1. Planetary Ionospheres
How is an ionosphere formed ?
1. What is an ionosphere ?
4. How are aurora and airglow related to ionospheric formation and decay ?
3. What determines the peak density and height of an ionosphere ?
5. What are the differences and similarities between the ionospheres of Earth, Mars and Venus ?

2. The Ionosphere is a Weakly Ionized Plasma
broad definition: “the ionosphere is that region of the atmosphere (or gaseous envelope) surrounding a solar system body where significant numbers of low-energy free electrons and ions are present”

3. Existence of ionosphere suggested -- by Gauss, Lord Kelvin and Stewart Balfour in the 19th century First direct verification of its existence -- Marconi succeeded in sending radio signals across the Atlantic ‘ionosphere’ coined by R.A. Watson in First direct evidence of an ionosphere on a planet other than earth -- radio occultation measurements by Mariner 5 as it flew by Venus on October 19, 1967

4. Formation of Ionospheres
Free electrons and positive
ions can be formed by
a) photoionization of neutrals
b) energetic particles knocking
electrons off neutrals

5. Photoionization in an Exponential Atmosphere
In the following we assume, for simplicity, and to get across the basic concepts:
• monochromatic radiation (i.e., small wavelength interval over which absorption and ionization cross-sections can be taken as constant)
• isothermal atmosphere
• horizontal stratification, i.e., n = n(z)
• curvature of planet neglected

6. z    =photons cm-2sec-1 (photon flux)
As z decreases, nj increases, so q
increases; however, at some point 
is diminished so much by absorption that
q begins to decrease as z decreases.
Therefore, there is always a height at
which q becomes maximum. Let us now
derive that height and obtain an expression
for the plasma production rate as a function
of height. z  
ds = -dz sec
Assume c ≤ 60º so that
curvature can be neglected
 =photons cm-2sec-1 (photon flux)
= intensity (erg cm-2sec-1)
h = Planck’s constant
c = speed of light
= wavelength
= (hc/l) 

7. Optical Depth and Unit Optical Depth
The attenuation of F along s due to absorption is given by (this relation basically defines sa, the absorption cross-section):
si = ionization cross-section
q = electron production rate

8. Peak Production of Plasma
We have just shown that the peak production occurs at the altitude of unit optical depth.
The level where F = F∞e-1 is a measure of penetration of a particular wavelength of radiation into the atmosphere.

9. Chapman Layer
q/qo y
By manipulating the previous equations, (see following page) it can be shown that
where y is the distance in scale heights from the level of peak production.
For secc
must be replaced
by the ‘Chapman
function’:
In reality, one must sum over several constituents and a range of l’s:

10. For Reference Only: Derivation of Chapman Layer

11. The ionosphere above 100 km is mainly produced by photoionization of O, O2 and N2
followed by
Fast
leaving

12. E and F-Region Chemistry
Photoproduction
fast
“rate-
limiting”
Charge transfer
Ion-atom Interchange
Dissociative Recombination
fast
Radiative Recombination
slow
metastable

13. Ionosphere Profile Shape: E-Region and Bottomside F-Region
To get across the essential ideas, for now consider only an O and O2 atmosphere.
Below some altitude near km
[O2] >> [O], and the primary processes are
Electron production rate (P) =
Production of e- due
to photoionization
of O2
Electron loss rate (L) =
Assuming photochemical equilibrium,
i.e., P = L, the electron density profile
follows that of the production rate:
peak
production
E-region

14. = Above the transition height, [O] >> [O2],
and the primary processes are
To first order, the last reaction “neutralizes” the O2+ as fast as it is produced.
Assuming photochemical equilibrium, i.e., production = loss:
Rate of Production of O+
Rate of Loss of O+ =

15. Note: Since , then the quantity in brackets is < 0, and hence the
e- concentration increases with height.
(loss decreases more rapidly with
height than production)