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The ionosphere is the part of the atmosphere that is ionized by solar radiation, and too tenuous to be cooled by contact with other air. It forms the inner edge of the magnetosphere and has practical importance because it reflects radio waves to distant places on Earth.
Geophysics
 the ionosphere is the region containing ions: approximately the mesosphere and thermosphere up to 550 km.
The lowest part of the Earth's atmosphere is called the troposphere and it extends from the surface up to about 10 km (6 miles). The atmosphere above 10 km is called the stratosphere, followed by the mesosphere. It is in the stratosphere that incoming solar radiation creates the Ozone layer. At heights of above 80 km (50 miles), in the thermosphere, the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The number of these free electrons is sufficient to affect radio propagation.This "ionized" region of the atmosphere is a plasma and is referred to as the ionosphere. Note. The ionosphere is not a layer in the Earth's atmosphere.
At the highest levels of the Earth's outer atmosphere, solar radiation is very strong but there are few atoms to interact with, so ionization is small. Solar radiation at ultraviolet (UV) and shorter 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. As the altitude decreases, more gas atoms are present so the ionization process increases. 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. 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 varys greatly with time (sunspot cycle, seasonally, and from day to day), with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions), and with certain solar-related ionospheric disturbances such as solar flares and the release of charged particles into the solar wind.
Ionosph.gif
Electron density (ionozation) during the day and night
The process outlined above, acting on the different compositions of the atmosphere with height, generates layers of ionization. The ionosphere is generally recognized to have three, sometimes four, layers. The D layer is the innermost layer (approximately 50 km to 95 km above the surface of the Earth), and mostly absorbs radio waves. The E layer is the middle layer and influences the propagation of radio waves. The F layer (or F region; approximately 160 km to 400 km above the surface of the Earth) consists of layers of increased free-electron density caused by the ionizing effect of solar radiation. 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, and are thickest and most reflective of radio on the side of the Earth facing the sun.
Experiments
The open system space tether, which uses the ionosphere, is being researched. The space tether uses plasma contactors and the ionosphere as parts of a circuit to extract energy from the Earth's magnetic field by electromagnetic induction.
Scientists also are exploring the structure of the ionosphere by bouncing radio waves of different frequencies from it, and using special receivers to detect how the reflected waves have changed from the transmitted waves. Project HAARP (High Frequency Active Auroral Research Program) investigation are focused to "understand, simulate, and control ionospheric processes that might alter the performance of communication and surveillance systems" and started in 1993 for a proposed twenty year experiment. CUTLASS (Co-operative UK Twin Located Auroral Sounding System) researches the ionosphere using radar.
Scientists are also examining the ionosphere by the changes to radio waves from satellites and stars transmitted through. The Arecibo radio telescope located in Puerto Rico, was originally intended to the study of Earth's ionosphere.
Radio
The ionosphere reflects HF radio waves quite well and is used for medium and long range terrestrial radio communication. Radio waves "hop" from the Earth to the ionosphere and back to the Earth.
The phenomenon of radio waves reflecting off of highly charged particles in the E-layer of the ionosphere, known as the E-skip, allows radio propagation to go thousands of miles or kilometers beyond their intended area of reception. Sporadic E propagation is a rare form of propagation where a radio wave bounces off a sporadic E cloud in the E layer of the ionosphere. Diurnal phase shift is the phase shift of electromagnetic signals associated with daily changes in the ionosphere.
The critical frequency determines the limiting frequency at or below which a wave component is reflected by, and above which it penetrates through, an ionospheric layer. The cutoff frequency is the frequency below which a radio wave fails to penetrate a layer of the ionosphere at the incidence angle required for transmission between two specified points by reflection from the layer.
DX communication, popular among amateur radio enthusiasts, enables communication over great distances using the ionosphere to refract the transmitted radio beam. The beam returns to the Earth's surface, and may then be reflected back into the ionosphere for a second bounce. FM DX is a term that means "distant reception" over FM. It is the search for faraway radio or television stations that can be received during unusual tropospheric conditions, or E-skip. Frequencies between approximately 1 MHz and 30 MHz can be reflected by the ionosphere, thus giving radio transmissions in this range a potentially global reach. Maximum usable frequency (MUF) describes, in radio transmission, using reflection from the regular ionized layers of the ionosphere, the upper frequency limit that can be used for transmission between two points at a specified time. This index is especially useful in regard to shortwave transmissions.
Shortwave frequencies are capable of reaching the other side of the planet by bouncing a signal off the ionosphere. Shortwave frequencies (in the 3 MHz to 30 MHz range) tend to bounce off the ionosphere and reflect back to earth and back again, and this enables shortwave frequencies to travel long distances. Short wave radio is used because it bounces between the ionosphere and the ground, giving a modest 1 kW transmitter (the standard power) a world-wide range. The Marine and mobile radio telephony or HF ship-to-shore operates on shortwave radio frequencies. Mediumwave signals have the properties of following the curvature of the earth (the groundwave) and reflecting or refracting off the ionosphere at night (skywave). High frequency waves (around 1 MHz) do not usually tend to hug the ground like lower frequency waves do, and this behavior lessens the higher the frequency is.
The Global Positioning System, usually called GPS, is used for determining location and providing time references. The system transmits the main positioning codes at two frequencies, 1575.42 MHz and 1227.6 MHz. These frequencies were chosen partly so the effect of the ionosphere and troposphere on the transmitted signal are minimized. However, during solar storms the charged particles in the ionosphere can 'slow down' the signals from the satellites, and may provide inaccurate readings.
INMARSAT, an international telecommunications, provides telephony and data services (via special digital radios called "terminals") but can be unreliable near the north and south poles, depending on the ionosphere.
The WWVB transmits longwave signal to travel along the ground, it requires a shorter and less turbulent path to get to the radio receivers than WWV's shortwave signal, which bounces between the ionosphere and the ground. This results in the WWVB signal having greater accuracy than the WWV signal as received at the same site. The WWVH broadcasts a directional signal on 5 MHz, 10 MHz, and 15 MHz, pointed primarily west. But despite this strategy, in certain places at certain times due to ionospheric conditions, the listener can actually hear both WWV and WWVH on the same frequency at the same time.
History
In 1899, Nikola Tesla researched ways to utilize the ionosphere to transmit energy wirelessly over long distances. In his experiments, he transmitted extremely low frequencies between the earth and ionosphere, up to what is called the Kennelly-Heaviside Layer.(Grotz, 1997) Tesla made mathematical calculations and computations based on his experiments. He predicted the resonant frequency of this area within 15% of modern accepted experimental value. (Corum, 1986) In the 1950s, researchers confirmed the resonant frequency was at the low range 6.8 Hz .
Guglielmo Marconi received the first trans-Atlantic radio signal on December 12, 1901, in St. John's, Newfoundland (now in Canada) using a 400-foot kite-supported antenna for reception. The transmitting station in Poldhu, Cornwall used a spark-gap transmitter to produce a signal with a frequency of approximately 500 kHz and a power of 100 times more than any radio signal previously produced. The message received was three dots, the Morse code for the letter S. To reach Newfoundland the signal would have to bounce off the ionosphere twice. Dr. Jack Belrose has recently contested this, however, based on theoretical work as well as an actual experiments. However, Marconi did achieve transatlantic wireless communications beyond a shadow of doubt in Glace Bay one year later.
In 1902, Oliver Heaviside proposed the existence of the Kennelly-Heaviside Layer of the ionosphere which bears his name. Heaviside proposal included means by which radio signals are transmitted around the earth's curvature. Heaviside's proposal, coupled with Planck's law of black body radiation, may have hampered the growth of radio astronomy for the detections of electromagnetic waves from celestial bodies till 1932 (and the development of high frequency radio transceivers). Also in 1902, Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties.
In 1912, Congress imposed The Radio Act of 1912 on amateur radio operators, limiting their operations to frequencies above 1.5 Mhz (wavelength 200 meters or smaller). The government thought those frequencies were useless. This lead to the discovery of HF radio propagation and confirmation of the ionosphere in 1923.
Edward V. Appleton was awarded, by Ernest Rutherford, a Nobel Prize for demonstrating the existence of the ionosphere. Lloyd Berkner first measured the height and density of the ionosphere. This permitted the first complete theory of short wave radio propagation. Maurice V. Wilkes researched the topic of radio propagation of very long radio waves in the ionosphere. Vitaly Ginzburg has developed a theory of electromagnetic wave propagation in plasmas such as the ionosphere.
References
- Corum, J. F., and Corum, K. L., "A Physical Interpertation of the Colorado Springs Data". Proceedings of the Second International Tesla Symposium. Colorado Springs, Colorado, 1986.
- Grotz, Toby, "The True Meaing of Wireless Transmission of power". Tesla : A Journal of Modern Science, 1997.
See also
External links
- Gehred, Paul, and Norm Cohen, "SEC's Radio User's Page (http://www.sec.noaa.gov/radio/radio.html)". sec.noaa.gov (http://www.sec.noaa.gov/). (Current data on the state of the ionosphere.)
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