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.
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.
The ionosphere is a region that contains ions: directly above the mesosphere and directly below the exosphere. Within the thermosphere layer, ultraviolet radiation causes ionization creating the ionosphere (up to 550 km in the Earth's atmosphere). The Earth's ionosphere, though protected from direct solar wind scouring by the magnetosphere (and the Earth's magnetic field), is a shield of layers that absorbs most energetic wavelengths in the atmosphere. The ionosphere state can be predicted by monitoring sunspots which increase the solar winds. The solar wind's stream of particles (mostly high-energy protons ~ 500 keV) are ejected from the Sun's upper atmosphere. The interactions between the solar wind and the ionosphere induces energy into the Earth's magnetic field (and effects the telluric currents). Scientists believe that the Schumann resonance is due to the space between the surface of the Earth and the ionosphere acting as a resonant cavity that is then excited by energy from lightning strikes.
The physics of the ionosphere and outer space plasmas where recombination and collisional excitation (i.e., radiative process es) occur are of interest currently because they are not completely understood: in particular, for the electron. The assumption of the Maxwell-Boltzmann distribution yields quantitatively wrong results and even prevent a correct qualitative understanding of the physics involved. 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.
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, which is used for determining location and providing time references is affected by the ionosphere as its radio signals move through it. 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.
In 1899, Nikola Tesla researched ways to utilize the ionosphere to transmit energy wirelessly over long distances. He transmitted extremely low frequencies through the earth and portions of the ionosphere, called the Kennelly-Heaviside Layer, in his experiments. Tesla made mathematical calculations and computations based on his experiments and discovered that the resonant frequency of this area was approximately 8 Hz. In the 1950s, researchers confirmed the resonant frequency was in this range.
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 reenactment of the experiment; he believes that Marconi heard only random atmospheric noise and mistook it for the signal.
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. The ionosphere was confirmed 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.
- Gehred, Paul, and Norm Cohen, "SEC's Radio User's Page ". sec.noaa.gov . (Current data on the state of the ionosphere.)