Meteor velocities: A new look at an old problem

Meteoroids that orbit the Sun encounter the Earth with speeds between 11 and 74 km/sec. However, the distribution of the velocities of meteoroids between these limits is not well known. The uncertainty is caused by the difficulty in measuring the true flux of meteors at the extrema of the velocity distribution. Whilst the most comprehensive measurements of meteor flux are those obtained using radio techniques, meteors with speeds > 50 km/sec occur at heights where the effects of initial radius of the trail and diffusion significantly reduce the radio reflection from the trails; on the other hand the high dependence of the collisional ionization probability on velocity (to the power 3.5) significantly inhibits the detection of meteors with speeds < 20 km/sec. Recent developments in meteor radar systems are now making it possible to measure the velocity of meteors at the extrema of the distribution. For meteoroids ablating at heights between 100 and 120 km the speed of entry can be measured at 2 and 6 MHz using a radar with a 1 km diameter array located near Adelaide; these observations will commence early in 1995. In the meantime a 54 MHz MST radar is being operated at a pulse repetition frequency of 1024 Hz to search for the presence of interstellar (speed > 74 km/sec) meteors. Both these radars exploit the phase information available prior to the closest-approach (t_o) point.     

Study of space debris burning in Earth’s atmosphere via television meteor monitoring

The observations on ground-based facilities miss multitudes of small fragments of space debris. However, the intrusion of fine particles into the atmosphere and their subsequent burning can be observed along with the usual meteor phenomena. Since the solar system meteoric body velocities at the entry into the atmosphere are over 11.2 km/s, and the velocity of the space debris objects does not exceed 11.2 km/s, the selection of meteors by velocity is a reliable criterion for separating these bodies. The paper describes a method of selecting the space debris fragments using a technique of television meteor monitoring. The technique was adapted on the material of real television observations on the FAVOR wide-field monitoring camera with high temporal resolution, conducted in 2006 in the Arkhyz station of the Institute for Precision Instrumentation (North Caucasus).     

OPTICAL PREDICTIONS FOR HIGH GEOCENTRIC VELOCITY METEORS

In this study we numerically modelled the atmospheric ablation and luminosity of cometary structure meteoroids with geocentric velocities from 71 to 200 km/s. We considered meteoroid masses ranging from 10⁻¹³ to 10⁻⁶ kg. Expected heights of ablation and maximum luminosity absolute magnitudes are determined. Height and trail length values are used to calculate the angle traversed in a single video frame. It is found that for pre-atmospheric meteoroid masses of greater than 10⁻⁸ kg, high geocentric velocity meteors should be detectable with current electro-optical technology if properly optimised.     

Meteoroid ablation models

The fate of entering meteoroids in atmosphere is determined by their size, velocity and substance properties. Material from ablation of small-sized meteors (roughly R≤0.01–1 cm) is mostly deposited between 120 and 80 km altitudes. Larger bodies (up to meter sizes) penetrate deeper into the atmosphere (down to 20 km altitude). Meteoroids of cometary origin typically have higher termination altitude due to substance properties and higher entry velocity. Fast meteoroids (V>30–40 km/s) may lose a part of their material at higher altitudes due to sputtering. Local flow regime realized around the falling body determines the heat transfer and mass loss processes. Classic approach to meteor interaction with atmosphere allows describing two limiting cases: – large meteoroid at relatively low altitude, where shock wave is formed (hydrodynamical models); – small meteoroid/or high altitudes – free molecule regime of interaction, which assumes no collisions between evaporated meteoroid particles. These evaporated particles form initial train, which then spreads into an ambient air due to diffusion. Ablation models should make it possible to describe physical conditions that occur around meteor body. Several self-consistent hydrodynamical models are developed, but similar models for transition and free molecule regimes are still under study. This paper reviews existing ablation models and discusses model boundaries.     

Meteoroid Bulk Density Determination Using Radar Head Echo Observations

We present the results of a study of meteoroid bulk densities determined from meteor head echoes observed by radar. Meteor observations were made using the Advanced Research Projects Agency Long-Range Tracking And Instrumentation Radar (ALTAIR). ALTAIR is particularly well suited to the detection of meteor head echoes, being capable of detecting upwards of 1000 meteor head echoes per hour. Data were collected for 19 beam pointings and are comprised of approximately 70 min. of VHF observations. During these observations the ALTAIR beam was directed largely at the north apex sporadic source. Densities are calculated using the classical physical theory of meteors. Meteoroid masses are determined by applying a full wave scattering theory to the observed radar cross-section. Observed meteoroids are predominantly in the 10⁻¹⁰ to 10⁻⁶ kg mass range. We find that the vast majority of meteoroid densities are consistent with low density, highly porous objects as would be expected from cometary sources. The median calculated bulk density was found to be 900 kg/m³. The orbital distribution of this population of meteoroids was found to be highly inclined.     

The hyperthermal ionization and high absolutemeteor velocities observed with HPLA radars

High Power Large Aperture (HPLA) radars generally observe very high meteor velocities averaging over 50 km s⁻¹. There are only a few events recorded around 30 km s⁻¹, while meteors at 20 km s⁻¹ or slower are very rare. This is a clear and debated contradiction to specular meteor radar results. A high plasma density condition contributes, but the dominating phenomenon is the hyperthermal ionization mechanism due to chemical dynamics of the ionization process. The observed high velocities can be explained in terms of high hyperthermal ionization cross-sections for collisions between ablated meteoroid metal atoms such as Na and/or Fe and atmospheric species.     

The initial train radius of sporadic meteors

The height distributions, velocity distributions and flux measurements of underdense echoes determined from meteor radar observations are significantly affected by the attenuation associated with the initial radius of meteor trains. Dual-frequency radar observations of a very large set of sporadic radar meteors at 29 and 38 MHz yield estimates of the initial train radius and its dependence on height and meteoroid speed as determined by the time-delay method. We provide empirical formulae that can be used to correct meteoroid fluxes for the effect of initial train radius at other radio frequencies.     

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.     

Meteor Outburst Profiles and Cometary Ejection Models

The spatial structure of meteor streams, and the activity profiles of their corresponding meteor showers, depend firstly on the distribution of meteoroid orbits soon after ejection from the parent comet nucleus, and secondly on the subsequent dynamical evolution. The latter increases in importance as more time elapses. For younger structures within streams, notably the dust trails that cause sharp meteor outbursts, it is the cometary ejection model (meteoroid production rate as a function of time through the several months of the comet’s perihelion return, and velocity distribution of the meteoroids released) that primarily determines the shape and width of the trail structure. This paper describes how a trail cross section can be calculated once an ejection model has been assumed. Such calculations, if made for a range of ejection model parameters and compared with observed parameters of storms and outbursts, can be used to constrain quantitatively the process of meteoroid ejection from the nucleus, including the mass distribution of ejected meteoroids.     

Detailed data for 259 fireballs from the Canadian camera network and inferences concerning the influx of large meteoroids

Abstract— We present data for 259 meteoric fireballs observed with the Canadian camera network, including velocities, heights, orbits, luminosities along each trail, estimates of preatmospheric masses and surviving meteorites (if any) as well as membership in meteor showers. Some 213 of the events comprise an unbiased sample of the 754 fireballs observed in a total of 1.51 × 10¹⁰ km² h of clear-sky observations. The number of fireballs and the amount of clear sky in which they were recorded are given for each day of the year. We find at least 37% of the unbiased sample are members of some 15 recognized meteor showers. Preatmospheric masses, based on an assumed luminous efficiency of 0.04 for velocities >10 km s^?1, range from 1 g for some very fast fireballs up to hundreds of kilograms for the largest events. We present plots and equations for the flux, as a function of initial mass, for the entire group of fireballs and for some subgroups: meteorite-dropping objects; meteor shower members; groups that appear to be mainly of asteroidal or cometary origin; and for very fast objects. For masses of a few kilograms, asteroidal objects outnumber cometary ones. Cometary objects attain greater peak brightness than asteroidal ones of equal mass largely due to higher velocity, but also because they fragment more severely. For 66 fireballs, we estimate the meteoroid density using photometric and dynamic masses. Presumed cometary objects have typical densities near 1.0, while asteroidal values show two groups that suggest meteoroids similar to carbonaceous and ordinary chondrites. Our basic data may be used by others for further studies or to reexamine our results using assumptions different from those employed in this paper.     

Plasma and Electromagnetic Simulations of Meteor Head Echo Radar Reflections

Recently, meteor head echo detections from high powered large aperture radars (HPLA) have brought new measurements to bear on the study of sporadic interplanetary meteors. These same observations have demonstrated an ability to observe smaller meteoroids without some of the geometrical restrictions of specular radar techniques. Yet incorporating data from various radar reflection types and from different radars into a single consistent model has proven challenging. We believe this arises due to poorly understood radio scattering characteristics of the meteor plasma, especially in light of recent work showing that plasma turbulence and instability greatly influences meteor trail properties at every stage of evolution. In order to overcome some of the unknown relationships between meteoroid characteristics (such as mass and velocity) and the resulting head echo radar cross-sections (RCS), we present our results on meteor plasma simulations of head echo plasmas using particle in cell (PIC) ions, which show that electric fields strongly influence early stage meteor plasma evolution, by accelerating ions away from the meteoroid body at speeds as large as several kilometers per second. We also present the results of finite difference time domain electromagnetic simulations (FDTD), which can calculate the radar cross-section of the simulated meteor plasma electron distributions. These simulations have shown that the radar cross-section depends in a complex manner on a number of parameters. In this paper we demonstrate that for a given head echo plasma the RCS as a function of radar frequency peaks at sqrt (2*peak plasma frequency) and then decays linearly on a dB scale with increasing radar frequency. We also demonstrate that for a fixed radar frequency, the RCS increases linearly on a dB scale with increasing head echo plasma frequency. These simulations and resulting characterization of the head echo radar cross-section will both help relate HPLA radar observations to meteoroid properties and aid in determining a particular radar facility’s ability to observe various meteoroid populations.     

Orbit characteristics of the tristatic EISCAT UHF meteors

The tristatic EISCAT 930-MHz UHF system is used to determine the absolute geocentric velocities of meteors detected with all three receivers simultaneously at 96 km, the height of the common radar volume. The data used in this study were taken between 2002 and 2005, during four 24-h runs at summer/winter solstice and vernal/autumnal equinox to observe the largest seasonal difference. The observed velocities of 410 tristatic meteors are integrated back through the Earth atmosphere to find their atmospheric entry velocities using an ablation model. Orbit calculations are performed by taking zenith attraction, Earth rotation as well as obliquity of the ecliptic into account. The results are presented in the form of different orbital characteristics. None of the observed meteors appears to be of extrasolar or asteroidal origin; comets, particularly short-period (<200 yr) ones, may be the dominant source for the particles observed. About 40 per cent of the radiants can be associated with the north apex sporadic meteor source and 58 per cent of the orbits are retrograde. There is evidence of resonance gaps at semimajor axis values corresponding to commensurabilities with Jupiter, which may be the first convincing evidence of Jupiter's gravitational influence on the population of small sporadic meteoroids surveyed by radar. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km s⁻¹ and a retrograde population peaking at 59 km s⁻¹. The EISCAT radar system is located close to the Arctic Circle, which means that the North Ecliptic Pole (NEP) is near zenith once every 24 h, i.e. during each observational period. In this particular geometry, the local horizon coincides with the ecliptic plane. The meteoroid influx should therefore be directly comparable throughout the year.     

Sputtering and high altitude meteors

Conventional ablation theory assumes that a meteoroid undergoes intensive heating during atmospheric flight and surface atoms are liberated through thermal processes. Our research has indicated that physical sputtering could play a significant role in meteoroid mass loss. Using a 4th order Runge-Kutta numerical integration technique, we tabulated the mass loss due to the two ablation mechanisms and computed the fraction of total mass lost due to sputtering. We modeled cometary structure meteoroids with masses ranging from 10⁻¹³ to 10⁻³ kg and velocities ranging from 11.2 to 71 km s⁻¹. Our results indicate that a significant fraction of the mass loss for small, fast meteors is due to sputtering, particularly in the early portion of the light curve. In the past 6 years evidence has emerged for meteor luminosity at heights greater than can be explained by conventional ablation theory. We have applied our sputtering model and find excellent agreement with these observations, and therefore suggest that sputtered material accounts for the new type of radiation found at great heights.     

Atmospheric behavior and extreme beginning heights of the thirteen brightest photographic Leonid meteors from the ground‐based expedition to China

Abstract— Precise atmospheric trajectories including dynamic and photometric data on thirteen of the brightest Leonid fireballs have been determined from the double‐station photographic observations of Leonid meteors during the ground‐based expedition to China in 1998 November. the expedition was organized as a collaboration between the dutch and chinese academy of sciences and was supported by the leonid multi‐instrument aircraft campaign (mac) program (jenniskens and butow, 1999). All data presented here were taken at Xinglong Observatory and at a remote station, Lin Ting Kou near Beijing, on the night of 1998 November 16/17. At the Xinglong station, photographic cameras were accompanied by an all‐sky television camera equipped with an image intensifier and 15 mm fish‐eye objective in order to obtain precise timings for all observed meteors up to magnitude +2. Whereas beginning heights of photographed meteors are all lower than 130 km, those observed by the all‐sky television system are at ～160 km, and for three brightest events, even > 180 km. Such high beginnings for meteors have never before been observed. We also obtained a precise dynamic single‐body solution for the Leonid meteor 98003, including the ablation coefficient, which is an important material and structural quantity (0.16 s² km^?2). From this and from known photometry, we derived a density of this meteoroid of 0.7 g/cm³. Also, all PE coefficients indicate that these Leonid meteors belonged to the fireball group IIIB, which is typical for the most fragile and weak interplanetary bodies. From a photometric study of the meteor lightcurves, we found two typical shapes of light curves for these Leonid meteors.     

Search for New Parent Bodies of Meteoroid Streams Among Comets. I. Showers of Comets 126P/1996 P1 and 161P/2004 V2 with Radiants on Southern Sky

We deal with theoretical meteoroid streams the parent bodies of which are two Halley-type comets in orbits situated at a relatively large distance from the orbit of Earth: 126P/1996 P1 and 161P/2004 V2. For two perihelion passages of each comet in the far past, we model the theoretical stream and follow its dynamical evolution until the present. We predict the characteristics of potential meteor showers according to the dynamical properties of theoretical particles currently approaching the orbit of the Earth. Our dynamical study reveals that the comet 161P/2004 V2 could have an associated Earth-observable meteor shower, although no significant number of theoretical particles are identified with real, photographic, video, or radar meteors. However, the mean radiant of the shower is predicted on the southern sky (its declination is about −23°) where a relatively low number of real meteors has been detected and, therefore, recorded in the databases used. The shower of 161P has a compact radiant area and a relatively large geocentric velocity of ∼53 km s⁻¹. A significant fraction of particles assumed to be released from comet 126P also cross the Earth’s orbit and, eventually, could be observed as meteors. However, their radiant area is largely dispersed (declination of radiants spans from about +60° to the south pole) and, therefore, mixed with the sporadic meteor background. An identification with real meteors is practically impossible.     

First definitive observations of meteor shower particles using a high-power large-aperture radar

We present the first clear observations of meteor shower activity from meteor-head echoes detected by a high-power large-aperture radar (HPLAR). Such observations have been performed at the Jicamarca VHF radar using its interferometric capabilities allowing the discrimination of meteor shower echoes from the much more frequent sporadic meteors. Until now, HPLARs were unable to distinguish meteor shower from the much more common sporadic meteor ones. In this work we have been able to detect and characterize the η-Aquariids (ETA) as well as the Perseids (PER) showers. The shower activity is more conspicuous for the ETA than for the PER shower due to the more favorable geometry. Namely, PER meteors come from low elevation angles, experiencing more filtering due to the combined Earth-atmosphere-radar instrument. In both cases, there is an excellent agreement between the measured mean velocity of the shower echoes and their expected velocity, within a fraction of 1 km s⁻¹. Besides the good agreement with expected visual results, HPLARs observe meteors with a variety of particles sizes and masses, not observed by any other technique. Taking into account the different viewing volumes, compare to optical observations Jicamarca observes more than 1000 times more ETA meteors. Our results indicate that Jicamarca and other HPLARs are able to detect the echoes from meteor showers, but without interferometric capabilities such populations are difficult to identify just from their velocity distributions, particularly if their velocity distributions are expected to be similar to the more dominant distributions of sporadic meteors.     

A dynamical model of the sporadic meteoroid complex

Sporadic meteoroids are the most abundant yet least understood component of the Earth's meteoroid complex. This paper aims to build a physics-based model of this complex calibrated with five years of radar observations. The model of the sporadic meteoroid complex presented here includes the effects of the Sun and all eight planets, radiation forces and collisions. The model uses the observed meteor patrol radar strengths of the sporadic meteors to solve for the dust production rates of the populations of comets modeled, as well as the mass index. The model can explain some of the differences between the meteor velocity distributions seen by transverse versus radial scatter radars. The different ionization limits of the two techniques result in their looking at different populations with different velocity distributions. Radial scatter radars see primarily meteors from 55P/Tempel-Tuttle (or an orbitally similar lost comet), while transverse scatter radars are dominated by larger meteoroids from the Jupiter-family comets. In fact, our results suggest that the sporadic complex is better understood as originating from a small number of comets which transfer material to near-Earth space quite efficiently, rather than as a product of the cometary population as a whole. The model also sheds light on variations in the mass index reported by different radars, revealing it to be a result of their sampling different portions of the meteoroid population. In addition, we find that a mass index of s=2.34 as observed at Earth requires a shallower index (s=2.2) at the time of meteoroid production because of size-dependent processes in the evolution of meteoroids. The model also reveals the origin of the 55° radius ring seen centered on the Earth's apex (a result of high-inclination meteoroids undergoing Kozai oscillation) and the central condensations seen in the apex sources, as well as providing insight into the strength asymmetry of the helion and anti-helion sources.     

Meteor head echo radar data: Mass-velocity selection effects

High-power, large-aperture (HPLA) radars detect the plasma that forms in the vicinity of a meteoroid and moves approximately at its velocity; reflections from these plasmas are called head echoes. For over a decade, HPLA radars have been detecting head echoes with peak velocity distributions >50 km/s. These results have created some controversy within the field of meteor physics because previous data, including spacecraft impact cratering studies, optical and specular meteor data, indicate that the peak of the velocity distribution to a set limiting mass should be <20 km/s [Love, S.G., Brownlee, D.E., 1993. Science 262, 550-553]. Thus the question of whether HPLA radars are preferentially detecting high-velocity meteors arises. In this paper we attempt to address this question by examining both modeled and measured head echo data using the ALTAIR radar, collected during the Leonid 1998 and 1999 showers. These data comprise meteors originating primarily from the North Apex sporadic meteor source. First, we use our scattering theory to convert measured radar-cross-section (RCS) to electron line density and mass, as well as to convert modeled electron line density and mass to RCS. We subsequently compare the dependence between mass, velocity, mean-free-path, RCS and line density using both the measured and modeled data by performing a multiple, linear regression fit. We find a strong correlation between derived mass and velocity and show that line density is approximately proportional to mass times velocity^3.1. Next, we determine the cumulative mass index using subsets of our data and use this mass index, along with the results of our regression fit, to weight the velocity distribution. Our results show that while there does indeed exist a bias in the measured head echo velocity distribution, it is smaller than those calculated using traditional specular trail data due to the different scattering mechanism, and also includes a bias against the low-mass, very high-velocity meteoroids.     

The beginning heights and light curves of high‐altitude meteors

Abstract— In this paper, we provide an overview of meteors with high beginning height. During the recent Leonid meteor storms, as well as within the regular double station video observations of other meteor showers, we recorded 164 meteors with a beginning height above 130 km. We found that beginning heights between 130 and 150 km are quite usual, especially for the Leonid meteor shower. Conversely, meteors with beginning heights above 160 km are very rare even among Leonids. From the meteor light curves, we are able to distinguish two different processes that govern radiation of the meteors at different altitudes. Light curves vary greatly above 130 km and exhibit sudden changes in meteor brightness. Sputtering from the meteoroid surface is the dominating process during this phase of the meteor luminous trajectory. Around 130 km, the process switches to ablation and the light curves become similar to the light curves of standard meteors. The sputtering model was successfully applied to explain the difference in the beginning heights of high‐altitude Leonid and Perseid meteors. We show also that this process in connection with high altitude fragmentation could explain the anomalously high beginning heights of several relatively faint meteors.     

Radar Backscatter from Underdense Meteors and Diffusion Rates

Many meteoroids burn up between about 120 km and 70 km, deposit metals and dust and form ionized trails which are detected by radars. Model studies about the influence of neutral or positively charged background dust on the ambipolar diffusion indicate that significant smaller decay times should be observed for weak meteor echoes compared to strong meteor echoes which can affect the estimation of temperatures. The variation of meteor decay times in dependence on echo strength, height, and season was studied using radar observations at 69° N, 22° S, and 67° S. Significantly reduced decay times were found for weak echoes below about 88 km at low latitudes throughout the year, and at high latitudes with the exception of summer. In summer at high latitudes, decreasing decay times of weak and strong meteors are observed at altitudes below about 85 km during the appearance of noctilucent clouds. The impact of reduced decay times on the estimation of neutral temperatures from decay times is discussed.