The role of thermionic emission in meteor physics

We solve the simultaneous equations for continuity of charge, mass, momentum and energy of a meteoroid entering the Earth's atmosphere to study its charging, deceleration, mass loss and heating along its path. In this analysis we also include the effects of thermionic emission from the frictionally heated meteoroid, which hitherto have not been considered. We have limited our analysis to the free-molecular regime which corresponds to meteoroids of radius less than or equal to1 cm in our altitude range of interest (H greater than or equal to 80 km). We have also limited our treatment to the night side of the Earth so as to avoid the complicating role of photoemission while emphasizing the role of thermionic emission. Our results show that, while the same meteoroid can change its charge polarity during flight, meteoroids with widely different material work functions could have opposite polarity at the same altitude. We find out that, while the escaping thermionic electrons do not constitute a significant source of energy loss from the surface, they could be a larger source of meteoric electrons at high altitude compared to electrons produced by the ablation process, particularly if the work function of the meteoroid is sufficiently small (i.e W less than or equal to 2 eV). Furthermore we find that the attitude range of meteoric ionization is larger than in the case when ionization is due only to collisions between sublimating molecules and background molecules. In addition the ionization has an earlier onset and a later shut-off. (C) 2001 Elsevier Science Ltd. All rights reserved.