Plasma in the Ionosphere Ionization and
Recombination
Agabi E. Oshiorenoya
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July, 2004
Space Physics 5P
Umeå Universitet
Department of Physics
Umeå, Sweden.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2. Ionization and Recombination . . . . . . . . . . . . . . . . . . . . . . .
8
3. Layer by layer .
3.1 D -Layer .
3.2 E Layer .
3.3 Es . . . .
3.4 F Layer .
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11
12
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13
13
List of Figures
1.1
Ionospheric profile . . . . . . . . . . . . . . . . . . . . . . . . . .
7
2.1
2.2
2.3
Ionization and recombination . . . . . . . . . . . . . . . . . . . .
Variation of reactions with day and night . . . . . . . . . . . . .
Ionization and recombination rates . . . . . . . . . . . . . . . . .
9
9
10
4
List of Tables
3.1
Summary of Ionization conditions . . . . . . . . . . . . . . . . . .
5
11
1
Introduction
The ionosphere is the region of the Earth’s atmosphere between 60 and
1000 km. In this region free electrons and ions can exist for a considerable
period of time, resulting in the formation of a plasma gas containing charged
particles. This plasma is not stationary, but is constantly circulating around.
Flow patterns develop which can, for example, carry plasma from the dayside
to the night side or vice-versa. The plasma flow pattern is related to the space
weather, and thus to the strength and direction of the interplanetary magnetic
field and the solar wind.
So where did the ionosphere come from?” Well it is created from the ambient neutral atmosphere by ionization. However, the source of magnetospheric
plasma is a much more complicated question: although it seems obvious that
both solar wind and ionosphere feed it, the relative importance of these two
sources is subject of ongoing research. Furthermore, it seems also that plasmasphere can provide part of the plasma in plasma sheet; although it originates,
in the end, from ionosphere, the transport mechanism is quite different than in
direct ionospheric outflow.
That issue of origin sorted, it is known based on electron density observations
that the ionosphere is subdivided into three layers: D-layer (60 to 90 km), the
E and Es -layers (90 to 140 km), and F1 and F2 -layer (above 140 km). Since
electron density increases nearly continuously with altitude to a maximum at
an average level close to 300 km, D- E-, F-layers are not discrete, rather differ
each other on the reflection of radio waves. The radio waves reflection being the
case as the ionosphere comes in handy for radio commucations.
Yes, it is easily construed that plasmas are only to be found in exotic places
(not only in the Ionosphere for the earth but also places like the plasmasphere
and frankly ¿99% of the cosmos is in the plasma state) but plasmas are just a gas
containing charged particles. This is obtainable in ordinary places just as well as
exotic places, in ordinary places plasmas can be created by collisions of energetic
particles, strong electric fields acting on bond electrons, or ionising radiation.
6
1. Introduction
7
Fig. 1.1: Ionospheric profile
So in order to demystify the exoticness of the plasma without trivializing the
nature of its phenomena, it is clear that plasmas are quite abundant and more
so in the ionosphere. So what happens to the plasma in ionosphere? What are
its characteristics as distinct or similar to other plasmas [1]?
2
Ionization and Recombination
Ionization and recombination is at the heart of the ionosphere, solar radiation
at ultraviolet (UV) and shorter X-Ray wavelengths is considered to be ionizing
since photons of energy at these frequencies are capable of dislodging an electron
from a neutral gas atom or molecule during a collision, this implying that the
temperatures are hot enough. At the same time, however, an opposing process
called recombination begins to take place in which a free electron is captured”
by a positive ion if it moves close enough to it. Ionization has a threshold energy,
recombination has not but is much less probable. However, not all radiations
from the sun (which is the main actor in the ionosphere variations) produce the
same effects on the ionosphere. But in all cases, charged particles and ionizing
radiations are events that affect the strongest the ionosphere, some carried by
geomagnetospheric currents to polar caps and to the equator via ring currents,
other striking directly the u pper atmosphere without embellishment.
Threshold is ionization energy (13.6eV, H) Xi . The Integral over Maxwellian
distribution gives rate coefficients (reaction rates). Because of the tail of the
Maxwellian distribution, the ionization rate extends below T = Xi and in equilibrium when
nions /nneutrals =< σi > v/< σr > v
The percentage of ions is in the range of 100% if electron temprature:
Te ≥Xi /10 e.g. H2 is ionized for Te ≥1eV .
As the gas density increases at lower altitudes, the recombination process
accelerates since the gas molecules and ions are closer together. The point of
balance between these two processes determines the degree of ionization present
at any given time. The ionization depends primarily on the Sun and its activity.
The amount of ionization in the ionosphere varies greatly with the amount of
radiation received from the sun.
After sunset the molecular ions e.g. H2 and He in the D, E and F1 layer
vanish quickly through dissociative recombination of the type;
8
2. Ionization and Recombination
Fig. 2.1: Ionization and recombination
Fig. 2.2: Variation of reactions with day and night
9
2. Ionization and Recombination
10
Fig. 2.3: Ionization and recombination rates
XY + + e− → X + Y
Which on yielding two atoms these processes have high reaction probabilities.
The F2 is a much harder process because
X + + e− → X + ~ω
and
X + + e− + M → X + M
for ~ω being a photon and M a third reactant which is much slower and
hence the F2 layer more stable at night.
Generally ionization and combination reactions play an important role in
conductivity of the ionosphere. With lower collision frequencies resulting in
lower conductivities but also depending on the proportion of neutral molecules
that abound.
Three kind of conditivities are distinguishable parallel conductivity, Pedersen
conductivity and Hall Conductivity which are determined by the electron and
ion collisions probabilities.
3
Layer by layer
It is clear up this point that one can summarize fig.3.1 [2] the ionosphere as a
region that is primarily bombarded by the ionizing rays of the sun (broadband”
radiation). However as expected this will vary from layer to though one can
essentialy say the entire ionosphere is built of a plasma, the reactions will vary.
Since we are earth bound lets imagine we are to fly into space with a very slow
rocket and a bag where we put the following premises;
• The earth has a magnectic field which is stronger at the poles, we know
this because we have measured it with compass needless.
• It also has a property called gravity, which we know because we are using
an anti-gravition generator to propel us into space.
• We know the air density decreases because we used weather baloons and
hence taking our own life support. We have also determined by advanced
spectroscopy the composition of the air around us.
• We know some quantum theory and electromagnectic theory because ...well
lets just say we know that...
As we journey upwards the A.I. on board which is tuned to verbose tells
of decreasing pressure which it explains as since more than half of the atmosphere’s molecules are located below an altitude of 5.5 km, atmospheric pressure
Layer
D
E
F1
F2
A
68-65
85-140
140-200
200-ca 1500
NED
< 102
2 X 103
2-5 X 105
3
5
5
DED
10
1-2 X 10
2-5 X 10
0.5-2 X 106
+
+
+
+
+
+
+ +
IS
NO O 2
NO O 2
N O O 2 O O He+ H +
CI Lmn.α (121.5nm)Lmn.β (102.5nm)X-Rays
UV
UV
Tab. 3.1: Summary of Ionization conditions
11
3. Layer by layer
12
decreases roughly 50% (to around 500 mb) within the lowest 5.5 km. Above 5.5
km, the pressure continues to decrease but at an increasingly slower rate. But
we are going much more higher than this soon we pass the ozone which the A.I
also explains away as being created by ultraviolet light striking oxygen molecules
containing two oxygen atoms (O2 ), splitting them into individual oxygen atoms
(atomic oxygen); the atomic oxygen then combines with unbroken O2 to create
ozone, O3. The ozone molecule is also unstable (although, in the stratosphere,
long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2
and an atom of atomic oxygen, a continuing process called the ozone-oxygen
cycle.
3.1 D -Layer
When we reach an altitude of about 50km we pay some attention to the range
of holographic data being displayed in the cockpit, the A.I explains that this is
the D layer which is the innermost layer, 50 km to 90 km above the surface of
the Earth. The Ionization here is due to Lyman series-alpha hydrogen radiation
at a wavelength of 121.5 nanometre(nm) ionizing nitric oxide (NO). In addition,
when the sun is active with 50 or more sunspots, Hard X-rays (wavelength ¡ 1
nm) ionize the air (N2 , O2 ).
During the night cosmic rays produce a residual amount of ionization. Recombination is high in this layer, thus the net ionization effect is very low and
as a result the high-frequency (HF) radio waves aren’t reflected by the D layer.
The frequency of collision between electrons and other particles in this region
during the day is about 10 million collisions per second! The D layer is mainly
responsible for absorption of HF radio waves, particularly at 10 MHz and below,
with progressively smaller absorption as the frequency gets higher. The absorption is small at night and greatest about midday. The layer reduces greatly after
sunset primarily due to recombination, but remains due to reionizing galactic
cosmic rays. A common example of the D layer in action is the disappearance
of distant AM broadcast band stations in the daytime.
3.2 E Layer
After covering a distance of about 30km we encounter the E layer which is the
middle layer, 90km to 120km above the surface of the Earth. Ionization is due
to Soft X-Ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of
molecular oxygen (O2 ).
This layer can only reflect radio waves having frequencies less than 10 MHz.
It has a negative effect on frequencies above 10 MHz due to its partial absorption
of these waves. During the daytime the solar wind presses this layer closer to
the Earth,
thereby limiting how far it can reflect radio waves. On the night side of
the Earth, the solar wind drags the ionosphere further away, thereby greatly
increasing the range which radio waves can travel by reflection.
3. Layer by layer
13
3.3 Es
The Es layer or sporadic E-layer. Sporadic E propagation is characterized by
small clouds of intense ionization.
3.4 F Layer
The F layer or region, also known as the Appleton layer, is 120km to 400km
above the surface of the Earth. Here extreme ultraviolet (UV) (10-100 nm) solar
radiation ionizes molecular oxygen (O2 ).
The F layer combines into one layer at night, and in the presence of sunlight
(during daytime), it divides into two layers, the F1 and F2. The F layers are
responsible for most skywave propagation of radio waves, and are thickest and
most reflective of radio on the side of the Earth facing the sun and merges with
the magnetosphere, whose plasmas are generally more rarefied but also much
hotter. The ions and electrons of the magnetospheric plasma come in part from
the ionosphere below, in part from the solar wind
References
[1] www.google.com. In most of what has followed i have found information
from the internet from numrous sources.
[2] Kjell Rönmark. Lecture notes on space physics. UmeåUniversitet, March
2003.
14