compatible with the given or measured projection data, yet each choice results in an image different from that of the others, with its own attendant artifacts and distortions. Collectively, the images produced by all the possible choices comprise an assembly of images. In this simulation study of one ionospheric model, 113 members of such an assembly are generated. All images look similar in gross features with a root-mean-square deviation not more than 29% from the mean. As expected, the largest deviation occurs near the region of highest gradients. By averaging all of the images in the assembly we show that the mean image is superior because of its smallest root-mean-square deviation from the true image. This conclusion, drawn on the simulation study of one model, may in fact have a general applicability, and we discuss why this may be so.

From analyzing nine months of airglow imaging observations of atmospheric gravity waves (AGWs) over Adelaide, Australia (35°S) [Walterscheid et al., 1999] have proposed that many of the quasi-monochromatic waves seen in the images were primarily thermally ducted. Here are presented 15 months of observations, from February 1996 to May 1997, for AGW frequency and propagation direction from a northern latitude site, Urbana Illinois (40°N). As Adelaide, Urbana is geographically distant from large orographic features. Similar to what was found in Adelaide, the AGWs seem to originate from a preferred location during the time period around summer solstice. In conjunction with these airglow data there exists MF radar data to provide winds in the 90 km region and near-simultaneous lidar data which provide a temperature climatology. The temperature data have previously been analyzed by States and Gardner [2000]. The temperature and wind data are used here in a full wave model analysis to determine the characteristics of the wave ducting and wave reflection during the 15 month observation period. This model analysis is applied to this and another existing data set recently described by Nakamura et al. [1999]. It is shown that the existence of a thermal duct around summer solstice can plausibly account for our observations. However, the characteristics of the thermal duct and the ability of waves to be ducted is also greatly dependent on the characteristics of the background wind. A simple model is constructed to simulate the trapping of these waves by such a duct. It is suggested that the waves seen over Urbana originate no more than a few thousand kilometers from the observation site.

A novel correlative technique is introduced for intercomparison of measurements of similar quantities made using different techniques. This process involves a generalized least-squares fitting method which can be used to estimate the slope of the best-fitting straight line that results when two separate data sets which are expected to be linearly correlated are compared via scatter-plots. The different data types may be subject to different uncertainties in their measurements. The algorithm determines and graphs the interrelationship between the errors in each method and the slope of the line of best fit, assuming Gaussian statistics.

The objective of this study is to compare observed mean meridional winds with those deduced from theory. The diabatic circulation is computed from High Resolution Dopper Imager (HRDI) mesospheric and lower thermospheric temperatures during January and July conditions. The meridional wind component is compared with HRDI Eulerian mean meridional winds near 95 km and with seasonal averages of meridional winds at a number of radar medium-frequency (MF) and meteor wind (MW) sites. The diabatic wind is directed from the summer toward the winter hemisphere. Peak values exceed 20 m s−1 and are observed at 105 km near 20° in the summer hemisphere. A secondary maximum of about 10 m s−1 is observed in the wintertime lower mesosphere during the July case. The diabatic wind is qualitatively consistent with HRDI 95-km mean meridional winds at latitudes equatorward of 50°. Time-averaged summertime radar winds are consistent with HRDI and diabatic winds between 50°S and 20°N. At winter midlatitudes, MF radar winds are directed oppositely to the diabatic wind, while one available MW measurement is directed with the diabatic wind. The zonal acceleration implied by the diabatic wind is about 150–200 m s−1 d−1 in the midlatitude summer lower thermosphere.

We examine the statistical estimation error for the dual coplanar beam momentum flux estimator in the presence of geophysical noise. Included in our derivation are the effects of measurement noise and finite spatial correlation of the wind fluctuations. We show that there is an optimal beam separation that minimizes the statistical estimation error at a value nearly equal to the irreducible error observed by an “ideal” anemometer measuring u and w at a point. For typical mesopause parameters this beam separation is approximately ±13°. The optimal angle is dependent in part on the ratio of vertical to horizontal wind variances and is therefore variable. For optimal momentum flux measurements a system, radar or lidar, with steerable beams becomes necessary.

A 50 MHz radar interferometer was used near Salinas, Puerto Rico, to probe the meter-scale E-region plasma density irregularities during two campaigns conducted in 1998. During the February–April period E-region echoing layers were primarily observed between 90 and 100 km heights. The layers were typically thin (∼1 km) and unstructured, although in several cases short period (∼90 s) layer oscillations were observed. During the June–July period E-region echoes showed more varied characteristics. In addition to low altitude layers, quasi-periodic structures with descending echoing layers were observed at altitudes above 100 km. Zonal motions detected during descending layer events were at times variable and oscillated between westward and eastward directions while the layer descent rates remained fixed.

The equatorial anomaly in ionization density has been imaged using the computerized ionospheric tomography technique applied to data from a low-latitude ionospheric tomography network. Examples of images representative of typical conditions during equinox and low solar flux are presented and shown to exhibit some characteristic features which have not been observed directly previously. The EA core, comprising the highest density region of the EA, is shown to exhibit a characteristic structure and asymmetry. These characteristics are quantified using measures of the ionospheric slab thickness and altitude, and they are discussed in light of the fountain mechanism which is responsible for formation of the EA.

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 gravity wave/quasi two-day wave interaction is an example of gravity wave/large-scale flow interaction, a subject of great interest but insufficient understanding. Here we present evidence, from MF radar data, for the quasi two-day wave modulation of gravity wave activity in the summer mesosphere over Urbana, Illinois (40° 10′ 10″ N, 88° 09′ 36″ W). A composite analysis of two-day wave events, identified using the meridional winds at 84 km, is performed on the four-hour mean winds and four-hour standard deviations about those means, the latter being representative of gravity wave activity. Our results show that quasi two-day variations in the winds modulate gravity wave activity at higher altitudes.

First observations of meter-scale equatorial 150-km plasma irregularities outside the American longitude sector are reported. The observations conducted at Pohnpei (western Pacific) with a wind profiling ST radar demonstrate the feasibility of making ionospheric electric field measurements with low cost instruments and also reveal a potentially important anisotropy in the angular distribution of 150-km VHF radar echoes.

The effect of geophysical noise on the precision of momentum flux measurements is examined. The dominant contribution to the uncertainty of momentum flux estimates scales with the geometric mean of the energy in horizontal and vertical wind fluctuations. To obtain statistically significant measurements of the momentum flux, long integration times are necessary since the flux is typically a small fraction of the geometric mean energy. For example, when the fraction is 1%, at least 16 days of stratospheric measurements will be needed for reliable flux estimation.

Sixty-five hours of Na lidar observations of vertical and horizontal winds, temperature, and Na density were obtained during eight different nights in 1994 and 1995 at the Starfire Optical Range, New Mexico, using a 3,5 m diameter telescope. The high-resolution data are used to study the spectra of gravity wave perturbations in the mesopause region. Wave activity was strong during the observations. The average variances of temperature, relative atmospheric density, horizontal wind, and vertical wind were 80 K 2 , 28 (%) 2 , 1100 m 2 /s 2 , and 4.3 m 2 /s 2 , respectively. The temperature, relative density, and horizontal wind spectra are generally consistent with the large body of published measurements and with the predictions of gravity wave theory. The observed temporal frequency (ω 0 ) and vertical wave number (m) spectra of vertical winds are both very shallow. The indices of the ω o spectra vary between -0.59±0.13 and -1.2±0.09, and the mean value is -0.76. The indices of the m spectra vary between -0.83±0.04 and -1.48±0.03, and the mean value is -1.1. In contrast, the indices of the horizontal wind m spectra vary between -2.8±0.10 and -3.2±0.13 with a mean of -3.0. These large differences imply that the underlying intrinsic spectra are not separable. However, the observed vertical wind m spectra are not consistent with the nonseparable theories which predict index values near +1. By using mathematical and numerical models, we show that the observed m spectra are distorted by Doppler and critical layer effects associated with the height-varying mean wind field. This distortion is greatest at high values of m and leads to observed vertical wind m spectra which are steeper than the underlying intrinsic spectra. Although the intrinsic spectra are definitely shallower (i.e., indices more positive) than the observations, it is not possible to determine if the measurements are entirely consistent with any of the nonseparable wave dissipation theories.

A 5-year (1991–1995) climatology of mesospheric gravity wave activity observed with the Urbana medium frequency (MF) radar is presented. We focus on the monthly mean horizontal wind variances associated with gravity waves with observed periods between 10 min and 8 hours. These results are compared with the seasonal characteristics from other gravity wave observations. The total wave variance, , displays a dominant semiannual variation (maxima summer/winter, minima spring/fall) below 87 km with little seasonal variation above 87 km. The zonal wave variance exceeds the meridional wave variance, , except around regions of zonal wind reversal. In all seasons the observed variance scale height is greater than that of a nondissipating wave field. A simple zero mean wind model based on linear saturation theory is found to be inadequate in reproducing the seasonal characteristics of the observed wave variances. An alternative model based on critical layer filtering by a height dependent wind field can reproduce the observed semiannual oscillation in the wave variance given a specific source input.

Na lidar and MF radar observations of the mesopause region (≈ 80–105 km) were made over a 2-year period at Urbana, Illinois (40°N, 88°W). The Na lidar data yielded both temperature profiles and Na density profiles which are used to derive independent estimates of the rms horizontal winds. Estimates of the rms horizontal winds from concurrent radar and lidar measurements are obtained on 37 nights, totaling 263 hours. These three sets of rms wind estimates are processed in a common fashion. This analysis yields vertical profiles of the rms winds that do not grow exponentially with altitude, indicating that the gravity wave field is saturated throughout this altitude region. All three techniques reveal an annual maximum in the summer rms horizontal winds. However, the Na density estimates also have a semiannual variation with a second maximum in winter, which was not produced by the other techniques. Estimates of the rms winds derived from lidar Na density and Na temperature data are comparable in magnitude. The radar estimates are systematically larger than both sets of lidar estimates at higher altitudes. The discrepancy between the radar and lidar estimates is found to be greatest at those altitudes where the saturation is most pronounced.

There exist two major mechanisms which are responsible for the fading phenomenon at HF frequencies. They are the multiple-mode interference and distortions due to the ionospheric irregularities. Fading time series produced by the first of these mechanisms alone should typically represent a multiple-periodical process. This kind of signal may also be produced by an autonomous dynamical system. The character of the time series produced by the second mechanism depends on the nature of the ionospheric irregularities. Recently, evidence has been accumulated to show that sometimes the ionospheric turbulence on equatorial and middle latitudes represents a low-dimensional deterministic chaotic process. These facts suggest that for both mechanisms the fading time series may have a deterministic nature and therefore is predictable. Accordingly we apply the nonlinear predicting technique proposed by Farmer and Sidorovich [1987] to the fading time series obtained by the University of Illinois HF sounder. In its application the prediction technique is modified to take into account specifics of the HF data. For comparison, the conventional linear autoregression prediction technique is also tested. It is found that, in general, the nonlinear prediction and the linear autoregressive forecasting allow prediction on a few correlation times and work with roughly the same success.

A multi-instrument global campaign involving incoherent scatter, medium frequency, and meteor wind radars was conducted during the period of January 20–30, 1993, to study the dynamics of the mesosphere and lower thermosphere. Data obtained from 15 radar stations covering a wide latitude range have been used to determine the global distribution of planetary and tidal waves during this 10-day campaign. Spectral analysis of the neutral winds measured by the radars in the altitude range from 80 to 130 km indicates the existence of a strong 48-hour wave near 90 km at latitudes between 40°N and 40°S that is present up to 108 km at 18°N. The semidiurnal tide is large at middle and high latitudes near 90 km and is predominant above 110 km, while the diurnal tide is observed to be particularly important in the upper mesosphere near 40° latitude. A least squares fit to the radar data is performed to obtain the amplitudes and phases of the tidal and 48-hour waves. Comparison with National Center for Atmospheric Research thermosphere-ionosphere-mesosphere general circulation model shows that the predictions from the model agree reasonably well with the observed global morphology of tidal wave amplitudes.

The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana Atmospheric Observatory during September 1992. The instrumentation included, among others, the Aerospace Corporation narrowband nightglow CCD camera, which observes the OH Meinel (6–2) band (hereafter designated OH) and the O2 atmospheric (0–1) band (hereafter designated O2) nightglow emissions; the University of Illinois Na density/temperature lidar; and the University of Illinois MF radar. Here we report on observations of small-scale (below 10-km horizontal wavelength) structures in the OH airglow images obtained with the CCD camera. These small-scale structures were aligned perpendicular to the motion of 30- to 50-km horizontal wavelength waves, which had observed periods of about 10–20 min. The small-scale structures were present for about 20 min and appear to be associated with an overturned or breaking atmospheric gravity wave as observed by the lidar. The breaking wave had a horizontal wavelength of between 500 and 1500 km, a vertical wavelength of about 6 km, and an observed period of between 4 and 6 hours. The motion of this larger-scale wave was in the same direction as the ≈30- to 50-km waves. While such small-scale structures have been observed before, and have been previously described as ripple-type wave structures [Taylor and Hapgood, 1990], these observations are the first which can associate their occurrence with independent evidence of wave breaking. The characteristics of the observed small-scale structures are similar to the vortices generated during wave breakdown in three dimensions in simulations described in Part 2 of this study [Fritts et al., this issue]. The results of this study support the idea that ripple type wave structures we observe are these vortices generated by convective instabilities rather than structures generated by dynamical instabilities.

New techniques are applied to measured Doppler velocity and angle of arrival to estimate horizontal wind vectors, variances, and momentum fluxes, from MF radar data. The approach used to estimate mean winds was first introduced as “time domain interferometry” (TDI) by Vandepeer and Reid [1995]. In the present paper, the algorithm is refined and used with data from the Urbana MF radar, which employs a single vertical antenna beam, to obtain a monthly mean wind climatology which is compared with the results from conventional spaced antenna full correlation analysis. The comparison validates the scattering model used in the development of the TDI technique and highlights instrumental and processing biases that differ between the two techniques. An extension of the TDI method that can provide estimates of the Reynolds stress tensor associated with propagating gravity waves is also proposed, and some preliminary results are presented.

We attempt to find the northern hemisphere zonal wavenumber for a striking quasi-2-day wave “event” or “burst” observed near 90 km altitude in the summer of 1992. A unique set of data on the upper atmosphere from nine radar sites is analysed (spacings ∼400– ∼ 12,000 km), and compared with expectations from models. The 2-day wave phase comparison, which finds zonal wavenumber m = 4, is conclusive. Determination of n, which defines the meridional wave amplitude structure, is not attempted, as the sites here have only a small latitude spread (21°N to 55°N). Also the amplitude seems to be unstable showing some sort of modulation which is not simultaneous at all sites. Finally, the radars have not been “calibrated” against each other in terms of wind speed. This calibration would have to be done before small differences in wave amplitude could be believed. A similar event in 1991 for which fewer sites are available is also discussed. Here the choice between m = 3 and 4 is not as clear.

The HWM90 thermospheric wind model has been revised in the lower thermosphere and extended into the mesosphere, stratosphere and lower atmosphere to provide a single analytic model for calculating zonal and meridional wind profiles representative of the climatological average for various geophysical conditions. Gradient winds from CIRA-86 plus rocket soundings, incoherent scatter radar, MF radar, and meteor radar provide the data base and are supplemented by previous data driven model summaries. Low-order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, local time (tides), and longitude (stationary wave 1), with a cubic spline interpolation in altitude. The model represents a smoothed compromise between the original data sources. Although agreement between various data sources is generally good, some systematic differences are noted, particularly near the mesopause. Overall root mean square differences between dar.a and model values are on the order of 15 m/s in the mesosphere and 10 m/s in the stratosphere for zonal winds, and 10 m/s and 5 m/s respectively for meridional winds.

This paper presents comparisons between wind measurements in the mesosphere and lower thermosphere by medium-frequency (MF) radars and by the high-resolution Doppler imager onboard the Upper Atmosphere Research Satellite (UARS). Because of distinctive observational patterns, time and space averaging, and data processing algorithms, satellites and radars observe the atmosphere from their own perspectives. Because thermal tides are an important part of dynamics in these regions of the atmosphere, tidal determinations from both techniques were given particular attention. Differences and similarities of the two methods are discussed and analyzed.

Horizontal wind fields in the mesosphere and lower thermosphere are obtained with the high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) by observing the Doppler shifts of emission lines in the O2 atmospheric band. The validity of the derived winds depends on an accurate knowledge of the positions on the detector of the observed lines in the absence of a wind-induced Doppler shift. Relative changes in these positions are readily identified in the routine measurements of onboard calibration lines. The determination of the absolute values relies on the comparison of HRDI observations with those obtained by MF radars and rockets. In addition, the degrees of horizontal and vertical smoothing of the recovered wind profiles have been optimized by examining the effects of these parameters both on the amplitude of the HRDI-derived diurnal tidal amplitude and on the variance of the wind differences with correlative measurements. This paper describes these validation procedures and presents comparisons with correlative data. The main discrepancy appears to be in the relative magnitudes measured by HRDI and by the MF radar technique. Specifically, HRDI generally observes larger winds than the MF radars, but the size of the discrepancy varies significantly between different stations. HRDI wind magnitudes are found to be somewhat more consistent with measurements obtained by the rocket launched falling sphere technique and are in very good agreement with the wind imaging interferometer (WINDII), also flown on UARS.

Numerical simulations are employed to investigate possible biases in Medium Frequency (MF) wind estimates. To explain apparent biases inferred from Arecibo Initiative in Dynamics of the Atmosphere (AIDA) campaign measurements, Hines et al. (1993) have suggested that MF radar estimates of horizontal winds should be biased toward the phase speed of perturbing gravity waves if the MF scattering process is modulated by the passage of such waves through the scattering volume. One of the proposed scenarios involves modulation of the tilts of anisotropic irregularities by the perturbation winds associated with gravity waves. We use a two-dimensional propagation model in which anisotropic scattering centers are represented by conducting line segments located at random positions inside the radar scattering volume. The scatterers are advected and tilted by the superposition of a mean wind and the perturbation winds associated with a gravity wave. Simulated complex time series, which would be recorded by a two-receiver antenna array with a typical MF radar system parameters, are analyzed to obtain spaced antenna (SA) wind estimates for comparison with the specified wind speed at the center of the scattering volume. Numerical simulations are carried out for many different sets of gravity wave parameters. Each simulation is performed twice, once with essentially isotropic scatterers (line segments of length λ/2) and again with anisotropic (“specular”) scatterers of length 10 λ. Numerical results indicate that individual SA wind estimates are accurately predicted by the wave-induced error model of Kudeki et al. (1993). This is true for both isotropic and specular scatterers as long as the gravity wave has a horizontal wavelength larger than the horizontal dimension of the radar range cell. The results also indicate that estimated winds averaged over one period of the perturbing gravity wave exhibit statistically significant biases when the scattering centers are highly anisotropic, but the bias is not necessarily toward the phase speed of the perturbing gravity wave and, more importantly, the magnitude of the bias exhibited in our simulations was always less than 15 m/s. Implications of the numerical results are discussed in light of observed discrepancies as large as 40 m/s in recent AIDA campaign comparisons between MF and incoherent scatter radar winds. In particular, it is argued that the simulation results support the wave-induced fluctuations model put forth by Kudeki et al. (1993) as an explanation for the AIDA results.

The two-frequency mutual coherence of HF electromagnetic field fluctuations caused by ionospheric irregularities in an oblique radio path is studied. The single scattering approximation is first developed and then extended to stronger field fluctuations by applying Rytov's approximation. Of considerable interest for wideband HF cpmmunication applications is the two-frequency, two-time correlation function of the channel 〈u* (ω,t1)u(ω + Ω,t1 + t)〉, where u(ω,t) is the complex amplitude of the radio wave transmitted at a frequency ω, measured by a receiver at time t. Our results show that this particular quantity behaves as though there were no diffraction effects (no Fresnel-filtering effects). Thus the correlation time τ0 is close to Λ /ν regardless of the ratio between the irregularity size and the Fresnel length. Here Λ is the irregularity size and ν is the component of the drift velocity perpendicular to the ray path. Typical scales for the two-frequency mutual correlation function are studied, and simple physical interpretations are developed. For example, the correlation bandwidth Ωc is determined by the condition that the rays connecting the transmitter and receiver at ω and ω + Ωc are separated from each other (in the vicinity of the reflection point) by a distance of the order of Λ. A procedure is described which allows the size of irregularities and components of plasma drift to be estimated from one of the mutual correlation functions and from the autocorrelation function. This procedure was applied to measurements from the University of Illinois sounder (transmitter is located in Platteville, Colorado, receiver in Urbana, Illinois). In the examples considered, irregularities with a size of the order of a few hundred meters drifting with a velocity 20–100 m s−1 can explain the fading. The observed magnitude of fading is consistent with an rms irregularity fluctuation of the order of a few tenths of one percent.