Aims. We study Fe and FeO emission in laser-induced plasma under reduced pressure to develop an approach for finding the conditions under which the laboratory plasma state matches the state of bolide wake plasma. Methods. To acquire spectra of laser-induced plasma, we ablated a target of Fe 3 O 4 in a vacuum chamber using a Q-switched laser. The Boltzmann plot method and Stark broadening of the emission lines were used to estimate the plasma temperature and electron number density. The intensity ratios of two Fe I lines (544.61 nm and 558.69 nm) to the intensity of FeO orange band at 587.1 nm were calculated to compare the conditions in laser-induced plasma and bolide wake plasma. Results. Several combinations of pressure (75-150 Torr) and delay (12 -15 µs) lead to the highest degree of similarity between laser-induced plasma and the Benešov bolide spectra at an altitude of 39 km. Importantly, the plasma parameters and pressure are consistent at these points. A detailed comparison of the spectra shows that the best-match conditions are 100 Torr and 15 µs. This pressure is ≈25 times higher than the ambient pressure at this altitude. Conclusions. We assume that the pressure in the bolide wake is higher than the ambient pressure by a factor of 20-30. This can be considered to be the upper bound estimate of the pressure in the bolide wake, and the developed approach would be beneficial to support the modeling of a meteoroid entry.

Laser-induced plasma is widely used as an emission source for laboratory simulations of different objects. The properties of laser plasma vary significantly depending on the pressure and composition of the environment, as well as during the evolution of the plasma plume. This property combined with the typical range of electron temperature (from 0.2 to 4 eV) and electron number density (10 15 -10 19 cm −1 ) make it a promising object for study of radiation from various plasma sources in atmosphere (combustion of meteors, airglow) and in outer space. The aim of the present work was to study structure and evolution of CaO and FeO bands in laser plasma under low pressure (0.16 to 32 Torr) that corresponds to the known spectra of Benešov bolide.We obtained emission spectra of laser plasma under different pressures and observation times while ablating samples of F e 3 O 4 , CaCO 3 and chondrite. In each case plasma temperature was calculated by Boltzmann plot method using 20 to 30 atomic lines. We also estimated electron number density by Saha-Boltzmann equation where possible. Study of spatial distribution of emitting atoms and molecules in plasma with precise plasma diagnostics allows to make conclusions about existence or absence of the local thermodynamic equilibrium. The obtained data show that the formation of FeO in plasma occurs with involvement of oxygen from the ablated material, but not from the surrounding atmosphere. On the contrary, CaO if formed primarily using oxygen from atmosphere. Therefore, abundance and of FeO and intensity of its emission in space objects may not depend on the pressure of the surrounding media, while CaO should have a strong dependency.

The scheme of CaO molecular fluorescence in laser-induced plasma involving the transitions between B1Π and X1Σ+ electron states was proposed and implemented. In fluorescence spectra CaO molecular bands were observed at wavelengths of 408.43 (band 0, 1) and 421 nm. The band at 421 nm was assigned to the (0, 3) transition using the energies of vibrational levels calculated by molecular constants. Selective excitation of rotational states was demonstrated, which is observed as a shift of the fluorescence intensity maximum in a spectrum with the change of the wavelength of an exciting laser within the vibrational band. The proposed scheme for CaO fluorescence was used in spatially resolved measurements to show the distribution of calcium oxide molecules in laser-induced plasma.

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