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| The '''ionosphere''' is a region of the upper [[Earth's atmosphere|atmosphere]], from about {{convert|85|km|mi|abbr=on}} to {{convert|600|km|mi|abbr=on}} altitude, and includes the [[thermosphere]] and parts of the [[mesosphere]] and [[exosphere]]. It is 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]].<ref name=rawer>K. Rawer. ''Wave Propagation in the Ionosphere''. Kluwer Acad.Publ., Dordrecht 1993. ISBN 0-7923-0775-5</ref>
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| [[Image:Atmosphere with Ionosphere.svg|thumb|400px|Relationship of the atmosphere and ionosphere]]
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| ==Geophysics==
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| The ionosphere is a shell of [[electron]]s and electrically charged [[atom]]s and [[molecule]]s that surrounds the Earth, stretching from a height of about {{convert|50|km|mi|abbr=on}} to more than {{convert|1000|km|mi|abbr=on}}. It owes its existence primarily to [[ultraviolet]] radiation from the [[Sun]].
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| The lowest part of the [[Earth's atmosphere]], the [[troposphere]] extends from the surface to about {{convert|10|km|mi|abbr=on}}. Above {{convert|10|km|mi|abbr=on}} is the [[stratosphere]], followed by the [[mesosphere]]. In the stratosphere incoming solar radiation creates the [[ozone layer]]. At heights of above {{convert|80|km|mi|abbr=on}}, 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 portion of the atmosphere is ''ionized'' and contains a [[Plasma physics|plasma]] which is referred to as the ionosphere. In a plasma, the negative free electrons and the positive ions are attracted to each other by the electrostatic force, but they are too energetic to stay fixed together in an electrically neutral molecule.
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| [[Ultraviolet]] (UV), [[X-Ray]] and shorter [[wavelength]]s of [[solar radiation]] are ''ionizing,'' since [[photon]]s at these frequencies contain sufficient energy to dislodge an [[electron]] from a neutral gas [[atom]] or [[molecule]] upon absorption. In this process the light electron obtains a high velocity so that the [[temperature]] of the created electronic gas is much higher (of the order of thousand K) than the one of ions and neutrals. The reverse process to [[ionization]] is [[Recombination (chemistry)|recombination]], in which a free electron is "captured" by a positive ion. Recombination occurs spontaneously, and causes the emission of a photon carrying away the energy produced upon recombination. As gas density increases at lower altitudes, the recombination process prevails, since the gas molecules and ions are closer together. The balance between these two processes determines the quantity of ionization present.
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| Ionization depends primarily on the [[Sun]] and its [[solar variation|activity]]. The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the Sun. Thus there is a [[Day|diurnal]] (time of day) effect and a seasonal effect. The local winter [[Earth|hemisphere]] is tipped away from the Sun, thus there is less received solar radiation. The activity of the Sun is associated with the [[sunspot cycle]], with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, [[auroral]] zones, [[mid-latitudes]], and equatorial regions). There are also mechanisms that disturb the ionosphere and decrease the ionization. There are disturbances such as [[solar flare]]s and the associated release of charged particles into the [[solar wind]] which reaches the Earth and interacts with its [[geomagnetic]] field.
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| ==The ionospheric layers==
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| [[File:Ionosphere Layers en.svg|thumb|250px|Ionospheric layers.]] At night the F layer is the only layer of significant ionization present, while the ionization in the E and D layers is extremely low. During the day, the D and E layers become much more heavily ionized, as does the F layer, which develops an additional, weaker region of ionisation known as the F<sub>1</sub> layer. The F<sub>2</sub> layer persists by day and night and is the region mainly responsible for the refraction of radio waves.
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| ===D layer===
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| The D layer is the innermost layer, {{convert|60|km|mi|abbr=on}} to {{convert|90|km|mi|abbr=on}} above the surface of the Earth. Ionization here is due to [[Lyman series]]-alpha hydrogen radiation at a [[wavelength]] of 121.5 [[nanometre]] (nm) ionizing [[nitric oxide]] (NO). In addition, with high [[Space weather|Solar activity]] hard [[X-ray]]s (wavelength < 1 nm) may ionize (N₂, O₂). During the night [[cosmic rays]] produce a residual amount of ionization. Recombination is high in the D layer, the net ionization effect is low, but loss of wave energy is great due to frequent collisions of the electrons (about ten collisions every msec). As a result high-frequency (HF) [[radio wave]]s are not reflected by the D layer but suffer loss of energy therein. This is the main reason for [[Ionospheric absorption|absorption of HF radio waves]], particularly at 10 MHz and below, with progressively smaller absorption as the frequency gets higher. The absorption is small at night and greatest about midday. The layer reduces greatly after sunset; a small part remains due to [[galactic cosmic ray]]s. A common example of the D layer in action is the disappearance of distant AM [[broadcast band]] stations in the daytime.
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| During [[solar proton event]]s, ionization can reach unusually high levels in the D-region over high and polar latitudes. Such very rare 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.
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| ===E layer===
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| The [[Kennelly–Heaviside layer|E layer]] is the middle layer, {{convert|90|km|mi|abbr=on}} to {{convert|120|km|mi|abbr=on}} 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). Normally, at oblique incidence, this layer can only reflect radio waves having frequencies lower than about 10 MHz and may contribute a bit to absorption on frequencies above. However, during intense [[Sporadic E]] events, the E<sub>s</sub> layer can reflect frequencies up to 50 MHz and higher. The vertical structure of the E layer is primarily determined by the competing effects of ionization and recombination. At night the E layer rapidly disappears because the primary source of ionization is no longer present. After sunset an increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer. | |
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| This region is also known as the [[Kennelly–Heaviside 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]].
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| ===E<sub>s</sub>===
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| The E<sub>s</sub> layer ([[wikt:sporadic#Adjective|sporadic]] E-layer) is characterized by small, thin clouds of intense ionization, which can support reflection of radio waves, rarely up to 225 MHz. Sporadic-E events may last for just a few minutes to several hours. [[Sporadic E propagation]] makes [[Amateur radio high bands|radio amateurs]] very excited, as propagation paths that are generally unreachable can open up. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs most frequently during the summer months when high signal levels may be reached. The skip distances are generally around {{convert|1640|km|mi|abbr=on}}. Distances for one hop propagation can be as close as {{convert|900|km|mi|abbr=on}} or up to {{convert|2500|km|mi|abbr=on}}. Double-hop reception over {{convert|3500|km|mi|abbr=on}} is possible.
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| ===F layer===
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| The [[F region|F layer]] or region, also known as the [[Edward Victor Appleton|Appleton]] layer, extends from about {{convert|200|km|mi|abbr=on}} to more than {{convert|500|km|mi|abbr=on}} above the surface of Earth. It is the densest point of the ionosphere, which implies signals penetrating this layer will escape into space. At higher altitudes, the number of [[oxygen]] ions decreases and lighter ions such as hydrogen and helium become dominant; this layer is the [[topside ionosphere]]. There, extreme ultraviolet (UV, 10–100 nm) solar radiation ionizes atomic oxygen. The F layer consists of one layer at night, but during the day, a deformation often forms in the profile that is labeled F₁. The F₂ layer remains by day and night responsible for most [[skywave]] propagation of [[radio]] waves, facilitating [[high frequency]] (HF, or [[shortwave]]) radio communications over long distances.
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| From 1972 to 1975 [[NASA]] launched the [[AEROS (satellite)|AEROS and AEROS B]] satellites to study the F region.<ref name="Yenne">{{cite book|author=Yenne, Bill|title=''The Encyclopedia of US Spacecraft''|publisher=Exeter Books (A Bison Book), New York|year=1985|isbn=0-671-07580-2}} p. 12 '''AEROS'''</ref>
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| ==Ionospheric model==
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| An ionospheric model is a mathematical description of the ionosphere as a function of location, altitude, day of year, phase of the sunspot cycle and geomagnetic activity. Geophysically, the state of the ionospheric [[Plasma (physics)|plasma]] may be described by four parameters: ''electron density, electron and ion [[temperature]]'' and, since several species of ions are present, ''ionic composition''. [[Radio propagation]] depends uniquely on electron density.
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| Models are usually expressed as computer programs. The model may be based on basic physics of the interactions of the ions and electrons with the neutral atmosphere and sunlight, or it may be a statistical description based on a large number of observations or a combination of physics and observations. One of the most widely used models is the [[International Reference Ionosphere]] (IRI)<ref>D.Bilitza:''International Reference Ionosphere 2000''.Radio Sci.36,#2,261-275 2001</ref> [[#References|(IRI 2007)]], which is based on data and specifies the four parameters just mentioned. The IRI is an international project sponsored by the [[Committee on Space Research]] (COSPAR) and the [[International Union of Radio Science]] (URSI).<ref>{{cite web|url=http://ccmc.gsfc.nasa.gov/modelweb/ionos/iri.html |title=International Reference Ionosphere |publisher=Ccmc.gsfc.nasa.gov |date= |accessdate=2011-11-08}}</ref> The major data sources are the worldwide network of [[ionosonde]]s, the powerful [[incoherent scatter]] radars (Jicamarca, [[Arecibo observatory|Arecibo]], Millstone Hill, Malvern, St. Santin), the ISIS and Alouette topside sounders, and in situ instruments on several satellites and rockets. IRI is updated yearly. IRI is more accurate in describing the variation of the electron density from bottom of the ionosphere to the altitude of maximum density than in describing the [[total electron content]] (TEC) .Since 1999 this model is "International Standard" for the terrestrial ionosphere (standard TS16457).
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| ==Persistent anomalies to the idealized model==
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| [[Ionogram]]s allow deducing, via computation, the true shape of the different layers. Nonhomogeneous structure of the [[electron]]/[[ion]]-[[Plasma (physics)|plasma]] produces rough echo traces, seen predominantly at night and at higher latitudes, and during disturbed conditions.
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| ===Winter anomaly===
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| At mid-latitudes, the F<sub>2</sub> 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 F<sub>2</sub> 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.
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| ===Equatorial anomaly===
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| [[Image:Diurnal ionospheric current.jpg|frame|right|Electric currents created in sunward ionosphere.]]
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| Within approximately ± 20 degrees of the ''magnetic equator'', is the ''[[equator]]ial anomaly''. It is the occurrence of a trough of concentrated ionization in the F<sub>2</sub> 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 plane|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''.
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| ===Equatorial electrojet===
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| The worldwide solar-driven wind results in the so-called Sq (solar quiet) current system in the E region of the Earth's ionosphere ([[ionospheric dynamo region]]) ({{convert|100|km|mi|abbr=on}} – {{convert|130|km|mi|abbr=on}} 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]].
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| ==Ephemeral ionospheric perturbations==
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| ===X-rays: sudden ionospheric disturbances (SID)===
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| When the Sun is active, strong [[solar flare]]s can occur that will hit the sunlit side of Earth with hard X-rays. The X-rays will penetrate to the D-region, releasing electrons that will rapidly increase absorption, causing a High Frequency (3 - 30 MHz) radio blackout. During this time Very Low Frequency (3 – 30 kHz) signals will be 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.
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| ===Protons: polar cap absorption (PCA)===
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| 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.
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| ===Geomagnetic storms===
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| A [[geomagnetic storm]] is a temporary intense disturbance of the Earth's [[magnetosphere]].
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| * During a geomagnetic storm the F₂ layer will become unstable, fragment, and may even disappear completely.
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| * In the Northern and Southern pole regions of the Earth [[polar aurora|aurora]]e will be observable in the sky.
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| ===Lightning===
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| [[Lightning]] can cause ionospheric perturbations in the D-region in one of two ways. The first is through VLF (Very Low 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.
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| 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.
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| In 1925, C. T. R. Wilson proposed a mechanism by which electrical discharge from lightning storms could propagate upwards from clouds to the ionosphere. Around the same time, Robert Watson-Watt, working at the Radio Research Station in Slough, UK, suggested that the ionospheric sporadic E layer (Es) appeared to be enhanced as a result of lightning but that more work was needed. In 2005, C. Davis and C. Johnson, working at the Rutherford Appleton Laboratory in Oxfordshire, UK, demonstrated that the Es layer was indeed enhanced as a result of lightning activity. Their subsequent research has focussed on the mechanism by which this process can occur.
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| ==Applications==
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| ===Radio communication{{anchor|Radio application}}===
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| <!-- This section is linked from [[Triple J]] --> | |
| [[DX communication]], popular among [[amateur radio]] enthusiasts, is a term given to communication over great distances. Thanks to the property of ionized atmospheric gases to [[refract]] high frequency (HF, or [[shortwave]]) radio waves, the ionosphere can be utilized to "bounce" a transmitted signal down to ground. Transcontinental HF-connections rely on up to 5 bounces, or [[Hop (telecommunications)|hop]]s. Such communications played an important role during [[World War II]]. [[Karl Rawer]]'s most sophisticated prediction method<ref name=rawer /> took account of several (zig-zag) paths, attenuation in the D-region and predicted the 11-year [[Sun|solar cycle]] by a method due to [[Solar variation|Wolfgang Gleißberg]].
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| ====Mechanism of refraction====
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| {{merge section|Earth–ionosphere waveguide|date=October 2013}}
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| 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-frequency energy is given up to this resonant oscillation. The oscillating electrons 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.
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| The [[critical frequency]] is the limiting frequency at or below which a radio wave is reflected by an ionospheric layer at vertical [[angle of incidence|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:
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| : <math>f_{\text{critical}} = 9 \times\sqrt{N}</math> | |
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| where N = electron density per m<sup>3</sup> and f<sub>critical</sub> is in Hz.
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| 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.
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| : <math>f_\text{muf} = \frac{f_\text{critical}}{ \sin \alpha} </math>
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| where <math>\alpha</math> = [[angle of attack]], the angle of the wave relative to the [[horizon]], and sin is the [[sine]] function.
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| 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. | |
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| ===Other applications===
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| The [[open system (systems theory)|open system]] [[electrodynamic 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]].
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| ==Measurements==
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| ===Overview===
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| Scientists also are exploring the structure of the ionosphere by a wide variety of methods, including passive observations of optical and radio emissions generated in the ionosphere, bouncing radio waves of different frequencies from it, [[incoherent scatter]] radars such as the [[EISCAT]], Sondre Stromfjord, [[Millstone Hill Observatory|Millstone Hill]], [[Arecibo Observatory|Arecibo]], and [[Jicamarca Radio Observatory|Jicamarca]] radars, coherent scatter radars such as the [[Super Dual Auroral Radar Network|Super Dual Auroral Radar Network (SuperDARN)]] radars, and using special receivers to detect how the reflected waves have changed from the transmitted waves.
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| 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.
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| 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.
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| 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.
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| ===Ionograms===
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| 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. Reduction rules are given in: "URSI Handbook of Ionogram Interpretation and Reduction", edited by [[William Roy Piggott]] and [[Karl Rawer]], Elsevier Amsterdam, 1961 (translations into Chinese, French, Japanese and Russian are available).
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| ===Incoherent scatter radars===
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| [[Incoherent scatter]] radars operate above the critical frequencies. Therefore the technique allows to probe the ionosphere, unlike ionosondes, also above the electron density peaks. The thermal fluctuations of the electron density scattering the transmitted signals lack [[Coherence (physics)|coherence]], which gave the technique its name. Their power spectrum contains information not only on the density, but also on the ion and electron temperatures, ion masses and drift velocities.
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| ===Solar flux===
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| 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 [[Dominion Radio Astrophysical Observatory]], Penticton, British Columbia, Canada.<ref>{{cite web|url=http://www.swpc.noaa.gov/forecast_verification/F10.html |title=F10.7 Solar Flux Forecast Verification |publisher=Swpc.noaa.gov |date=2007-10-01 |accessdate=2011-11-08}}</ref> 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.
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| * The ''[[A-index|A]]'' and ''[[K-index|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 [[Boulder Geomagnetic Observatory]] {{Coord|display=inline|40.137558|-105.237875|format=dms}}.
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| * The geomagnetic activity levels of the Earth are measured by the fluctuation of the Earth's magnetic field in [[SI]] units called [[tesla (unit)|tesla]]s (or in non-SI [[gauss (unit)|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).
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| ===[[GPS]]/[[GNSS]]===
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| {{stub section|date=October 2013}}
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| {{see also|Total electron content}}
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| ==Ionospheres on other planets and Titan==
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| The [[atmosphere of Titan]] includes an ionosphere that ranges from about {{convert|1100|km|mi|abbr=on}} to {{convert|1300|km|mi|abbr=on}} in altitude and contains carbon compounds.<ref>[http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=1498 NASA/JPL: Titan's upper atmosphere] Accessed 2010-08-25</ref>
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| Planets with ionospheres (incomplete list): [[Atmosphere of Venus#Upper atmosphere and ionosphere|Venus]],
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| [[Atmosphere of Uranus#Thermosphere and ionosphere|Uranus]].
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| ==History== | |
| [[Guglielmo Marconi]] received the first trans-Atlantic radio signal on December 12, 1901, in [[St. John's, Newfoundland]] (now in [[Canada]]) using a {{convert|152.4|m|ft|abbr=on}} 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 [[Kilohertz|kHz]] and a power of 100 times more than any radio signal previously produced. The message received was three dits, 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.<ref>John S. Belrose, "[http://www.ieee.ca/millennium/radio/radio_differences.html 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.</ref> However, Marconi did achieve transatlantic wireless communications beyond a shadow of doubt in [[Glace Bay, Nova Scotia]] one year later.
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| 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.
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| 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. | |
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| In 1926, Scottish physicist [[Robert Watson-Watt]] introduced the term ''ionosphere'' in a letter published only in 1969 in ''[[Nature (journal)|Nature]]'':
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| {{Quotation|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.}}
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| [[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.
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| In 1962 the [[Canada|Canadian]] satellite [[Alouette 1]] was launched to study the ionosphere. Following its success were [[Alouette 2]] in 1965 and the two [[ISIS (satellite)|ISIS]] satellites in 1969 and 1971, further AEROS -A and -B in 1972 and 1975, all for measuring the ionosphere.
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| ==See also==
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| {{Portal|Atmospheric sciences}}
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| {{col-float}}
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| * '''[[Geophysics]]'''
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| ** [[International Reference Ionosphere]]
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| ** [[ionospheric dynamo region]]
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| ** [[Magnetospheric electric convection field]]
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| ** [[Schumann resonances]]
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| ** [[Van Allen radiation belt]]
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| * '''[[Radio]]'''
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| ** [[Earth-Ionosphere waveguide]]
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| ** [[Fading]]
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| ** [[Ionospheric absorption]]
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| ** [[Ionospheric scintillation]]
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| ** [[Line-of-sight propagation]]
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| ** [[Sferics]]
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| {{col-float-break}}
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| * '''Related'''
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| ** [[Canadian Geospace Monitoring]]
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| ** [[High Frequency Active Auroral Research Program]]
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| ** [[International Geophysical Year]]
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| ** [[Ionospheric heater]]
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| ** ''[[New Horizons]]''
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| ** [[Nozomi (probe)|Nozomi]]
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| ** [[Pioneer Venus project]]
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| ** [[Soft gamma repeater]]
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| ** [[Sprite halo|Upper Atmospheric Lightning]]
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| ** [[Sura Ionospheric Heating Facility]]
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| ** [[Tether propulsion]]
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| ** [[TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)]]
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| * '''Lists'''
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| ** [[List of astronomical topics]]
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| ** [[List of electronics topics]]
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| ** [[List of plasma (physics) articles]]
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| {{col-float-end}}
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| ==Notes==
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| {{Reflist}}
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| ==References==
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| * {{cite book |author=Davies, Kenneth |title=Ionospheric Radio |isbn= 0-86341-186-X |series=IEE Electromagnetic Waves Series #31
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| |year=1990 |publisher=Peter Peregrinus Ltd/The Institution of Electrical Engineers |location=London, UK }}
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| * Hargreaves, J. K., "''The Upper Atmosphere and Solar-Terrestrial Relations''". Cambridge University Press, 1992,
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| * Kelley, M. C, and Heelis, R. A., "''The Earth's Ionosphere: Plasma Physics and Electrodynamics''". Academic Press, 1989.
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| * Leo F. McNamara. (1994) ISBN 0-89464-804-7 "''Radio Amateurs Guide to the Ionosphere''".
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| * Rawer,K.:"''Wave Propagation in the Ionosphere''". Kluwer Academic Publ., Dordrecht 1993 ISBN 0-7923-0775-5.
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| * D. Bilitza, "''International Reference Ionosphere 2000,''".Radio Science '''36''', #2, pp 261–275, 2001.
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| * J. Lilensten et P.-L. Blelly: ''Du Soleil à la Terre, Aéronomie et météorologie de l'espace'', Collection Grenoble Sciences, Université Joseph Fourier Grenoble I, 2000. ISBN 978-2-86883-467-6
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| * P.-L. Blelly and D. Alcaydé, ''Ionosphere'', in: Y. Kamide/A. Chian, ''Handbook of the Solar-Terrestrial Environment'', Springer-Verlag Berlin Heidelberg, pp. 189–220, 2007. {{doi|10.1007/11367758_8}}
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| * H. Volland, ''Atmospheric Electrodynamics'', Springer Verlag, Berlin, 1984.
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| ==External links==
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| {{Commons category|Ionosphere}}
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| {{Wiktionary|ionosphere}}
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| {{Refbegin}}
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| * Gehred, Paul, and Norm Cohen, ''[http://www.sec.noaa.gov/radio SWPC's Radio User's Page]''.
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| *[http://www.esa-spaceweather.net/sda/ionosfera/ Amsat-Italia project on Ionospheric propagation (ESA SWENET website)]
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| *[http://www.wcflunatall.com/nz4o1.htm NZ4O Solar Space Weather & Geomagnetic Data Archive]
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| *[http://www.wcflunatall.com/nz4o5.htm NZ4O 160 Meter (Medium Frequency)Radio Propagation Theory Notes] Layman Level Explanations Of "Seemingly" Mysterious 160 Meter (MF/HF) Propagation Occurrences
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| *[http://geomag.usgs.gov USGS Geomagnetism Program]
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| *[http://www.britannica.com/eb/article-9042708/ionosphere-and-magnetosphere Encyclopaedia Britannica, Ionosphere and magnetosphere]
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| *[http://www.sec.noaa.gov/SWN/ Current Space Weather Conditions]
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| *[http://www.sec.noaa.gov/rt_plots/xray_1m.html Current Solar X-Ray Flux]
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| *[http://superdarn.jhuapl.edu/ Super Dual Auroral Radar Network]
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| *[http://www.eiscat.se/ European Inchorent Scatter radar system]
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| *[http://haystack.mit.edu/atm/mho/index.html Millstone Hill incoherent scatter radar]
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| <!-- *[http://www.kuittho.edu.my/waras Equatorial Ionosonde Station] -->
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| {{Refend}}
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| {{Earth's atmosphere}}
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| [[Category:Ionosphere| ]]
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| [[Category:Radio frequency propagation]]
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| [[Category:Radio terminology]]
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| {{Link FA|hr}}
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