Geoscience Reference
In-Depth Information
Fig. 8
Ionization of an
Oxygen molecule due to
solar radiation (modified from
Partially absorbed
by an atom
Producing ion and
a free electron
Solar Radiation
e
-
O
+
Oxygen Atom
Initially Neutral
4.1.1 Ionization
There are three different sources for ionization within the ionosphere, which are
extreme UV and X-ray radiation, high-energy particles, and cosmic rays.
Extreme UV and X-ray radiation: At the highest altitudes of the Earth's
atmosphere, solar radiation is very strong but a smaller number of atoms exist to
interact with, and so the amount of ionization is limited. At somewhat lower alti-
tudes, more gas atoms are present, increasing the ionization process. In the ionizating
process, a neutral atom
X
absorbs energy
hf
, to produce a positive ion
X
+
and a free
electron
e
−
(Fig.
8
)
X
+
+
e
−
.
X
+
hf
→
(41)
High-energy particles: Those particles could reach up to several thousands of
electron volts (
eV
) in the auroral zone due to magnetosphere activities.
Cosmic rays: The Ultra-high energy particles with cosmic origin can penetrate
deep into the atmosphere and cause ionization within the lower ionosphere.
4.1.2 Recombination
Meanwhile an opposing process begins to take place in which a free electron moves
close enough to a positive ion to be captured by it. This recombination process also
accelerates at lower altitudes as the density of gas molecules and ions increases and
they are closer to each other. The balance between ionization and recombination
determines the degree of ionization present at any time.
If ionization was undone by recombination, the atmosphere would become fully
ionized. However, instead, the production and destruction of ions reach an equilib-
rium. In this recombination process, negative electrons
e
−
, and positive ions
X
+
,
combine to produce neutral particles. There are two basic types of recombination.
Radiative recombination: An electron combines with an atomic ion producing a
neutral atom and a photon
h
ν
:
e
−
+
X
+
→
X
+
h
ν.
(42)
Dissociative recombination: An electron combines with a molecular ion
XY
+
and
produces two neutral atoms
X
and
Y
:
