干旱区生态环境知识资源中心

A total of 18 image intensified CCD detectors were deployed at 6 locations (two in Negev Desert, Israel, and one in each of the Canary Islands, Long Key in Florida, Haleakala in Hawaii, and the Kwajalein Atoll) to provide a real-time reporting system, as well as data for subsequent detailed analysis, for the 1999 Leonid shower. Fields of view ranged from 9 to 34 degrees, with apparent limiting stellar magnitudes from about +7 to +9. In addition, a dual frequency (29.850 and 38.15 MHz) automated meteor radar with directional determination capability was located in the Canadian Arctic at Alert, Nunavut and provided continuous monitoring of the shower from a location where the radiant was constantly above the horizon. Both the radar and electro-optical systems successfully recorded the activity of the shower in real time, and typical real-time activity plots are presented. Post-event analysis has concentrated on the Israel electro-optical wide field cameras and the time interval centered around the peak of the storm. About 2700 meteors have been digitized, with 680 measured for this analysis. Of these 371 were well enough determined to permit a single-station technique to yield approximate heights. Light curves and photometric masses were computed for these 371 Leonids which form the basis of the preliminary results reported in this paper. These cameras recorded Leonid meteors with peak luminosity in the magnitude range -3 to +5, corresponding to the photometric mass range 10(-4) to 10(-7) kg. A regression plot of photometric mass with magnitude did not indicate any change in light curve shape over the interval studied here. The peak flux as determined by the electro-optical observations was 1.6 +/- 0.1 Leonid meteors of magnitude +6.5 or brighter falling on a one square kilometer area (oriented perpendicular to the Leonid radiant) per hour. This peak flux occurred at approximately 2:07 +/-: 06 UT on Nov 18 1999, corresponding to solar longitude lambda (o) = 235.248 (epoch 2000.0). The radar results were consistent with this maximum flux rate and time. There was not a strong change in mass distribution over the few hours around maximum, although there is some indication that the peak interval was stronger in fainter meteors. Height histograms are provided for beginning, maximum luminosity and ending heights. It was found that maximum luminosity and ending heights were completely independent of mass, consistent with a dustball model in which the meteoroids are fragmented into constituent grains prior to ablation of the grains. However, the beginning height increases sharply (9.1 km per decade of photometric mass change) with increasing mass. This is possibly indicative of a volatile component which ablates early in the atmospheric trajectory.