Model assumptions The radiation originates in a finite slab of gas (plasma) with a cross section P Atomic level population is described by the Boltzmann law for an excitation temperature T Self-absorption is taken into account (the gas is not optically thin)
Free parameters Excitation temperature, T Column densities of observable atoms, N j Meteor cross-section, P Damping constant,
Two components in meteor spectra The spectra can be explained by the superposition of two components with different temperatures The main component, T = 4500 K - present in all spectra - temperature does not depend on velocity! - originates from a relaxed vapor cloud near and behind the meteoroid
The second component, T = 10 000 K - present in bright and fast meteors (vapor lines – air lines present also in faint fast meteors) - temperature does not depend on velocity (or only slightly) - originates from a transition zone in the front of the vapor cloud - typical lines: Ca II, Mg II, Si II
Determination of elemental abundances Estimation of electron density Use of Saha equation Determine ionization degree Recompute neutral atom abundances to total abundances
Estimation of electron density 1.From meteor size and atom column densities + neutrality condition 2.From CaII/CaI ratio (if the high temperature component is absent) 3.By combining both components podivat se podrobneji !
Three phases of train evolution 1.Initial rapid decay of intensity, dominated by atomic line emission (the afterglow) 2.Atomic emissions persisting for about 30 seconds (the line phase) 3.Continuous emission emerging about 20 s after train formation and persisting for minutes (the continuum phase)
METEORAFTERGLOW Contains high excitation/ionization lines: Ca +, Mg +, Si +, Fe +, H (10,000 K component) Contains high excitation atmospheric lines: N, O Contains low excitation semi-forbidden (intercombination) lines: *Fe, *Mg, *Ca Contains forbidden green oxygen line COMMON: low excitation allowed transitions: Na, Fe
Afterglow explanation The line decay rate is proportional to the excitation potential Rapid cooling of gas under non-equilibrium conditions Low electron density causes non-Boltzmann level populations
Afterglow “physics” Line intensity: Level population from statistical equilibrium: radiative deexcitation + collision deexcitation = collisional excitation
LINE PHASE AFTERGLOW The Mg line at 517 nm of medium excitation (5 eV) is strong and persisting Mg lines of even higher excitation are present and persisting Lines of medium excitation are much fainter than low excitation lines and decay much more rapidly Different spectra, different physical mechanisms
What is the physical mechanism behind the line radiation? A mechanism to populate high levels (up to 7 eV) needed Thermal collisions absolutely insufficient because of low temperature Chemical reaction are not so exothermal Recombination suggested though previously discarded (Cook & Hawkins 1956)
What causes the continuum? The continuum is probably produced by molecular emissions excited by chemical reactions We need to identify the molecules Various sources suggested: –FeO (Jenniskens et al. 2000) –NO 2 (Borovicka & Jenniskens 2000) –OH (Clemensha et al. 2001) for IR radiation
Comments on identifications FeO is likely present but does not explain all radiation FeO bands are not well pronounced and the observed radiation is stronger in red and near-infrared (a ~750 nm maximum?) Possible additional contributors: OH, NO 2, CaO
Conclusion Conclusion Three phases of Leonid train evolution: 1.Afterglow = cooling phase 2.Line phase= recombination 3.Continuum phase = chemiluminescence All phases are relatively well separated in time
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