is the region of charged particles in the uppermost layer of the earth's atmosphere
that is created by interaction with ultraviolet radiation
produced by the sun. It begins at an altitude
of about 80 km and continues up into space.
As altitude over the Earth's surface increases, atmospheric density varies, creating layers in the atmosphere. Gas becomes increasingly rarefied at higher altitudes, and at heights of roughly 80 km above the surface of the earth, gases are so thin that free electrons can exist for short intervals before being captured by nearby positive ions. The existence of these particles at this altitude signals the beginning of the ionosphere, a part of the atmosphere that has properties of both a gas and a plasma.
The following is a representation of the positioning of the ionospheric layer. Note that the D LAYER is the first layer of the ionosphere, located at roughly 80 km above the surface.
D LAYER | |
← F LAYER E LAYER of ionosphere __ __
| | | | | | |
km 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
Radiation from the sun hits the outer layers of the atmosphere with a power density of 1370 watts per meter2 or 0.137 walls per cm2. This radiation is spread across a spectrum of radio frequencies, infrared and visible light, and x-rays. Radiation at ultraviolet levels and shorter wavelengths are considered to be ionizing since photons at these levels are able to dislodge an electron from a neutral gas atom during collisions.
At the outermost levels of the atmosphere, radiation is very strong, but there are hardly any atoms there for ionization to take place. As altitude decreases, more are available so ionization increases. However, there is an opposing process occuring, known as recombination, in which free electrons are captured by positive ions if they are close enough together. As densities of gases increase at lower altitudes, recombination accelerates since molecules and ions are closer. It is the examination of the balance between these two processes that determines the ionization in the ionosphere.*
At even lower altitudes in the ionosphere, the number of gas atoms increases and there is an increased chance for absorption of energy from a photon of ultraviolet radiation. However, the intensity of said radiation is smaller at these levels because of absorption in part in higher regions. There is a point, therefore, where lower radiation, greater gas density and greater recombination balance and ionization rates decrease with decreasing altitude. This leads to heaviside layers of ionization.
These ionization peaks are called the D, E, and F layers, as indicated in the diagram above.
At the surface of the earth, atmospheric gases are mostly in molecular form, i.e., O2 and N2. In the ionosphere, however, O is much more prevalent than O2. In ionospheric physics, these non-ionized gases are known as "neutrals."
In the highest levels of the ionosphere, at the F2 layer level (about 250 km above the surface), these gases interact with Extreme Ultraviolet Radiation. These "neutrals" are vital for providing a sort of filter for this radiation when the atom absorbs the radiation and becomes an ion as an electron is freed. This is the essence of ionization.
Also interesting about the ionosphere is that radio signals can be "bounced" off this layer of the atmosphere, allowing for communication by radio "over the horizon," so to speak. Long, medium, and short wave radio broadcasts reach long distances because of this principle. Since the ionosphere isn't a perfect mirror, the signal is often scattered in many different directions which causes a loss of signal strengths and interference. Particularly in auroral regions, during magnetic sub-storms, the ionosphere will be disturbed. Short wave radio signals pass through the ionosphere, and are used by satellites for imaging the earth, and the ionosphere does affect these signals much in the same way that the atmosphere overall produces the "twinkling" appearance of stars.
*I stared at that phrase for awhile. "the ionization in the ionosphere." It's one of those things where you begin to question whether the hell you can spell anything correctly. Or something.