Visual Meteors Research Articles

An observational synthesis of the Taurid meteor complex

ABSTRACTWe provide an overview of the observational properties of the four major Taurid showers, namely the Northern and Southern Taurids (#017 NTA and #002 STA), the β Taurids (#173 BTA), and the ζ Perseids (#172 ZPE). Analysing more than two decades of meteor observations from visual, optical, and radar measurements, we present the Taurids average activity, annual variations in strength, radiant drift, and orbital variations as a function of solar longitude and particle size. The Taurid showers are detected over several weeks in the spring and autumn, but their annual activity level is generally low (less than 15 visual meteors per hour). We find the STA to be pre-dominant in autumn, while its twin, the ZPE, dominates over the BTA in spring. Due to their long duration, the position of each shower’s radiant and orbital elements are variable with time. Optical measurements have previously recorded enhanced STA activity and increased fireball rates caused by the return of a swarm of meteoroids trapped in the 7:2 mean motion resonance with Jupiter. However, we find no presence of the swarm in radar data, suggesting that small meteoroids are removed from the resonance faster than fireball-producing meteoroids. We also find the STA to be enriched in smaller particles early in their activity period. The differences we identify in our analysis between the showers at different particle sizes provide strong observational constraints to future dynamical modelling of the Taurid Meteoroid Complex.

Activity of the Eta-Aquariid and Orionid meteor showers

Aims.We present a multi-instrumental, multidecadal analysis of the activity of the Eta-Aquariid and Orionid meteor showers for the purpose of constraining models of 1P/Halley’s meteoroid streams.Methods.The interannual variability of the showers’ peak activity and period of duration is investigated through the compilation of published visual and radar observations prior to 1985 and more recent measurements reported in the International Meteor Organization (IMO) Visual Meteor DataBase, by the IMO Video Meteor Network and by the Canadian Meteor Orbit Radar (CMOR). These techniques probe the range of meteoroid masses from submilligrams to grams. Theη-Aquariids and Orionids activity duration, shape, maximum zenithal hourly rates values, and the solar longitude of annual peaks since 1985 are analyzed. When available, annual activity profiles recorded by each detection network were measured and are compared.Results.Observations from the three detection methods show generally good agreement in the showers’ shape, activity levels, and annual intensity variations. Both showers display several activity peaks of variable location and strength with time. Theη-Aquariids are usually two to three times stronger than the Orionids, but the two showers display occasional outbursts with peaks two to four times their usual activity level. CMOR observations since 2002 seem to support the existence of an ~12 yr cycle in Orionids activity variations; however, additional and longer term radar and optical observations of the shower are required to confirm such periodicity.

Orionids and Eta Aquariids in the IAU MDC database

The structure of the Orionids and the Eta Aquariids meteor streams was studied using the IAU MDC Photographic Orbits Database. Both the streams are associated with one of the most active comet known in the cometary population – 1P/Halley. The stream is therefore constantly supplemented by new particles, what creates conditions for a formation of a complex internal structure. We found that two frequency maxima of the photographic Orionids identified at the solar longitudes λ⊙=208.5° and λ⊙=210.5° coincide with the positions of the maximum activity of the visual meteors and bolides. One of five identified associations (O1) probably does not belong to the Orionids, i.e. meteor stream 008 ORI, but regards the September omicron Orionids - 479 SOO. The orbital parameters of the filament O4 correspond to the mean orbit of the Orionids resulting from the meteor radar observations in the period 2002–2012. There is no spatial relation between meteors observed in May (031 ETA) and October (008 ORI). While Eta Aquariids are in fact the core of the stream, the orbits of Orionids are much further from the cometary orbit.

THE RETURN OF THE ANDROMEDIDS METEOR SHOWER

The Andromedid meteor shower underwent spectacular outbursts in 1872 and 1885, producing thousands of visual meteors per hour and described as `stars fell like rain' in Chinese records of the time. The shower originates from comet 3D/Biela whose disintegration in the mid-1800's is linked to the outbursts, but the shower has been weak or absent since the late 19th Century. This shower returned in December 2011 with a zenithal hourly rate of approximately 50, the strongest return in over a hundred years. Some 122 probable Andromedid orbits were detected by the Canadian Meteor Orbit Radar. The shower outburst occurred during 2011 Dec 3-5. The radiant at RA +$18\degree$ and Dec +$56\degree$ is typical of the `classical' Andromedids of the early 1800's, whose radiant was actually in Cassiopeia. The orbital elements indicate that the material involved was released before 3D/Biela's breakup prior to 1846. The observed shower in 2011 had a slow geocentric speed (16 km s$^{-1}$) and was comprised of small particles: the mean measured mass from the radar is $\sim5 \times 10^{-7}$ kg corresponding to radii of 0.5 mm at a bulk density of 1000 kg/m$^3$. Numerical simulations of the parent comet indicate that the meteoroids of the 2011 return of the Andromedids shower were primarily ejected during 3D/Biela's 1649 perihelion passage. The orbital characteristics, radiant, timing as well as the absence of large particles in the streamlet are all consistent with simulations. Predictions are made regarding other appearances of the shower in the years 2000-2047 based on our numerical model. We note that the details of the 2011 return can, in principle, be used to better constrain the orbit of 3D/Biela prior to the comets first recorded return in 1772.

Dynamical resonant structures in meteoroid stream orbits

Herein we use the Canadian Meteor Orbit Radar (CMOR) data set to search for evidence of a resonant swarm in the Taurid meteoroid stream at the 7:2 Jovian resonance. We use a numerical method to estimate the reduction in radar orbit measurement uncertainty required to detect this feature in a data set. This is highly dependent on the proportion of observed particles that are members of the resonant swarm. However, we find that a meteor radar with uncertainties a factor of 10 lower than those of the current CMOR will be sufficient for detection to be possible, if the meteor shower consists of more than 5 per cent resonant particles (considered likely given the results of visual meteor studies). Such an improvement will require accurate removal of deceleration errors from pre-atmospheric meteor velocities, and improvement to the robustness of echo inflection point identification algorithms and interferometric measurements.

The Orionid Meteor Shower Observed Over 70 Years

Visual Orionid meteor data dating back to 1944 were transformed into the standard format of the Visual Meteor Data Base (VMDB) of the International Meteor Organization (IMO) for systematic analysis. The strong 2006 Orionid return with a very low population index (r = 1.6) and a peak ZHR of 60 (about 2.5 of the average peak strength) resembled meteor showers connected with the returns of resonant meteoroids. An investigation of data dating back to 1928 yielded similar rate enhancements in 1936, further supporting the assumption that meteoroids trapped in the 1:6 resonance with Jupiter caused the unusual 2006 Orionid return.

Taurid resonant-swarm encounters from two decades of visual observations

Enhanced Taurid activity in terms of visual meteor and fireball rates has been found in 1988, 1991, 1995, 1998 and 2005 data. The years of heightened activity are shown to be unequivocally linked to the encounters of swarms of resonantly trapped particles in the Taurid meteoroid stream according to the model proposed by Asher & Clube. While the annual activity level of the Taurid meteor shower in terms of zenithal hourly rate (ZHR) is 7.8 ± 1.2, swarm year activity typically reaches ZHRs of 12‐17. The annual fraction of fireballs is below 1 per cent; in swarm years, this fraction is as high as 2.4‐4.6 per cent near the maximum of the Taurid activity period.

The activity of the 2004 Geminid meteor shower from global visual observations

A comprehensive set of 612 h of visual meteor observations with a total of 29 077 Geminid meteors detected was analysed. The shower activity is measured in terms of the Zenithal Hourly Rate (ZHR). Two peaks are found at solar longitudes and with ZHR = 126 ± 4 and ZHR = 134 ± 4, respectively. The physical quantities of the Geminid meteoroid stream are the mass index and the spatial number density of particles. We find a mass index of s≈ 1.7 and two peaks of spatial number density 234 ± 36 and 220 ± 31 particles causing meteors of magnitude +6.5 and brighter in a volume of 109 km3, for the two corresponding ZHR maxima. There were 0.88 ± 0.08 and 0.98 ± 0.08 particles with masses of 1 g or more in the same volume during the two ZHR peaks. The second of the two maxima was populated by larger particles than the first one. We compare the activity and mass index profiles with recent Geminid stream modelling. The comparison may be useful to calibrate the numerical models.

Temporal evolution of the vertical content of metallic ion and neutral species

Temporal changes in the vertical column contents of the meteoric metals Mg +, Mg, Fe +, and Fe have been measured at ∼1030 LT for all longitudes in the latitude zone of 10°N–30°N during November 1996, encompassing the period of the Leonid meteor shower. The column contents were obtained using UV radiances measured by the GOME instrument on the ERS-2 satellite. Throughout the month there are several interesting prominent content enhancements of all species. After the Leonid shower peak on 17 November the ion species contents increase before the contents of the neutrals. However, it is difficult to confirm that this is a shower effect given the other similar variations seen earlier in the contents and in the sporadic meteor flux enhancements. The peaks in visual meteors on 17 November are not observed in the total metal column amounts. The GOME instrument has been in continuous operation since April 1995 and is an excellent resource for studying the temporal behavior of meteoric metals on a global scale.

Radar and optical leonids

Abstract. We present joint optical-radar observations of meteors collected near the peak of the Leonid activity in 2002. We show four examples of joint detections with a large, phased array L-band radar and with intensified video cameras. The general characteristic of the radar-detected optical meteors is that they show the radar detection below the termination of the optical meteor. Therefore, at least some radar events associated with meteor activity are neither head echoes nor trail echoes, but probably indicate the formation of "charged clouds" after the visual meteor is extinguished.

The D Discriminant and Near-Earth Asteroid Streams

A method of separating an association of orbits, a stream, from the local background using the traditional D discriminant is introduced. Applying this procedure to all 708 asteroids with perihelia less than 1.3 AU known on April 22, 1999 suggests that the orbits of Earth-approaching asteroids may be rather organized. By searching the volume around every orbit in the sample space, 14 associations are found with from 4 to 25 members that statistically stand out from the local background. A more traditional search, where orbits are gathered into strings with every orbit connected to at least one other orbit by D< D c , reveals eight additional three-member strings. Using Poisson statistics seven pairs are identified as having exceptionally similar orbits; three of these pairs are a part of an association or string. On the other hand, comparing the results of a volume search around randomized orbits indicates that perhaps 50 to 75% of these groupings are due to chance, indicating that observational effects may severely affect the background and therefore the reality of the associations that are separated out. The lack of compositional or taxonomic information about most of the 155 asteroids, 22% of the sample, hinders interpretation of the nature of the streams. Of the three associations found in an earlier study (Drummond 1991, Icarus 89, 14–25), only one remains intact as a result of the stricter statistical requirements adopted here. Three of four fireball streams (Halliday et al. 1990, Meteoritics 25, 93–99), previously found to be related to the old associations, can be loosely associated with three of the new associations, but none of the four recovered meteorites with known orbits can be associated with them. Nevertheless, theoretical meteor radiants are listed, all of which are so large and diffuse, and have such slow geocentric velocities, that it is doubtful if visual meteor showers can be linked to the new streams. However, these associations, strings, and pairs of asteroids are good candidates for meteorite sources, especially where organized falls are recognized, since they generally satisfy the immediate dynamical requirements for delivery, namely, gentle geocentric velocities produced by overtaking the Earth around sunset.

Mixed particle stream model of the main peak of 1998 Leonids

On the basis of the radar observational data of the Leonids obtained in China in November of 1998, we found that the characteristics of the meteor population index gamma during the main peak could not be explained well with a one-component magnitude distribution. Moreover, according the visual observational data obtained in China and other districts of Asia, gamma also presents the property of non-uniformity. In this paper, we set forth a mathematical model of mixed particle streams. From this, the observed phenomenon of rarity of visual meteors and abundance of radar meteors during the main peak is interpreted.

The 1998 outburst and history of the June Bootid meteor shower

The June Bootid meteor shower (sometimes referred to as the Draconids) surprised a number of regular and casual observers by an outburst with maximum zenithal hourly rates (ZHRs) near 100 on 1998 June 27 after a quiescent period of several decades. A total of 1217 June Bootid meteors were recorded during regular visual meteor observations throughout this outburst. An average population index of r=2.2±0.10 was derived from 1054 shower magnitude estimates. The broad activity profile with ZHR>40 lasting more than 12 h and the large spread of apparent radiants in 1998 resemble the 1916 and 1927 outbursts. The peak time is found to be at about λ⊙=95°.7 (2000.0); peak ZHRs are of the order of 200, whereas reliable averages reach only 81±7. The period of high ZHRs covered by a single observer implies a full width at half-maximum of 3–4 h. The resulting maximum flux of particles causing meteors brighter than +6.5 mag is between 0.04 and 0.06 km−2 h−1. The average radiant from photographic, radar and visual records is α=224°.12, δ=+47°.77. The observed activity outbursts in 1916, 1927 and 1998 are not related to the orbital period or the perihelion passages of the parent comet 7P/Pons–Winnecke. These are probably a consequence of the effects of the 2:1 resonance with Jupiter.

Satellite impact probabilities: annual showers and the 1965 and 1966 Leonid storms

The stream meteoroid impact probability for space platforms is reviewed and found to be very low under normal circumstances. While the literature contains numerous accounts of spacecraft apparently suffering damage and/or interference during meteoroid stream encounters, we find that there is, in fact, very little evidence to support such claims. This conclusion may not be valid, however, during meteor storms, when the flux of visual meteors can increase by factors in excess of 10 3 to 10 4 of that from the sporadic background. Special attention is directed towards the Leonid meteor storms of 1965 and 1966—the only meteor storms since the dawn of the space age. The space platform impact probabilities during the 1966 storm were small but none negligible, being of order 1% for an exposed surface area of 2 m 2 at a limiting meteoroid mass of 10 −7 g (and assuming a stream mass index s=2.0). The circumstances surrounding the possible encounters of the Pegasus II and III, and Mariner 4 spacecraft with Leonid stream meteoroids are discussed in some detail. While the 1966 Leonid meteor storm is the strongest on record (in the sense of the highest visual meteor rates) no apparent meteoroid inflicted damage to a spacecraft can be unambiguously linked to it. This result is mostly a consequence of the small number and small size of spacecraft in Earth-orbit at the time of the 1966 storm.

The Leonid Meteor Shower and the Lunar Sodium Atmosphere

Observations of lunar sodium were made on November 16–18, 1997, at Asiago and Mount Lemmon Observatories. A small enhancement was observed, and we suggest that it was related to the Leonid shower. Visual observations by members of the Association of Lunar and Planetary Observers and the North American Meteor Network show a peak, a few hours wide, at about 12:50 UT on the 17th. The possible enhancement of the lunar Na appears to last at least 3 days, and must be associated with a considerably more extended cloud of particles than that responsible for the bright visual meteors. The circumstances of the 1997, 1998, and 1999 events are discussed. Large showers may occur in 1998 and 1999; the 1998 shower occurs too close to new moon for good atmospheric observations, but if the extended, faint shower is also enhanced the corresponding lunar Na should be observable for a considerably longer period.

Visual observations of the Perseid meteor shower 1988–1994

Visual meteor observations are used to determine the flux profile of the Perseid stream from 1988 to 1994. The position and peak flux of recently detected outburst maxima are found for each year as is the location and magnitude of the regular maximum. The Perseid stream is found to consist of three primary components; a long-lived and relatively weak background component, a core component which is active for 1–2 days near the main peak of the shower and an outburst component which is active for only a few hours. The outburst peaks show a systematic variation in terms of the solar longitudes of the peak. The position of the outburst shifted backwards in solar longitude (J2000.0) from λ ⊙ = 139°.78 in 1988 to λ ⊙ = 139°.48 in 1992, when it occurred closest to the node of 109P/Swift-Tuttle (139°.44). In subsequent years the peak position shifted forward again to λ ⊙ = 139°.59 in 1994. The magnitude of the peak flux shows a significant difference between returns before 1991 as compared to those after this year. This may indicate an ejection origin other than 1862 for outburst material encountered before 1991. The particle composition of the outburst maximum and core maximum are indistinguishable. A double maximum in the population index associated with these activity maxima has been detected in the average Perseid profile. High temporal resolution study of the flux profile of the ascending branches of the outburst profiles in 1993 and 1994 suggest that this component may consist of sub-components, possibly of differing ages.

The mass distribution of comets and meteoroid streams and the shower/sporadic ratio in the incident visual meteoroid flux

Summary The large dust particles that comets emit as they decay produce meteoroid streams. If the Earth passes close to the centre of a meteoroid stream, a shower of meteors is produced in the atmosphere and the intensity of this shower can be quantified by the maximum zenithal hour rate (ZHR) of the meteors that are observed. If, as seems reasonable, the dust/snow mass ratio and the mass distribution of the dust particles are similar in all comets, then comets and meteoroid streams are expected to have similar mass distribution indices. This expectation has been confirmed for the more massive comets and streams by comparing the distribution of shower ZHRs with the distribution of cometary masses. The later distribution is also used to predict the expected numbers of minor meteor showers, with low ZHRs, that are present in what was previously referred to as the sporadic background. It is concluded that on a typical ‘non-shower’ night, (20 ±3) per cent of the observed visual meteors belong to minor showers, and the remaining percentage are truly sporadic. Statistical variations in the numbers of massive short-period comets and thus massive meteoroid streams will lead to significant variations in the decay products of these objects. It is thus expected that this effect will cause the sporadic meteor flux and the brightness of the zodiacal cloud to vary by around 30-40 per cent over time periods of the order of 103-105 yr.

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.

The luminosity functions of the 1969 Perseid and Orionid meteor showers

Visual counts of the 1969 Perseid and Orionid meteor showers are presented, comprising 288 Perseids and 56 Orionids. On the basis of the maximum-likelihood method of determining the power law luminosity function index, we derive s ≡ 1 + 2.5 log( r) = 1.56 ± 0.06 for the Perseids with m v = +1 to −5, and s ≈ 1.85 ± 0.1 for the Orionids with m v = +2 to −3. These values are somewhat lower than those found by other observers, but we confirm the approximate power law nature of the luminosity functions. Under the assumption that the masses of visual meteors are proportional to a power law function of the luminosities, this implies power law mass functions. If mass is directly proportional to luminosity, we have power law mass functions with the indices s given above.

Mass distribution of massive meteoroids determined by occurrence of brilliant fireballs

DETERMINATION of the mass distribution function of bodies is of prime importance for an understanding of the evolutionary processes that determine the population of the Solar System. We report here a method for measuring mass distribution of meteoroids, by determining the occurrence frequency of very brilliant fireballs. In a certain mass regime the number distribution can be described by a power law dn = (const.)m−s dm where dn is the number of particles having a mass between m and m+dm and s is the mass distribution index. s is a slowly varying function of mass, and values of s have been well established for meteoroid particles 10−12< m ≲ 101 g by various techniques, and reasonable estimates are available for large bodies m ≳ 105 g (meteorites and asteroids). There is, however, a lack of information in the mass range 101 ≲ m ≲ 105 g corresponding1 to meteoroids which yield meteors of visual magnitude − 2 ≳ Mv □ − 12 ranging from bright visual objects to very brilliant fireballs (the term fireball generally refers to any meteor brighter than Venus, Mv ≃ −4). The problem is one of sampling: while meteor (radar, visual, optical) counts over periods of hours at a single station are suitable for estimates of s for m ≲ 1 g (Mv ≳ 0), such a procedure yields a prohibitively low count for larger objects. This difficulty would be overcome by observing over a large area of the Earth's surface for many years. We determine here the occurrence frequency of very brilliant fireballs, as a method of s measurement. A fireball of magnitude equal to that of the full Moon is an outstanding spectacle likely to be well reported in an area with a high population density. With good communications and active research groups analysing reports and recording data, a reliable estimate of the influx of such large objects could be secured. In the British Isles, extensive records of fireball events covering more than 40 yr, are available2,3. Most observers' estimates were based on the great illumination produced on the landscape or inside buildings, and the personnel analysing the reports were experienced workers in the field. A detailed examination of the reports reveals a consistent rate of about two lunar fireballs (apparent visual magnitude equal to or brighter than that of the full Moon) seen in the British Isles per year. Specifically, in the period 1900–36 the average occurrence frequency was 2.4 yr−1. This frequency can be compared with that of visual meteors to obtain an estimate of s.