The characteristics of VHF echoes from the summer mesopause region at mid-latitudes

Radar observations have been carried out at Aberystwyth (52.4°N, 4.1°W) during the period May–August 1990 to search for echoes analogous to the Polar Mesosphere Summer Echoes commonly observed at high latitudes. Signal strength measurements in the vertical and at 12° from the vertical are used to examine the aspect sensitivity of echoes, and Doppler measurements at 6° from the vertical and in the vertical to estimate wind velocities. The observations show the presence of two types of moderately strong echoes from heights above 80 km. On most days a spectrally broad echo is observed with characteristics consistent with isotropic turbulence scatter. On certain days between mid-June and mid-July, stronger spectrally narrow echoes are observed with characteristics similar to Polar Mesosphere Summer Echoes.     

A study of signal statistics of VHF radar echoes from clear air

Signal statistics of VHF radar returns from vertical pointing observations of the clear air are investigated. In particular, the signal signature, its Doppler spectrum and statistical distribution are examined. It is found that the most important factors that characterize the statistics of the signals are the width of the spectrum and the Nakagamim-coefficient for the intensity distribution. Using these factors as criteria, two types of signals are found. One corresponds to volume scattering arising from turbulence or multiple thin laminae or sheets. The other corresponds to return from a single sheet. Examples of the different scattering/reflection processes will be shown. Numerical modeling is used to simulate the scattering/reflection processes. From the simulation, it is demonstrated that echo signals from some range gates are consistent with the picture of reflection from a single, diffuse sheet, causing focusing and defocusing of the signals.     

On the relation between diffusion coefficients and height from radar meteor echoes

Diffusion coefficients derived from radar meteor echoes and corrected for atmospheric wind shears have been used to investigate a linear relation between diffusion and height in the atmosphere. The results of a least square fit and of a method in which both variables are considered subject to errors are presented and discussed. The present results seem to indicate that the relation between diffusion coefficient and height is not as simple as supposed. Scale height values derived from these data are not consistent with accepted values in the atmosphere.     

A study of the VHF radar data via a random variance model

Data sets from some VHF radars have been analysed. Gaussian distributions with random variance are proposed for the signal's quadrature components. The suggested distributions explain the data sets satisfactorily, especially as the length of the data series increases. Non-stationarity of the signals will also be interpreted using the proposed model. Moreover, a χ²-goodness of fit test for the proposed model has been conducted and its results are persuasive. We suggest that it is better to use the proposed distribution for the quadrature components than to use the Nakagami distribution for the amplitude distribution or the regular Gaussian distribution for the quadrature components. In addition, the sampling time should be less than 4 min to guarantee the stationarity of the data.     

VHF radar observations of nonlinear interactions in the summer polar mesosphere

VHF radar measurements of velocities and echo power in the summer polar mesosphere have been analysed using maximum entropy, bispectral and cross-spectral methods in order to study wave-wave interactions. The results show nonlinear interactions of second and even third order between the diurnal and semidiurnal tides and planetary waves with periods of 2 and 3 days in the velocity field. Similar analyses of time series of echo power suggest corresponding variations of the temperature field.     

First results with the Adelaide VHF radar: spaced antenna studies of tropospheric winds

The first results from a VHF radar of the ST type located at Buckland Park near Adelaide, Australia (35°S, 138°E), are presented. The radar is designed to be versatile and can be used to measure velocities in the lower atmosphere using both the spaced antenna (SA) and Doppler beam-swinging (DBS) techniques. Here studies of irregularities and motions made with the spaced antenna technique are discussed. It is shown that the scale of the diffraction pattern formed by the backscattered radiation varies with altitude, with the mean pattern scale being smaller in the troposphere than in the stratosphere. The observations are consistent with the backscattered energy decreasing as a function of off-vertical angle by 1.5 dB per degree in the troposphere and by about 2.8 dB per degree in the lower stratosphere. An intercomparison of zonal velocities measured with the SA and DBS methods shows good agreement. In May and August 1984 an extensive comparison was made between the velocities measured by the SA method and winds determined from over 80 balloon-borne radiosondes released from Adelaide Airport, situated some 36 km to the south of the radar. The velocities were compared on a statistical basis and showed excellent agreement, although the SA speeds tended to be 1–2 m s⁻¹ smaller in magnitude than the radiosonde velocities. Overall, the rms differences between the two sets of measurements was only 3–4ms⁻¹ throughout the troposphere, a result which is consistent with the random errors inherent in each technique, as well as the spatial separation between the radar and balloon observations. The utility of the SA method for meteorological observations is illustrated by a study of both the horizontal and vertical wind fields during the passage of a cold front made in November 1984. The high time resolution available with the radar allows detailed studies of the development of the pre-frontal jet, the wind convergence into the front and associated vertical motions.     

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromsø, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 10⁴ cm⁻³. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.     

Absolute reflectivities and aspect sensitivities of VHF radio wave scatterers measured with the SOUSY radar

By accurately calibrating the SOUSY radar in West Germany it has been made possible to measure absolute values of effective reflection coefficients and turbulence structure constants. Some typical values of these parameters as a function of altitude are presented. Such profiles are presented for both a vertically directed beam, and also for two beams directed 7° off-vertical. Comparisons of powers on the vertical and off-vertical beams show that scatter became more aspect sensitive at the tropopause and in the lower stratosphere, but, unexpectedly, scatter was observed to become considerably more isotropic in the higher regions of the stratosphere (above 15–18 km) on this occasion. An enhancement of signal from the tropopause occurred not only on the vertical beam, but also on the off-vertical beams.Comparisons of signal strengths scattered from the mesosphere and measured with the vertical and off-vertical beams showed that for the present observations mesospheric scatter was close to isotropic. The backscatter cross-sections at VHF have been compared with other measurements at medium and high frequencies at other locations, and these comparisons help set some limits on the scales of turbulent and specular scatterers in the mesosphere.     

The meteorological jet stream as a source of medium scale gravity waves in the thermosphere: an experimental study

Results are reported of a co-ordinated experimental study of medium scale gravity waves involving use of a multistatic incoherent scatter system together with two networks of polarimeters monitoring transmissions from a geostationary satellite during a six-day period in July 1974. The latter observations give access to the horizontal propagation parameters of the waves while the former technique yields information on the vertical structure of both waves and the ambient atmosphere. A reverse ray tracing analysis, through the atmosphere, for each gravity wave is described and its validity and accuracy discussed in detail. For the majority of the waves the reverse group path can be followed down to tropopause level and comparison with meteorological data has shown that many of the possible source regions of the observed waves appear to lie in proximity to the jet stream, with some evidence to suggest a preference for the polar side of the jet. Other wave sources are found to be close to regions of convectively unstable cold polar air. Wave energetics are discussed together with possible generation mechanisms, the latter in the form of non-linear interaction of shear flow instabilities in the jet stream and penetrative convection.     

New observational techniques for studying the dynamics of the middle atmosphere using the Chung Li VHF radar

Taking advantage of the newly developed volume scattering model for MST radar and the unique features of the Chung Li VHF radar, several novel observational techniques have been developed and implemented. Techniques such as oblique spaced antenna (OSA), the frequency domain analysis of spaced antenna data, the full spectrum analysis (FSA) and the multifrequency frequency domain interferometer (FDI) will be discussed and experimental results will be presented. Potential applications of the new techniques to study the dynamics of the middle atmosphere will be discussed.     

Evaluation of the vertical flux of energy into the thermosphere from medium scale gravity waves generated by the jet stream

Since the paper of Hines (1960), the heating of the thermosphere by gravity waves is well known. Until recently there were no statistical evaluations of their energy. The present paper is a study of this subject. From experimental data of a Faraday rotation experiment and with the help of a measurement theory and a propagation model in a dissipative atmosphere, we evaluated the mean energy flux for the medium scale gravity waves detected during July 1974. We found 0.11 erg cm⁻² s⁻¹ for waves with horizontal phase velocity between 100 and 180 m s⁻¹. We also evaluated the heights of energy deposition which are from 120 to 180 km. This study shows that the heating by gravity waves may be important for the thermospheric equilibrium.     

First low-power VHF radar observations of tropospheric,stratospheric and mesospheric winds and turbulence at the Arecibo Observatory

First VHF radar measurements with height resolution of 300 m and angular resolution of 1.7° were carried out in low latitudes at the Arecibo Observatory, Puerto Rico. A short outline is given of the experimental set-up which consisted of a 160W average power radar-transceiver and a self-contained digital radar control and data acquisition unit. The new VHF feed system of the Arecibo dish is described shortly. Reliable radar echoes were detected from the troposphere, lower stratosphere and from some heights in the mesosphere, indicating that the described VHF radar is capable of proper investigations of dynamical processes in the low latitude middle atmosphere. The angular dependence of aspect sensitive tropospheric and stratospheric turbulence structures was measured to be 1.5–2.5 dB degree⁻¹. Echoes from the mesosphere indicate a patchy structure of turbulence. The analysis of the signal-to-noise ratio shows considerably high reflectivity in the upper troposphere, which can be caused by high-reaching tropical cumulus convection. Wind profiles measured with the VHF radar between 7.5 and 19.5 km with a height resolution of 300m are very similar to radiosonde wind profiles. Mesospheric VHF radar winds are roughly consistent in amplitude with tidal winds.     

Characteristics of polar mesosphere summer echoes (PMSE) observed with the EISCAT 224 MHz radar and possible explanations of their origin

We describe experiments carried out with the EISCAT VHF radar during the MAC/SINE campaign. These experiments included observations of the polar mesosphere summer echoes (PMSE), which were studied with a high spectral resolution program. The fine structure of the spectra imply that very thin and non-random transient structures of reflectivity occur frequently in the mesopause region. We find no clear relation between the echo power and the coherence time which could support the hypothesis of scatter from turbulence or partial reflection. In addition, the estimates of radar reflectivity let us discard incoherent scatter and pure turbulence scatter as the cause of the PMSE. We also discuss the relation of the PMSE and cluster ions, electric fields, charge accumulation and atmospheric gravity waves.     

The use of the phasor display in studying ionospheric radio echoes

The phase and amplitude of a radio pulse reflected from the ionosphere usually vary during the pulse. It is convenient to observe these variations using the X-Y mode of an oscilloscope to display the phasor of the echo. The variations are then seen as an oval or spiral shape traced out by the end point of the phasor. These shapes provide a sensitive method of detecting the presence of more than one echo, and are useful as a measure of dispersion.     

Steepening of reflectivity structures detected in high-resolution Doppler spectra of polar mesosphere summer echoes (PMSE) observed with the EISCAT 224 MHz radar

Polar mesosphere summer echoes observed with the EISCAT 224 MHz radar frequently exhibit significant discontinuous offsets or jumps in the Doppler frequency. We can explain these frequency jumps as a result of a lifting of partially reflecting or scattering layers, which are distorted by bumps. These bumps can be caused by steepened refractivity variations, i.e. reflectivity structures. These suggestions are supported by model computations. We also notice that a relation exists between these structure shapes and gravity waves, which are steepened, but which do not necessarily break into enhanced turbulent velocity fluctuations.     

Hemispheric properties of the two-day wave from mesosphere-lower-thermosphere radar observations

Measurements of the pseudo 2-day wave have been made at many of the mesosphere-lower-thermosphere radars. Comparisons are made here between measurements taken at Saskatoon MF radar (52°N, 107°W) and two meteor radars, one at Christmas Island (2°N, 157°W) and the other at Durham (43°N, 71°W). Although results averaged for 10 days or longer agree with previous measurements (i.e. larger amplitudes and more phase stability in late summer), when the wave is analyzed over 2–4 days as a 48 h component, interesting phase properties emerge and the wave is seen over more of the year. The wave is amplitude and phase modulated, making the interpretation of results obtained over long time frames (20 days or more) difficult. There is strong evidence of solar influence on the 2-day wave.     

新疆吐鲁番胜金店墓地出土煤精制品的科技分析

煤精独特的质地与光泽使其成为制作装饰品和工艺品的重要原料之一,自新石器时代中期便在我国作为艺术载体而出现。目前煤精制品的鉴定主要依据外观形貌、划痕颜色和破坏程度较大的煤岩学分析,无损或微损的科技分析甚少。本研究利用气相色谱-质谱联用、红外光谱及同步辐射显微CT技术对新疆吐鲁番西汉时期胜金店墓地出土的一枚黑色珠子开展了无损和微损分析。结果表明,珠子为煤精制品,结构均匀,制作时采用了对向钻孔工艺并经过了打磨、修整处理。煤精制品自西汉以来逐渐出现在低阶人群墓葬中,这种消费大众化现象可能与汉代煤的首次大规模利用有关;而煤精制品自汉代开始在新疆地区零星出现,推测新疆煤精制品的利用很可能是汉文化影响的结果。     

Vertical propagation characteristics of internal gravity waves around the mesopause observed by the Arecibo UHF radar

A high resolution wind observation of the mesosphere and lower thermosphere (73–95 km) was conducted with the aid of the high power UHF Doppler radar at Arecibo (18.4°N, 66.8°W). Zonal wind velocities were continuously observed during day-time hours on 1–15 August 1980. We discuss here the observed wind fluctuations with periods of 1–4 h in the light of internal gravity waves. The phase propagation associated with these fluctuations is, on average, shown to be downward, indicating an upward energy flux. A space-time spectral analysis shows that waves with vertical wavelengths shorter than 10 km disappear around the mesopause (about 85km), while those with longer vertical wavelengths exist throughout the observational height. This result is explained in terms of wave absorption at a critical layer where the mean zonal wind has a westerly shear with height. This feature is consistent with the behavior expected for internal gravity waves around the summer mesopause in order to explain general circulation models.