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.

P

AA

z

P

z

absP

CA

FN

dt

dT

A

Nn

FnFeFdt

dTC

eFFdz

dF

dt

dTC

dt

VQd

0

01

The temperature change goes linearly with the flux!

12hinincreaseetemperaturKTh

K

s

K

dt

dT

s

K

Kmole

Jms

J

mole

partm

dt

dT

Kkg

J

mole

gm

W

mole

partcm

CA

FN

dt

dT

P

A

960801084.2

1084.2

1007.29

137010022.610

005.129

137010022.610

5

5

3

2

23216

2

23220

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.

This is comparable to the observed day and night variation of the thermosphere temperature of T ≈ 1000K.

The Ionosphere

Ionosphere reflector of long range radio waves.

upper range of thermosphere

Earth’s ionosphere

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.

~200-600km

eNOnmhNO

eNnmhN

eOnmhO

eNnmhN

eOnmhO

1.134

2.85

0.91

6.79

6.102

22

22

UV < 150 nm

Network of ionization processes

z

cr dznFIonY )(

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

Transition selection rules: ℓ=+/-1 m=0, +/-1

ℓ=0 =1 =2 =3 =4

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!

nmmE

hc

hceVE

nZE

Zn

ZEE

nZ

nSZE

K

K

nn

nn

nn

86.11086.1

6661

6.131

6.131

6.131

6.131

6.13

9

12

2

2

2

2

2

2

2

2

2

Energy of electron on orbit n:

Ionization energy for electron from orbit n :

absorbed light with wavelength:

Energetic x-ray wavelengths generated by solar radiation

n=1

n=2

n=3

Kα

Kβ Lα

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!

nmeVeVE

neVZE

nmeVeVE

nmeVeVE

ZO

neVZE

L

i

L

K

K

i

K

4.136.929

1

4

16.137

1

2

16.131

09.24.5929

116.137

48.28.4994

116.137

8:

116.131

2

22

2

2

2

2

2

Gerhard Riessbeck

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!

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.

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

Configuration Term J Level cm−1

1s2 2s2 2p4 3P 2 0.000

1 158.265

0 226.977

1s2 2s2 2p4 1D 1D 2 15867.86

1s2 2s2 2p4 1S 0 33792.58

1s2 2s2 2p3(4S)3s 5S 2 73768.20

630 nm

558 nm

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:

K

Kskgm

smkgT

m/s smvwithOfor

Kskgmk

k

vmTvmTkE

291101038.12

/1031066.116

)11000: velocityescape (half /5500

1038.1

22

1

12223

2727

16

12223

22

Lower mass particle at same temperature have higher velocity and escape easier.

smHm

TkHv /22000

1066.1

291101076.2

)(

2)(

27

23

twice the escape velocity

smHem

TkHev /11000

1066.14

291101076.2

)(

2)(

27

23

escape velocity

Exosphere densities The mean free path of a gas particle in the (lower range of the) exosphere is equal to the scale height. Energetic particles therefore can escape easily into outer space with a 50% escape chance. Below observed escape for Titan and Saturn.

section cross scattering theis1

15501055.1

81.9016.0

29110314.81

2

1 6

2

nH

kmm

s

m

mole

kg

KKmole

J

gm

TRH

nllf

1

2

l

No escape velocity – – falling back to Earth

Clouds Humidity Condensation Aerosols

Gravity

Gravity