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.     

Residual mass from ablation of meteoroid grains detached during atmospheric flight

Previous studies of the residual masses resulting from ablation of small meteoroid grains have been concerned with the ablation of particles which enter the atmosphere independently. There is widespread evidence that fragmentation is a common occurrence for meteors ranging from bright fireballs to the smallest meteors recorded with optical techniques. According to a widely accepted model, meteoroids can be considered to be a collection of tiny grains, with these grains being detached from the meteoroid during atmospheric flight. This investigation numerically solves the differential equations governing ablation of grains detached at different heights. Initial velocities from 12 to 70km s⁻¹, and initial masses from 10⁻⁵ to 10⁻¹³kg, are considered. The ablation equations allow for thermal heating prior to the onset of intensive evaporation, and thermal reradiation throughout. The atmospheric density profile used is one based on the U.S. Standard Atmosphere (1962, U.S. Government Printing Office, Washington). Calculations were completed for grains detached at 120, 100, 95, 90, 85, 80 and 75km height. For the purposes of the ablation model it is assumed that grains are ejected with an initial temperature of 1300 K, and that intensive grain evaporation begins at 2100 K. These values are consistent with grains emitted according to the model of Hawkes and Jones (1975a, Mon. Not. R. astr. Soc. 173, 339; Mon. Not. R. astr. Soc. 185, 727). For comparison purposes, calculations were also completed for grains entering the atmosphere independently (initial height 140km and beginning temperature 280 K assumed).

It is found that particles ejected at heights of 100km and above behave essentially as independent particles incident from infinity. Hence the results of earlier studies (e.g. Nicol et al., 1985, Planet. Space Sci.33, 315) can be applied. For ejection at lower heights the resultant residual mass is somewhat less than that corresponding to grains of the same initial mass and velocity. The difference is greatest for high velocity, low mass meteors. For initial masses near 10⁻⁵kg, residual mass is almost independent of ejection height, at least down to an ejection height of 75km. The significant finding of Nicol et al. (1985, Planet. Space Sci.33, 315) that residual mass is almost independent of initial mass for a fairly wide range of initial masses is only loosely followed when in-flight ejection of particles at heights below about 95 km is considered.

Typical calculations are presented to show that in-flight fragmentation of dustballs can be an important source of macroscopic ablation product micrometeorites. The astronomical and atmospheric implications of this finding are briefly discussed.     

Effects of sensor placement on acoustic vector-sensor arrayperformance

We consider the role played by the sensor locations in the optimal performance of an array of acoustic vector sensors, First we derive an expression for the Cramer-Rao bound on the azimuth and elevation of a single far-field source for an arbitrary acoustic vector-sensor array in a homogeneous wholespace and show that it has a block diagonal structure, i.e., the source location parameters are uncoupled from the signal and noise strength parameters. We then derive a set of necessary and sufficient geometrical constraints for the two direction parameters, azimuth and elevation, to be uncoupled from each other. Ensuring that these parameters are uncoupled minimizes the bound and means they are the natural or “canonical” location parameters for the model. We argue that it provides a compelling array design criterion. We also consider a bound on the mean-square angular error and its asymptotic normalization, which are useful measures in three-dimensional bearing estimation problems. We derive an expression for this bound and discuss it in terms of the sensors' locations. We then show that our previously derived geometrical conditions are also sufficient to ensure that this bound is independent of azimuth. Finally, we extend those conditions to obtain a set of geometrical constraints that ensure the optimal performance is isotropic     

Flux of very faint Leonid meteors observed with a 3 meter liquid mirror telescope intensified charge‐coupled device system

Abstract— We have used a 3.0 m diameter liquid mirror telescope (LMT) coupled to a microchannel plate image‐intensified charge‐coupled device (CCD) detector to study the 1999 Leonid meteor shower. This is the largest aperture optical instrument ever utilized for meteor detection. While the observing system is sensitive down to stars of +18 astronomical magnitude under optimum conditions, when corrections for meteor motion are applied the majority of the meteors collected fall in the absolute magnitude range from +5 to +10, corresponding to photometric masses from about 10^?7 to 10^?9 kg. This is largely due to the fact that the field of view of the LMT was only 0.28°, so that only a small portion of the luminous meteor trail was recorded. While the flux of these small (1.4 times 10^?9 kg) Leonid meteors is low (on the order of one Leonid meteor per hour per square kilometer perpendicular to the Leonid), we do have clear evidence that the Leonid stream contains particles in the mass range studied here. The data showed a possibly significant peak in Leonid flux (9.3 ± 3.5) for the 1 h period from 11:00 to 12:00 u.t. 1999 November 17 (solar longitude 234.653 to 234.695, epoch 2000.0), although the main trend of these results is a broad low‐level Leonid activity. There is evidence that small meteoroids are more widely distributed in the Leonid stream, as would be expected from cometary ejection stream models. As would be expected from an extrapolation of mass distribution indices for brighter meteors, the vast majority of meteors at this size are sporadic. The LMT is a powerful detector of sporadic meteors, with an average non‐Leonid detection rate of more than 140 meteor events per hour.     

Searching for Light Curve Evidence of Meteoroid Structure and Fragmentation

We used light curve analysis to search for evidence of the dustball meteoroid model. Leonid, Taurid, Alpha Monocerotid and sporadic meteors from November 2003 were observed and analyzed using uniform methodology. Meteors from these four sources were examined for evidence of fragmentation by examining light curve shape and searching for light curve irregularities. Differences in meteoroid structure should be reflected by differences in meteor light curves. The resulting meteor light curve F-parameter values showed no statistically significant differences between the meteors from the various cometary showers or the sporadic meteors. The F-parameter values also suggest that the meteoroids from these sources do not follow a single body ablation model, which suggests that all four sources produce meteoroids with a dustball structure.