* Astronomy

Title: Ablation process of the 1999 Kobe meteorite inferred from shockwave data
Authors: Ishihara, Y.; Takahashi, Y.; Hiramatsu, Y.; Furumoto, M.

Fireballs or meteoroids passing through the atmosphere with high velocities generate strong shockwaves. The shockwaves are often detected by ground instruments such as a seismometer. The seismological records provide two kinds of information, the shockwave arrival time and the amplitude of the ground motion generated by the shockwave at each seismic station. The shockwave arrival time data enable us to determine the trajectories of the fireballs. For example, Ishihara et al. (2003) determined the trajectory of the 1998 Miyako fireball using the arrival times. In this report, we show that the amplitude data are also useful to study fireball phenomena. The amplitude of the shockwave depends on the energy release rate or the reduction rate of the mass and velocity of the fireball. Therefore, it is expected that the amplitude data can be used to study the ablation process of a meteoroid in the atmosphere.
We investigate the ablation process of the 1999 Kobe meteorite using the seismological records. The amplitudes of the ground motions are converted to the amplitudes of the atmospheric shockwaves using a conversion formula given by an experiment and a theoretical consideration (Ben-Menahem and Singh, 1981). The obtained amplitudes of the shockwaves on the ground are in a pressure range from 1 to 30Pa. The shockwave amplitudes and representative source dimensions at the sources in the upper atmosphere, whose locations have been determined by the analysis of the arrival times of the shockwaves (Ishihara et al., 2001), are calculated by the formula of ReVelle (1976). Then we estimate the radius of the meteorite, adopting a relation R=Md, where R is the representative dimension, d is the radius, and M is the Mach number of the meteorite motion. The results show that the diameter of the Kobe meteorite has changed from 2 m at 70 km to 0.7m at 30 km and that at 30 25 km the size has rapidly decreased. This rapid size change is caused by a fragmentation of the meteoroid.

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