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

Spectra of meteors and meteor trains

Jiří BorovičkaDepartment 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

zeroorder

second order

detail of grating spectrum

Detail of a Perseid spectrum

almost head-on meteorblue 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, Nj

• Meteor cross-section, P

• Damping constant,

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

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

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

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

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

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

--> 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

Incomplete evaporation

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

Ca/Fe model evaporation

Schaefer & Fegley (2005)

LOW RESOLUTION VIDEO SPECTRA OF METEORS

Spectral and direct cameras in Ondřejov

LEONID METEOR SPECTRUMNovember 18, 2001 10:24:14 UT Mt. Lemmon

Meteor magnitude: –1.5

frame 21Pheight 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

Oh=109 km

Mg

Na

O

h=101.5 km

MgO

h= 98.5 km

Na O

h=117 km

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

“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

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

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

iji ANhvI ~

ii AN~

ieiii QnNCN 0~

kTEiei

ieQnNCN /000 ~~

Afterglow level populations

1~

/0

ie

i

kTE

i

Qn

AeN

Ni

)s 10( 15 Qne

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

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

ii AN~

ieiii QnNCN 0~

0~

),(~ ie ETNn DE

eieTNn /

0 )(

empirical factor

Level populations for recombination

iei

DEe

i QnA

eTNnN

i

/0 )(

~

14 s 10 Qne kD K / 9800 eV 84.0~

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

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)– NO2 (Borovicka & Jenniskens 2000)– OH (Clemensha et al. 2001) for IR radiation

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

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

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