Power and duration of impact flashes on the Moon : Implication for the cause of radiation
Résumé
Meteoroid falls on the Moon produce transient luminous events usually named impact flashes. These emissions have been reported by several independent observers using ground-based telescopes over the last decade. We present here a compendium of these observations for the period 1999-2007, including apparent flash magnitudes, durations and the origin of the bolide (meteor shower or sporadic impact). Impact flashes appear on 1-10 camera frames corresponding to durations ranging from ∼10 ms to ∼1 s. The analysis of these data reveals a correlation between duration and intensity, with the exception of Leonid meteors. The difference between Leonids and other meteoroids are likely explained by the higher velocity of this swarm. For the other events, the observed trend implies that impact flash detections are at present limited by the frame rate which is generally equal or less than 60 frames par second (f/s). The durations of these transient events are typically longer than predictions based on expanding plasma-gas clouds. We thus argue that these luminous events correspond to radiation emitted by a cloud composed of gas and small ejected melt droplets. A simple model considering the black body radiation of cooling droplets provides a time scale commensurable with the observations. In addition, such modeling is useful for optimizing the specifications of monitoring equipment. In particular, the inferred range of effective temperatures implies that near-infrared observations would efficiently increase the number of detections, whereas multi-spectral observations are essential to progress in the understanding of the nature of these luminous events.