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Encyclopedia > Ionosphere
Relationship of the atmosphere and ionosphere

The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the Thermosphere. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... Air redirects here. ... ... Cloud to ground Lightning in the global atmospheric electrical circuit. ... A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... Radio propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated from one point on the Earth to another. ... The thermosphere is the layer of the earths atmosphere directly above the mesosphere and directly below the exosphere. ...

Solar radiation at ultraviolet (UV) and shorter X-Ray wavelengths is considered to be ionizing since photons at these frequencies are capable of dislodging an electron from a neutral gas atom or molecule during a collision. 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. Solar irradiance spectrum at top of atmosphere. ... For other uses, see Ultraviolet (disambiguation). ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... For other uses, see Wavelength (disambiguation). ... In modern physics the photon is the elementary particle responsible for electromagnetic phenomena. ... For other uses, see Electron (disambiguation). ... For other uses, see Atom (disambiguation). ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... Recombination usually refers to the biological process of genetic recombination and meiosis, a genetic event that occurs during the formation of sperm and egg cells. ...

## The ionospheric layers

Solar radiation, acting on the different compositions of the atmosphere with height, generates layers of ionization:

### D layer

During solar proton events, ionization can reach unusually high levels in the D-region over the high and polar latitudes. Such events are known as Polar Cap Absorption (or PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region. In fact, absorption levels can increase by many tens of dB during intense events, which is enough to absorb most (if not all) transpolar HF radio signal transmissions. Such events typically last less than 24 to 48 hours. A Solar Proton Event occurs when high-energy protons, ejected from the suns surface during a solar flare, get caught by the Earths magnetic field and cause ionization in the ionosphere. ...

### E layer

The E layer is the middle layer, 90 km to 120 km 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 about 10 MHz. It has a negative effect on frequencies above 10 MHz due to its partial absorption of these waves. The vertical structure of the E layer is primarily determined by the competing effects of ionization and recombination. At night the E layer begins to disappear because the primary source of ionization is no longer present. This results in an increase in the height where the layer maximizes because recombination is faster in the lower layers. Diurnal changes in the high altitude neutral winds also plays a role. The increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer. The Kennelly-Heaviside Layer is also known as the E region or just as Heaviside Layer (after Oliver Heaviside). ... This article is about the chemical element and its most stable form, or dioxygen. ...

This region is also known as the Kennelly-Heaviside Layer layer or simply the Heaviside layer. Its existence was predicted in 1902 independently and almost simultaneously by the American electrical engineer Arthur Edwin Kennelly (1861-1939) and the British physicist Oliver Heaviside (1850-1925). However, it was not until 1924 that its existence was detected by Edward V. Appleton. The Kennelly-Heaviside Layer is also known as the E region or just as Heaviside Layer (after Oliver Heaviside). ... Arthur Edwin Kennelly (December 17, 1861 - June 18, 1939), was an American engineer in electricity. ... Oliver Heaviside (May 18, 1850 â€“ February 3, 1925) was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, developed techniques for applying Laplace transforms to the solution of differential equations, reformulated Maxwells field equations in terms of electric and... Sir Edward Victor Appleton (September 6, 1892 – April 21, 1965) was an English physicist. ...

### F layer

From 1972-1975 NASA launched AEROS and AEROS B satellites to study the F-region.[1] For other uses, see NASA (disambiguation). ... AEROS[1] satellite was to study the science of the upper atmosphere and ionospheric F regions solar ultraviolet radiation. ...

### Ionospheric model

The atmospheric physics community contributes to the definition and maintenance of an ionospheric model: the International Reference Ionosphere, through a series of academic committees and conferences. As discoveries are made and generally accepted, the model is improved. (IRI85-6)

## Anomalies to the ideal model

The statements above assumed that each layer was smooth and uniform. In reality the ionosphere is a lumpy, cloudy layer with irregular patches of ionization.

### Winter anomaly

At mid-latitudes, the F2 layer daytime ion production is higher in the summer, as expected, since the sun shines more directly on the earth. However, there are seasonal changes in the molecular-to-atomic ratio of the neutral atmosphere that cause the summer ion loss rate to be even higher. The result is that the increase in the summertime loss overwhelms the increase in summertime production, and total F2 ionization is actually lower in the local summer months. This effect is known as the winter anomaly. The anomaly is always present in the northern hemisphere, but is usually absent in the southern hemisphere during periods of low solar activity.

### Equatorial anomaly

Electric currents created in sunward ionosphere.

Within approximately ± 20 degrees of the magnetic equator, is the equatorial anomaly. It is the occurrence of a trough of concentrated ionization in the F2 layer. The Earth's magnetic field lines are horizontal at the magnetic equator. Solar heating and tidal oscillations in the lower ionosphere move plasma up and across the magnetic field lines. This sets up a sheet of electric current in the E region which, with the horizontal magnetic field, forces ionization up into the F layer, concentrating at ± 20 degrees from the magnetic equator. This phenomenon is known as the equatorial fountain. Schematic diagram of the electric-current pattern in the ionosphere driven by diurnal heating from the Sun. ... World map showing the equator in red For other uses, see Equator (disambiguation). ... For the indie-pop band, see The Magnetic Fields. ... This article is about tides in the ocean. ... In astronomy, geography, geometry and related sciences and contexts, a plane is said to be horizontal at a given point if it is locally perpendicular to the gradient of the gravity field, i. ...

### Equatorial electrojet

The worldwide solar-driven wind results in the so-called Sq (solar quiet) current system in the E region of the Earth's ionosphere (100-130 km altitude). Resulting from this current is an electrostatic field directed E-W (dawn-dusk) in the equatorial day side of the ionosphere. At the magnetic dip equator, where the geomagnetic field is horizontal, this electric field results in an enhanced eastward current flow within ± 3 degrees of the magnetic equator, known as the equatorial electrojet.

## Ionospheric perturbations

### X-rays: sudden ionospheric disturbances (SID)

When the sun is active, strong solar flares can occur that will hit the Earth with hard X-rays on the sunlit side of the Earth. They will penetrate to the D-region, release electrons which will rapidly increase absorption causing a High Frequency (3-30 MHz) radio blackout. During this time Very Low Frequency (3 - 30 kHz) signals will become reflected by the D layer instead of the E layer, where the increased atmospheric density will usually increase the absorption of the wave, and thus dampen it. As soon as the X-rays end, the sudden ionospheric disturbance (SID) or radio black-out ends as the electrons in the D-region recombine rapidly and signal strengths return to normal. A solar flare observed by Hinode in the G-band. ... A sudden ionospheric disturbance (SID) is an abnormally high plasma density in the ionosphere caused by an occasional sudden solar flare, which often interrupts or interferes with telecommunications systems. ...

### Protons: polar cap absorption (PCA)

Associated with solar flares is a release of high-energy protons. These particles can hit the Earth within 15 minutes to 2 hours of the solar flare. The protons spiral around and down the magnetic field lines of the Earth and penetrate into the atmosphere near the magnetic poles increasing the ionization of the D and E layers. PCA's typically last anywhere from about an hour to several days, with an average of around 24 to 36 hours.

### Geomagnetic storms

A geomagnetic storm is a temporary intense disturbance of the Earth's magnetosphere. A geomagnetic storm is a temporary disturbance of the Earths magnetosphere. ... A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ...

• During a geomagnetic storm the F2 layer will become unstable, fragment, and may even disappear completely.
• In the Northern and Southern pole regions of the Earth aurora will be observable in the sky.

Aurora borealis Polar aurorae are optical phenomena characterized by colorful displays of light in the night sky. ...

### Lightning

Lightning can cause ionospheric perturbations in the D-region one of two ways. The first is through VLF frequency radio waves launched into the magnetosphere. These so-called "whistler" mode waves can interact with radiation belt particles and cause them to precipitate onto the ionosphere, adding ionization to the D-region. These disturbances are called Lightning-induced Electron Precipitation (LEP) events. Not to be confused with lighting. ... A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ...

Additional ionization can also occur from direct heating/ionization as a result of huge motions of charge in lightning strikes. These events are called Early/Fast.

DX communication, popular among amateur radio enthusiasts, is a term given to communication over great distances. When using High-Frequency bands, the ionosphere is utilized 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. DX communication is communication over great distances using the ionosphere to refract the transmitted radio beam. ... Amateur radio station with modern solid-state transceiver featuring LCD and DSP capabilities Amateur radio, often called ham radio, is both a hobby and a service that uses various types of radio communications equipment to communicate with other radio amateurs for public service, recreation and self-training. ... This article refers to refraction in waves. ...

Radio waves "hop" from the Earth to the ionosphere and back to the Earth. When a radio wave reaches the ionosphere, the electric field in the wave forces the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio wave energy is given up to this mechanical oscillation. The oscillating electron will then either be lost to recombination or will re-radiate the original wave energy. Total refraction can occur when the collision frequency of the ionosphere is less than the radio frequency, and if the electron density in the ionosphere is great enough. In telecommunication, the term hop has the following meanings: 1. ... In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. ... Oscillation is the variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. ...

The critical frequency is the limiting frequency at or below which a radio wave is refracted by an ionospheric layer at vertical incidence. If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. It is calculated as shown below: In telecommunication, the term critical frequency has the following meanings: In radio propagation by way of the ionosphere, the limiting frequency at or below which a wave component is reflected by, and above which it penetrates through, an ionospheric layer. ... Fig. ... In physics, plasma oscillations, often referred to as Langmuir waves or plasma waves, are periodic oscillations of charge density in conducting media such as plasmas or metals. ...

$f_{critical} = 9 times 10^{-3} sqrt{N}$

where N = electron density per cm3 and fcritical is in MHz.

The Maximum Usable Frequency (MUF) is defined as the upper frequency limit that can be used for transmission between two points at a specified time.

$f_{muf} = frac{f_{critical}}{ sin alpha}$

where α = angle of attack, the angle of the wave relative to the horizon, and sin is the sine function. In this diagram, the black arrow represents the direction of the wind. ... Horizon. ... In mathematics, the trigonometric functions are functions of an angle, important when studying triangles and modeling periodic phenomena. ...

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 refraction from the layer. A bode plot of the Butterworth filters frequency response, with corner frequency labeled. ...

## Other applications

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. An open system may refer to: Open system (computing), one of a class of computers that provides some combination of interoperability, portability and open software standards, particularly Unix and Unix-like systems Open system (computer science), in the computer sciences a collection of interacting software, hardware, and human components with... Artists conception of satellite with a tether Tether propulsion uses long, strong strings (known as Tethers) to change the orbits of spacecraft. ... For magnetic induction, see Magnetic field. ...

## Measurements

### Ionograms

Ionograms show the virtual heights and critical frequencies of the ionospheric layers and which are measured by an ionosonde. An ionosonde sweeps a range of frequencies, usually from 0.1 to 30 MHz, transmitting at vertical incidence to the ionosphere. As the frequency increases, each wave is refracted less by the ionization in the layer, and so each penetrates further before it is reflected. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma, or critical, frequency of the layer. Tracings of the reflected high frequency radio pulses are known as ionograms. A ionosonde is a special radar for the examinatation of the ionosphere. ...

### Solar flux

Solar flux is a measurement of the intensity of solar radio emissions at a frequency of 2800 MHz made using a radio telescope located in Ottawa, Canada. Known also as the 10.7 cm flux (the wavelength of the radio signals at 2800 MHz), this solar radio emission has been shown to be proportional to sunspot activity. However, the level of the sun's ultraviolet and X-ray emissions is primarily responsible for causing ionization in the earth's upper atmosphere. We now have data from the GOES spacecraft that measures the background X-ray flux from the sun, a parameter more closely related to the ionization levels in the ionosphere. The 64 meter radio telescope at Parkes Observatory A radio telescope is a form of directional radio antenna used in radio astronomy and in tracking and collecting data from satellites and space probes. ... -1... Goes is a municipality and a city in the southwestern Netherlands, in Zuid-Beveland. ...

• The A and K indices are a measurement of the behavior of the horizontal component of the geomagnetic field. The K index uses a scale from 0 to 9 to measure the change in the horizontal component of the geomagnetic field. A new K index is determined at the Table Mountain Observatory, north of Boulder, Colorado.
• The geomagnetic activity levels of the earth are measured by the fluctuation of the Earth's magnetic field in SI units called tesla (unit)s (or in non-SI gauss, especially in older literature). The Earth's magnetic field is measured around the planet by many observatories. The data retrieved is processed and turned into measurement indices. Daily measurements for the entire planet are made available through an estimate of the ap index, called the planetary A-index (PAI).

The A-index provides a daily average level for geomagnetic activity, based on the K-index. ... The K-index quantifies disturbances in the horizontal component of earths magnetic field with an integer in the range 0-9 with 1 being calm and 5 or more indicating a geomagnetic storm. ... This article is about Table Mountain in South Africa. ... Boulder is a Home Rule Municipality that is the county seat and most populous city of Boulder County, Colorado, in the United States. ... Official language(s) English Demonym Coloradan Capital Denver Largest city Denver Largest metro area Denver-Aurora Metro Area Area  Ranked 8th in the US  - Total 104,185 sq mi (269,837 kmÂ²)  - Width 280 miles (451 km)  - Length 380 miles (612 km)  - % water 0. ... Look up si, Si, SI in Wiktionary, the free dictionary. ... SI unit. ... The gauss, abbreviated as G, is the cgs unit of magnetic flux density (B), named after the German mathematician and physicist Carl Friedrich Gauss. ...

### Scientific research on ionospheric propagation

A variety of experiments, such as HAARP (High Frequency Active Auroral Research Program), involve high power radio transmitters to modify the properties of the ionosphere. These investigations focus on studying the properties and behavior of ionospheric plasma, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and military purposes. HAARP was started in 1993 as a proposed twenty year experiment, and is currently active near Gakona, Alaska. There is concern among many members of the scientific community regarding the dangers involved in disturbing the ionosphere. HAARP redirects here. ...

The SuperDARN radar project researches the high- and mid-latitudes using coherent backscatter of radio waves in the 8 to 20 MHz range. Coherent backscatter is similar to Bragg scattering in crystals and involves the constructive interference of scattering from ionospheric density irregularities. The project involves more than 11 different countries and multiple radars in both hemispheres.

Scientists are also examining the ionosphere by the changes to radio waves from satellites and stars passing through it. The Arecibo radio telescope located in Puerto Rico, was originally intended to study Earth's ionosphere. The Arecibo Observatory is located in Arecibo, Puerto Rico on the north coast of the island. ...

## History

Guglielmo Marconi received the first trans-Atlantic radio signal on December 12, 1901, in St. John's, Newfoundland (now in Canada) using a 152.4 m (500 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 and experimental work.[2] However, Marconi did achieve transatlantic wireless communications beyond a shadow of doubt in Glace Bay one year later. For the inventor of radio, see the competing claims in history of radio and the invention of radio. ... is the 346th day of the year (347th in leap years) in the Gregorian calendar. ... Year 1901 (MCMI) was a common year starting on Tuesday (link will display calendar) of the Gregorian calendar (or a common year starting on Monday [1] of the 13-day-slower Julian calendar). ... St. ... Poldhu is a small area in south Cornwall, UK, situated on the Lizard Peninsula it comprises Poldhu Point and Poldhu Cove. ... A kilohertz (kHz) is a unit of frequency equal to 1,000 hertz (1,000 cycles per second). ... 1922 Chart of the Morse Code Letters and Numerals Morse code is a method for transmitting telegraphic information, using standardized sequences of short and long elements to represent the letters, numerals, punctuation and special characters of a message. ...

In 1902, Oliver Heaviside proposed the existence of the Kennelly-Heaviside Layer of the ionosphere which bears his name. Heaviside's 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 detection of electromagnetic waves from celestial bodies until 1932 (and the development of high frequency radio transceivers). Also in 1902, Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties. Oliver Heaviside (May 18, 1850 â€“ February 3, 1925) was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, developed techniques for applying Laplace transforms to the solution of differential equations, reformulated Maxwells field equations in terms of electric and... Year 1902 (MCMII) was a common year starting on Wednesday (link will display calendar) of the Gregorian calendar (or a common year starting on Tuesday [1] of the 13-day-slower Julian calendar). ... Arthur Edwin Kennelly (December 17, 1861 - June 18, 1939), was an American engineer in electricity. ...

In 1912, the U.S. 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 led to the discovery of HF radio propagation via the ionosphere in 1923. The Congress of the United States is the legislative branch of the federal government of the United States of America. ... Following the 1912 disaster of the RMS Titanic, which was exacerbated by inept radio operators, the United States Congress passed the Radio Act of 1912. ...

In 1926, Scottish physicist Robert Watson-Watt introduced the term ionosphere in a letter published only in 1969 in Nature: Sir Robert Alexander Watson-Watt, ca. ... Nature is a prominent scientific journal, first published on 4 November 1869. ...

We have in quite recent years seen the universal adoption of the term ‘stratosphere’..and..the companion term ‘troposphere’... The term ‘ionosphere’, for the region in which the main characteristic is large scale ionisation with considerable mean free paths, appears appropriate as an addition to this series.

Edward V. Appleton was awarded a Nobel Prize in 1947 for his confirmation in 1927 of 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 and J. A. Ratcliffe 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. Sir Edward Victor Appleton (September 6, 1892 – April 21, 1965) was an English physicist. ... The Nobel Prize (Swedish: ) was established in Alfred Nobels will in 1895, and it was first awarded in Physics, Chemistry, Physiology or Medicine, Literature, and Peace in 1901. ... Lloyd V. Berkner (born February 1, 1905, in Milwaukee, died June 4, 1967, in Washington, D.C.) was the U.S. physicist and engineer who first measured the height and density of the ionosphere. ... Maurice V. Wilkes Maurice Vincent Wilkes (born June 26, 1913 in Dudley, Staffordshire, England) is a British computer scientist, credited with several important developments in computing. ... John Ashworth Ratcliffe, FRS (12 December 1902 â€“ 25 October 1987), JAR or Jack, was an influential British radio physicist. ... Vitaly Lazarevich Ginzburg (Russian: ; born October 4, 1916 in Moscow) is a Russian (formerly Soviet) theoretical physicist and astrophysicist, a member of the Russian Academy of Sciences, and the successor to Igor Tamm as head of the Department of Theoretical Physics of Academys physics institute (FIAN). ...

In 1962 the Canadian satellite Alouette 1 was launched to study the ionosphere. Following its success were Alouette 2 in 1965 and the two ISIS satellites in 1969 and 1971, all for measuring the ionosphere. Alouette 1 was Canadas first satellite, and the first satellite operated by a country other than the USSR or the United States. ... Alouette 2 was launched on November 29, 1965. ... ISIS-I ISIS I and II (International Satellites for Ionospheric Studies) were the third and fourth satellites that were launched in a series of Canadian satellites sent up to study the ionosphere. ...

## References

1. ^ Yenne, Bill (1985). The Encyclopedia of US Spacecraft. Exeter Books (A Bison Book), New York. ISBN 0-671-07580-2. p.12 AEROS
2. ^ John S. Belrose, "Fessenden and Marconi: Their Differing Technologies and Transatlantic Experiments During the First Decade of this Century". International Conference on 100 Years of Radio -- 5-7 September 1995.
• Corum, J. F., and Corum, K. L., "A Physical Interpretation of the Colorado Springs Data". Proceedings of the Second International Tesla Symposium. Colorado Springs, Colorado, 1986.
• Davies, K., 1990. Peter Peregrinus Ltd, London. ISBN 0-86341-186-X Ionospheric Radio.
• Grotz, Toby, "The True Meaning of Wireless Transmission of power". Tesla : A Journal of Modern Science, 1997.
• Hargreaves, J. K., "The Upper Atmosphere and Solar-Terrestrial Relations". Cambridge University Press, 1992,
• Kelley, M. C, and Heelis, R. A., "The Earth's Ionosphere: Plasma Physics and Electrodynamics". Academic Press, 1989.
• Leo F. McNamara. (1994) ISBN 0-89464-804-7 Radio Amateurs Guide to the Ionosphere.
• K.Rawer and Y.V.Ramanamurty (eds) (1 January 1986). "International Reference Ionosphere - Status 1985/86". Advances in Space Research 5 (10). ISBN 0-08-034026-1 (Publisher: Pergamon Press), ISSN 0273-1177.

is the 1st day of the year in the Gregorian calendar. ... Year 1986 (MCMLXXXVI) was a common year starting on Wednesday (link displays 1986 Gregorian calendar). ... ISSN, or International Standard Serial Number, is the unique eight-digit number applied to a periodical publication including electronic serials. ...

Results from FactBites:

 NGDC/STP - Introduction to the Ionosphere (312 words) The ionosphere is that part of the upper atmosphere where free electrons occur in sufficient density to have an appreciable influence on the propagation of radio frequency electromagnetic waves. ionospheric structures and peak densities in the ionosphere vary greatly with time (sunspot cycle, seasonally, and diurnally), with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions), and with certain solar-related ionospheric disturbances. The ionosphere is a dynamic system controlled by many parameters including acoustic motions of the atmosphere, electromagnetic emissions, and variations in the geomagnetic field.
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