Radio Meteor Echoes Research Articles

Effective mass function of radio meteoroids as a modulating factor in determining atmospheric scale height

Abstract A long-standing problem in meteor science has been the persistent presence of bias in the measured value of atmospheric scale heights obtained from radio meteor echoes. A common practice of fitting a linear function for bias correction follows the assumption that the systematic bias is devoid of seasonal asymmetry. This would be true if the mass and the velocity distribution of meteoroids remain invariant, both spatially and temporally. But so far no such convincing evidence has been published. On the contrary, fundamental arguments suggest that an universal mass function of radio meteoroids is counter-intuitive. This parameter cannot remain invariant due to the intrinsic variability in the meteor response function resulting from the Earth’s motion on the plane of ecliptic. In this paper, we show that an inverse relation exists between the width of meteor height distribution, expressed in unit of atmospheric scale height, and the exponent of the mass function. The overall mean of this exponent for the Sodankylä radar is 1.91 ± 0.02, modulated by a seasonal variation from the mean of the order of ∼±0.1. The stated inverse relation is applied to correct for the effect of mass distribution on the height distribution. Allowing for variable mass correction effectively removes the nonlinear bias in the measured scale heights in the meteor ionisation region.

Radar meteor decay rate variability and atmospheric consequences

Abstract. The reasons for scatter in plots of log(inverse decay times) vs. height for radio meteor echoes are examined, and an explanation for the characteristics is offered. Effects like temperature variability, pressure variation, angular detection accuracy, pulse length, phase errors, plasma processes and variation in meteoroid metallic content are considered. Using computer simulations the observed scatter is reproduced to good accuracy, and then these results are utilized to develop a new procedure that can be used to determine temperatures in the meteor region. These same studies also permit determination of some limited information about the nature of the diffusive expansion process and the variability in the metallic content of meteors. The impact of the quality of phase calibration of interferometric radars on accurate reproduction of atmospheric temperatures is also examined.

Particle mass distribution in the new Perseid 1987–1994 filament

Observations of the Perseid meteor stream in the period 1987–1994 were characterized by an appearance of a new filament the maximum activity of which was found near solar longitude L ⊙ = 138.9° (eq. 1950.0). Its activity culminated in 1993 while 1991 and 1995 observations showed no abnormal increase of meteors in that region. The mass distribution within the new filament has been studied in the interval 138.62° ⩽ L ⊙ < 139.10° during the period of its activity and compared with analogous results from 1958–1986, 1991, and 1995 within the same L ⊙ interval. The mass distribution indices, s, for overdense radio-meteor echoes associated with the new filament show slightly lower values comared with those determined during the activity of the regular stream.

Wind velocity and temperature fluctuations due to a 2‐day wave observed with radio meteor echoes

We study in this paper characteristics of both wind velocity and temperature fluctuations due to a 2‐day wave near the mesopause, by means of meteor echo measurements with the MU (middle and upper atmosphere) radar in Shigaraki, Japan (35°N, 136°E) and a meteor wind radar, newly constructed near Jakarta, Indonesia (6°S, 107°E). A 2‐day wave was conspicuously enhanced on July 12–19, 1993, at both Shigaraki and Jakarta. Fluctuations of temperature and northward wind were inphase in Shigaraki, and they became antiphase in Jakarta, consistent with a prediction assuming a Rossby‐gravity wave. The phase lag of the fluctuations between the two sites suggested that this wave propagated westward with a zonal wavenumber of 3. Seasonal variations of the 2‐day wave in Jakarta were examined by using a frequency spectrum and complex demodulation technique for temperature and wind fields, finding clear enhancements during periods corresponding to summer months in each hemisphere, especially in January 1993. The dominant wave period was fairly stable and was confined in a narrow range centered around 47 and 49 hours during summer months of the southern and northern hemispheres, respectively. The phase variations of temperature preceded those for the northward wind by 180°–225° when an enhanced 2‐day wave was seen in the northward component, indicating their antiphase relation in the southern hemisphere. These results strongly support the interpretation of the 2‐day wave near the mesopause as the (3,0) mode of a Rossby‐gravity wave with zonal wavenumber of 3.

The Annual Variation of Radio Meteor Echoes Observed from 1981 to 1985

AbstractThe radio observations of meteor echoes using FM broadcasting were carried out from January, 1981 to June, 1985. The annual variation of meteor rates in the 5 years was obtained from the observations made from 25h through 29h in local time (16h-20h UT).Some features in this annual variation can be explained by the occurrence of the major meteor showers. After the effects of these major showers are removed there remains a significant annual variation which shows higher rates in the winter half of the year (October to March). This seems to be caused by the annual variation of non-shower meteors radiating from the apex region.

Ozone and the duration of overdense radio meteors

This paper describes a study of the duration distribution of overdense radio meteor echoes using combined radio and visual data gathered over many years. We have paid particular attention to the wellknown change of slope which has previously been attributed to electron attachment to the atmospheric molecules. Using visual and photographic meteor data to estimate the heights, we have determined the height variation of the time constant associated with the electron loss process and we find that it corresponds closely to that of a reaction of meteoric ions with ozone to form oxide ions which quickly recombine dissociatively with the free electrons. We suggest that the duration distribution of overdense radio meteors could constitute a valuable diagnostic for monitoring the ozone concentration in the 80–100 km height range.

On apparent changes in the ambipolar diffusion coefficient determined from radio-meteor echoes

The predicted cooling times for meteor train electrons in a recent calculation ( Baggaley and Webb, 1977) are an order of magnitude smaller than the experimental values of Delov (1975). Delov's results are based on the apparent variation with time of the ambipolar diffusion coefficient deduced from the decay rate of radio-meteor echoes. An alternative explanation for this apparent variation is presented as being due to radio wave reflection processes. Additional experiments are suggested which may be used to determine the source of this effect.

The de-ionizatton of dense meteor trains

Contrary to the predictions of simple models, H.F. radio-meteor echoes only very rarely endure for more than a few minutes. The factors important in limiting echo durations are examined and model duration-height characteristics are compared with a unique record of long-lived echoes from enduring meteoric ionization.

The Effect of Wind Shear Gradients on Underdense Radio Meteor Decay Times

It is shown that wind shear gradients in the meteor region can cause a scatter in the decay time of underdense radio meteor echoes. This model agrees well with the experimental observations of forward scatter radio meteors, but in the case of backscatter echoes the wind shear gradient component of the dispersion in the decay times is much smaller than that observed. It is shown that for backscatter echoes the dispersion in decay times is probably almost completely due to cosmic noise.

On the effect of small-scale wind shears on radio meteor echoes

A theory of the effects of small scale wind shears, that is, those wind shears with vertical wavelengths less than about 6 km, on radio meteor echoes has been developed. It has been found that these wind shears can result in the premature decay of overdense radio meteor echoes and can also cause a scatter in the decay times of underdense radio meteors. Comparison of the theory with observational data indicates that in the meteor region the root-mean-square turbulent velocity associated with these small scale wind shears is <1.6 m sec −1; scatter in the decay times of underdense radio meteors due to these wind shears is estimated to be <20 per cent.

On the mass distribution of faint sporadic meteors

Forward scatter and backscatter determinations of the amplitude distributions of underdense sporadic radio meteor echoes are described. The experimental results lead to an ionization distribution index of 3.13 and are shown to be consistent with an initial train radius of about 80 cm at a height of 100 km. Comparison of the ionization and luminosity distributions of meteor trains indicates a small mass dependence of the geocentric meteor velocity, which we suggest is due to the mass distribution index of meteoroids in retrograde orbits being higher than that of the meteoroid population as a whole.

Wind structure from the amplitude fluctuations in persistent radio meteor echoes

The amplitude fluctuations which are almost always present in persistent radio echoes from meteor trains can be interpreted in terms of the wind structure in the meteor region of the atmosphere. Measurement of parameters describing the onset of the fluctuations are in agreement with a horizontal wind velocity profile which is a sinusoidal function of height, the amplitude of the sinusoid increasing exponentially with height.

On the apparent mass distributions of underdense radio meteors

A method of obtaining the mass distribution index of meteoroids from the amplitude distribution index of underdense radio meteor echoes which takes into account the effects of the initial train radius and the finite velocity has been developed.Application of this method to existing data has enabled a new value for the initial radius of meteor trains to be obtained which is in good agreement with other determinations.