spectra of meteors and meteor trains jiří borovička department of interplanetary matter
TRANSCRIPT
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Spectra of meteors and meteor trains
Jiří BorovičkaDepartment of Interplanetary Matter
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Meteor photograph
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All-sky image
Kouřim bolide(– 13 mag)
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Bolide – 18 mag
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Double-station video meteor
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Meteor speeds
11 – 73 km/s
Faint meteors: 110 – 80 km
Fireballs: 200 – 20 km
Meteor heights
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HIGH RESOLUTION PHOTOGRAPHIC SPECTRA
OF FIREBALLS
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Battery of six photographic grating cameras with rotating shutter in Ondřejov
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Example of a photographic
prism spectrum of a bright
Perseid meteor
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Detail of the prism spectrum
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Example of photographic grating spectrum of a slow sporadic fireball
first order
zeroorder
second order
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detail of grating spectrum
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Detail of a Perseid spectrum
almost head-on meteorblue part shown (3700–4600 Å)
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Radiative transfer in spectral lines
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Assuming thermal equilibrium
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Emission curve of growth
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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)
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Free parameters
• Excitation temperature, T
• Column densities of observable atoms, Nj
• Meteor cross-section, P
• Damping constant,
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Total number of Fe atoms
2.00 2.20 2.40 2.60 2.80
Tim e [s]
1E+21
1E+22
1E+23
Num
ber
of F
e at
oms
Fl ight c urve
EN 270200
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Temperature
2.00 2.20 2.40 2.60 2.80
Tim e [s]
3500
4000
4500
5000
Tem
pera
ture
[K]
Fl ig ht c urve
EN 270200
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Cross-section
2.00 2.20 2.40 2.60 2.80
Tim e [s]
0E+0
1E+6
2E+6
3E+6
Cro
ss s
ectio
n [c
m2 ]
Fl ig ht c urve
EN 270200
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Electron density
2.00 2.20 2.40 2.60 2.80
Tim e [s]
1E+11
1E+12
1E+13
1E+14
Ele
ctro
n de
nsity
[cm
-3]
Fl ight c urve
EN 270200
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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
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• 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
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Two components
Example of a Perseid fireball
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Determination of elemental abundances
• Estimation of electron density
• Use of Saha equation
• Determine ionization degree
• Recompute neutral atom abundances to total abundances
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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 !
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Electron density from atom densities
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Abundances in meteor vapors
--> increasing volatility
-3.0
-2.0
-1.0
0.0
1.0
Log
(rat
io to
CI a
bund
ance
)no
rmal
ized
to M
g
Al Ca N i M g Fe S i C r M n Na
asteroidal
Gem inids
Taurid
L eonids
P erseids
1P / H alley
incompleteevaporation
lowcometaryFe/Mg
Cr ??
volatiledepletionin Geminids
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Incomplete evaporation
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Abundances along the trajectory
2.00 2.20 2.40 2.60 2.80
Tim e [s]
1E-4
1E-3
1E-2
1E-1
1E+0
Ele
men
t/Fe
ratio
EN 270200 M g
N a
C r
C a
A l spike
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Ca/Fe model evaporation
Schaefer & Fegley (2005)
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LOW RESOLUTION VIDEO SPECTRA OF METEORS
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Spectral and direct cameras in Ondřejov
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LEONID METEOR SPECTRUMNovember 18, 2001 10:24:14 UT Mt. Lemmon
Meteor magnitude: –1.5
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frame 21Pheight 109 km
O
Na
Mg
[O] 557nm
blue end
IR end
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Mg Na
O
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Mg Na
O
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Mg Na
O
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Mg Na
O
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Mg Na
Oh=109 km
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Mg
Na
O
h=101.5 km
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MgO
h= 98.5 km
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Na O
h=117 km
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Meteor spectral classes
M g I - 2 N a I - 1
Fe I - 15
Other
Irons
N a free
N a rich
M ainstream
N orm al
N a poor
Fe poor
Enhanced N a
4 03 0 2 0
1 5
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“All-wavelength” spectrum
From Carbary et al. (2003)
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SPECTRA OF METEOR TRAINS
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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)
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The meteor and afterglow spectrum
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METEOR AFTERGLOW
• 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
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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
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Afterglow “physics”
Line intensity:
Level population from statistical equilibrium:
radiative deexcitation+
collision deexcitation=
collisional excitation
iji ANhvI ~
ii AN~
ieiii QnNCN 0~
kTEiei
ieQnNCN /000 ~~
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Afterglow level populations
1~
/0
ie
i
kTE
i
Qn
AeN
Ni
)s 10( 15 Qne
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Train initial cooling
0.0 0.5 1.0 1.5 2.0Tim e [s]
0
1000
2000
3000
4000
5000T
empe
ratu
re [K
]
1999 tra in (Borovicka & Jenniskens 2000)
2001 Train 1
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The spectrum in the line phase
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The spectrum in the line phase (2)
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The spectrum in the line phase (3)
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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
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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)
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Recombination “physics”
radiative deexcitation
+
collision deexcitation
=
collisional excitation (negligible)
+
direct recombination &
downward cascade
ii AN~
ieiii QnNCN 0~
0~
),(~ ie ETNn DE
eieTNn /
0 )(
empirical factor
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Level populations for recombination
iei
DEe
i QnA
eTNnN
i
/0 )(
~
14 s 10 Qne kD K / 9800 eV 84.0~
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Fitting the spectrum with the recombination formula
4000 4500 5000 5500 6000 6500
W avelength [A ]
Inst
rum
enta
l int
ensi
ty
- observed
- com putedM g
N a
* F e * M g* F e
* C a
M g
F e
M gN a N a
* F e , C a
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Transition to the continuum phase
• Animation of train 6• Time 24 – 60 s
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The continuum phase
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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)– NO2 (Borovicka & Jenniskens 2000)– OH (Clemensha et al. 2001) for IR radiation
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Comparison with laboratory FeO
5000 5500 6000 6500 7000W avelength [A ]
TR AIN 6
(Jenniskens et. a l. 2000)
(40 - 60 s)
not ca libra ted
observed
laboratory FeO
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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, NO2, CaO
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ConclusionConclusion
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