R. L. Hawkes

Are meteoroids really dustballs

Abstract Analysis of light curves of faint meteors seem to suggest that most meteoroids are collections of hundreds to thousands of fundamental grains at least some of which are released prior to the onset of intensive ablation. We would expect these grains, unless extremely uniform in physical properties, to be aerodynamically separated during flight, and therefore to produce wake, which is defined as instantaneous meteor light production from an extended spatial region. We present here theoretical results for wake production as a function of grain mass distribution, height of separation, zenith angle and velocity. In addition, we have obtained observational results from a new study which used short duration intensified CCD detectors to search for wake in sporadic meteors. The system employed coaxial intensified CCD cameras at each of two separated stations, one camera utilizing a rotating shutter and one not at each station. The majority of these nonshower meteors showed no statistically significant wake. However, several examples of apparent transverse separation of the light production regions were found. We also present results of two interesting Leonid meteors from the Mount Allison light curve experiment in NASAs 1998 Leonid Multi-instrument Aircraft Campaign (MAC) program. One of these provide support for the idea of transverse spread in the light production region, up to hundreds of meters, while the other provides a clear case of extreme wake in one Leonid meteor which can only be successfully modeled with very small (≈10−16– 10 −17 kg ) constituent grains.

Multi-station electro-optical observations of the 1999 Leonid meteor storm

Abstract Single- and double-station video observations of the 1999 Leonid shower made from Israel are presented. A total of 232 double-station Leonids had trajectories computed. Additionally, some 2500 single-station Leonids were used to measure the Leonid storm flux and mass distribution in the interval from 0.5– 3 UT 18 November 1999. The height distribution for storm Leonids of average mass ∼10−6– 10 −7 kg indicates that the ablation zone is approximately Gaussian-shaped with best-fit mean begin, maximum brightness and end heights of 123.3±0.7, 107.3±0.42 and 95.0±0.56 km respectively. The peak flux at the time of the storm was found to be 0.81±0.06 meteoroids km −2 hr −1 Mv using 15 min binning and 0.99±0.11 meteoroids km −2 hr −1 Mv for 3 min intervals. The smaller temporal resolution reveals a broad plateau in flux lasting from approximately λ0=235.276–235.285° (J2000.0). At least one significant feature in the rate curve is apparent near 235.272°, which we suggest is associated with material released in 1932. The video mass distribution index over the course of the Leonid storm was found to be constant near s=1.75. The peak time of the storm estimated from 15 min sampling of the flux profile is near 235.283±0.005° (1h58m±7m) while 3 min resolution data place the maximum at 235.281±0.003° (1h55m±4m). The mean radiant position at the time of the storm was found to be α=153.1±0.1° and δ=21.5±0.2° (J2000), with some hint of a more compact radiant grouping within the range α=153–154° and δ=21–22°. We do not find evidence for any significant high altitude Leonid population at video masses despite biasing one camera pair to an intersection altitude of 160 km .

Residual mass from atmospheric ablation of small meteoroids

Abstract Numerical solutions of the equations of meteor ablation in the Earths atmosphere have been obtained using a variable step size Runge-Kutta technique in order to determine the size of the residual mass resulting from atmospheric flight. The equations used include effects of meteoroid heat capacity and thermal radiation, and a realistic atmospheric density profile. Results were obtained for initial masses in the range 10−7–10−2 g, and for initial velocities less than 24 km s−1 (results indicated no appreciable residual mass for meteors with velocities above 24 km s−1 in this mass range). The following function has been obtained to provide the logarithm of the ratio of the residual mass following atmospheric ablation to the original preatmospheric mass log r = 4.7 −0.33v ∞ −0.013v ∞ 2 + 1.2 log m ∞ + 0.08 log 2 m ∞ −0.083v ∞ log m ∞ M The pre-atmospheric mass and velocity are represented by m∞ and v∞. When the results are expressed in terms of the size of the residual mass following atmospheric ablation as a function of the initial mass and velocity, it is found that the final residual mass is almost independent of the original mass of the meteoroid, but very strongly dependent on the original velocity. For example, the residual mass is very nearly 10−7 g for a meteoroid with velocity 18 kms−1 for initial masses from 10−7 to 10−3 g. On the other hand, a slight change in the initial velocity to 20 km s−1 will shift the residual mass to approx. 10−8 g. This strong velocity dependence coupled with the weak dependence on the original mass has important consequences for the sampling of ablation product micrometeorites.

Residual mass from ablation of meteoroid grains detached during atmospheric flight

Abstract 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.

The size of meteoroid constituent grains: Implications for interstellar meteoroids

The most widely accepted model for the structure of cometary meteoroids is a dustaball with grains bound together by a more volatile substance [1]. In this paper we estimate the size distribution of dustball grains from meteor flare duration, using image intensified CCD or 1998 Leonid meteors. Upon the assumption of simultaneous release of dustball grains at the beginning of the flare, numerical atmospheric ablation models suggest that the dustball grains in these Leonids are of the order of 10 −5 to 10 −4 kg, which is somewhat larger than estimates obtained by other methods. If the dustball grain sizes determined here are representative of cometary meteoroid structure in general, only the most massive (O and BO) type stars could eject these grains into interstellar space by radiation pressure forces.