W.G. Elford

Observations of the quasi 2-day wave near 90 km altitude at Adelaide (35°S)

Abstract A quasi 2-day oscillation in the meridional winds near 90 km altitude has been observed at Adelaide (35°S) during late summer of the years 1966–1975. The mean amplitude in mid-January is 48 m s−1, and the phase variation with height is indicative of a wave with downward phase propagation and a vertical wavelength greater than 100 km.

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

Abstract 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 height distribution of radio meteors: observations at 2 MHz

Abstract Conventional meteor radars, operating at wavelengths of around 5–15 m, are unable to detect high-altitude meteors due to the wavelength-dependent echo ceiling. It is suggested that the ‘missing mass’ in the 10 −6 –10 −2 g range of interplanetary material is in fact a high-velocity component which is normally undetected since it ablates at high altitude. This contention is supported by previous work. In this paper we describe measurements of the heights of radio meteors (limiting magnitude about +7) at a wavelength of 150 m (frequency 2 MHz), for which the echo ceiling is above 140 km. The resultant true height distribution is found to peak at ~ 104 km, about 10 km above the peak found by conventional meteor radars. The majority of meteors are detected at or above this peak, and substantial numbers are seen right up to 140 km. It is therefore concluded that the ‘missing mass’, comprising the vast majority of the meteoric input to the atmosphere, ablates well above 100 km.

The height distribution of radio meteors: comparison of observations at different frequencies on the basis of standard echo theory

Abstract A comparison is made between the observed height distributions of underdense radio meteor echoes measured at frequencies of 2, 6, 26 and 54 MHz and a simple model based on standard theory. This theory takes account of the initial radius of the train, the finite formation time due to the meteor velocity, and diffusion in the time between radar sweeps. The main features of the measured VHF (26 and 54 MHz) height distributions are predicted by this model, with peaks below 100 km and few meteors detected above 105 km. The main features of the measured HF (2 and 6 MHz) distributions are also successfully predicted, with peaks at ~105km. The model indicates few 6 MHz echoes from above 115km, in line with the observational data, although for the data used there is an instrumental cut-off just above this height. It is suggested that even at 2 MHz perhaps 50% of all meteor trains above 105km remain undetected. A comparison of the model and the measured 2 MHz distribution, which displays many meteors to at least 130 km, reveals that the true height distribution continues to rise above 105 km and would peak above 110 km were it not for the limited detectability of such high altitude meteors, even at such low frequencies.

Meteoroid orbital element distributions at 1 AU deduced from the Harvard Radio Meteor Project observations

The orbital element distributions of meteoroids detected during the Harvard Radio Meteor Project, 1968–69 Synoptic Year Program, have been reanalysed to remove selection effects associated with the radar observations. Corrections are made for the observing schedule, antenna beampatterns, the radio diffusion ceiling, speed dependence of ionization production, the flux enhancement due to the Earth’s gravity and the probability of encounter with the Earth. These render the eccentricity, aphelion distance, and inclination distributions for meteoroids larger than 10−4 g (radius ∼200 μm), with orbits that cross the ecliptic near 1 AU.

Response of high frequency radar to meteor backscatter

Abstract 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 Earths 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.

A new method for the measurement of meteor speeds: The pre-to phase technique

In this paper we describe a new technique for measuring the speeds of individual meteors based upon the variation of the phase received in specular reflections from the meteor train prior to the meteoroid reaching the point of closestapproach to the observation site. The technique is characterized by high precision, high yield, and is less subject to selection effects than the techniques used heretofore. About 75% of observed meteors are able to have a speed determined, compared with only 10% previously. It has application to the measurement of the velocities of meteoroid streams and the distribution of speeds of sporadic meteors. The latter is vital to the modeling of the response of HF/VHF radars to meteor backscatter [Elford, 1964; Thomas et al., 1988], as well as having astronomical implications. Results of observations with our narrow-beam VHF radar are presented. Data collection carried out during the epoch of the θ Ophiuchid meteor shower, whose radiant passes through the response function of the VHF radar, renders speeds which are in accord with the previously known speed for that shower. The speed distribution of sporadic meteors is determined and compared with the distributions obtained in previous radar and optical programs

Novel applications of MST radars in meteor studies

Abstract MST radars have been used to detect and study meteors since the mid-1980s. By exploiting the narrow beamwidth, high-pulse repetition rate and measurement of echo phase inherent in such systems, a number of new techniques have been developed specifically for meteor studies. These include: measurements of the speed of entry of meteoroids to a precision of 0.3%; measurements of the deceleration of meteoroids during ablation; observation of the fragmentation of meteoroids. The measurement of meteor drifts and hence the study of the dynamics of the atmosphere in the ‘meteor region’ is now a routine procedure at many MST radar sites. A serendipitous adjunct to MST meteor studies is the measurement of electron density profiles in the E-region using the Faraday effect.

Implications for meteoroid chemistry from the height distribution of radar meteors

Abstract We have gathered substantial evidence from novel radar meteor observations that most meteoroids start to ablate at altitudes considerably in excess of 100 km (at heights up to ∼140 km). This beginning height hb is a strong function of the melting point of the meteoroid. Theoretical modelling indicates that for stony meteoroids observed as faint radio meteors these heights range from hb = 116 km (for entry speeds v∞ = 70 km s−1) to 90 km (for v∞ = 15 km s−1). Meteoroids that begin ablating above these heights must have lower temperature melting points. We interpret the observations in terms of a population of the meteoroids largely constituted of heavy organic compounds that are susceptible to gross-fragmentation as they heat and encounter the Earth at speeds greater than ∼30 km s−1.

Measurement of Faraday rotation of radar meteor echoes for the modelling of electron densities in the lower ionosphere

Abstract Narrow beam VHF radars are able to detect transient ionization at heights between 80 and 120 km produced by meteoroids entering the atmosphere ‘down-the-beam’. Using a vertically directed beam, observations of this new type of meteor echo on two orthogonal receiving arrays enable the orientation of the plane of polarization to be measured. Significant Faraday rotation is observed to occur during daytime at heights above 80 km. Such observations make possible the measurement of ionospheric electron density profiles between heights of 80 and 120 km.