Thermosphere
The thermosphere begins about 80 km above the earth and ranges up to the
exosphere at 500 km altitude. The layer is also called the ionosphere, satellite
orbits are located in the medium to upper range of the thermosphere .
Mass and Composition
Most of the gas mass in the atmosphere (80%) is concentrated within the
troposphere. The mass of the thermosphere above about 85 km is only
0.002% of the total mass. No mixing and no significant physical feedback
from the thermosphere to the lower atmospheric regions is expected.
The composition of the thermosphere
changes from predominantly N2 and O2 to
atomic O, atomic N; remnant gas particles
collide so infrequently that the gases
become somewhat separated based on
the types of chemical elements they
contain. Energetic ultraviolet and X-ray
photons from the Sun break apart
molecules in the thermosphere. Towards
the upper thermosphere, ionized gas
components as well as H and He become
the main components .
Composition of Thermosphere
Variation of particle density, composition and
ionization level with altitude in thermosphere.
Rapid decline of H2O content, slow decline of
molecular components with increase in the
fraction of atomic and ionic components.
Tidal winds and electric currents
Winds and the overall circulation in the thermosphere are largely driven by tides
and waves. Moving ions, dragged along by collisions with the electrically neutral
gases, produce powerful electrical currents in some parts of the thermosphere.
Two kinds of tidal waves, one driven by solar radiation impact (moves westwards) and
the second influenced by radiation impact from earth with strong longitudinal variances.
Thermosphere temperatures increase with
height due to absorption of highly
energetic solar radiation by the small
amount of residual oxygen still present at
high altitudes. Temperatures are highly
dependent on solar activity, and rise up to
1,500 °C to 2,500 °C during the day. The
temperature increase and the temperature
level depends on the incoming flux of solar
radiation.
dQ V
dT dF
   CP 

Fabs  1  F0  e  z
dt
dt
dz
dT
  CP 
    F0  e  z    F    n  F
dt
N
dT   N A  F
The temperature change
n A

goes linearly with the flux!
A
dt
A  CP
part
W
10 cm  6.022  10
 1370 2
dT   N A  F
mole
m


g
J
dt
A  CP
29
 1.005
mole
kg  K
J
16 2
23 part
10 m  6.022  10
 1370
2
dT
K
mole
s

m

 2.84  105
J
dt
s
29.07  103
mole  K
dT
K
5 K
 2.84  10
 80  T  960 K temperatur e increase in 12h
dt
s
h
This is comparable to the observed day and night variation
of the thermosphere temperature of T ≈ 1000K.
 20
2
23
For this we adopted a rather arbitrary average absorption cross section
of 10,000 barn. This indeed depends very critical on the wavelength of
the incoming radiation due to the variety of excitation and ionization
processes in a specific atmospheric layer. Also neglected are potentially
remaining cooling processes that might compensate the heating.
The Ionosphere
upper range of thermosphere
Ionosphere reflector of long range radio waves.
Earth’s ionosphere
~200-600km
This range of the upper atmosphere is characterized by ionization effects of
incoming high energy cosmic radiation from UV light to x rays from the sun! This
generates a large flux of free high energetic electrons which can cause secondary
ionization and dissociation effects on high altitude gases and molecules.
O2  h   102.6nm   O2  e 
N 2  h   79.6nm   N 2  e 
O  h   91.0nm   O   e 
N  h   85.2nm   N   e 
NO  h   134.1nm   NO   e 
UV < 150 nm
Network of ionization processes
Y ( Ion)  Fcr     n  dz
z
The conversion rate of molecules to atoms
and atoms to ions depends on the cosmic
radiation flux Fcr, the cross section for the
break-up or ionization processes  (mostly
inelastic scattering) and the particle density n.
Level schemes
Energy levels for different
main quantum numbers n
Energy levels for different orbital
momentum quantum numbers ℓ
ℓ=0 =1 =2
=3 =4
Transition selection
rules: ℓ=+/-1
m=0, +/-1
The Oxygen ionization energy
Electron transitions and photon emission in a multi-electron
atomic system depends on the charge number Z and the shielding
Sn of the attractive nuclear potential by inner orbit electrons!
Energy of electron on orbit n:
Ionization energy for electron
from orbit n :
En   Z  S n  
13.6
2 13.6




Z

1
 2
2
n
n
2 13.6
2 13.6
En   En  Z  1  2  Z  1  2
n

2
 1 
hc


En  Z  1 13.6  2
En 1  666eV 
n 
K
 
hc
absorbed light with wavelength: K 
 1.86 10 9 m  1.86nm
E
2
Energetic x-ray wavelengths generated by solar radiation

1
2
EK  Z  1  13.6 eV  1  2 
 ni 
O: Z 8
E K
EK 
 1
 7  13.6 eV  1    499.8 eV 
 4
 1
 72  13.6 eV  1    592.4 eV 
 9
2
1
1
2
EL  Z  1  13.6 eV   2  2 
 2 ni 
1 1
EL  72  13.6 eV      92.6 eV 
4 9
n=3
Kβ
  2.48nm 
Lα
  2.09nm 
n=2
Kα
  13.4nm 
n=1
High energy radiation is absorbed by excitation and ionization
processes of atomic gases in the upper atmosphere range! The
light is emitted by recombination and de-excitation processes!
Oxygen atoms and molecules are excited by
interaction with cosmic ray flux, de-excitation
of excited oxygen is caused by collisions with
nitrogen molecules in lower altitudes causing
the emission of characteristic green light!
Gerhard Riessbeck
Aurora Borealis (Australis) effect
Aurora occurs at time of high cosmic ray flux from the sun. Charged particles are
funneled by the earth magnetic field and interact with atmospheric gases, causing
excitation and de-excitation under the emission of characteristic light in
approximately 80 to 250 km altitude. Also involved are recombination effects of
free electrons with ions and charged molecules. Dominant emission wavelengths:
• 557.7 nm green line from O
• 630.0 nm red line from O
• 636.3 nm red line from O
• UV light from N2 molecule
At high altitude (z>150km) excitation takes place through interaction with cosmic radiation
followed by de-excitation of molecular scattering causing red light. At lower altitudes
molecular scattering dominates excitation and de-excitation, triggering green light emission.
Configuration
1s2 2s2 2p4
Term
3P
1s2 2s2 2p3(4S)3s
Level cm−1
2
0.000
1
158.265
0
226.977
1D 1D
2
15867.86
1S
0
33792.58
5S
2
73768.20
630 nm
1s2 2s2 2p4
558 nm
2
2
1s 2s 2p4
J
Only a few transitions possible in few electron system with each quantum state
defined by an electron positioned in an orbital around the nuclear core of atom
Multiple transitions possible because of the increased complexity of the quantum
configurations, including electron orbitals, vibrational motion, and rotational modes.
Futzing around with the ionosphere
Aurora light induced by high altitude nuclear missile tests in the 1960ies
Exosphere
The theoretical top boundary of the exosphere is the point at which the solar particle flux is
not influenced anymore by the Earth’s gravitational pull on the atmospheric particles. This has
been detected to about 190,000 km from the surface of the Earth. Empirically, 10,000 km is
considered the official boundary between the Earth’s atmosphere and interplanetary space.
Exosphere temperature
Exosphere is nearly absolute vacuum, the remaining particles move with
high velocity. Temperature is defined in terms of kinetic energy or velocity:
1
m  v2
2
E  k T   m  v
T
2
2k
k  1.38 10  23 m 2 kg s  2 K 1
for
O with v  5500 m / s
16
(half escape velocity : 11000 m/s )
16 1.66 10  27 kg  3 107 m / s 
T
 29110 K
 23
2
2
1
2 1.38 10 m kg s K
2
Lower mass particle at same temperature have higher velocity and escape easier.
2k  T
2.76 10 23  29110
v( H ) 

 22000m / s
 27
m( H )
1.66 10
2k  T
2.76 10 23  29110
v( He) 

 11000m / s
m( He)
4 1.66 10  27
twice the escape velocity
escape velocity
Exosphere densities


1
The mean free path l  2   of a gas particle in the (lower range
of the) exosphere is equal to the scale height.
J
 29110 K
1
1
R T
mole  K
llf 

 H

 1.55 106 m  1550 km
kg
m
m g
2  n
0.016
 9.81 2
mole
s
 H   n  1
 is the scattering cross section
8.314
Energetic particles therefore can escape easily into outer space with
a 50% escape chance. Below observed escape for Titan and Saturn.
No escape velocity –
– falling back to Earth
Clouds
Humidity
Condensation
Aerosols
Gravity
Gravity