A “shocking” Leonid meteor at 1000 fps

Abstract

A bright, estimated magnitude −3, Leonid meteor was observed from the University of Alaska Fairbanks Poker Flat Research Range north of Fairbanks, Alaska, at 10:42 UT on 18 November 2001. Images of the meteor were recorded at 1000 frames per second. The millisecond time resolution allows spatial structures in the ablation cloud to be recorded without “smearing” due to the ∼72 km/s Leonid entry velocity. The meteor was first observed at an altitude of about 123 km. As it descended deeper into the atmosphere it brightened and a tail developed. In addition, a bright structure developed in front of the meteor, much resembling a shock, reaching its maximum brightness at an altitude of 104 km. The spatial scale-size of the shock-like structure is large, several 100 m. Observation of such structure has, to our knowledge, not been reported before. We do not have an explanation for its cause. If its formation is similar to the shock observed around re-entry vehicles it would indicate the size of the meteor to be ∼100 m, which is clearly not realistic. The meteor will create ionization, but at 105 km the ion mean-free path is only ∼1 m and it is difficult to see how ionospheric plasma instabilities driven by the ionization caused by the meteor can result in ∼100 m structures. It is more likely that the structure is associated with photo-chemical processes driven by the intense UV radiation from the hot meteor. A simple computer simulation appears to be able to reproduce the observations. However, the process cannot just be photo ionization of the ambient molecular oxygen or nitrogen as the required total photon flux is too large to be energetically possible.