solar emittance spectrum c onsl/lectures/phys20054/15lecture 4... · 2015-01-27 · each element...
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Absorption in Solar Atmosphere
A black body spectrum emitted from solar surface causes excitation processes on atoms in the solar atmosphere. This in turn causes absorption of characteristic wavelengths corresponding to those atoms that are the most abundant ones in the solar atmosphere – absorption spectroscopy.
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Solar Emittance Spectrum
Spectrum is characterized by many atomic absorption lines,
primarily by H-Balmer series of highly abundant hydrogen.
infrared visible
ultraviolet
Wavenumber: 11
cm
ck
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The Hydrogen Atom
E ZE
nE eV
E
Z Z
n 2 0
2 0
0
136.
is energy necessary to ionize atom
is charge of nucleus, = 1
n : main quantum number, n=0, 1, 2, ...
ℓ : orbital momentum quantum number, ℓ = 1, 2, 3, ...n-1
m: magnetic quantum number , 2ℓ+1 values from -ℓ, -ℓ+1 ...0... ℓ+1, ℓ
s = ½ spin quantum number, s=1/2, -1/2
http://www.walter-fendt.de/ph14e/bohrh.htm
http://www.falstad.com/qmatom/
n=1
ℓ=0
n=2
ℓ=0
n=2
ℓ=1
r n m
En
eV
n
n
2 9
2
0529 10
13606
. [ ]
.[ ]
Electron transition between orbits requires or releases energy:
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Transition between Energy Orbitals
Possible emission or absorption of light with fixed wavelengths by transitions of electrons between orbits! Wavelength depends on energy difference between orbits!
Hydrogen emission spectrum
Balmer Series
n=1
n=2
En = -13.6/n2
N=3⇒2
Hα: =656.3nm
N=4, N shell
L transitions
N=2, L shell
N=3, L shell
N=1, K shell
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Energy and Wavelength
of emitted or absorbed Photons
eVnnch
chhE
if
22
116.131
2 2
eVnn
EEEif
nn fi
22
116.13
. J s
. m s
6 626 10
2 987 10
34
8
h Planck Co nstant: ]
c Speed of L ight:
[
[ / ]
HR
mhc
eV
110097.1
6.13 7
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Multi-Electron Atoms Z is the charge and determines the chemical characteristics of the element, e.g. Z=1(Hydrogen), Z=2 (Helium) Z=8 (Oxygen). Electron transitions and photon emission is more complex and depends on Z and the shielding Sn by inner electron shells.
1
2
2
2
2
2
2
1
2
2
2
2
116.131
1
6.131
6.131
6.131
6.13
EE
hc
E
hc
hc
nZE
Zn
ZEEE
nZ
nSZE
n
K
K
K
nK
nn
K-transitions
to n=1
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Example: Calculate the K and L UV-Ray Lines for He
nmE
cheVeVE
neVZE
nmE
cheVeVE
nmE
cheVeVE
ZHe
neVZE
L
L
i
L
K
K
K
K
i
K
8.656889.19
1
4
16.131
1
2
16.131
6.10209.129
116.131
6.1212.104
116.131
2:
116.131
22
2
2
2
n=1
n=2
n=3
Kα
Kβ Lα
Kα and K transitions in the ultraviolet range! L α and other L and M transitions in visible range
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Each element has its own characteristic transitions
depending on the orbit quantum numbers and the
charge Z (number of electrons, protons). These
transitions can be analyzed by spectroscopy of the
light to determine the elemental abundance!
Characteristic Spectrum of Elements
ultraviolet visible infrared
Solar Spectrum
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Absorption in the Earth Atmosphere
Absorption and scattering of radiation in the atmosphere depends on the
chemical and physical composition of the atmospheric layers. This includes
dust and molecular composition, moisture, temperature as well as density. It
also depends on the nature of the interaction processes of the incoming (and
exiting) radiation (photons) with the atomic and molecular gas components!
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Solar Radiation in Atmosphere
dxx
d
eFF
eII
),(
0
0
)()(
F() transmitted radiation
(,x) absorption coefficient
x: thickness of atmosphere layer
The absorption coefficient
includes absorption probabilities
by molecular rotational or
vibrational excitation processes,
and photon scattering, it depends
on the composition of the
atmosphere and its atomic and
molecular components: N2, O2,
O3, CO2, CH4, H2O
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Absorption Probabilities
The particle density ni(x)
depends on the overall radial
density dependence and the
mass fractions of the gas
components in the different
atmospheric layers.
The cross section i is the interaction
probability of light (photons) with a
certain wavelength or energy E=h·
with various atmospheric gases and
dust particles (excitation and
scattering)
A
i
ii
i
ii
dxx
NA
xXxn
xnx
eF
F
)()(
),(
)(
)( ),(
0
ni(x): particle density (cm-3)
i(): cross section (cm2)
dxx
abs eF
F
F
F ),(
00
)(1)(
1)(
Transmission Absorption (heat production in absorbing material)
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Atmospheric Composition
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Physical and Chemical Characteristics
Large variation in ni(x) with altitude for different gaseous elements and
molecules I. Enrichment of H2O vapor in lower atmosphere, the
troposphere. CO2, O2, Ar fairly continuous, O3 break up in stratosphere!
mass fraction
Ar
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Rayleigh Scattering
4
257.44)(
Volume polarizability: gas =1.7·10-24 cm3
Rayleigh scattering is the elastic scattering of light by particles of size d much
smaller than the wavelength of the light , such as molecules or dust particles.
Rayleigh scattering is a function of the electric polarizability of the particles.
Scattering cross section increases with lower
wavelengths, blue light gets more scattered than
red light. It gives the sky its typically blue color
(otherwise sky would have solar white spectrum
color). Looking at sunset, scatter has increased
because of increased thickness of atmospheric
layer, blue is removed from line of sight to sun.
blue red
nmdd 100
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Example Calculate Rayleigh scattering cross section on air for blue
b=450 nm and red light r=650 nm; gas =1.7·10-24 cm3
224
3
227
227
4
44
4
25
101
22.2)(
1022.2)(
64.9)(
1064.9)(
10953.37.44)(
cmbarn
mbarn
cm
mbarn
cm
cm
r
b
b
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A
i
ii
i
ii NA
xXxnxnx
)()(),(
110101.1),(
00
410100.9),(
0
1033.3)(
)(
1023.1)(
)(
56
2
55
eeeF
F
eeeF
F
ddxx
H
ddxx
air
xi αi Ai ni [cm-3] σi [cm2] i cm1
N2 0.78 1.77 28 1.6776E+22 4.67145E-27 7.8366E-05
O2 0.19 0.8 32 3.5756E+21 9.543E-28 3.4122E-06
Ar 0.01 1.66 38 1.5847E+20 4.10886E-27 6.5115E-07
H2O 0.01 1.48 18 3.3456E+20 3.26609E-27 1.0927E-06
CO2 0.003 2.63 44 4.1059E+19 1.03137E-26 4.2347E-07
SO4 0.001 4.34 98 6.1449E+18 2.80856E-26 1.7258E-07
Air 0.994 1.7 2.0891E+22 4.30926E-27 9.0026E-05
everything scattered
66% scattered
Wave length =550 nm, Density 1g/cm3, Thickness of atmosphere layer d10 km
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Rayleigh Scattering
1.00E-10
1.00E-09
1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
13000 15000 17000 19000 21000 23000 25000
Scattering cross section in barn
Transmission through
10 km air (vertically)
Transmission through
30 km air (horizontally)
blue red
ddxx
air
eeF
F
),(
0 )(
)(
4257.44)( kk
Wave number k=1/
σ(
) [b
arn
], T
()
769 666 588 526 476 435 400 [nm]
[cm-1]
0/ FF