The ionosphere is a region of our atmosphere that begins approximately
at an altitude of 65 kilometers and extends out beyond 1000 km. It
essentially extends as far as our atmosphere does. Yet, since particle
densities slowly drop off as you travel away from Earth, how can you choose
a point that constitutes an end?
HISTORY
Karl Freidrich Gauss first proposed the idea of our atmosphere containing
a conducting layer in it in 1839. He believed currents in this layer
were responsible small daily variations in the geomagnetic field.
In 1864 James Clerk Maxwell put forth the theory that electric and magnetic
waves can propagate through free space. Heinrich Rudolf Hertz proved
24 years later Maxwell’s theory.
Marchese Guglielmo Marconi is credited with sending the first long range radio wave transmission, in 1899, across the English Channel. It was believed that radio-wave propagation relied on "line of sight" transmission and reception. Another distance attempt by Marconi was an experiment from the coast of Nova Scotia to a ship that was carrying a radio receiver. The distance over which he was able to communicate with the ship surprised Marconi. It was much further than he had anticipated. Two years later he achieved the first transatlantic communication. From Cape Cod Massachusetts to Poldhu, on the English coast at Cornwall. More information on this historic event and the text of the message from President Roosevelt to , the King of England, Edward VII, can be found on this related link. http://www.stormfax.com/wireless.htm
In 1902 Oliver Heaviside, an English Physicist, and Arthur Kennelly, an American Electrical Engineer proposed the idea that our atmosphere contained " a conducting Layer" that reflected these radiowaves. The term "ionosphere was first used in 1926 by a British Physicist who is credited with the invention of radar, Sir Robert Watson-Watt. The term "ionosphere' was commonly accepted in 1932.
THE IONOSPHERE and ENERGY ABSORPTION
In the ionosphere the molecules and atoms that compose our atmosphere
can be found in both gaseous and plasmatic states. Remember that
plasmas have more energy than gasses. The energy levels of these
two phases of matter vary with the influx on solar radiant energy.
Plasma is the fourth state of matter, in which some atoms are ionized,
having lost one or more electrons. Thus plasma is gas consisting of positive
ions and electrons, as well as neutral atoms. On Earth all matter is solid,
liquid, or gas, with no plasma, except in the magnetosphere. But in a star
the bulk of the matter is in the plasma state. Even after the big bang
the original material of the universe was plasma, and still today the majority
of the matter in the universe is plasma. Plasma is electrically conducting,
which is why eruptions on the surface of the Sun are warped by the Sun's
magnetic field.
On the daylight side of the Earth atmospheric gasses absorb some of
the sun’s energy. The most immediate result of this absorption is
an increase in the temperature of the particle. This increase might
cause the particle to vibrate or rotate faster. The rotation referred
to here is that of the valence electrons. Generally, valence electrons
exist at energy levels that are called a "ground state". Yet, when
energy is absorbed, if the electron goes through a large enough change
in orbiting speed it may jump from its ground state to a higher energy
level.
This might be more simply understood as the electron revolving around the
nucleus at a greater distance, or radius, than its normal ground state
radius. In the average high school chemistry class these radii are
known as the K, L, M, N rings. Bound electrons can only exist on
these energy rings.
As a molecule or atom absorbs energy it is referred to as being "excited". In an excited state the electrons that revolve around the nucleus of the particle are at higher energy levels than their ground state. For example, a K level electron jumps up to an L level. Or an L level electron absorbs a lot of energy and jumps up two steps to an N level. When a particle’s valence electrons have more energy than their ground state the particle to tries to get rid of this extra energy.
The rules of modern physics state that the release of energy by an atom can only be done in specific energy packets. There are different levels of energy packets, or quanta of energy, that can be released by atoms or molecules. Essentially, if a particle does not have enough extra energy to radiate it off as a complete packet of energy then the atom has to hold onto that energy.
Energy absorption by a molecule, di-atomic or even tri-atomic particle can also causes a process called "photodiscossiation’. In this process there is a separation of the constituent pieces of the particle resulting in the production of ions. “Photoionization” happens when a single atom absorbs energy and releases an electron. These are the two main processes by which the ionosphere is created. The main source of energy for all of this is the Sun. Other sources of electromagnetic radiation for these processes include deep space sources and our planet as both a producer and a reflector.
PROTECTION BY OUR ATMOSPHERE
Our
atmosphere absorbs virtually all of the ionizing shorter wavelengths of
the electromagnetic spectrum. Gamma rays penetrate to an altitude
of 20 km. The shortest of X-rays reach to an altitude of 55 km.
Far ultra-violet waves are partially absorbed at even higher levels.
The Ozone layer is our best protector from ultra-violet radiation.
It extends from about 30 to 80 kilometers in altitude and decreases in
thickness as you move in latitude from equator to pole. The near
ultraviolet waves are the longest of ultraviolet waves and they penetrate
all the way to the surface of the Earth.
While Ultra-violet waves are considered non-ionizing they are harmful to life here on Earth. The rest of the non-ionizing parts of electromagnetic spectrum are not harmful to life. The parts that reach the surface include the entire visible spectrum, and all but the shortest of radio waves. Only a small part of the infrared range reaches the surface. Most of it is absorbed by the two lowest parts of the atmosphere the stratosphere and the Troposphere.
The two molecules most responsible for absorption of incident solar
radiation are water vapor and carbon dioxide. Other greenhouse gasses,
in addition to ozone, which absorb a lot of incident solar radiation, are
methane and nitrous oxide. Oxygen gas, which composes 20.9% of our
atmosphere, absorbs most strongly in the microwave range. The most
common gas in our atmosphere Nitrogen (78%) does very little in the way
of protecting us.
DIVISIONS OF THE IONOSPHERE
The ionosphere is divided into four regions, C, D, E, and F. Some of these, the C and D regions, exist only during the daylight hours. The E and F regions separate further into Esporadic and F1 and F2 layers during the day as well. The differences between these regions are based largely on electron density as you move up in altitude. Electron density is the critical factor that dictates how radio waves propagate in each region. Changes in each region and its subdivisions occur with subsequent changes in latitude, season, and solar cycle variations. These are in addition to the daily changes within the ionosphere that occur from day to night.
C REGION
The most dense region of the ionosphere. Its altitude is between
60 and 65 kilometers.
D REGION
This is a daytime layer that extends in altitude from a height of 65
to 100 kilometers in altitude. Particle density is relatively high
which creates a lot of collisions between particles. Most of these
are electrons colliding with neutral particles or ions colliding with neutrals
Scale Height is one of the means of measuring differences between ionospheric
layers. Scale height is a measure of the rate of change in density
of atmospheric molecules and atoms as you go up in altitude. Below
100 km scale height is the same for all atmospheric gasses due to the high
rate of mixing. Scale height is also a fair approximation of the
distance between particles at any given altitude. As you go up in
altitude the number of collisions decreases exponentially. Conversely,
the number of electrons found per cubic centimeter, or electron density,
increases with height. Electron densities vary from as many as 10,000
to as few as 100 per cubic centimeter. Electron number or density
is a second means of measuring differences between ionospheric layers.
The D region absorbs high frequency (HF) waves between 3 and 30 megahertz
or wavelengths between 10 and 100 m. This region reflects or refracts
frequencies in the range of 3 - 30 kilohertz. This is considered
very low frequency (VLF).
As I stated this is a daytime layer. Absorption of ultra-violet
and visible light radiation creates more negative ions than electrons during
the day. At night these ions combine with other ionic particles.
This is only possible because of the high number of particles found in
this layer. This expansion and contraction between day and night
can cover a range of 20 kilometers, or from 70 to 90 kilometers in altitude.
Surprisingly, while this layer absorbs radiation during the day it actually
decreases in height.
E REGION
This is a region of the ionosphere that extends from 100 - 110 km in
altitude. The air is considerably thinner from the layers below it.
Electron densities are approximately 105 electrons per cubic centimeter.
As a result of this rarified air there are fewer collisions of ions and
electrons, resulting in a population of molecular ions, O2+, NO+.
Due to differences in their masses and velocities electrons are forced
to follow magnetic field lines while ions float freely. Interestingly,
some metal ions (Fe, Mg, and Ca) can be found in this layer. It is
believed that they were deposited as the fallout of meteors entering and
burning up in the Earth's atmosphere.
This is a highly variable layer from day to night, and also with respect
to latitudinal changes. At mid-latitudes wind shears combine with
the effect of the geomagnetic field to cause metal ions to compress into
thin layers, perhaps as thick as one kilometer, yet several hundred kilometers
horizontally (Rishbeth 5.3 p S218). During the daytime large
currents are created above equatorial regions due to ion and electron drift.
These currents occur at a region in the atmosphere where the velocity of
the ions and electrons equals, or is much greater than the gyrofrequency
of the particles. (Gyrofrequency is the motion of a charged particle
around or about a magnetic field.) The net motion of the electrons
is along the geomagnetic field lines at that altitude. This "Hall
Current" (Rishbeth 4.5 p S216) is the main part of the total ionospheric
current. Conversely, ions move in a direction opposite the electrons,
and peak where the ion velocity is equal to its gyrofrequency. This
"Pederson Current" (Rishbeth 4.5 p S216) reaches a maximum at an altitude
of 125 km and extends in the lower part of the F layer. Very little
current flows below an altitude of 100 km or above 125 km. Below
this altitude Electron densities are too low due to the large number of
collisions with ions and the resulting recombination. Above 125 km
velocities are much greater than the gyrofrequency so no net current can
be started.
Again, this layer is highly variable from day to night. An influx
of electromagnetic radiation provides the energies needed to separate valence
electrons from their atom or molecule. At night particle energy levels
drop to a level where more collisions occur, allowing recombination, producing
neutral atoms. At night, during geomagnetic storms this layer wakes
up as solar radiations penetrates our atmosphere along the magnetic field
lines that outline the polar cusps of our magnetosphere.
F REGION
The F region is the largest part of the ionosphere. It extends from 110 km up through the end of our atmosphere. Since particle densities decrease as you travel away from Earth it is difficult to say exactly where our atmosphere ends. Similarly, our atmosphere is in a constant state of change as it puffs up with an influx of solar radiation, or contracts inward as incident radiation decreases. The point where interplanetary space begins is an arbitrary one. It is generally accepted that our atmosphere ends at an altitude of 1000 km.
Since it is such a large region the F layer is divided into two sections. This division includes a daytime layer F1 and the denser F2 region. In the F2 layer the number of electrons increases with altitude up to approximately 105 0r 106 per cubic centimeter. In the upper reaches of the ionosphere gravity has a lessening effect on particles. As a result particles create different layers depending on their mass. The heavier particles sink to the bottom of the F region and the lighter ones rise to the top. This explains why electron density increases with altitude. The F1 layer, below 200 km, molecular ions dominate. Mainly these are NO+ and O2+.
The production of NO+ comes from two reactions;
O+ + N2 ----> NO+ + N
N2+ + O ----> NO+ + N
Molecular oxygen ions are the product of a charge transfer between molecular oxygen and an oxygen ion.
O+ + O2 ----> O2+ + O
Since gravity has a lessened effect on the constituents of the ionosphere the charge particles tend to position along the magnetic field lines.
Along the day night meridian electrons numbers rise and fall. At sunset electrons numbers decrease, resulting in recombination of these particles with ions in the F1 layer during the night. On the sunrise meridian electron numbers increase as neutral molecules and atoms absorb solar radiation, mostly ultra-violet. Two examples of this are:
O + energy ----> O+ + e-
N2 + energy ----> N2+ + e-
The F2 region is predominantly O+. Electron densities, as high as one million per cubic centimeter, drop off rapidly above a height of 300 km. This is an important point in this layer. All ground based research of the atmosphere that depend on electron densities to reflect radio waves back become ineffective above this height. The radio waves with which they probe the atmosphere simply pass through it. Similarly, incoherent scatter techniques also become useless. Incoherent Scatter, or Thompson Scattering is a technique that measures the waves of energy emitted from an electron after that electron is stimulated with some form of energy. The idea is to hit an electron with a radio wave and then measure the energy that it gives back. Above the peak level of the F2 region electron densities are too low. The incident wave doesn't stimulate enough electrons to radiate the power necessary to read.
Again, particles settle into layers largely due to their mass. The upper
reaches of the ionosphere, above 700km, are further portioned into the
heliosphere and protonosphere. Probes of these levels in the ionosphere
are done with satellites.