Discoveries from Observations and Modeling of the 1998/99 Leonids
The Leonid meteor storm of November 1999 entered the history books as the second in the space age, but was the first to be well observed. This was a rare encounter with fresh comet ejecta that can be precisely dated to the 1899 return of the parent comet.
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SETI Institute, NASA Ames Research Center, Moffett Field, California, USA
Abstract. The Leonid meteor storm of November 1999 entered the history books as the second in the space age, but was the first to be well observed. This was a rare encounter with fresh comet ejecta that can be precisely dated to the 1899 return of the parent comet. Earth's atmosphere acted as a giant detector for measuring composition and morphology of large mm-cm sized grains during what was in effect a comet mission to 55P /Tempel-Tuttle. The high meteor flux and preponderance of persistent trains enabled the deployment of modern observing techniques to probe the chemistry and physics of the interaction of meteoric matter with the Earth's atmosphere, leading to many discoveries of interest to a wide array of science fields such as astrobiology, planetary astronomy, and the atmospheric sciences. Moreover, the correct timing of the 1999 Leonid storm by meteor stream modeling has ushered meteor storms into the modern era of forecasting space weather. This paper summarizes the discoveries that resulted from these observations and subsequent theoretical modeling.
I. INTRODUCTION
The return of possible Leonid storms was announced in the form of enhanced Leonid activity in 1994 (Jenniskens 1994, 1996). The Leonid parent comet 55P /Tempel-Tuttle was recovered in November of 1997, several months prior to its return to perihelion on February of 1998 (Hainaut et al. 1998). The historic importance of past Leonid storms is explained in the review paper on meteors and meteoroid studies by Ceplecha et al. (1998) and, for example, in the popular book on the Leonid showers by Littmann (1998). Ceplecha et al. also provide a summary of the state of the meteor and meteoroids field just before the new season of Leonid activity. Now, shortly after the 1999 Leonid meteor storm, we are looking back on a flurry of activity using modern observing techniques and numerical modeling.
II. METEOROID STREAMS AND METEOR STORMS Numerical techniques to study the orbital dynamics of meteoroids and the formation of meteoroid streams came of age only in the past ten years with the advance of computing facilities. Gradually, the large parameter space for variables such as ejection processes, effects of radiation pressure, and specific E. Grün et al. (eds.), Interplanetary Dust © Springer-Verlag Berlin Heidelberg 2001
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aspects of orbital dynamics under planetary perturbations have been probed and many ideas are now postulated that can affect the formation and evolution of meteoroid streams. It is an ongoing struggle to identify what mechanism is responsible for what. Reviews are found in Lovell (1954), Lewin (1961), Williams (1993), Steel (1994), and Jenniskens (1998). Because meteor storms represent relatively recent ejecta, they offer our best hope of identifying the correct mechanisms offormation and evolution (Kresak 1993; Jenniskens 1995). The Leonid storms are among the most intense in recent history. Early expectations of enhanced Leonid rates duri
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