Coordinated observations of two large Leonid meteor fireballs over northern New Mexico,and computer model comparisons

Abstract— This paper describes the coordinated results of several sets of measurements of two Leonid meteor fireballs over northern New Mexico at 1:32 and 3:06 MST, respectively, on the night of 1998 November 17. The measurements included visible band photometry on both events, as well as filtered 5890 Å all-sky images of the Na airglow. Also, for the 3:06 a.m. event, we obtained an infrasound measurement of the hydrodynamic yield. For the 1:32 a.m. event, we obtained a set of visible band charge-coupled device (CCD) camera images of the meteor train for times extending to 30 min after the initial impact. The measurement results have been combined to derive an optical efficiency for the intense early-time optical flash, and the total explosion yields and masses for both of the meteors. We have also done a set of numerical radiation, hydrodynamic, and chemistry computations to investigate the nature and distribution of the long-lasting airglow. We attribute the brightest visible airglow to atomic O 5577 Å line emission, with additional contributions from atomic Na emission and NO₂ chemiluminescence. The near-infrared atmospheric bands of molecular O₂ should be very strong as well. All of the band emissions are expected to show a hollow limb-brightened structure.     

Which fireballs are meteorites? A study of the Prairie Network photographic meteor data

Plausible meteorite mass distributions imply that in the Prairie Network data there must be many fainter fireballs produced by meteorites with physical properties that, except for mass, are very similar to the recovered ordinary chondrite (H5) Lost City. Four criteria are proposed for identifying these other meteorites among the fireballs. These are: deceleration to final velocity ≤8 km/sec; a photometric mass/dynamic mass ratio within a factor of 2 of that of Lost City, agreement of the observed and theoretical single body end heights (calculated using dynamic mass), and a lightcurve no more irregular than those of the three recovered fireballs. These criteria can be related to the PE criterion of Ceplecha and McCrosky, but include a wider range of observational data, and also differ from the PE criterion in avoiding inclusion of data not helpful to the particular problem of identifying ordinary chondrite fireballs. By use of our criteria, 27 Prairie Network fireballs are identified as being meteorites comparable to or greater in strength and density to Lost City, most of these should be ordinary chondrites. The orbital element distributions of these objects span a wide range, include those of recovered fireballs, and show that the 4.0-AU aphelion of the Pribram meteorite is not unusually large. Perihelia are concentrated near 1.0 AU, in agreement with previous inferences from time-of-fall and radiant distributions, demonstrating the usefulness of these data based on visual observations.     

流星电声响研究

对有关电声响(Electrophonic sounds)研究的历史和现状做一综述。早在公元前，我国就有关于流星声音的记录。近20年来，对流星电声响的报道与研究有了很大进展，电声响被定义为由电磁辐射直接转换成的可听见的声音。据报道，大的流星火球、非常明亮的北极光、附近的强闪电、地震以及核爆炸等，都可能产生电声响。但是，由于这些事件的罕见性、偶发性以及物理过程的特殊性，电声响无论在观测上，还是在理论研究方面都有很大的难度和许多至今仍未解决的问题。     

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.     

Spectrum of cosmic fireballs

A progress report on cosmic fireballs is presented. The main new results are: (a) the phenomenon should be almost universal, and most explosive -ray sources should show the characteristic fireball spectrum; (b) even if the radiation density is insufficient, pair production in electron-proton or electron-electron scattering might start the fireball; (c) some computed fireball spectra are shown. They all have in common a 1/E low-energy behaviour, a 100 keV flattening, and a 0.5 MeV cut-off.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts, held at Toulouse, France, 26–29 November, 1979.     

On the visibility of bright Venusian fireballs from Earth

The possibility of observing Venusian fireballs from Earth is examined. We estimate the steady-state flux of large, fireball-producing meteoroids at the orbit of Venus, and find that the prospects for observing such events from Earth with small, amateur-sized telescopes are not unreasonable.     

Monopole gravitational waves from relativistic fireballs driving gamma-ray bursts

Einstein's general relativity predicts that pressure, in general stresses, plays a similar role to energy density,  ε=ρ c ²  (with ρ being the corresponding mass density), in generating gravity. The source of gravitational field, the active gravitational mass density, sometimes referred to as Whittaker's mass density, is  ρ_grav=ρ+ 3 p / c ²  , where p is pressure in the case of an ideal fluid. Whittaker's mass is not conserved, hence its changes can propagate as monopole gravitational waves. Such waves can be generated only by astrophysical sources with varying gravitational mass. Here we show that relativistic fireballs, considered in modelling gamma-ray burst phenomena, are likely to radiate monopole gravitational waves from high-pressure plasma with varying Whittaker's mass. Also, ejection of a significant amount of initial mass-energy of the progenitor contributes to the monopole gravitational radiation. We identify monopole waves with   h ¹¹+ h ²²  waves of Eddington's classification which propagate (in the z -direction) together with the energy carried by massless fields. We show that the monopole waves satisfy Einstein's equations, with a common stress-energy tensor for massless fields. The polarization mode of monopole waves is  Φ₂₂  , i.e. these are perpendicular waves which induce changes of the radius of a circle of test particles only (breathing mode). The astrophysical importance of monopole gravitational waves is discussed.     

Electrophonic Sound Generation by the Chelyabinsk Fireball

Simultaneous, also called electrophonic sounds were widely reported by eye-witnesses to the Chelyabinsk fireball. The available data indicate that such sounds were heard at ranges to at least ~100 km from the fireball’s atmospheric path. We estimate that the fireball may have generated of order 625 W of energy in the form of very low frequency radiation, and we find some tentative evidence to indicate that the acoustic conversion efficiency at a 100 km range was of order 0.1 %. Numerical simulations of the atmospheric flight path indicate that electrophonic sounds should have commenced some 5 s after the fireball first became luminous and would have lasted for some 7.5 s prior to the moment of catastrophic break-up.     

VLF detection of fireballs

The results from 80 hours of simultaneous visual/video and VLF recordings made during the Lyrids, Perseids, Orionids, Leonids and Geminids are presented. All meteor magnitudes from –11 to +4 have been sampled at least once during these observations. The only positive VLF fireball detection was made at 19:57:32 UT on August 11, 1993 from the South of France. We present a Fourier transform analysis of this event and we also derive a lower limit to the electrical field strength produced by the fireball at the antenna. Our present observations suggest a lower limit of M_v –10 ± 1 for a fireball to produce a VLF signal.     

Photographic observations of fireballs in Tajikistan

Since 2006, systematic double-station photographic observations of fireballs using all-sky cameras equipped with Zeiss Distagon “fisheye” objectives (f/3.5, f = 30 mm) with a 180° field of view have been carried out at two observatories, Gissar (GisAO) and Sanglokh (IAOS), of the Institute of Astrophysics of the Tajik Academy of Sciences. In the method of astrometric reduction of fireball photographs, the empirical formulae for converting the measured coordinates to horizontal celestial coordinates are used. These formulae contain 12 unknown constants to be determined by the least-squares method and the iteration method. Such an approach enables the determination of the coordinates of an object at any point of the celestial hemisphere with a precision close to the theoretical limit whose value is quite comparable with the measurement errors. In the photometric reduction, the dependence of the measured width of the diurnal star trails on their magnitudes was used. As a result of astrometric and photometric reduction of the double-station photographs of five fireballs, the data on atmospheric trajectories, the coordinates of radiants, orbits in interplanetary space, light curves, and photometric masses of meteoroids which produced fireballs were obtained, and the belonging of fireballs to the known meteor showers was determined as well.     

Leonid meteor spectrum from 110 to 860 nm

During the Leonid meteor shower on 18 November 1999, the five spectrographic imagers onboard the Midcourse Space Experiment (MSX) satellite recorded the first complete meteor spectra from 110 to 860 nm. The observation occurred at 00:23:36.2 UT, at which time the satellite was pointed at a tangent altitude of 100 km over 37.2°N and 78.2°E. The spectrograph slits were oriented approximately parallel to the horizon at a tangent altitude of 100 km, and the meteor passed approximately perpendicular through the slits’ fields of view. All five spectrographic imagers observed the passage of a bright object (m_v < −2.8 at 100 km) and each recorded several frames of data. In the visible, common meteor emissions were observed from iron, sodium, and oxygen. However, the ultraviolet spectrum displayed a wealth of more intense features, some of which actually caused saturation in the spectrographs. The most intense features appeared between 220 and 300 nm and are attributed to neutral and singly ionized iron and ionized magnesium. Some unknown emissions, possibly from an unidentified molecular species such as iron oxide, appear between 180 and 220 nm. In the far ultraviolet from 110 to 130 nm, oxygen and nitrogen features appear in the spectrum, with some features from ionized iron and magnesium. In particular, the FUV spectrum showed an intense emission from hydrogen Lyman alpha and a much weaker emission from what appeared to be neutral carbon. The atmospheric emissions can be associated with the heating within the meteor shock, while the metallic emissions originate from the fireball of the meteor proper. The ultraviolet emissions were much stronger than those in the visible and near-infrared parts of the spectrum. The energy of emissions in the ultraviolet (110 < λ < 337 nm) exceeded the energy of the visible (337 < λ < 650 nm) by a factor of at least 5.     

Separation of meteor streams from the sporadic background

A cluster analysis procedure has been used to estimate the fraction of the sporadic interlopers (sporadis bias) identified as stream members among the observed meteor orbits. Using the artificial meteor orbits with the same distribution as the observed one, the sporadic bias is estimated for the given threshold value of the orbital similarityD _c. It has been shown that in case of the radio meteor catalogues theD _c values given by the formula proposed in Southworth and Hawkins (1963)and in Lindblad (1971) correspond to the sporadic bias of 8–21%. For the five radio meteor catalogues the values ofD _c corresponding to the fixed bias equal to 10% and 15% are given.     

Deconvoluting measurement uncertainty from the meteor speed distribution

Debiasing the velocity distribution of meteors observed by the Canadian Meteor Orbit Radar (CMOR) yields a distribution with large numbers of slow meteors. The distribution also contains significant numbers of hyperbolic meteors, in conflict with the expectation that interstellar meteors should be rare. In Moorhead et al. (2017a), we noted that measurement uncertainties were possibly smoothing the speed distribution and redistributing meteors to the extreme ends of the speed distribution. In this report, we use techniques analogous to image sharpening to remove the blurring caused by measurement uncertainties. The deconvolved speed distribution appears to have no meteors slower than 14 km s⁻¹ and none faster than 74 km s⁻¹. The result is to substantially raise the characteristic velocity of incoming meteoroids from 12.9 to 20.0 km s⁻¹.     

A comparison of detection sensitivity between ALTAIR and Arecibo meteor observations: Can high power and large aperture radars detect low velocity meteor head-echoes

Meteor head-echo observations using High Power and Large Aperture (HPLA) radars have been routinely used for micrometeor studies for over a decade. The head-echo is a signal from the radar-reflective plasma region traveling with the meteoroid and its detection allows for very precise determination of instantaneous meteor altitude, velocity and deceleration. Unlike specular meteor radars (SMR), HPLA radars are diverse instruments when compared one to another. The operating frequencies range from 46 MHz to 1.29 GHz while the antenna configurations changes from 18,000 dipoles in a 300 m×300 m square array, phase arrays of dipoles to single spherical or parabolic dishes of various dimensions. Hunt et al. [Hunt, S.M., Oppenheim, M., Close, S., Brown, P.G., McKeen, F., Minardi, M., 2004. Icarus 168, 34-42] and Close et al. [Close, S., Brown, P., Campbell-Brown, M., Oppenheim, M., Colestock, P., 2007. Icarus, doi:10.1016/j.icarus.2006.09.07] recently showed, by utilizing a head-echo plasma-based model, the presence of instrumental biases in the ALTAIR VHF radar system against detecting meteors produced by very small particles (<1 μg) moving at slow (∼20 km/s) velocities due to the low head echo radar cross-section (RCS) associated with these particles. In this paper we apply the same methodology to the Arecibo 430 MHz radar and compare the results with those presented by Close et al. [Close, S., Brown, P., Campbell-Brown, M., Oppenheim, M., Colestock, P., 2007. Icarus, doi:10.1016/j.icarus.2006.09.07]. We show that, if the methodology applied by Hunt et al. [Hunt, S.M., Oppenheim, M., Close, S., Brown, P.G., McKeen, F., Minardi, M., 2004. Icarus 168, 34-42] and Close et al. [Close, S., Brown, P., Campbell-Brown, M., Oppenheim, M., Colestock, P., 2007. Icarus, doi:10.1016/j.icarus.2006.09.07] is accurate, for particles at least 1 μg or heavier, while the bias may exist for the ALTAIR measurements, it does not exist in the Arecibo data due to its greater sensitivity.     

Orthos' meteor showers

Probably the majority of meteor showers has a cometary origin. Investigation of Near-Earth asteroids' orbital evolution to determine whether they have related meteor showers are necessary to determine which asteroids evolved from comets. The results of calculations show that asteroid Orthos' orbit is an octuple Earth-crosser. Therefore, if Orthos has an old meteoroid stream it may produce eight meteor showers observable on the Earth. The existence of four Orthos' Northern meteor showers is confirmed by our search in the published catalogues of meteor radiants and orbits or in the archives of the IAU Meteor Data Center (Lund, Sweden).     

Non-linear meteor trails

In this essay an attempt is made to not only review but reopen the debate on non-linear meteor trails. On the basis of data culled from various, now historical, sources it is found that approximately one in every two hundred of the visual meteors is likely to show a non-linear trail, and that of such trails about 60% will be continuously curved and 40% sinusoidal. It is suggested that two mechanisms may explain the various trail types: the continuously curved trails being a manifestation of the classical Magnus effect, and the sinusoidal trails resulting from torque-free precession.     

Large fireballs: Evaporation and fragmentation

The main physical processes during the entry of natural 0.01-to 100-m large cosmic bodies into planetary atmospheres are fragmentation and evaporation. The relative role of evaporation can be characterized by the mass-loss parameter, which is proportional to the ratio of the kinetic energy per unit mass of the body to the effective enthalpy of evaporation. Examples of actual large fireballs are given for various values of this parameter. The proposed general approach helps in understanding the extensive observational data of various level of reliability and also makes it possible to preliminarily separate real hypotheses from unrealistic ones.     

The observed characteristics of radio-echoes from overdense meteor trains

Observational data relating to the duration of radio-echoes from overdense meteor trains are presented and discussed, with particular reference to the differences between daytime and night-time conditions. Experimental distributions of echo-duration for different meteor showers observed at various radio frequencies have been collected from several sources. When interpreted in terms of duration distribution theory (which, as developed here, represents an important revision of previous theories) these data are shown to be consistent with the assumption that free electrons are removed by an apparent three-body attachment mechanism, at a rate which is approximately three times higher at night than during the day. The absolute rates which emerge from this analysis depend on the exact meteor ionization model adopted, but for the most probable range of models our values are found to be lower than those obtained by other workers by an order of magnitude. The duration of echoes from bright meteors associated with the Quadrantid shower has been observed to increase considerably some 30 min after sunrise in the meteor region. This phenomenon is associated with a decrease in the mean range of the echoes. The results indicate that deionization is particularly rapid below ˜80 km at night, and may no longer appear as a three-body attachment in this region.

It is shown that the observed echo characteristics cannot be satisfactorily explained in terms of the well-known processes of direct recombination or simple attachment to atmospheric molecules, indicating that a much more complex ion-chemistry is required.     

Structure of sporadic meteor radiant distributions from radar observations

An analysis of sporadic meteor flux distributions over the celestial sphere is given. Symmetry of the radiant distribution relative to the plane crossing the ecliptic pole and the Earth's apex and antapex was identified.     

Observations of the 2001 Leonid meteor shower using VHF meteor radar

Every year the Earth crosses or passes near one of the dust trails left by Comet 55P/Tempel-Tuttle in its pass through the Solar System every 33.2 years. This produces a meteor shower Commonly called the Leonid. The 2001 Leonid meteor shower is one of the strongest in recent years. We present observations made by the 50 MHz all-sky meteor radar located at the Platteville Atmospheric Observatory in Colorado (40° N, 105° W). The spatial and temporal distributions of the meteor activity detected by the radar during the 2001 Leonid shower differs from the observed sporadic activity detected by VHF radars. Estimation of the radiant flux of the meteor shower of the shower by a well-known methodology is presented, and the intensity of the phenomena is discussed.