Radio meteor wind analysis using finite element approximation

A method for decomposing radio meteor wind data into the instantaneous velocity component functions is introduced. The technique employs finite element least squares approximation, and is spectrally unbiased. It is also limited in resolution essentially only by the data. The decomposition is demonstrated on known test function and on actual meteor wind data.     

The Grahamstown all-sky meteor radar

A coherent-wave meteor radar system which has been in operation at Grahamstown since April 1986 is described, and some results obtained to date briefly reviewed. The radar is of the monostatic type, operating at a fixed frequency near 28 MHz, square-wave modulated at 500 Hz. The effective antenna pattern, produced by an array of simple half-wave dipoles, consists of two oppositely directed broad beams which provide coverage of all azimuths. The direction of arrival and doppler shift of each meteor echo is determined, enabling wind motions in the meteor region to be deduced. 8, 12 and 24 hr periodicities are present at most times, superimposed on a nearly eastward prevailing wind. A “2-day” oscillation was strongly present in the meridional wind early in 1987. Observations of meteor shower activity are also presented, with some emphasis on the Comet Halley stream. Plans for future improvements to the system are briefly discussed.     

Response of high frequency radar to meteor backscatter

The response of a High Frequency (HF) Radar System to echoes backscattered from underdense meteor trails is calculated. Three propagation modes are identified according to whether the echo is received along the direct (line-of-slight) path, or along two possible paths from beyond the horizon involving ionospheric reflection. The system response contours in terms of meteor radiant position on the sky are presented in the altitude-azimuth and celestial ecliptic coordinate systems. Diurnal echo rate curves are deduced for point radiants, which correspond to meteor showers and for a density distribution of radiants which is appropriate to sporadic meteors.The calculations are compared with observations of integrated meteor echo power from sporadic meteors made with an experimental radar system at frequencies throughout the HF band. Satisfactory agreement is reached between predictions and observations as functions of time of day, radar frequency and range. The extension of observations to include ionospherically propagated echoes permits meteor echo rates to be simultaneously monitored over an area of the Earth's surface of the order of 10⁶ km² with a single radar system.A greater than normally accepted echo rate is required to explain our observations. However, we believe that this enhanced rate is consistent with the true echo height distribution and the attenuating effects of trail initial radius and diffusion, which are particularly severe at the radar frequencies normally used for meteor detection near and beyond the top of the HF band. Our echo rate is consistent with the meteoroid cumulative mass distribution which may be inferred from a simple interpolation between satellite and visual measurements.     

Long period wind oscillations observed by the Kyoto meteor radar and comparison of the quasi-2-day wave with Adelaide HF radar observations

We have detected wind oscillations with periods ranging from 1.4 to 20 days at 80–110 km altitude using Kyoto meteor radar observations made in 1983–1985. Among these oscillations, the quasi-2-day wave is repeatedly enhanced in summer and autumn. We found that the period of the quasi-2-day wave ranges from 52 to 55 h in summer, and becomes as short as 46 to 48 h in autumn in 1983 and 1984. The change in the wave period seems to coincide with a decrease in the amplitude of the zonal mean wind. A quasi-2-day wave event was simultaneously observed in January 1984 at Kyoto (35° N, 136°E) and Adelaide (35° S, 138° E), which are located at conjugate points relative to the geographic equator. Amplitudes of the meridional component at Adelaide are approximately four times larger than those observed at Kyoto. Comparison observations clearly show that the meridional component is in phase and the zonal component is out of phase, respectively, implying antisymmetry of the quasi-2-day wave between the northern and southern hemispheres. Relative phase progressions with height are similar between the Kyoto and Adelaide results for both meridional and zonal components, and indicate the presence of an upward energy propagating wave with a vertical wavelength of about 100 km.     

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 meteor radar winds from two locations in the British Isles

Winds and tides have been measured by a two-station meteor radar system which has increased spatial resolution compared with single station radars used in the past. Narrow radar beams, pointing SW from Sheffield (53.5°N, 1.6°W) and 30°N of W from Shrivenham (51.5°N, 1.6°W), are arranged to converge over the U.K. MST radar site near Aberystwyth, thus defining a unique atmospheric volume in which meteor wind components are simultaneously measured from the two radar sites. The resultant ‘true’, or local, wind vector is compared with the spatially averaged vector obtained with the aid of beams pointing SW and NW from Sheffield only. It is found that the ‘true’ and averaged tidal winds are in good agreement, as expected from their large scale sizes, and that the main advantages of the dual station technique lie in the resolution of a small scale structure such as that related to internal atmospheric gravity waves. By the simultaneous deployment of two-station meteor radar, MST radar and LIDAR, such waves may now be studied through a large vertical section of the atmosphere in a geographically localized area.     

Seasonal and latitudinal variations of the lower thermospheric tidal winds from meteor radar measurements in the U.S.S.R.

Meteor wind results obtained at 93–95 km altitude at seven sites, six in the Soviet Union and one in Antarctica, between 1965 and 1985 are reported. Attention is focussed on the amplitudes and phases of the semi-diurnal tide, showing a 22-yr oscillation, and of the generally weaker diurnal tide. The measurement results are compared with the results of theoretical models.     

Thermospheric wind measurements with EISCAT

Thermospheric wind measurements with the EISCAT UHF radar around the evening Harang discontinuity are presented both in the E- and F-layers. Within the E-layer auroral oval the Lorentz and Coriolis force are shown to be more or less in balance. The neutral velocity is a factor of the order of two smaller than the ion velocity and is on average advanced 90° in a clockwise direction compared to the ion velocity. In the low electron density region just before the Harang discontinuity and outside the auroral oval a large (~250 m s⁻¹), thermally dominated neutral wind is closely followed by the ion wind in the antisolar direction. There is also a large downward flow present just before the Harang discontinuity. In the F-layer the neutral wind approximately follows the ion convection pattern, except for a couple of hours after the sudden change in the ion convection just after the passage of the evening Harang discontinuity. The close resemblance between the equilibrium ion and neutral flow when the neutral-ion collision frequency is close to twice the Earth's angular velocity may be connected to back pressures created by Joule heating in the case of an appreciable ion-neutral velocity difference.     

Determination of the vertical neutral temperature and wind profiles using EISCAT and HF Doppler radar

Simultaneous observations by EISCAT and an HF Doppler system of a TID are presented. Crossspectral analysis of the data allows the vertical variation of the neutral temperature and horizontal wind in the thermosphere to be determined.     

Some aspects of Doppler radar measurements of the mean and fluctuating components of the wind field in the upper middle atmosphere

The basic assumptions made when a Doppler radar is used to measure the mean and fluctuating components of the wind field in the middle atmosphere with various beam configurations are examined. Particular reference is made to the measurement of the various components of the Reynolds stress tensor associated with short period internal gravity waves. It is shown that it is not generally possible to measure the upward flux of horizontal momentum with the conventional Doppler radar beam configuration in the upper middle atmosphere and that an optimum beam configuration is that in which beams are directed at +θ,0 and − θ to the zenith in both the zonal and meridional planes. This allows five of the six components of the Reynolds stress tensor (all those except the horizontal transport of momentum) to be obtained directly from the mean square radial velocities. In addition, the mean wind components and, in principle, the horizontal divergence and stretching deformations may be obtained. The power spectrum of the horizontal velocity may also be calculated using only the assumption that the statistics of the motions are horizontally homogeneous.     

Ionospheric wind measurements and activities in Swansea

Some of Sir Granville Beynon's contributions as a person and as an eminent physicist during 1956/57 at the University College of Swansea are considered. In particular, the spaced-receiver measurements of winds in the lower ionosphere are discussed; events and activities in that era are recalled.     

Summertime stratospheric wind measurements above the South Pole

Observations of the mean wind flow and wave motions in the stratosphere at the South Pole are presented. The atmospheric motions are determined from the tracking of a high altitude, zero-pressure balloon launched from Amundsen-Scott Station during the austral summer of 1985–1986. The balloon position was precisely monitored by an optical theodolite for a large portion of the flight so that small scale motions could be resolved. The mean flow above the pole was approximately 3ms⁻¹. Atmospheric motions characteristic of internal gravity waves were observed with an intrinsic period of approximately 4.5 h and vertical and horizontal wavelengths of approximately 2.5km and 125km, respectively. The horizontal perturbation velocity of the observed waves was large compared to the mean horizontal flow velocity. The implication is that wave motions play a dominant role in the transport of stratospheric constituents in regions where the mean winds are light, such as over the South Pole during austral summer.     

Seasonal variation in mesospheric semi-diurnal tides. Comparison of meteor radar observations and results from an excitation source model

The seasonal variation of the semi-diurnal tide is well established in the upper mesosphere from meteor radar observations, such as those made at Garchy (France). A classical propagation model, using a realistic excitation source from ozone and water vapour solar heating, can account for most of the seasonal variation characteristics, and in particular the strong difference between summer and winter features.     

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.     

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.     

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.     

One full-day radar measurement of lower stratospheric winds over Jicamarca

The lower stratospheric wind field is measured simultaneously at several altitudes for 24 h on 23–24 May 1974 using the Jicamarca radar in Peru. Maximum entropy spectral analysis is used to estimate echo power spectra from the measured autocorrelation functions which are truncated beyond the maximum time lag of 6.65 s. The fluctuating wind velocity is resolved into steady and tidal components. A vertical shear of 2–3 m s⁻¹ km⁻¹ is observed throughout the time of observation in the zonal wind. The mean zonal wind is in good agreement with the Lima-Callao radiosonde data obtained in the same month. A very large diurnal component, of amplitude 1–5 m s⁻¹ is observed in the zonal wind in the lower stratosphere. However it appears that a semidiurnal oscillation below the tropopause is predominant.