Spectra of meteors and meteor trains Jiří Borovička Department of Interplanetary Matter

Meteor photograph

All-sky image Kouřim bolide (– 13 mag)

Bolide – 18 mag

Double-station video meteor

Meteor speeds 11 – 73 km/s Faint meteors: 110 – 80 km Fireballs: 200 – 20 km Meteor heights

HIGH RESOLUTION PHOTOGRAPHIC SPECTRA OF FIREBALLS

Battery of six photographic grating cameras with rotating shutter in Ondřejov

Example of a photographic prism spectrum of a bright Perseid meteor

Detail of the prism spectrum

Example of photographic grating spectrum of a slow sporadic fireball first order zero order second order

detail of grating spectrum

Detail of a Perseid spectrum almost head-on meteor blue part shown (3700–4600 Å)

Radiative transfer in spectral lines

Assuming thermal equilibrium

Emission curve of growth

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, 

Total number of Fe atoms

Temperature

Cross-section

Electron density

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 = 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

Two components Example of a Perseid fireball

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 !

Electron density from atom densities

Abundances in meteor vapors incomplete evaporation low cometary Fe/Mg Cr ?? volatile depletion in Geminids

Incomplete evaporation

Abundances along the trajectory

Ca/Fe model evaporation Schaefer & Fegley (2005)

LOW RESOLUTION VIDEO SPECTRA OF METEORS

Spectral and direct cameras in Ondřejov

LEONID METEOR SPECTRUM November 18, :24:14 UT Mt. Lemmon Meteor magnitude: –1.5

frame 21P height 109 km O Na Mg [O] 557nm blue end IR end

Mg Na O

Mg Na O

Mg Na O

Mg Na O

Mg Na O h=109 km

Mg Na O h=101.5 km

Mg O h= 98.5 km

NaO h=117 km

Meteor spectral classes

“All-wavelength” spectrum From Carbary et al. (2003)

SPECTRA OF METEOR TRAINS

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)

The meteor and afterglow spectrum

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

Afterglow level populations

Train initial cooling

The spectrum in the line phase

The spectrum in the line phase (2)

The spectrum in the line phase (3)

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)

Recombination “physics” radiative deexcitation + collision deexcitation = collisional excitation(negligible) + direct recombination & downward cascade empirical factor

Level populations for recombination

Fitting the spectrum with the recombination formula

Transition to the continuum phase Animation of train 6 Time 24 – 60 s

The continuum phase

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

Comparison with laboratory FeO

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