Meteor Trails Research Articles

Evidence of Meteoroid Fragmentation in Specular Trail Echoes Observed Using Gadanki MST Radar

Meteoroids are responsible for deposition of thousands of kilograms of annual mass flux in the Earth’s upper atmosphere but the disintegration mechanisms of these bodies, and hence their composition, still remains a subject of debate in the meteor radar community. The role and significance of fragmentation as a meteoroid disintegration mechanism has been of particular interest in the past few years but in contrast to the head echoes, relatively little work has been done to study the effect and extent of fragmentation on trail echoes observed by the high power large aperture radars. Using the 53 MHz Gadanki MST radar, we present examples of radar meteor trails whose evolution cannot be explained with just the aid of classical meteor ablation theory. These examples are analyzed and discussed on a case-by-case basis and it is reported that the evolution of these trails can be explained with the help of fragmentation. This study will form the basis for future modeling efforts of fragmenting meteor trails and has important implications on the form in which the meteoroid mass is deposited in the upper atmosphere.

Structural evolution of long‐duration meteor trail irregularities driven by neutral wind

AbstractAn experiment on spatial domain interferometry observations of meteor trail irregularities at a low‐latitude location in China was conducted during August 2013 using the Sanya VHF coherent radar (18.4°N, 109.6°E). More than 3 thousand range‐spread meteor trail echoes (RSTEs) were observed. Among the trail echoes, the spatial structure of meteor trail irregularities responsible for a single long‐duration RSTE event persisting for ~4 min was reconstructed. This RSTE was found to be initially generated at 90–115 km altitudes and aligned along the radar beam boresight. After about the first minute of the trail lifetime, the trail echo appeared only in a narrow altitude range of 94–98 km. An analysis on the spatial pattern of the long‐duration RSTE showed that the trail irregularities at lower range gates moved away from the region perpendicular to the geomagnetic field. The eastward drifts of the RSTE irregularities were found to decrease with increasing altitude, e.g., from 80 ms−1 at ~94 km to 20 ms−1 at ~100 km. Simultaneous horizontal neutral wind measurements made with the Fuke all‐sky meteor radar (located north of Sanya) recorded a similar velocity profile. We suggest that the neutral wind could drive the spatial structural evolution of the RSTE irregularities, and help to determine the altitudes where the longest portion of the RSTE was located.

Meteor Trail Echo Rejection in Atmospheric Phased Array Radars Using Adaptive Sidelobe Cancellation

AbstractStrong meteor trail echoes are interferences in the wind velocity estimates made from mesosphere radar observations. Contaminated spectra are detected by their discontinuity and are removed at the risk of greater fluctuations of spectra, leading to a severe reduction of the signal-to-noise ratio (SNR) and inaccurate wind estimates for weak atmospheric echoes. This paper presents an adaptive signal processing technique for the suppression of spectral contaminations by meteor trail echoes. The method is based on the norm-constrained and directionally constrained minimization of power (NC-DCMP), which balances the capability of canceling the clutter and the robustness of beam shaping, at the cost of a slight decrease in the SNR, which can be determined in advance. Simulation results show that with a 3-dB decrease of the SNR being allowed, the method improves the signal-to-interference ratio (SIR) by 15 dB, giving wind estimates that are about 8 m s−1 better in terms of root-mean-square error and providing 4 times as wide an observable range when compared with the results of the ordinary nonadaptive beamforming method. The results for an actual observation show that the improvement of both the SIR and the observable range are achieved as in the simulations, which implies that the method should provide the simulated accuracy for the estimation of wind velocity from actual observations.

Meteor trail characteristics observed by high time resolution lidar

Abstract. We report and analyse the characteristics of 1382 meteor trails based on a sodium data set of ~ 680 h. The observations were made at Yanqing (115.97° E, 40.47° N), China by a ground-based Na fluorescence lidar. The temporal resolution of the raw profiles is 1.5 s and the altitude resolution is 96 m. We discover some characteristics of meteor trails different from those presented in previous reports. The occurrence heights of the trails follow a double-peak distribution with the peaks at ~ 83.5 km and at ~ 95.5 km, away from the peak height of the regular Na layer. 4.7% of the trails occur below 80 km, and 3.25% above 100 km. 75% of the trails are observed in only one 1.5 s profile, suggesting that the dwell time in the laser beam is not greater than 1.5 s. The peak density of the trails as a function of height is similar to that of the background sodium layer. The raw occurrence height distribution is corrected taking account of three factors which affect the relative lifetime of a trail as a function of height: the meteoroid velocity (which controls the ratio of Na/Na+ ablated); diffusional spreading of the trail; and chemical removal of Na. As a result, the bi-modal distribution is more pronounced. Modelling results show that the higher peak corresponds to a meteoroid population with speeds between 20 and 30 km s−1, whereas the lower peak should arise from much slower particles in a near-prograde orbit. It is inferred that most meteoroids in this data set have masses of ~ 1 mg, in order for ablation to produce sufficient Na atoms to be detected by lidar. Finally, the evolution of longer-duration meteor trails is investigated. Signals at each altitude channel consist of density enhancement bursts with the growth process usually faster than the decay process, and there exists a progressive phase shift among these altitude channels.

Observational evidence of high‐altitude meteor trail from radar interferometer

AbstractWhether radar meteor echoes occur at high altitudes (above ~130 km) in the Earth's atmosphere is a long‐standing question within the meteor radar community. Using observations from the Sanya VHF coherent radar interferometer during 11 July to 10 August 2013, we have found a new class of range‐spread high‐altitude meteor trail echoes (HAMEs), some of which appeared at ~170 km altitude lasting more than 10 s. A statistical analysis on the local time dependence of the identified HAME events shows a maximum around 00–04 LT. The results imply that there could be much more meteor mass input due to meteoroid sputtering at high altitudes in the Earth's atmosphere than previously thought.

The effects of deionization processes on meteor radar diffusion coefficients below 90 km

AbstractThe decay times of VHF radar echoes from underdense meteor trails are reduced in the lower portions of the meteor region. This is a result of plasma neutralization initiated by the attachment of positive trail ions to neutral atmospheric molecules. Decreased echo decay times cause meteor radars to produce erroneously high estimates of the ambipolar diffusion coefficient at heights below 90 km, which affects temperature estimation techniques. Comparisons between colocated radars and satellite observations show that meteor radar estimates of diffusion coefficients are not consistent with estimates from the Aura Microwave Limb Sounder satellite instrument and that colocated radars operating at different frequencies estimate different values of the ambipolar diffusion coefficient for simultaneous detections of the same meteors. Loss of free electrons from meteor trails due to attachment to aerosols and chemical processes were numerically simulated and compared with observations to determine the specific mechanism responsible for low‐altitude meteor trail plasma neutralization. It is shown that three‐body attachment of positive metal ions significantly reduces meteor radar echo decay times at low altitudes compared to the case of diffusion only that atmospheric ozone plays little part in the evolution of low‐altitude underdense meteor trails and that the effect of three‐body attachment begins to exceed diffusion in echo decay times at a constant density surface.

Meteor radar wind over Chung-Li (24.9°N, 121°E), Taiwan, for the period 10-25 November 2012 which includes Leonid meteor shower: Comparison with empirical model and satellite measurements

The neutral winds in the mesosphere and lower thermosphere (MLT) region are measured by a newly installed meteor trail detection system (or meteor radar) at Chung-Li, Taiwan, for the period 10–25 November 2012, which includes the Leonid meteor shower period. In this study, we use the 3 m field-aligned plasma irregularities in the sporadic E (Es) region in combination with the International Geomagnetic Reference Field model to calibrate the system phase biases such that the true positions of the meteor trails can be correctly determined with interferometry technique. The horizontal wind velocities estimated from the radial velocities of the meteor trails and their locations by using a least squares method show that the diurnal tide dominates the variation of the MLT neutral wind with time over Chung-Li, which is in good agreement with the horizontal wind model (HWM07) prediction. However, harmonic analysis reveals that the amplitudes of the mean wind, diurnal, and semidiurnal tides of the radar-measured winds in height range 82–100 km are systematically larger than those of the model-predicted winds by up to a factor of 3. A comparison shows that the overall pattern of the height-local time distribution of the composite radar-measured meteor wind is, in general, consistent with that of the TIMED Doppler Interferometer-observed wind, which is dominated by a diurnal oscillation with downward phase progression at a rate of about 1.3 km/h. The occurrences of the Es layers retrieved from fluctuations of the amplitude and excess phase of the GPS signal received by the FORMOSAT-3/COSMIC satellites during the GPS radio occultation (RO) process are compared with the shear zones of the radar-measured meteor wind and HWM07 wind. The result shows that almost all of the RO-retrieved Es layers occur within the wind shear zones that favor the Es layer formation based on the wind shear theory, suggesting that the primary physical process responsible for the Es layer events retrieved from the scintillations of the GPS RO signal is very likely the plasma convergence effect of the neutral wind shear.

Optical observations of meteors generating infrasound—I: Acoustic signal identification and phenomenology

We analyse infrasound signals from 71 bright meteors/fireballs simultaneously detected by video to investigate the phenomenology and characteristics of meteor-generated near-field infrasound (<300km) and shock production. A taxonomy for meteor generated infrasound signal classification has been developed using the time–pressure signal of the infrasound arrivals. Based on the location along the meteor trail where the infrasound signal originates, we find most signals are associated with cylindrical shocks, with about a quarter of events evidencing spherical shocks associated with fragmentation episodes and optical flares. The video data indicate that all events with ray launch angles >117° from the trajectory heading are most likely generated by a spherical shock, while infrasound produced by the meteors with ray launch angles ≤117° can be attributed to both a cylindrical line source and a spherical shock. We find that meteors preferentially produce infrasound toward the end of their trails with a smaller number showing a preference for mid-trail production. Meteors producing multiple infrasound arrivals show a strong infrasound source height skewness to the end of trails and are much more likely to be associated with optical flares. We find that about 1% of all our optically-recorded meteors have associated detected infrasound and estimate that regional meteor infrasound events should occur on the order of once per week and dominate in numbers over infrasound associated with more energetic (but rarer) bolides. While a significant fraction of our meteors generating infrasound (~1/4 of single arrivals) are produced by fragmentation events, we find no instances where acoustic radiation is detectable more than about 60° beyond the ballistic regime at our meteoroid sizes (grams to tens of kilograms) emphasizing the strong anisotropy in acoustic radiation for meteors which are dominated by cylindrical line source geometry, even in the presence of fragmentation.

Methods of the active radiointerference protection of the meteoric adiolocation system

Two methods of active protection of the meteor automated radiolocation system with interference, caused by backscatter sounding (BSS) of the atmosphere were developed in the paper. Such interference occurs when the signal, reflected from the meteor trail (useful signal) and the signal, reflected from the ionosphere (BSS interference) come simultaneously to the meteor radiolocation system receiver input. This interference does not allow to conduct studies of meteors since the useful signal can be absolutely invisible on their background, and most of the important information is lost. During studies, two methods for active BSS interference protection were proposed. The main purpose of these methods is to distinguish the useful signal from interference, provided that the useful signal arrival time is known. The first method suggests using several frequencies for signal transmission and reception. The second method is based on applying noise-like signals (NLS) with several different code sequences.As a result of the investigation, using simulation, it was found that the most appropriate signal is NPS, based on the Walsh signal, taken from the Hadamard matrix. These studies are of value when using in meteor radiolocation systems as their information about meteor studies will not be distorted. Proposed methods can also be used in meteor communication systems that will allow to improve the communication quality.

Nonspecular meteor trails from non‐field‐aligned irregularities: Can they be explained by presence of charged meteor dust?

AbstractNonspecular meteor echoes have been associated with field‐aligned irregularities and have been observed at low‐latitude and midlatitude sites. We present observations obtained at high latitudes with range‐time features that resemble those at lower latitudes. However, these echoes cannot come from field‐aligned irregularities, since the radar‐pointing angles are almost parallel to the magnetic field. Using interferometry, we have been able to discriminate space and time features. Our echoes could be qualitatively explained by the presence of charged dust forming from the meteoric material immersed in a turbulent flow. This can lead to a high Schmidt number plasma that can sustain meter‐scale turbulence just as it does for the polar mesospheric summer echoes. These rare events require relatively large meteoroids. The result emphasizes the importance of charged dust in understanding all long‐duration nonspecular meteor echoes. This dust will extend their diffusion times and will affect temperature estimations from specular echoes.

All‐sky imaging of meteor trails at 55.25 MHz with the first station of the Long Wavelength Array

A new capability for high‐sensitivity, all‐sky monitoring of VHF meteor trail reflections with the first station of the Long Wavelength Array, or “LWA1,” is described. LWA1 is a ∼100 m diameter HF/VHF array of 256 crossed‐dipole antennas with a unique transient buffer mode that allows it to monitor for meteor trails via all‐sky imaging with the same sensitivity as a single‐dish antenna in diameter. To demonstrate this capability, we have used a 2 h observing run conducted in August 2012 aimed at detecting and characterizing meteor trail reflections of analog TV transmissions at 55.25 MHz. The analysis techniques described here allowed for a detection rate of ∼9500 trails per hour, including the detection of two meteor streams with radiants in the Aries/Perseus and Aquila/Hercules regions that were not previously reported in the literature. In addition, we have found a population of relatively long‐duration (∼1 to a few minutes), typically faint trails. These trails have implied horizontal speeds of 15–130 m s−1, with a typical speed of ∼30 m s−1. We have also used high‐resolution time series of the brightest trails to characterize decay times over a relatively large geographical area (10°×7° in longitude and latitude) and on short (∼5 min) time scales. Potential enhancements that could be enabled by the addition of more LWA stations are discussed.

Plasma turbulence of nonspecular trail plasmas as measured by a high‐power large‐aperture radar

AbstractHigh‐power, large‐aperture radars have been used to characterize plasmas formed as meteoroids ablate in Earth's atmosphere. These plasmas are referred to as heads, the plasmas surrounding the meteoroids, and trails, the plasmas left behind by the meteoroids. A subset of trails is nonspecular trails, which are detected when the radar beam is quasi‐perpendicular to the magnetic field. Radar returns from trail plasma are thought to originate from field‐aligned irregularity reflections that form due to turbulence within the trail. In this paper, we present theory and analysis of plasma trail diffusion using nonspecular trails detected by the Advanced Research Project Agency Long‐range Tracking and Identification Radar. These data include dual frequency, dual polarized, and high‐range resolution in‐phase and quadrature returns with azimuth and elevation data. We present turbulence onset times for nonspecular trails and derive comparisons to models. We compare diffusion coefficients calculated from the decay in signal return with ambipolar diffusion coefficients derived for specular meteor trails. These results, in conjunction with an analysis of the diffusion perpendicular and parallel to the magnetic field, demonstrate that the ambipolar diffusion coefficient is not a sufficient description of the turbulent diffusion in nonspecular trails and that other influences, such as external electric fields and anomalous cross‐field diffusion, must be considered when calculating the diffusion coefficients of nonspecular trails. In addition, we examined these results with respect to the polarization of the returns and found similar trends between all polarizations with slight differences for the right circular return.

VHF meteor radar at King Sejong Station, Antarctica

Since 2002, we have been observing the mesosphere and lower thermosphere (MLT) region over King Sejong Station (KSS; 62.22°S, 58.78°W), Antarctica, using various instruments such as the Spectral Airglow Temperature Imager (SATI), All Sky Camera (ASC) and VHF meteor radar. The meteor radar, installed in March 2007, continuously measures neutral winds in the altitude region 70-110 km and neutral temperature near the mesopause 24 h·d-1, regardless of weather conditions. In this study, we present results of an analysis of the neutral wind data for gravity wave activity over the tip of the Antarctic Peninsula, where such activity is known to be very high. Also presented is temperature estimation from measurement of the decay times of meteor trails, which is compared with other temperature measurements from SATI and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere Ionosphere Mesosphere Energy and Dynamics (TIMED) satellite.

Comparison of sporadic sodium layer characteristics observed at different time resolutions

Abstract. Sporadic sodium (Nas) layers, occurring in roughly the same height range as ionospheric sporadic-E layers, were first detected by lidar some 30 yr ago. Nas layers have a typical thickness of a few hundred meters to a few km, with peak atom concentrations several times that of the background layer. Despite a great deal of excellent work over the past decades, the source of Nas layers is still not altogether clear, partly as a result of our incomplete knowledge of Nas layer characteristics. In this paper we concentrate on some typical case studies chosen from the ~127 h of sporadic sodium layer observations made at a time resolution of 1.5 s at Yanqing (115.97° E, 40.47° N), Beijing, China. This is a much better time resolution than what has been employed in most earlier measurements. The results show that the Nas layer peak heights are dispersed at slightly different although adjacent heights. When averaged over several minutes, as has been the case with most earlier measurements, the height scatter results in an apparent layer thickness of a few km. We conclude, therefore, that these dispersed peaks at different but adjacent heights constitute the 5 min Nas layer. Similar to the observations of sporadic-E-ion (Es) layers and meteor rate, we observe quasi-periodic fluctuations on a timescale on the order of several minutes in the peak height and the peak density of sporadic layers, which is a universal feature but concealed by the lower temporal resolution previously adopted. Spatially localized multiple scatterers and multiple thin layers with similar apparent movement in Nas layers are also found. We discuss the possible formation mechanism by the direct deposition of large swarms of micrometeoroids and demonstrate a typical example of meteor trails evolving into a Nas layer, which suggests that this mechanism might indeed occur.

The diffusion of multiple ionic species in meteor trails

Meteor trails are composed of a number of different types of ions that are produced during the meteoroid ablation process. The diffusion of meteoric plasma is usually presented in terms of the diffusion of a single ionic species, but this ignores the possibility of non-linear diffusion due to complex meteor trail composition. This study uses numerical simulations to investigate what effect multi-ion diffusion has on the time-decay of meteor radar echoes, and whether multi-ion diffusion could be responsible for the anomalous diffusion coefficient estimates produced by radars operating at different frequencies. It is found that the diffusion of different species of ions in meteor trails does not produce the same discrepancies seen in estimates of the ambipolar diffusion coefficient made using meteor radar.

Simulation of the parameters of the meteor radar system in order to optimize its operation

Since 1966 the Meteor Automated Radar System (MARS) has been operating at the Balakleya scientific testing ground of the Kharkov National University of Radio Electronics, which had provided with the unique scientific data on meteors and the earth's atmosphere.For a long time of operation the equipment has undergone considerable moral and physical obsolescence. The need to continue the meteoric research on the one hand, and the inability to do so on the available equipment on the other hand, requires the modernization of the complex with regard of the new objectives and new opportunities.Simple switch to the new element base without taking into account the changing of the working conditions and new research objectives is not reasonable. It is necessary to revise a number of parameters of the available equipment. A large number of factors can be found by means of modeling of the improved complex. However, the simulation can not be carried out without a major component - the model of the meteor-trail reflections. Therefore, the primary problem is to develop a model that would include the ground-based research equipment parameters and the model of the processes occurring in the meteor zone as well.The article proposes the structure of this model, presents the analytical concepts for some modeling stages and their graphic interpretation. The above model is implemented as a computer program. The above model is implemented as a computer program. It differs by the change of the signal amplitude in the meteor trail formation process; the ability to model meteor propagation to the "ultra-short" routs - between the main and offset points; as well as to determine the trail coordinates, which is necessary for further determination of the meteor radiant.

Eureka, 80° N, SKiYMET meteor radar temperatures compared with Aura MLS values

Abstract. The meteor trail echo decay rates are analysed on-site to provide daily temperatures near 90 km. In order to get temperatures from trail decay times, either knowledge of the pressure or the background temperature height gradient near 90 km is required (Hocking, 1999). Hocking et al. (2004) have developed an empirical 90 km temperature gradient model depending only on latitude and time of year, which is used in the SKiYMET on-site meteor temperature analysis. Here we look at the sensitivity of the resulting temperature to the assumed gradient and compare it and the temperatures with daily AuraMLS averages near Eureka. Generally there is good agreement between radar and satellite for winter temperatures and their short-term variations. However there is a major difference in mid-summer both in the temperatures and the gradients. Increased turbulence in summer, which may overwhelm the ambipolar diffusion even at 90 km, is likely a major factor. These differences are investigated by generating ambipolar-controlled decay times from satellite pressure and temperature data at a range of heights and comparing with radar measurements. Our study suggests it may be possible to use these data to estimate eddy diffusion coefficients at heights below 90 km. Finally the simple temperature analysis (using satellite pressures), and a standard meteor wind analysis are used to compare mean diurnal variations of temperature (T) with those of zonal wind (U) and meridional wind (V) in composite multi-year monthly intervals.

Double laminar and turbulent meteor trails observed in space and simulated in the laboratory

One of nature's most interesting and beautiful sights is meteor trails that are visible to the naked eye. The first published sketches of trails appeared in 1869 after the Leonids storm of 1868. One of the perplexing features of these trails is that very often they are double trails and seldom triple or quadruple, which rules out breaking up or fractioning as a source. The original observations were visual and reproduced in artwork. The photographs used now reveal the accuracy of the original 1868 drawings. Since 1907, scientific explanations have ruled out physical breakup as a source. The long‐standing explanation by Trowbridge (1907) has also been ruled out using modern cameras, and several new explanations have been proposed. Here we modify the explanation by Zinn and Drummond (2005), who modeled the event as an explosion followed by buoyancy, and instead, explain the effect by showing that the trails are severely convectively unstable, leading to the vortices proposed by these authors. This idea is supported by lidar‐based measurements of the temperature gradient in the trail. We also show laboratory experiments that are remarkably similar to the meteor trails in appearance, although we argue that buoyancy is not the effect in the natural case.

Optical trail widths of faint meteors observed with the Canadian Automated Meteor Observatory

The optical trail widths of 30 faint meteors (average magnitude ∼+3, m ∼ 10 −4 kg) were resolved and measured using the Canadian Automated Meteor Observatory (CAMO). CAMO is an automated, high-resolution intensified digital video system capable of detecting meteors as faint as magnitude +5. The observatory’s two stations, separated by a baseline of 50 km, and each comprised of a narrow- and wide-field camera, observed the meteors for this survey. Data from the wide-field cameras were used to derive a trajectory solution for each meteor with the software package METAL, while data from the narrow-field cameras were used to measure trail widths with the image analysis software IMAGEJ. Raw widths were measured as a function of distance along the meteor trail for each frame of the video. To ensure that instrumental bloom did not artificially increase trail widths, the width of a star imaged with the narrowfield camera, with equivalent brightness to the meteor trail at the point of measurement, was subtracted from each observed raw width. Corrected trail widths up to 100 m at heights above 110 km were observed, varying with height as the inverse of atmospheric density. 14 of the 30 events were observed with both narrow-field cameras and each showed good agreement of trail width values after bloom correction. This suggested that the widths were true physical sizes, and not instrumental artefacts. Preliminary investigation suggests collisional de-excitation of energetic atoms is a plausible process for the formation of these wide trails.

The 2011 October Draconids outburst – II. Meteoroid chemical abundances from fireball spectroscopy

On October 8, 2011 the Earth crossed dust trails ejected from comet 21P/Giacobini-Zinner in the late 19th and early 20th Century. This gave rise to an outburst in the activity of the October Draconid meteor shower, and an international team was organized to analyze this event. The SPanish Meteor Network (SPMN) joined this initiative and recorded the October Draconids by means of low light level CCD cameras. In addition, spectroscopic observations were carried out. Tens of multi-station meteor trails were recorded, including an extraordinarily bright October Draconid fireball (absolute mag. -10.5) that was simultaneously imaged from three SPMN meteor ob-serving stations located in Andalusia. Its spectrum was obtained, showing a clear evolution in the relative intensity of emission lines as the fireball penetrated deeper into the atmosphere. Here we focus on the analysis of this remarkable spectrum, but also discuss the atmospheric trajectory, atmospheric penetration, and orbital data computed for this bolide which was probably released during 21P/Giacobini-Zinner return to perihelion in 1907. The spectrum is discussed together with the tensile strength for the October Draconid meteoroids. The chemical profile evolution of the main rocky elements for this extremely bright bolide is compared with the elemental abundances obtained for 5 October Draconid fireballs also recorded during our spectroscopic campaign but observed only at a single station. Significant chemical heterogeneity between the small meteoroids is found as we should expect for cometary aggregates being formed by diverse dust components.