Image-intensified video results from the 1998 Leonid shower: I. Atmospheric trajectories and physical structure

Two-station electro-optical observations of the 1998 Leonid shower are pres ented. Precise heights and light curves were obtained for 79 Leonid meteors that ranged in brightness (at maximum luminosity) from +0.3 to +6.1 astron omical magnitude. The mean photometric mass of the data sample was 1.4 x 10 (-6) kg. The dependence of astronomical magnitude at peak luminosity on pho tometric mass and zenith angle was consistent with earlier studies of faint sporadic meteors. For example, a Leonid meteoroid with a photometric mass of similar to1.0 x 10(-7) kg corresponds to a peak meteor luminosity of abo ut +4.5 astronomical magnitudes. The mean beginning height of the Leonid me teors in this sample was 112.6 km and the mean ending height was 95.3 km. T he highest beginning height observed was 144.3 km. There is relatively litt le dependence of either the first or last heights on mass, which is indicat ive of meteoroids that have clustered into constituent grains prior to the onset of intensive grain ablation. The height distribution, combined with n umerical modelling of the ablation of the meteoroids, suggests that silicat e-like materials are not the principal component of Leonid meteoroids and h ints at the presence of a more volatile component. Light curves of many Leo nid meteors were examined for evidence of the physical structure of the ass ociated meteoroids: similar to the 1997 Leonid meteors, the narrow, nearly symmetric curves imply that the meteoroids are not solid objects. The light curves are consistent with a dustball structure.