Evidence for transverse spread in Leonid meteors

We report here evidence for significant transverse spread of the light production region in bright Leonid meteors. One Leonid meteor has an apparent spread in the light production region of about 600 m perpendicular to the flight path for the meteor, that transverse spread persisting for at least 0.3 s. We have also detected short-duration, jet-like features emanating from a bright Leonid meteor recorded in 1998. These jet-like features have maximum spatial dimensions up to 1.9 km. While we cannot definitively rule out instrumental artefacts as a cause for these jet-like features, they may be evidence of motion contributing to the observed spatial spread in the light production region.     

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.     

Annual variation of sporadic radar meteor rates

The activity of sporadic meteors has been measured from 2002 to 2005 with the Canadian Meteor Orbit Radar. Single-station data at 29.85 MHz have been used to find the positions of the five sporadic sources visible from the Northern hemisphere, and the activity measured for each source has been corrected for observing biases. Relative fluxes and the annual variation of each source are well determined. The variations in activity are of great interest in determining the history and origin of the sporadic meteoroid population.     

The 2007 Aurigid meteor outburst

A rare outburst of the Aurigid meteor shower was predicted to occur on 2007 September 1 at 11:36 ± 20 min  ut due to Earth's encounter with the one-revolution dust trail of long-period comet C/1911 N1 (Kiess). The outburst was predicted to last ∼1.5 h with peak zenithal hourly rate of ∼200 h⁻¹, which is ∼20 times higher than the annual Aurigid shower. Three members of Armagh Observatory observed this outburst from the general area of San Francisco, CA, USA, where the shower was anticipated to be best seen. Observed radiant, velocity and activity peak time were consistent with the predictions, whereas the zenithal hourly rate was about half of the predicted value. Five Aurigids were observed by two stations simultaneously, enabling their spatial trajectory to be worked out. The orbits of these double station meteors are in good agreement with that of their parent comet Kiess. The outburst was abundant in bright (−2 to +1 mag) meteors. The first high-altitude Aurigid, with a beginning height of 137.1 km, was recorded.     

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.     

A determination of the strengths of the sporadic radio-meteor sources

We have calculated the response functions for the Springhill (Ottawa) and Christchurch (New Zealand) radars. Using the sizes and locations of each of the six sporadic radio-meteor sources found in our previous study (Jones and Brown, 1993), we have determined the response of each radar to each of the sources. Then, using least-squares fit to data collected over several years, we have determined the strength of each source. Excellent agreement with the non-radar data is obtained for the total flux after application of a correction for the initial train radius. The results from the two radars show good consistency for the sources which are visible to both of them. For individual years the fits are good to between 5 and 10% but there does appear to be some variability from year to year which may be due to atmospheric effects. Presented at theInternational Conference on Meteoroids 28–31 August 1994 in Bratislava.Submited to Earth, Moon and Planets     

The relationship between the semi-major axis, mass index and age of the Leonid meteor shower

Owing to sublimation of ice, comet nuclei eject dust particles when they are near to the sun. Those particles assume velocities and then vary their orbits to ones similar to that of the comet. The most notable difference between the orbit of the parent comet and those of the particles is their semi-major axes. This difference (Δ a ) has been widely used in modern meteor shower predictions. Observational evidence of the distribution showed that it is a function of Δ a , and the age of the dust trail. However, the relation is not well known. In this paper, a simplified relation between Δ a , the mass index ( s ) and the age of the dust trail is presented, taking the instance of a recent Leonid meteor shower.     

Television spectra of meteors

The first results of the television observations of meteors at the Ondejov Observatory are presented. It is shown that three spectral components may be distinguished in meteors: cool meteoric, hot meteoric and hot atmospheric. The intensity ratio of these components varies strongly even in meteors of the same velocity and within the records of single meteors. This is evidence for variations in the ablation process and in the formation of the shock wave. The so called calcium anomaly is in fact only a demonstration of these variations.     

On the shape of meteor light curves, and a fully analytic solution to the equations of meteoroid ablation

The shape and characteristics (beginning and end heights, and height of maximum brightness) of meteor light curves are investigated under the constraint that the surface area S that a meteoroid presents to the oncoming air flow varies as a power law in the meteoroid mass m such that   S ∼ m ^α  . We investigate the meteoroid ablation for a range of values of α, and find that the  α= 1  condition allows for a fully analytic solution to the coupled differential equations of meteoroid ablation when the density profile is that of an isothermal atmosphere. The possible geometrical properties of Geminid meteoroids are discussed in terms of the  α= 1  ablation model and it is shown that they are consistent with being derived from an asteroidal, rather than cometary, parent body.     

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.     

Meteoroid rotation and fireball flickering: a case study of the Innisfree fireball

Some 5 per cent of bright meteors show rapid, quasi-periodic brightness variations. It is argued that this effect, observationally known as flickering, is a manifestation of the rotational modulation of surface mass loss through ablation of a non-spherical meteoroid. We develop a set of time-dependent, single-body ablation equations that include the effect of cross-section area modulation. We present a discussion of the effects that the rotation of a non-spherical meteoroid has on the resultant meteor light curve, and we look in depth at the data related to the fireball associated with the fall of the Innisfree meteorite. We find that the parent object to the Innisfree meteorite was spinning at a rotation frequency of 2.5 Hz when it encountered the Earth's upper atmosphere. We also find that the Innisfree parent body had an initial mass of about 20 kg and that the ratio of its semiminor and semimajor axes was about 0.5.