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.     

Gyro line observations with the EISCAT VHF radar

This paper reports the first successful gyro line experiment with the EISCAT VHF (224 MHz) radar. The incoherent scatter gyro line (also known as ‘resonance line’ and ‘whistler line’ in the literature) corresponds to the electrostatic wave mode ω ≈ Ω cos α known to be present in a weakly magnetized plasma (Ω is the electron gyro frequency and α is the angle between the scattering wave vector and the magnetic field). The line is very weak, but has the great advantage from an observational point of view that its position in the scattered spectrum is only marginally dependent on the electron density and temperature. This means that filter offsets can be easily predicted and that a long pulse and long integration times can be used in the experiment. Measurements were made at angles of 55 and 69° with the geomagnetic field where the gyro line frequencies are approximately 800 and 500 kHz, respectively. The line was seen in the altitude region 100–220 km, being most intense at 160–170 km. The strong dependence of the gyro line on the magnetic field may be used to study variations in the field. Other interesting aspects of the line to be investigated in future experiments are the effects of suprathermal particles, the possible effects of stimulated scattering, and the heating effects in an ionospheric modification experiment.     

Measurement of vertical phase and group velocities of atmospheric gravity waves by VHF radar

A systematic method of deriving from MST radar data the group velocity and phase velocity of the atmospheric wave along the radar beam direction is proposed and verified by a series of numerical simulations. We apply the method to two data sets measured by Chung-Li radar under different background wind conditions. It is found that the vertical group velocity and phase velocity are mostly in the opposite direction when the background wind is weak. The energy source of downgoing wave packets was evidently related to the instability in the upper height range (10.5–11.7 km) where strong wind shear existed. When the background wind and wind shear are stronger, the vertical group and phase velocities may propagate in the same direction. We also found from numerical simulation and data analysis that the wave packet of gravity waves following power law spectrum are short-lived. A by-product of the group velocity measurement is that the horizontal wavelength may also be deduced from a vertical radar beam measurement from the dispersion relation if it is valid.     

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.     

Night-time equatorial F-region zonal plasma drifts from VHF backscatter radar observations

Night-time equatorial F-region plasma drifts are deduced from VHF backscatter radar observations of F-region irregularities. The zonal drifts reveal large vertical shears. It is found that the irregularity polarization electric field (though small compared to the ambient field) is significant in affecting the observed zonal drifts.     

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.     

Beam-generated electrostatic electron waves: relevance to EISCAT VHF radar measurements

The generation by an electron beam of (slowly) growing electrostatic waves, detectable with the EISCAT VHF radar, is investigated. A set of graphs is derived from which it is possible to estimate, by interpolation, frequency, growth rate and resonant beam velocity for most situations of interest. The dependence of the mentioned variables on propagation direction, beam temperature and density and on ionospheric density over the part of parameter space that is of practical interest in the auroral and subauroral ionosphere is presented and discussed.     

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.     

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.     

The characteristics of VHF radar echoes from the tropopause region near a jet stream

Measurements carried out with the SOUSY radar system to the south-east of a jet stream during a night in November 1981 are described. Particular attention is paid to the association between power and spectral width of echoes returned from the vicinity of the tropopause located near 12 km. A negative association is found between power and spectral width of echoes from 150 m height channels between 10.65 km and 11.70 km, both with the radar beam directed vertically and at 7° to the vertical in the northerly and easterly directions. Evidence for a positive association is only found for height channels centred at 11.85 and 12.15 km and then chiefly with the beam directed vertically. A partial correlation analysis suggests that the possible influences of the finite beamwidth of the radar system and of wind shear on the spectral width do not affect the associations found. The results are discussed in terms of the scattering and partial reflection mechanisms giving rise to the echoes.     

MST radar studies of wind and turbulence in the middle atmosphere

A survey is presented of recent developments in the observation of wind and turbulence in the stratosphere and mesosphere using MST radars. One of the highlights of these developments is the growing recognition that the MST/ST radar is a valuable tool for routine monitoring of the atmospheric wind field. Furthermore, preliminary observations have shown the feasibility of monitoring atmospheric turbulence as well. Recent observations of mesospheric turbulence support theoretical models that emphasize the role of propagating waves in coupling the lower and middle atmospheres. Scientific groups in several countries are now planning or constructing MST radars so that within a few years observations should be available from diverse geographical locations spanning the globe.     

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.     

Intense turbulence in the polar mesosphere : rocket and radar measurements

Simultaneous observations of polar mesospheric summer echoes (PMSE) have been made with two different frequency radars during the launch of a sounding rocket designed to measure the fluctuations in the electron density in the same height range. The cross-section for radar backscatter deduced from the rocket probe data under the assumption of isotropic turbulence is in reasonable agreement with the measured signals at both 53.5 MHz with the mobile SOUSY radar and 224 MHz with the EISCAT VHF radar, which correspond to backscatter wavelengths of about 3 and 0.75 m, respectively. Some controversy exists over the relative roles of turbulent scatter vs specular reflections in PMSE. A number of characteristics of the data obtained in this experiment are consistent with nearly isotropic, intense meter-scale turbulence on this particular day. Since equally compelling arguments for the importance of an anisotropic-type mechanism have been presented by other experimenters studying PMSE, we conclude that both isotropic and anisotropic mechanisms must operate. We have found the inner scale for the electron fluctuation spectrum, which corresponds to the diffusive subrange for that fluid, and have compared it to the inner scale for the neutral gas. The latter was found from the Kolmogorov microscale, which in turn depends on the energy dissipation rate in the gas. We found the dissipation rate from the spectral width of the 53.5 MHz backscatter signal and from the rocket electron density fluctuation data. The diffusive subrange was found to occur at a wavelength a factor of about 10 times smaller than the viscous subrange. This corresponds to a Schmidt number of about 100. High Schmidt numbers have been reported in recent measurements of the diffusion coefficient of the electrons in this height range made with the EISCAT incoherent scatter radar. About 15 min after the rocket flight an extremely high radar reflectivity was found with the SOUSY system. We have been able to reproduce this high level theoretically by scaling the rocket data with an increase in the neutral turbulence energy dissipation rate by a factor of 14 as deduced from the SOUSY spectral width, an increase in the electron density which is consistent with riometer data, and a 33% decrease in the electron density gradient scale length which is hypothesized. We also estimate the radar reflectivity at 933 MHz and conclude that signals in excess of thermal scatter levels would have occurred at the peak of the event studied, provided that the electron fluctuation spectrum decreases as k⁻⁷ in the viscous subrange. If the spectrum has an exponential form, however, a turbulent source cannot explain the enhanced 933 MHz echoes reported by EISCAT.     

Studies of high latitude mesospheric turbulence by radar and rocket 1: energy deposition and wave structure

During early spring, 1985, the MAE-3 (Middle Atmospheric Electrodynamics) Program was conducted at Poker Flat Research Range, Alaska to study the origin of wintertime mesospheric echoes observed with the Poker Flat MST radar there, by probing the mesosphere with in situ rocket measurements when such echoes occurred. Pre-launch criteria required the appearance of echoes exhibiting some wave structure on the MST radar display; these could be met even under weak precipitation conditions with riometer absorption near or above 1.0 dB. Two morning rockets were launched under such conditions, the first (31.048) on 29 March 1985, at 1703 UT and the second (31.047) on 1 April 1985, at 1657 UT. Both payloads were deployed on a high altitude parachute near a 95 km apogee to provide a stable platform for data acquisition within the mesosphere (below 80 km). Each payload carried a solid state detector to measure energetic electrons between 0.1 and 1.0 MeV and an NaI crystal detector to measure x-rays from >5 to >80 keV. Payload 31.048 also carried a positive ion ‘turbulence’ probe which measured ion density changes (ΔN_i/N_i) during payload descent, whereas 31.047 carried a nose tip ‘turbulence’ probe designed to measure electron density changes (ΔN_e/N_e) during upleg ram conditions plus a Gerdien condenser for the measurement of bulk ion properties during downleg. The energy deposition curves for each event exhibited peak deposition rates between 75 and 80 km with a half width of 16–18 km, almost exclusively induced by precipitating relativistic electrons. They also showed a maximum bottomside gradient between 65 and 75 km. Radar echoes and atmospheric turbulence were observed in the same altitude domain, consistent with the anticipated need for adequate free thermal electron gradients to make such phenomena visible on the radar. The vertical wave structure from radar echoes was found to be consistent with that observed in horizontal wind and temperature profiles measured by Datasondes flown shortly after each large rocket. An analysis of the wave structure from radar data has shown that although large scale waves (λ_z ~ 7 km) were found to be present, a higher frequency shorter wavelength (∼ 1–3 km) component probably played a more significant role in modulating the signal-to-noise structure of the radar echoes.     

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.     

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.     

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.     

Consistency of rocket and radar electron density observations : implication about the anisotropy of mesospheric turbulence

We report about a quantitative comparison of rocket observations of electron density fluctuations and simultaneous 53.5 MHz radar measurements that were obtained during the MAC/SINE campaign in northern Norway in summer 1987. Out of three rockets launched during the Tur-bulence/Gravity Wave salvo on 14 July 1987, two were flown during conditions that allowed a detailed investigation. For a large part of the data from these rocket flights it is found that the radar reflectivity is about 10 dB, enhanced over what would be expected from the rocket observations in the case of isotropic electron density fluctuations. The observations can be reconciled under the assumption of an anisotropic turbulence. Assuming a simple model spectrum for the electron density fluctuations, we derive a relation between the rocket and radar observations that covers the whole range from isotropic turbulent scatter to Fresnel scatter at horizontal density stratifications. For the observed dataset, an anisotropy which typically corresponds to a ratio of the horizontal to the vertical coherence length of about 10 is consistent with the comparison of rocket and radar observations. A similar anisotropy is found also from the observed aspect sensitivity of the radar echoes. The variation of the anisotropy with height and time shows an anticorrelation with the turbulence level of the mesosphere as deduced from the spectral width of the radar echoes. The anisotropy is found to maximize in heights where the electron density displays deep ‘bite-outs’. These depletions in the electron density were independently observed by a Langmuir and an admittance probe on board two of the rockets.