Assessment of meteoroid pre-atmospheric diameter from brightness measurements prior to fragmentation

A relationship is developed for evaluating the initial meteoroid diameter based on the measured absolute visual magnitude prior to fragmentation. This approach provides an alternative to the luminous efficiency approach for evaluating the size of a meteoroid from a measured lightcurve. The developed relationship, which is valid for asteroidal meteoroids with entry velocities below 30 km/s and initial diameters greater than 10 cm, is written as ??????= 0.0556 x 10-0.182 ??????60 ????????????-0.0562 ??????60 ????????????, where ?????? is the initial meteoroid diameter in m, ??????60 ????????????is the absolute visual magnitude measured at an altitude of 60 km, and ??????60 ????????????is the velocity in km/s at 60 km altitude. This relationship is enabled by the insights provided by recent computational fluid dynamics and radiation simulations, which show that for an unfragmented meteoroid, the ??????60 ????????????value is a much stronger function of meteoroid diameter than the meteor ablation rate or meteoroid composition. This relationship is also enabled by the recent increase in the number of meteor events with calibrated lightcurves and recovered meteorites. In addition to providing the meteorite density, which enables a more accurate conversion between meteoroid mass and size, the recovered meteorites enable a diameter based on radionuclide or noble gas analyses. This provides an alternative to the dynamic mass and infrasound-based diameters, which results in three different meteor diameter assessments that are independent of the luminous efficiency. These three approaches are used to develop the present relationship between the meteor diameter and ??????60 ????????????. The altitude of 60 km is chosen for developing this relationship because it is high enough to minimize the potential for fragmentation and low enough for ablation to have reached a steady-state. This altitude is also low enough to avoid significant measurement noise in the lightcurve, which may be present at higher altitudes due to weaker emission and a longer measurement distance.