Ionospheric Chemistry

CHEMICAL REACTIONS IN THE UPPER ATMOSPHERE

Many chemical reactions take place in the upper atmosphere to maintain the ionosphere - a balance between the production of free electrons and their removal. There are also intermediate reactions that change the composition of the atmosphere above the homopause (about 100 km altitude). In the generic reactions below, upper case letters represent a molecule, γ represents a photon of electromagnetic energy (light, UV, EUV or X-ray), and e^- is an electron.

Electron production:

X + γ → X⁺ + e^-
X^- + Y → XY + e^-

photoionisation
associative detachment

Intermediate reactions:

XY + γ → X + Y
X⁺ + YZ → YX⁺ + Z

photodissociation
charge exchange

Electron loss:

X⁺ + e^- → X
X⁺ + e^- → X + γ
YX⁺ + e^- → Y + X
X + e^- → X^-

simple recombination
radiative recombination
dissociative recombination
electron attachment

At different altitudes in the atmosphere, different reactions become more important. depending on reaction rates which are determined by species concentrations, temperatures, and availability of photons of the right energy.

The species that provide most of the ions in the ionosphere are oxygen (in both the diatomic and monatomic forms) and nitric oxide (specifically in the lower ionosphere).

The last two of these species are produced by the following reactions:

O₂ + γ → O + O
N + O₂ → NO + O
NO + N → N₂ + O

photodissociation
production of nitric oxide
removal of nitric oxide

The monatomic nitrogen in the above reactions may be produced by photoionisation followed by dissociative recombination or by charge exchange reactions:

N₂ + γ → N₂ + e^-
N₂⁺ + e^- → N + N
O⁺ + N₂ → NO⁺ + N
O + N₂⁺ → NO⁺ + N

photoionisation
dissociative recombination

charge exchange
charge exchange

Past the peak of the F₂ layer, hydrogen also becomes important:

H⁺ + O ↔ H + O⁺

charge exchange

In the D-region, negative ions produced by electron attachment are found:

O₂ + e^- + A → O₂^- + A

electron attachment (with catalyst)

And it is found that associative detachment is more efficient than photoionisation in producing electrons, by the equation:

O₂^- + O → O₃ + e^-

associative detachment

It should be noted however, that the above reaction is not the mechanism by which the ozone layer is produced at lower altitudes.

The above reactions form part of the photochemistry that churns daily above our heads as the solar electromagnetic influx varies in its diurnal cycle. In bulk they give rise to the changing ionosphere that makes possible long distance radio propagation but that is also the bane of radio astronomy and satellite navigation and communication.

ASA Australian Space Academy