Meteor Heights Research Articles

Atmospheric trajectories and light curves of shower meteors

Double station data on 496 meteors belonging to several meteor showers were obtained within the program of the video meteor observations during years 1998-2001. Analyzed meteors cover a range of photometric masses from 10 -7 to 10 -4 kg with a corresponding range of maximum brightness from +4.7 to -2.1 absolute magnitude. Atmospheric trajectories of Perseid, Orionid and Leonid meteors are analysed. These typical cometary high velocity meteors are compared to Geminid meteors with probable asteroidal origin and Taurid meteors - another cometary shower with significantly lower entry velocity. The light curses of the studied meteors vary widely, but generally are nearly symmetrical with the point of maximum brightness located close the to middle of the luminous trajectory. Small differences between showers are reported. We found that the height data are in good agreement with the dust-ball model predictions. The only difference is the beginning height behaviour. The beginning heights of cometary meteors increase with increasing photometric mass. These meteoroids probably contain a volatile part which starts to ablate before we are able to detect the meteors. The Geminid meteors are a different case. They start to ablate suddenly and their beginning height is almost constant in the whole range of studied meteoroid masses. In this case we observe real beginnings of meteor ablation.

Meteor radar observation of circulation near mesopause over Wuhan

The Wuhan meteor radar is the first all-sky meteor radar in China. The circulation near the mesopause from February to September 2002 is studied based on the data obtained by the Wuhan meteor radar. The zonal wind is usually eastward in winter from 80 to 100 km. The mean zonal wind become westward from March to early of May, when the winter circulation reverses to the summer circulation. But at the meteor heights, the zonal circulation usually becomes eastward after the middle of May. The meridional circulation is always equatorward. The peaks of the meridional wind move downward with the height, and the peak value reaches its maximum of 21 ms−1 in July. Having made a comparison between monthly mean wind and HWM93 model wind, the value of southward wind, the maximal value of eastward wind as well as the reversal height of zonal wind are found to be quite different from each other.

MF radar observations of mean winds over Yamagawa (31.2°N, 130.6°E) and Wakkanai (45.4°N, 141.7°E)

Continuous MF radar measurements of mesospheric mean winds are in progress at the observatories in Yamagawa (31.2°N, 130.6°E) and Wakkanai (45.4°N, 141.7°E). The observations at Yamagawa and Wakkanai were started in August 1994 and September 1996, respectively. The real-time wind data are used for the study of major large scale dynamic features of the middle atmosphere such as mean winds, tides, planetary waves, and gravity waves, etc. In the present study of mean winds, we have utilized the data collected until June 1999, which include the simultaneous observation period of little more than two and a half years, for the two sites. The database permits us to draw conclusions on the characteristics of mean winds and to compare the mean wind structure over these sites. The mean prevailing zonal winds at both sites are dominated by westward/eastward motions in summer/winter seasons below 90 km. Meridional circulation at meteor heights is generally southward during most times of the year and it extends to lower mesospheric heights during summer also. The summer westward jet at Wakkanai is consistently stronger than those at Yamagawa. However, the winter eastward winds have identical strength at both locations. Meridional winds also show larger values at Wakkanai. The mean wind climatology has been examined and compared with the MU radar observations over Shigaraki (34.9°N, 136.1°E). The paper also presents the results of the comparison between the MF radar winds and the latest empirical model values (HWM93 model) proposed by Hedin et al. (1996. Journal of Atmospheric and Terrestrial Physics 58, 1421–1447). Hodograph analyses of mean winds conducted for the summer and winter seasons show interesting similarities and discrepancies.

Ozone concentration at meteor heights determined from forward-scatter radar echoes

The ozone concentration has been measured in the meteor zone using a method of radar echo duration distribution suggested by Jones and Šimek (1995). Data obtained by the forward-scatter system during the Lyrid meteor shower period in five consecutive years 1992–1996 agree with the Solar Mesosphere Explorer results for the same heights of 95 km. The mean value of the ozone concentration [O 3]=(1.56±0.05)×10 14 m −3 has been derived from a total of 6596 radar meteor echoes with duration ≥1 s. The results for individual years indicating a possible decrease in [O 3] are discussed.

Super Dual Auroral Radar Network observations of meteor echoes

Radar echoes from ranges less than 500 km are routinely observed by the Super Dual Auroral Radar Network (SuperDARN) on most days. Many of these echoes have properties which are markedly different from what one would expect from E or F region irregularities. We show that these unusual short‐range HF echoes are due to scattering off meteor trails. This explains why, among other things, the Doppler shift from the short‐range echoes taken from the SuperDARN Saskatoon antenna are consistent with the mesospheric winds observed by the Saskatoon MF radar. This means that the SuperDARN radars can be used to study neutral winds at meteor heights, a result which is especially interesting since it opens up the capability for a global coverage of mesospheric winds using the worldwide distribution of SuperDARN radars.

Fragmenting Particles During 1989-1994 Perseid Radar Observations

AbstractThe occurrence of fragmenting particles was investigated from radar observations of the Perseids at Ondřejov Observatory. Diurnal patterns of fragmenting meteoroids are presented. Their minimum occurrence coincides with the culmination of the Perseid radiant while the maximum appears at the time of sunset at meteor heights. Solar longitude of the main contribution of fragmenting meteoroids at L⊙ = 138°86±0°04 (equinox 1950.0) fits with the peak activity of the new filament in the stream. This supports the concept of the low age of the filament. The maximum proportion of fragmenting particles occurred in 1991.

A note on the determination of ozone concentrations from radio-meteor duration distributions.

Hajduk, et al (1992) have drawn attention to a possible source of error in our earlier analysis of visual and radar observations of shower meteors from which we calculated the ozone concentration at meteor heights. We have considered their main criticism that the features of our data which we used to calculate the ozone concentration may have been due to the constraint that our meteors had to be observed by both methods. We have shown that observations collected from radar-only systems show very similar characteristics and that these can be analysed without recourse to visual data to produce very similar results to those from our original analysis.

Perseid meteor stream 1991 – 1993 from TV observations in Kiev

Some results of the double station television meteor observations provided in Kiev during the Perseid shower period in years 1991 – 1993 are presented. The dependence of the beginning hights of meteors on their initial velocities based on 57 best reduced double station meteors is constructed. Exceptionaly great beginning heights of meteors are discussed, as well as the exceptional case of a Perseid bolide.

The Heights of Radar Meteors

Previous measurements of the heights of radio meteors using VHF radars have distributions peaking at around 95km with few meteors detected above 105km. Using HF radars (frequencies 2 and 6MHz) near Adelaide, South Australia, we have now shown that there are many more meteors than previously believed ablating at above 100km, with the peak being at 110-120km, and meteors being detected to at least 140km. The gross difference from VHF measurements, as typified by our own observations at 54MHz, is because the higher frequencies are severely biased by the so-called “echo ceiling” effect. The HF height distributions have been confirmed by co-workers using the Jindalee over-the-horizon radar in central Australia, which operates at frequencies of 6-30MHz. These results have implications for a number of different areas of atmospheric research; for example the existence of metallic ions at altitudes of 140km and above, since ablation is apparently occurring much higher than was previously thought to be the case.

The characteristics of the mesospheric two-day wave as observed at Grahamstown (33.3°S, 26.5°E)

The characteristics of the quasi two-day wave, as observed in meteor wind data recorded at Grahamstown (33°19′S, 26°30′E) between April 1986 and April 1989, are described and discussed in the context of existing knowledge. As is typical, the wave amplitude has been largest during the summer months December–February with a maximum in late January, but this amplitude has differed markedly from year-to-year. The period and phase are found to be variable and, where possible, have been obtained as functions of date. January 1987 and 1989 were both characterised by clear drifts toward longer periods, with corresponding drifts in phase. At each of the summer maxima observed the period was found to be close to 48 h, with the phase of the meridional component within 3 h of local midnight. Direct comparison of local data for January 1987 with published data obtained simultaneously in Australia and Antarctica confirms the well-established westward propagation of the wave, but with an apparent zonal wavenumber somewhat smaller than the expected value of 3. It is shown that the discrepancy could arise from a combination of observational effects and a northward tilt to the wave velocity. The best result of an attempt to detect a horizontal phase gradient from local data alone is, however, more consistent with a southward tilt. There is theoretical support for both conclusions, but the matter cannot be resolved with the data presently available. It is also concluded that the background circulation at meteor heights has little influence on the wave parameters, and that indications of wave activity outside the summer season are of doubtful significance.

Tides in the middle atmosphere

Some recent progress in the study of tides in the middle atmosphere are reviewed, with special emphasis placed on radar observations at high latitudes, as well as data analysis methods used in the calculation of tidal structures. Observations carried out outside the meteor zone with MST radars and satellites are also presented. Theoretical and numerical advances on the diurnal tide are extensively discussed. Finally, some outstanding problems, which we hope will be solved in the near future are raised: the existence of hemispheric asymmetries in tidal structure; the role played by non-migrating modes at meteor heights and short time scales variations of tides.

The height distribution of radio meteors: observations at 2 MHz

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.

Wind gradients at meteor heights over mid-latitude stations

The gradients of amplitude and phase of diurnal and semidiurnal components of zonal and meridional winds in the height intervals of 80–90 km and 90–100 km are studied using the meteor wind radar data collected at Atlanta (34°N, 84°W) during the period August 1974–March 1978. The results are compared with those at Adelaide (35°S, 139°E). It is found that the gradients vary in an opposite manner between the two height intervals.

Numerical simulations of atmospheric tides for solstice conditions

Numerical simulations are made to study the behavior of atmospheric tides in an atmosphere with arbitrary distribution of mean temperature and zonal wind. Owing to their importance at meteor heights the parametrization of diffusive processes is discussed, and two kinds of models are used, depending on the particular mode studied. Specific attention is given to the (1, 1) mode of the diurnal tide because we want to take into account the modification of its latitudinal‐vertical structure due to the background atmosphere. Simulations of semidiurnal and diurnal tides are presented and compared with previous theoretical results and experimental data. However, our main goal is an improved theoretical description of the diurnal tide. In particular, (1, 1) mode “distortions” due to mean wind are shown to be important in accounting for the seasonal variation of the amplitude of the diurnal tide existing in experimental data. The action of turbulent diffusion on the diurnal tide in our numerical results is also analyzed.

Comparison observations between the MU radar and the Kyoto meteor radar

We have performed a comparison between the MU radar and the Kyoto meteor radar on August 14–15, 1984. The MU radar provided vertical, zonal and meridional wind components in the 65‐ to 85‐km region during daylight hours when echoes from the turbulent scattering were intense. Meteor echoes were also received by the MU radar, and utilized to measure winds at meteor heights. Comparisons among these MU radar measurements and Kyoto meteor radar observations have shown fairly consistent wind profiles at 65‐ to 105‐km altitudes. To interpret differences in the wind fields observed in 2 successive days, it was necessary to consider the behavior of quasi 2‐day oscillation deduced from long‐duration Kyoto meteor radar observations.

Seasonal variation of solar atmospheric tides at meteor heights.

Systematic observation of the Kyoto meteor radar (35°N, 136°E) has been continuing since the end of 1977. A seasonal variation of diurnal and semidiurnal solar atmospheric tides at meteor heights has been studied by using meteor radar data. Monthly average behaviours of these tidal components have been summarized for the period from March 1979 to November 1980. Vertical profiles and harmonic dials of these components have been investigated in comparison with other meteor radar data. Complicated behaviour of diurnal tide can be explained by an interference between propagating and evanescent modes. The evanescent mode becomes dominant in summer, while the propagating mode appears in winter. As for semidiurnal tide, the S2, 2 mode is fundamental in summer, and the S2, 4 mode becomes apparent in winter.

Evidence of planetary-scale waves in the lower thermospheric winds

Significant oscillations in the zonal wind at meteor heights with periods between 2 and 10 days have been found at Budrio (450N, 120 E) during four campaigns carried out in 1976, 1978, 1979 and 1980. These oscillations have now been established as the effect of zonally travelling planetary-scale waves. Long-period oscillations of nearly 2 and 5 days essentially occur in summer and winter, whereas, for equinoctial campaigns, waves of longer period are observed. Quasi-2-day oscillations represent probably an atmospheric free mode which exhibits considerable amplification after the summer solstice. Preliminary comparisons on this summer anomalous aspect have been made with the results obtained at Sheffield (U. K.) in the course of two cooperative campaigns performed in 1979 and 1980.

Numerical simulation of semidiurnal atmospheric tides

Numerical modeling of the solar and lunar semidiurnal atmospheric tides has been performed by invoking a comprehensive approach that includes both algebraic manipulation and numerical solution of the primitive equation system. The effect of latitudinally varying temperature and associated mean zonal wind in a modeled atmosphere with realistic forcing are successfully incorporated to reproduce tidal variabilities at meteor heights delineated by varieties of radar observations.

Inertio‐gravity wave induced accelerations of mean flow having an imposed periodic component: Implications for tidal observations in the meteor region

The semidiurnal harmonic exhibits great day‐to‐day variability in amplitude and phase. In addition, the variability appears to be substantially local and random, suggesting a connection with gravity wave activity. We suggest that a significant contribution to the observed semidiurnal harmonic at meteor heights might result from inertio‐gravity wave induced accelerations of the mean flow. The rate of wave forcing of the mean wind is related to the Doppler‐shifted phase velocity, so that during alternate phases of an imposed mean wind oscillation interactions with waves that accelerate the mean wind in opposite senses may be favored. Thus the imposed mean wind may modulate the mean‐flow acceleration at the imposed frequency. In this view, the semidiurnal character of the acceleration is a manifestation of the modulation of the interaction process by the semidiurnal tide. The variability of the semidiurnal harmonic would then reflect the local variability of inertio‐gravity wave fluctuations and also nonlinear feedback on the waves. Calculations with a simple time‐dependent wave‐mean‐flow model indicate that a wave‐induced component of the semidiurnal harmonic with amplitudes comparable to the semidiurnal tide itself is possible.

Observations of lower ionospheric wind by the Kyoto Meteor Radar

This paper describes the results of meteor radar observations which have been carried out at Kyoto University since December 1977. The radar, a coherent pulse Doppler radar with transmitting frequency of 31.57 MHz and nominal peak power of 10 kW, measures wind fields at meteor heights of 80–110 km. The solar semidiurnal tide generally dominates the wind oscillations at these heights. The less regular diurnal tide has comparable magnitude. The variability of the tidal structure suggests the possibility of interaction between respective waves or winds, particularly the influence of disturbed conditions in the lower atmosphere. Other components of winds as mean flow or a quasi 2‐day planetary wave have also been observed. The combination of meteor wind radar and other observational techniques will further the understanding of the global dynamics of atmospheric waves.