Tackling the FeO orange band puzzle in meteor and airglow spectra through combined astronomical and laboratory studies

Abstract

ABSTRACT The iron oxide ‘orange arc’ bands are unambiguously detected in persistent meteor trains, meteor wakes, and clouds, as well as in the terrestrial airglow. In contrast to the majority of other astronomically important diatomic molecules, theoretical simulation of the FeO rovibronic spectra is not feasible due to the extremely condensed and strongly perturbed multiplet structure of its excited states. In this work, the time-evolution of the laser-induced breakdown spectra (LIBS) of high-purity iron recorded in air at high temperature and impact conditions is used to mimic the FeO pseudo-continuum emission observed during meteor events and the terrestrial night airglow. The relative intensity distributions in the structural continuum of the LIBS spectra are measured at 530–660 nm and a plasma temperature of 1500–6500 K. The anomalous increase of the intensity observed at 620–640 nm and temperature < 2000 K could be attributed to the emission of higher oxides of iron as explained by the conducted thermodynamic and kinetic modelling of iron burning in the atmosphere.