Resonant meteoroids from Comet Tempel-Tuttle in 1333: the cause of the unexpected Leonid outburst in 1998

Recent observations of an unexpectedly high incidence of bright Leonid mete ors about 16 h before the predicted maximum of the main shower are explaine d by the ejection of dust grains into the 5/14 mean-motion resonance with J upiter, principally during the perihelion passage of Comet 55P/Tempel-Tuttl e in 1333. The dynamical evolution of resonant grains has the following pro perties: first, the grains do not spread uniformly around the orbit, but in stead librate about a resonance centre within the main stream; secondly, th ese resonant zones contain a much higher space density of particles than th e background stream, with the particle density approaching that of recently ejected cometary grains; thirdly, differential precession between the come tary orbit and the orbits of resonant particles may lead to meteor storms a t unexpected times, possibly far removed from that of the normal shower. Th e presence of resonant dust grains leads to a complex structure within the Leonid meteoroid stream, and is an important general feature of meteoroid s treams associated with Halley-type comets, themselves often trapped for lon g periods in mean-motion resonances.