Observational Data And Numerical Modelling

The 1998 Leonid shower was rich in bright fireballs, some of which produced intense flares. We use observational data collected in Mongolia for 316 Leonid meteors observed with microchannel plate image intensified CCD detectors (see [14] for more details on the equipment and observations). Four of these meteors had intense flares - see Figure 1. The duration of meteor flares can be used to estimate the size of the constituent grains if one assumes that a rapid commencement flare is the result of simultaneous detachment of many grains [15].

Figure 1. Leonid meteor recorded at 22:37:48 UT on Nov. 16, 1998. These two images are only 5 video frames (0.167 s) apart. This meteor displayed a single intense flare.

These flares were so bright that precise absolute photometry is impossible. Although the CCD auto-gain circuitry was turned off during observations, several of these events were bright enough to enable the protection circuitry in the microchannel plate image intensifiers (which then reduced the intensifier gain momentarily). If we extrapolate techniques used for image intensified CCD meteor photometry [11,13,16,17] we can determine light curves for these events. We demonstrate in Figure 2 the light curve for the early part of the 22:37:48 UT Nov 16 1998 event. It is clear that there was a well defined meteor light curve which suddenly brightened to produce an intense flare. A single station technique which utilizes the known radiant and velocity and the apparent angular velocity from the video data [18] can be used to estimate the heights of these meteors to a precision of about 2.0 km. The data is shown in Table 1.

If we assume that the flares are a consequence of simultaneous detachment of a large number of meteoroid grains we can match the observed flare duration with predictions based on numerical modeling of the atmospheric ablation of these grains [12]. We assume that the grains are spherical, with an average bulk density of 1000 kg m3, and with a sum of latent heat of vaporization plus fusion of 6xl06 J kg"1. The grain mass which best matches the height of maximum luminosity of the flare is given in the final column of Table 1.

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