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.     

Neutral wind measurements over Waltair using meteor wind radar

A coherent pulse Doppler Radar System at Waltair (17°43′N, 83°18′E) has been used to measure neutral winds in the altitude range 80–110 km. Preliminary results obtained during July and August 1979 are presented. The diurnal component is found to be the dominant tidal mode for most of the time. The results are compared with meteor wind observations at other stations.     

Interpretation of elevation-scan HF backscatter data from Losquet Island radar

Most methods using HF ground backscatter radar data to estimate the ionospheric bottomside electron density profile rely upon multi-frequency measurements of the minimum group delay. However, information of the same nature can also be extracted at a single frequency if the elevation angle can be precisely controlled. We outline the analysis of this technique, known as elevation-scan backscatter sounding. The relevant parameter estimation problem is studied using a Bayesian approach. We report on an experiment using the Losquet Island radar to illustrate this method. The performance is compared to ionosonde data. This technique provides a method of teledetection of the bottomside F-region electron density profile hundreds of km from the radar site: however, further development is needed to provide increased reliability of the estimates.     

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.     

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.     

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.     

Incoherent scatter radar contributions to high latitude D-region aeronomy

Recent aeronomical work on the high latitude D-region is reviewed, restricting the discussion to observations of the D-region by the incoherent scatter technique. Emphasis is given to chemical aeronomy, which governs part of the coupling between the neutral atmosphere and the ionosphere, and forms the basis for the global role of the high latitude D-region. Details of the dynamics of the high latitude D-region, and thus the actual coupling with regions below and above, are, however, not discussed in this context. The aeronomical consequences of special high-latitude phenomena are discussed. These include the effects of the polar summer, precipitation of high energy electrons during auroral substorms and high ionization of the D-region during solar proton events. A detailed discussion is given on selected studies concerning the series of solar proton events that occurred in 1989. Problems of ion and neutral chemistry are readily accessible with incoherent scatter measurements through chemical modelling of the D-region. In this way the continuous nature of incoherent scatter measurements can be utilized to expand our knowledge of the D-region, which earlier was mainly based on momentarily sounding rocket experiments. However, it is pointed out how the interpretation of incoherent scatter data from the D-region strongly benefits from multi-instrument configurations. An outline is given of a possible new development based on the combined use of the Tromsø heating facility and the EISCAT incoherent scatter radars.     

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.     

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.     

The amplitude of auroral backscatter: II. Topology of the backscatter range-azimuth distribution

The range-azimuth distribution of auroral backscatter echoes received at Essoyla at frequencies of 93 and 45 MHz is predicted for a model which includes the effects of electron density, magnetic aspect angle, and the azimuth of current flow and also takes into account ionospheric refraction. The distribution is in the form of an arc with maxima of backscatter in both eastern and western wings. As the electron density increases, the intensity of the backscatter increases more rapidly and the azimuths of maximum backscatter separate even further. For currents flowing along the L-shells, the backscatter is strongest in the eastward wing. This asymmetry is intensified if the current flow is rotated anti-clockwise but if the current flow is rotated sufficiently in a clockwise direction the backscatter is stronger in the westward wing. These predictions are supported by observations made at Essoyla.     

The excitation of spectral lines in faint meteor trains

The dissociative recombination of meteoric oxide ions in meteor trains has been examined with a view to determining the spectral characteristics of the associated radiation. Probabilities for the excitation of specific product states have been obtained on a simple statistical basis leading to approximate relative line strengths. For a typical stony meteor the main emitters are found to be five mulitplets of Fe, three of Mg and one of Ca with smaller contributions from several weaker multiplets of Fe and forbidden transitions of Si and Fe. The weighted mean wavelength of these lines is ∼440 nm (or, as seen by the dark-adapted eye, ∼ 490 nm) and, on average, ∼0.25 visible photons arc emitted per recombination. With the possible exception of the sodium D-lines (due to a non-ionic process) recombination lines probably dominate in the emission from trains of meteors fainter than visual magnitude ∼−5. Radiation arising from mutual neturalization of meteoric ions by O⁻ ions is probably due to four of the above mentioned multiplets (two of Mg and two of Fe) and the forbidden oxygen line at 557.7 nm might also be detectable. At ∼90 km the photon output due to neutralization is expected to be an order of magnitude less than that due to recombination. At lower heights neutralization by O₂⁻ becomes more significant, giving rise to many visible multiplets and possibly molecular bands of O₂.     

Angle-of-arrival oscillations in the mesosphere as seen by medium frequency (MF) radar

The Saskatoon MF radar operates continuously in the spaced antenna mode with 5 min resolution, primarily for horizontal wind analysis at mesospheric heights. Mean Doppler frequency and antenna-pair phase difference are available as byproducts. Clear oscillations are sometimes seen in the latter. Spectral analysis shows that these also occur in the Doppler and extend over a range of heights. The two cases shown are almost certainly perturbations in specular reflecting layers moving with horizontal velocity close to that of the wind. Downward phase propagation is apparent, and when the oscillations are present, the wind appears to be decelerating. The tentative explanation of a breaking gravity wave decelerating the mean flow is put forward, although the relations between frequency, and horizontal and vertical wavelengths, contradict linear gravity wave theory.     

Sky-wave backscatter observations of sporadic-E over Japan

Observations of sporadic-E (Es) activity have been carried out during 1970–1974 by using a 25 MHz sounder operating at Iioka, Japan (36°N). Earth backscatter echoes were presented on range-azimuth-intensity plots and recorded on facsimile paper.Examination of individual Es clouds has shown the variations in sizes and lifetimes with time of day and season. Es clouds drifted predominantly toward the west-southwest. Analysis of drift vectors revealed the presence of a north-south variation of the drift direction with a semi-diurnal period. The observed Es cloud motions are consistent with the interpretation that Es clouds move with the neutral winds in the lower E-region. The seasonal dependence of azimuthal occurrence of Es echoes revealed the north-south and east-west difference over Japan. The north-south difference is found to be due to the seasonal movement of the latitudinal position of the maximum Es activity. The east-west difference which is mainly caused by the difference in backscatter coefficient between land and sea was found during only a few non-summer months.