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

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Presentation on theme: "Spectra of meteors and meteor trains Jiří Borovička Department of Interplanetary Matter."— Presentation transcript:

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

2 Meteor photograph

3 All-sky image Kouřim bolide (– 13 mag)

4 Bolide – 18 mag

5 Double-station video meteor

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

7 HIGH RESOLUTION PHOTOGRAPHIC SPECTRA OF FIREBALLS

8 Battery of six photographic grating cameras with rotating shutter in Ondřejov

9 Example of a photographic prism spectrum of a bright Perseid meteor

10 Detail of the prism spectrum

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

12 detail of grating spectrum

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

14 Radiative transfer in spectral lines

15 Assuming thermal equilibrium

16 Emission curve of growth

17 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)

18 Free parameters Excitation temperature, T Column densities of observable atoms, N j Meteor cross-section, P Damping constant, 

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27 Total number of Fe atoms

28 Temperature

29 Cross-section

30 Electron density

31 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

32 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

33 Two components Example of a Perseid fireball

34 Determination of elemental abundances Estimation of electron density Use of Saha equation Determine ionization degree Recompute neutral atom abundances to total abundances

35 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 !

36 Electron density from atom densities

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

38 Incomplete evaporation

39 Abundances along the trajectory

40 Ca/Fe model evaporation Schaefer & Fegley (2005)

41 LOW RESOLUTION VIDEO SPECTRA OF METEORS

42 Spectral and direct cameras in Ondřejov

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

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

51 Mg Na O

52 Mg Na O

53 Mg Na O

54 Mg Na O

55 Mg Na O h=109 km

56 Mg Na O h=101.5 km

57 Mg O h= 98.5 km

58 NaO h=117 km

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60 Meteor spectral classes

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

62 SPECTRA OF METEOR TRAINS

63 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)

64 The meteor and afterglow spectrum

65 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

66 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

67 Afterglow “physics” Line intensity: Level population from statistical equilibrium: radiative deexcitation + collision deexcitation = collisional excitation

68 Afterglow level populations

69 Train initial cooling

70 The spectrum in the line phase

71 The spectrum in the line phase (2)

72 The spectrum in the line phase (3)

73 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

74 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)

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

76 Level populations for recombination

77 Fitting the spectrum with the recombination formula

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

79 The continuum phase

80 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

81 Comparison with laboratory FeO

82 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

83 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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