SPA-GRL Issue Highlights

The 1998 Leonid shower was the best since the storm of 1966. The high fluxes of visible meteors provided a rare opportunity to study the effects of meteors on the upper atmosphere. The nights of November 16-18, 1998, were particularly rich in bright fireballs, some as bright as the full Moon. Many left in their wakes spectacular chemiluminescent trails that remained visible for as long as 30 minutes. The four papers in the special section describe high resolution lidar and imager observations of these ablation trails from a research aircraft flying out of Okinawa, Japan, and at the Starfire Optical Range near Albuquerque, New Mexico. These unique measurements provide new insights into the chemical processes responsible for the persistent ablation trails and the effects of eddy diffusion and gravity wave dynamics on this region of the upper atmosphere.

The lifetimes of the ablation trails observed by the Fe and Na lidars are directly related to the chemical lifetimes of the species below 90 km and to molecular diffusion above 95 km. Chu, Pan et al. use a molecular diffusion model to determine that the average age of the trails is just over 10 minutes and is strongly altitude dependent. The shortest ages were observed below 92 km and above 98 km. Kelly et al. measure for the first time the sodium densities in the chemiluminescent trails produced by the Leonid meteor storm event, and also image the trails in sodium light.

Chu, Liu et al. probe seven persistent trails associated with bright fireballs with a steerable sodium wind/temperature lidar in New Mexico during the shower. They suggest that the masses of the meteors range from 1 g to 1 kg. The persistent trails are especially rich in Na airglow emissions. The fine structure of these emissions, recorded by CCD imagers, suggests that the airglow is confined to the walls of a tube which increases in diameter over time in response to molecular diffusion. Lidar profiles show clearly that temperatures are warmest at edges of the persistent trails where the airglow emissions are strongest. Current models of chemical heating, however, predict heating rates that are about 100 times smaller than observed. Grime et al. acquire and track meteor trails with a Doppler wind/temperature sodium resonance lidar, and use the spatial tracking to estimate the region's neutral wind to infer the local small-scale diffusivity.

On two close flybys (September 17, 1997 and August 14, 1999), the spacecraft Galileo flew directly through the geometric wake of the Jovian moon Callisto. Gurnett et al. present measurements of the plasma density in Callisto's vicinity and conclude that Callisto is the source. The authors propose photoionization and electron impact ionization as mechanisms for producing the high plasma densities near Callisto.