View source for Ionosphere

{{short description|Ionized part of Earth's upper atmosphere}}
[[File:1 atmosphere layers.png|thumb|upright=2|Relationship of the atmosphere and ionosphere]]

The '''ionosphere''' ({{IPAc-en|aɪ|ˈ|ɒ|n|ə|ˌ|s|f|ɪər}}){{refn|{{cite encyclopedia |last=Jones |first=Daniel |author-link=Daniel Jones (phonetician) |title=ionosphere |encyclopedia=English Pronouncing Dictionary |editor1=Peter Roach |editor2=James Hartmann |editor3=Jane Setter |location=[[Cambridge]] |publisher=[[Cambridge University Press]] |orig-year=1917 |year=2003 |isbn=978-3-12-539683-8}}}}{{refn|{{cite Merriam-Webster|ionosphere}}}} is the [[ionization|ionized]] part of the [[upper atmosphere]] of Earth, from about {{cvt|48|km|mi}} to {{cvt|965|km|mi}} [[height above sea level|above sea level]],<ref>{{cite web |last=Zell|first=Holly |title=Earth's Atmospheric Layers |url=https://www.nasa.gov/mission_pages/sunearth/science/atmosphere-layers2.html |publisher=NASA |date=2 March 2015 |access-date=2020-10-23}}</ref> a region that includes the [[thermosphere]] and parts of the [[mesosphere]] and [[exosphere]]. The ionosphere is ionized by [[solar irradiance|solar radiation]]. It plays an important role 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 [[Earth]].<ref name=rawer>{{cite book |first=K.|last=Rawer |title=Wave Propagation in the Ionosphere |publisher=[[Springer Science+Business Media|Kluwer Academic]] |location=[[Dordrecht]] |year=1993 |isbn=0-7923-0775-5}}</ref> Travel through this layer also impacts [[GPS signals]], resulting in effects such as deflection in their path and delay in the arrival of the signal.<ref>{{Citation |last=Lopez |first=Ericson D. |title=Preliminary mapping of ionospheric total electron content (TEC) over Ecuador using global positioning system (GPS) data |date=2024-03-27 |url=https://arxiv.org/abs/2403.19053 |access-date=2024-12-05 |doi=10.48550/arXiv.2403.19053 |last2=Ubillus |first2=Bryan A. |last3=Meza |first3=Ariel A.}}</ref>

==History of discovery==
As early as 1839, the German mathematician and physicist [[Carl Friedrich Gauss]] postulated that an electrically conducting region of the atmosphere could account for observed variations of Earth's magnetic field.<ref>{{cite book |last1=Gauss |first1=Carl Friedrich |editor1-last=Gauss |editor1-first=Carl Friedrich |editor2-last=Weber |editor2-first=Wilhelm |title=Resultate aus den Beobachtungen des Magnetischen Vereins im Jahre 1838 |trans-title=Findings from the Observations of the Magnetic Society in the Year 1838 |date=1839 |publisher=Weidmanns' Bookshop |location=Leipzig, (Germany) |pages=1–57 |url=https://books.google.com/books?id=TYI5AAAAcAAJ&pg=PA50 |language=German |chapter=Allgemeine Theorie des Erdmagnetismus [General theory of terrestrial magnetism]}} Gauss speculated that magnetic forces might be generated not only by electrical currents flowing through the Earth's interior but also by some sort of electrical current(s) flowing through the atmosphere. From p. 50:  ''"§ 36. Ein anderer Theil unserer Theorie, über welchen ein Zweifel Statt finden kann, ist die Voraussetzung, … zu untersuchen, wie die aus denselben hervorgehende magnetische Wirkung auf der Erdoberfläche sich gestalten würde."''  (Another part of our theory about which doubt may arise is the assumption that the agents of terrestrial magnetic force have their source exclusively in the interior of the Earth.   If the immediate causes [of terrestrial magnetism] should be sought entirely or in part outside [the Earth's interior], then we can — in so far as we exclude baseless fantasies and we want to restrict ourselves to the scientifically known [facts] — consider only galvanic currents. Atmospheric air is not a conductor of such currents; empty space also is not: thus our knowledge fails us when we seek a carrier for galvanic currents in the upper regions [of the atmosphere]. Only the enigmatic phenomena of the northern lights — in which by all appearances electricity in motion plays a major role — prohibits us from simply denying the possibility of such currents just on account of that ignorance, and in any case it remains interesting to investigate how the magnetic effect resulting from [those currents] would manifest itself on the Earth's surface.)  
*  English translation: {{cite book |last1=Gauss |first1=Carl Friedrich |last2=Sabine |first2=Elizabeth Juliana, trans. |editor1-last=Taylor |editor1-first=Richard |title=Scientific Memoirs, Selected from the Transactions of Foreign Academies of Science and Learned Societies, and from Foreign Journals |date=1841 |publisher=Richard and John E. Taylor |location=London, England |pages=184–251 |chapter-url=https://books.google.com/books?id=Nn9HAQAAMAAJ&pg=PA229 |chapter=General theory of terrestrial magnetism}} See p. 229.
*  English translation: {{cite journal |last1=Glassmeier |first1=K.-H |last2=Tsurutani |first2=B. T. |title=Carl Friedrich Gauss – General Theory of Terrestrial Magnetism – a revised translation of the German text |journal=History of Geo- and Space Sciences |date=2014 |volume=5 |issue=1 |pages=11–62|doi=10.5194/hgss-5-11-2014 |bibcode=2014HGSS....5...11G |doi-access=free }}</ref> Sixty years later, [[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.<ref>{{Cite journal |last=Marconi |first=Guglielmo |date=January 2002 |title=Wireless telegraphic communication |url=http://dx.doi.org/10.1007/bf02836176 |journal=Resonance |volume=7 |issue=1 |pages=95–101 |doi=10.1007/bf02836176 |issn=0971-8044}}</ref> The transmitting station in [[Poldhu]], Cornwall, used a [[spark-gap transmitter]] to produce a signal with a [[frequency]] of approximately 500&nbsp;[[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 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] {{webarchive|url=https://web.archive.org/web/20090123214652/http://www.ieee.ca/millennium/radio/radio_differences.html |date=2009-01-23 }}". International Conference on 100 Years of Radio, 5–7 September 1995.</ref> However, Marconi did achieve transatlantic wireless communications in [[Glace Bay, Nova Scotia]], one year later.<ref>{{Cite journal|title=Marconi and the History of Radio|journal=IEEE Antennas and Propagation Magazine|volume=46}}</ref>

In 1902, [[Oliver Heaviside]] proposed the existence of the [[Kennelly–Heaviside layer]] of the ionosphere which bears his name.<ref>{{cite encyclopedia  | last = Heaviside  | first = Oliver | title = Telegraphy |encyclopedia= Encyclopaedia Britannica |year=1902 | edition = 10th | volume = 33 | pages = 213–235 | url = https://digital.nls.uk/encyclopaedia-britannica/archive/193470565#?c=0&m=0&s=0&cv=238&xywh=2459%2C398%2C2561%2C2108 }} Speaking of wireless telegraphy, Heaviside speculated about the propagation of Hertzian (radio) waves through the atmosphere. From p. 215:  "There may possibly be a sufficiently conducting layer in the upper air. If so, the waves will, so to speak, catch on to it more or less. Then the guidance will be the sea on one side and the upper layer on the other."</ref> Heaviside's proposal included means by which radio signals are transmitted around the Earth's curvature. Also in 1902, [[Arthur Edwin Kennelly]] discovered some of the ionosphere's radio-electrical properties.<ref>{{cite journal |last1=Kennelly |first1=A.E. |title=On the elevation of the electrically conducting strata of the earth's atmosphere |journal=The Electrical World and Engineer |date=15 March 1902 |volume=39 |issue=11 |page=473 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015048964426&view=1up&seq=497&skin=2021}}</ref>

In 1912, the [[U.S. Congress]] imposed the [[Radio Act of 1912]] on [[amateur radio operators]], limiting their operations to frequencies above 1.5&nbsp;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.<ref>[https://worldradiohistory.com/BOOKSHELF-ARH/On-The-Short-Waves.pdf worldradiohistory.com: Broadcast listening in the pioneer days of radio on the short waves, 1923 1945 Jerome S. Berg] Quote: "...In addition to having to obtain licenses - a constraint to which they adapted only slowly - the amateurs were, with some exceptions, restricted to the range below 200 meters (that is, above 1500 kc.), bands that were largely unexplored and thought to be of little value. The navy attributed most interference to the amateurs, and was happy to see them on the road to a hoped - for extinction. From the amateurs' point of view, their development of the shortwave spectrum began less as a love affair than a shotgun marriage. However, all that would change...It took several years before experimenters ventured above 2-3 mc. and started to understand such things as shortwave propagation and directionality. The short waves, as they were called, were surrounded with mystery...Also in 1928 Radio News publisher Hugo Gernsback began shortwave broadcasting on 9700 kc. from his station, WRNY, New York, using the call W2XAL. "A reader in New South Wales, Aus- tralia," reported Gernsback, "writes us that while he was writing his letter he was listening to WRNY's short-wave transmitter, 2XAL, on a three-tube set; and had to turn down the volume, otherwise he would wake up his family. All this at a distance of some 10,000 miles! Yet 2XAL ...uses less than 500 watts; a quite negligible amount of power. "6...The 1930s were the golden age of shortwave broadcasting...Shortwave also facilitated communication with people in remote areas. Amateur radio became a basic ingredient of all expeditions...The term shortwave was generally taken to refer to anything above 1.5 mc., without upper limit...", [https://web.archive.org/web/20210703104647/https://worldradiohistory.com/BOOKSHELF-ARH/On-The-Short-Waves.pdf backup]</ref>

In 1925, observations during [[Solar eclipse of January 24, 1925|a solar eclipse]] in New York by [[Alfred Norton Goldsmith|Dr. Alfred N. Goldsmith]] and his team demonstrated the influence of sunlight on radio wave propagation, revealing that short waves became weak or inaudible while long waves steadied during the eclipse, thus contributing to the understanding of the ionosphere's role in radio transmission.<ref>{{cite news |title=Sun Affects Radio, Observations Show |url=https://timesmachine.nytimes.com/timesmachine/1925/01/25/101635261.html |access-date=25 January 2024 |work=[[The New York Times]] |issue=24473  |date=25 January 1925 |pages=1, 4}}</ref>

In 1926, Scottish physicist [[Robert Watson-Watt]] introduced the term ''ionosphere'' in a letter published only in 1969 in ''[[Nature (journal)|Nature]]'':<ref>The letter, dated 8 November 1926, was addressed to the Secretary
of the Radio Research Board.
*  The letter was quoted in: {{cite journal |last1=Gardiner |first1=G. W. |title=Origin of the term Ionosphere |journal=Nature |date=13 December 1969 |volume=224 |issue=5224 |page=1096|doi=10.1038/2241096a0 |bibcode=1969Natur.224.1096G |s2cid=4296253 |doi-access=free }}
*  See also:  {{cite journal |last1=Ratcliffe |first1=J.A. |title=Robert Alexander Watson-Watt |journal=Biographical Memoirs of Fellows of the Royal Society |date=1975 |volume=21 |pages=549–568}} See p. 554.</ref>

{{Blockquote|We have in quite recent years seen the universal adoption of the term 'stratosphere'..and..the companion term 'troposphere'...&nbsp;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.}}

In the early 1930s, test transmissions of [[Radio Luxembourg]] inadvertently provided evidence of the first radio modification of the ionosphere; [[HAARP]] ran a series of experiments in 2017 using the eponymous [[Luxemburg–Gorky effect|Luxembourg Effect]].<ref name="Gakona HAARPoon 2017">{{cite web |url=https://sites.google.com/alaska.edu/gakonahaarpoon/operations-news |title=Gakona HAARPoon 2017 |date=2017-02-19 |url-status=live |archive-url=https://web.archive.org/web/20170220175950/https://sites.google.com/alaska.edu/gakonahaarpoon/operations-news |archive-date=2017-02-20 }}</ref>

[[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.

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.

On July 26, 1963, the first operational geosynchronous satellite Syncom 2 was launched.<ref>{{cite web|url=http://harveycohen.net/crcss/history.html|title=Firsts in the Space Race. From an Australian perspective|website=harveycohen.net|access-date=8 May 2018|url-status=live|archive-url=https://web.archive.org/web/20170911162138/http://harveycohen.net/crcss/history.html|archive-date=11 September 2017}}</ref> On board radio beacons on this satellite (and its successors) enabled – for the first time – the measurement of [[total electron content]] (TEC) variation along a radio beam from geostationary orbit to an earth receiver. (The rotation of the plane of polarization directly measures TEC along the path.)  Australian geophysicist [[Elizabeth Essex-Cohen]] from 1969 onwards was using this technique to monitor the atmosphere above Australia and Antarctica.<ref>{{cite web|url=http://harveycohen.net/essex|title=Elizabeth A. Essex-Cohen Ionospheric Physics Papers etc|website=harveycohen.net|access-date=8 May 2018|url-status=live|archive-url=https://web.archive.org/web/20170911205109/http://harveycohen.net/essex/|archive-date=11 September 2017}}</ref>

==Geophysics==
The ionosphere is a shell of [[electron]]s and electrically charged [[atom]]s and [[molecule]]s that surrounds the Earth,<ref>{{Cite web |title=The Ionosphere {{!}} Center for Science Education |url=https://scied.ucar.edu/learning-zone/atmosphere/ionosphere |access-date=2023-04-05 |website=scied.ucar.edu}}</ref> stretching from a height of about {{convert|50|km|mi|sigfig=1|abbr=on}} to more than {{convert|1000|km|mi|sigfig=1|abbr=on}}. It exists primarily due to [[ultraviolet]] radiation from the [[Sun]].

The lowest part of the [[Earth's atmosphere]], the [[troposphere]], extends from the surface to about {{convert|10|km|mi|sigfig=1|abbr=on}}. Above that 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|sigfig=1|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 partially ''ionized'' and contains a [[Plasma physics|plasma]] which is referred to as the ionosphere.

[[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.

Ionization depends primarily on the Sun and its Extreme Ultraviolet (EUV) and X-ray irradiance which varies strongly with [[solar variation|solar activity]]. The more magnetically active the Sun is, the more [[sunspot]] active regions there are on the Sun at any one time. Sunspot active regions are the source of increased [[coronal heating]] and accompanying increases in EUV and X-ray irradiance, particularly during episodic magnetic eruptions that include [[solar flares]] that increase ionization on the sunlit side of the Earth and [[solar energetic particle]] events that can increase ionization in the polar regions. Thus the degree of ionization in the ionosphere follows both a [[Day|diurnal]] (time of day) cycle and the 11-year [[solar cycle]]. There is also a seasonal dependence in ionization degree since the local winter [[Earth|hemisphere]] is tipped away from the Sun, thus there is less received solar radiation. 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.

[[Sydney Chapman (mathematician)|Sydney Chapman]] proposed that the region below the ionosphere be called '''''neutrosphere'''''<ref name="Chapman1950">{{cite journal|last1=Chapman|first1=Sydney|title=Upper atmospheric nomenclature|journal=Journal of Geophysical Research|volume=55|issue=4|year=1950|pages=395–399|issn=0148-0227|doi=10.1029/JZ055i004p00395|bibcode=1950JGR....55..395C}}</ref>
(the '''''neutral atmosphere''''').<ref>{{Cite book|url=https://books.google.com/books?id=x9lECgAAQBAJ&q=neutral%20atmosphere&pg=PA47|title = Atmospheric and Space Sciences: Neutral Atmospheres: Volume 1|isbn = 9783319215815|last1 = Yiğit|first1 = Erdal|date = 27 July 2015| publisher=Springer }}</ref><ref>{{cite web|url=http://glossary.ametsoc.org/wiki/Neutrosphere |title=Neutrosphere - Glossary of Meteorology |publisher=Glossary.ametsoc.org |date=2012-01-26 |access-date=2022-08-12}}</ref>

{{anchor|layers_anchor}}

==Layers of ionization==
{{more citations needed|section|date=December 2024}}
[[File:Ionosphere Layers en.svg|thumb|upright=1.4|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}} layer. The F{{sub|2}} layer persists by day and night and is the main region responsible for the refraction and reflection of radio waves.

[[File:Ionospheric layers from night to day.png|thumb|Ionospheric sub-layers from night to day indicating their approximate altitudes]]
[[File:Lightning sprites.jpg|thumb|[[Sprite (lightning)|Lightning sprites]].]]

===D layer===
The D layer is the innermost layer, {{convert|48|to|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 121.6 [[nanometre]] (nm) ionizing [[nitric oxide]] (NO). In addition, [[solar flares]] can generate hard X-rays (wavelength {{nowrap|< 1 nm}}) that ionize N{{sub|2}} and O{{sub|2}}. Recombination rates are high in the D layer, so there are many more neutral air molecules than ions.

Medium frequency (MF) and lower high frequency (HF) [[radio wave]]s are significantly attenuated within the D layer, as the passing radio waves cause electrons to move, which then collide with the neutral molecules, giving up their energy. Lower frequencies experience greater absorption because they move the electrons farther, leading to greater chance of collisions. This is the main reason for [[Ionospheric absorption|absorption of HF radio waves]], particularly at 10&nbsp;MHz and below, with progressively less absorption at higher frequencies. This effect peaks around noon and is reduced at night due to a decrease in the D layer's thickness; only a small part remains due to [[cosmic rays]]. A common example of the D layer in action is the disappearance of distant AM [[broadcast band]] stations in the daytime.

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.<ref name="Rose1962">{{cite journal|last1=Rose|first1=D.C.|last2=Ziauddin|first2=Syed|title=The polar cap absorption effect|journal=Space Science Reviews|date=June 1962|volume=1|issue=1|page=115|doi=10.1007/BF00174638|bibcode=1962SSRv....1..115R|s2cid=122220113}}</ref> 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.

===E layer===
{{main|Kennelly–Heaviside layer}}

The [[Kennelly–Heaviside layer|E layer]] is the middle layer, {{convert|90|to|150|km|mi|sigfig=2|abbr=on}} above the surface of the Earth. Ionization is due to soft X-ray (1–10&nbsp;nm) and far ultraviolet (UV) solar radiation ionization of molecular [[oxygen]] (O{{sub|2}}). Normally, at oblique incidence, this layer can only reflect radio waves having frequencies lower than about 10&nbsp;MHz and may contribute a bit to absorption on frequencies above. However, during intense [[sporadic E]] events, the E{{sub|s}} layer can reflect frequencies up to 50&nbsp;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 weakens 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.

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). In 1924 its existence was detected by [[Edward V. Appleton]] and [[Miles Aylmer Fulton Barnett|Miles Barnett]].

===E{{sub|s}} layer===
The E{{sub|s}} layer ([[wikt:sporadic#Adjective|sporadic]] E-layer) is characterized by small, thin clouds of intense ionization, which can support reflection of radio waves, frequently up to 50&nbsp;MHz and rarely up to 450&nbsp;MHz. Sporadic-E events may last for just a few minutes to many hours. [[Sporadic E propagation]] makes VHF-operating by [[Amateur radio high bands|radio amateurs]] very exciting when long-distance propagation paths that are generally unreachable "open up" to two-way communication. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs every day during June and July in northern hemisphere mid-latitudes when high signal levels are often reached. The skip distances are generally around {{convert|1640|km|mi|abbr=on}}. Distances for one hop propagation can be anywhere from {{convert|900|to|2500|km|mi|abbr=on}}. Multi-hop propagation over {{convert|3500|km|mi|abbr=on}} is also common, sometimes to distances of {{convert|15000|km|mi|abbr=on}} or more.

===F layer===
{{main|F region}}

[[File:Msis atmospheric composition by height.svg|thumb|350px|Main gases of the ionosphere (about {{convert|50|km|mi|abbr=on|disp=semicolon}}and above on this chart) vary considerably by altitude]]

The [[F region|F layer]] or region, also known as the Appleton–Barnett layer, extends from about {{convert|150|km|mi|sigfig=2|abbr=on}} to more than {{convert|500|km|mi|sigfig=2|abbr=on}} above the surface of Earth. It is the layer with the highest electron density, which implies signals penetrating this layer will escape into space. Electron production is dominated by [[extreme ultraviolet]] (UV, 10–100&nbsp;nm) radiation ionizing atomic oxygen. The F layer consists of one layer (F{{sub|2}}) at night, but during the day, a secondary peak (labelled F{{sub|1}}) often forms in the electron density profile. Because the F{{sub|2}} layer remains by day and night, it is responsible for most [[skywave]] propagation of [[radio]] waves and long distance [[high frequency]] (HF, or [[shortwave]]) radio communications.

Above the F layer, the number of [[oxygen]] ions decreases and lighter ions such as hydrogen and helium become dominant. This region above the F layer peak and below the [[plasmasphere]] is called the topside ionosphere.

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|date=1985|isbn=978-0-671-07580-4}} p. 12 '''AEROS'''</ref>

=={{anchor|Ionospheric model}}Ionospheric model==
{{broader|Upper-atmospheric model}}

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.

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>Bilitza, 2001</ref> 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 |access-date=2011-11-08 |url-status=live |archive-url=http://archive.wikiwix.com/cache/20110223151912/http://ccmc.gsfc.nasa.gov/modelweb/ionos/iri.html |archive-date=2011-02-23 }}</ref> The major data sources are the worldwide network of [[ionosonde]]s, the powerful [[incoherent scatter]] radars (Jicamarca, [[Arecibo Telescope|Arecibo]], Millstone Hill, Malvern, St Santin), the ISIS and Alouette topside [[Atmospheric infrared sounder|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).

==Persistent anomalies to the idealized model==
{{Unreferenced section|date=July 2024}}
[[File:Ionosphere-Thermosphere Processes.jpg|thumb|Overview of ionosphere phenomena]]
[[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.

===Winter anomaly===
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.

===Equatorial anomaly===
[[Image:Diurnal ionospheric current.jpg|thumb|upright=1.25|Electric currents created in sunward ionosphere.]]
Within approximately ± 20 degrees of the ''magnetic equator'', is the ''[[equator]]ial anomaly.''<ref name=":0">{{Cite journal |last=Andreeva |first=E. S. |last2=Franke |first2=S. J. |last3=Yeh |first3=K. C. |last4=Kunitsyn |first4=V. E. |date=2000-08-15 |title=Some features of the equatorial anomaly revealed by ionospheric tomography |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/1999GL003725 |journal=Geophysical Research Letters |language=en |volume=27 |issue=16 |pages=2465–2468 |doi=10.1029/1999GL003725 |issn=0094-8276}}</ref><ref>{{Cite journal |last=Wu |first=Tsung-Yu |last2=Liu |first2=Jann-Yenq |last3=Chang |first3=Loren C. |last4=Lin |first4=Chien-Hung |last5=Chiu |first5=Yi-Chung |date=2021-07-19 |title=Equatorial ionization anomaly response to lunar phase and stratospheric sudden warming |url=https://www.nature.com/articles/s41598-021-94326-x |journal=Scientific Reports |language=en |volume=11 |issue=1 |pages=14695 |doi=10.1038/s41598-021-94326-x |issn=2045-2322|pmc=8289839 }}</ref> It is the occurrence of a trough in the ionization in the F<sub>2</sub> layer at the equator and crests at about 17 degrees in magnetic latitude.<ref name=":0" /> The [[Earth's magnetic field]] lines are horizontal at the magnetic equator. Solar heating and [[Tide|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''.<ref name=":1">{{Cite web |last=Bridgman |first=Tom |date=2018-01-31 |title=NASA Scientific Visualization Studio {{!}} Interface to Space: The Equatorial Fountain |url=https://svs.gsfc.nasa.gov/4617/ |access-date=2024-09-30 |website=NASA Scientific Visualization Studio |language=english}}</ref>

===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 ([[ionospheric dynamo region]]) ({{convert|100–130|km|mi|sigfig=1|abbr=on}} altitude).{{Citation needed|date=September 2024}} Resulting from this current is an electrostatic field directed west–east (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]].<ref name=":1" />

==Ephemeral ionospheric perturbations==
{{Unreferenced section|date=July 2024}}

===X-rays: sudden ionospheric disturbances (SID)===
{{Main|Sudden ionospheric disturbance}}
When the Sun is active, strong [[solar flare]]s can occur that hit the sunlit side of Earth with hard X-rays. The X-rays penetrate to the D-region, releasing electrons that rapidly increase absorption, causing a high frequency (3–30&nbsp;MHz) radio blackout that can persist for many hours after strong flares. During this time very low frequency (3–30&nbsp;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 steadily declines as the electrons in the D-region recombine rapidly and propagation gradually returns to pre-flare conditions over minutes to hours depending on the solar flare strength and frequency.

===Protons: polar cap absorption (PCA)===
{{Further|Solar particle event#Polar cap absorption events}}
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. [[Coronal mass ejection]]s can also release energetic protons that enhance D-region absorption in the polar regions.

===Storms===
{{Main|Geomagnetic storm|Ionospheric storm}}
Geomagnetic storms and ionospheric storms are temporary and intense disturbances of the Earth's [[magnetosphere]] and ionosphere.

During a geomagnetic storm the F₂ layer will become unstable, fragment, and may even disappear completely. In the Northern and Southern polar regions of the Earth [[polar aurora|aurora]]e will be observable in the night sky.

===Lightning===
{{Further|Lightning}}
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.

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.

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 (E<sub>s</sub>) 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 E<sub>s</sub> layer was indeed enhanced as a result of lightning activity. Their subsequent research has focused on the mechanism by which this process can occur.

==Applications==
===Radio communication{{anchor|Radio application}}===
<!-- This section is linked from [[Triple J]] -->
Due to the ability of ionized atmospheric gases to [[refract]] high frequency (HF, or [[shortwave]]) radio waves, the ionosphere can reflect radio waves directed into the sky back toward the Earth. Radio waves directed at an angle into the sky can return to Earth beyond the horizon. This technique, called "skip" or "[[skywave]]" propagation, has been used since the 1920s to communicate at international or intercontinental distances. The returning radio waves can reflect off the Earth's surface into the sky again, allowing greater ranges to be achieved with multiple [[Hop (telecommunications)|hop]]s. This communication method is variable and unreliable, with reception over a given path depending on time of day or night, the seasons, weather, and the 11-year [[sunspot cycle]]. During the first half of the 20th century it was widely used for transoceanic telephone and telegraph service, and business and diplomatic communication. Due to its relative unreliability, shortwave radio communication has been mostly abandoned by the telecommunications industry, though it remains important for high-latitude communication where satellite-based radio communication is not possible. Shortwave broadcasting is useful in crossing international boundaries and covering large areas at low cost. Automated services still use [[shortwave radio]] frequencies, as do [[radio amateur]] hobbyists for private recreational contacts and to assist with emergency communications during natural disasters. Armed forces use shortwave so as to be independent of vulnerable infrastructure, including satellites, and the low latency of shortwave communications make it attractive to stock traders, where milliseconds count.<ref>{{cite journal | url=https://ieeexplore.ieee.org/document/9311865 | doi=10.1109/TWC.2020.3046475 | title=Receiver Designs for Low-Latency HF Communications | year=2021 | last1=Arikan | first1=Toros | last2=Singer | first2=Andrew C. | journal=IEEE Transactions on Wireless Communications | volume=20 | issue=5 | pages=3005–3015 | s2cid=233990323 }}</ref>

====Mechanism of refraction====
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.

A qualitative understanding of how an electromagnetic wave propagates through the ionosphere can be obtained by recalling [[geometric optics]]. Since the ionosphere is a plasma, it can be shown that the [[refractive index]] is less than unity. Hence, the electromagnetic "ray" is bent away from the normal rather than toward the normal as would be indicated when the refractive index is greater than unity. It can also be shown that the refractive index of a plasma, and hence the ionosphere, is frequency-dependent, see [[Dispersion (optics)]].<ref>{{cite book|last1=Lied|first1=Finn|title=High Frequency Radio Communications with Emphasis on Polar Problems|date=1967|publisher=Advisory Group for Aerospace Research and Development|pages=1–6}}</ref>

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 (optics)|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:

: <math>f_{\text{critical}} = 9 \times\sqrt{N}</math>

where N = electron density per m<sup>3</sup> and f<sub>critical</sub> is in Hz.

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.

: <math>f_\text{muf} = \frac{f_\text{critical}}{ \sin \alpha} </math>

where <math>\alpha</math> = [[angle of arrival]], the angle of the wave relative to the [[horizon]], and sin is the [[sine]] function.

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.

===GPS/GNSS ionospheric correction===
{{expand section|date=October 2013}}
{{see also|Total electron content}}
There are a number of models used to understand the effects of the ionosphere on global navigation satellite systems. The '''Klobuchar model''' is currently used to compensate for ionospheric effects in [[GPS]]. This model was developed at the '''US Air Force Geophysical Research Laboratory''' circa 1974 by '''John (Jack) Klobuchar'''.<ref>{{cite web|url=https://www.ion.org/awards/2003-ionfellow-Klobuchar.cfm|title=ION Fellow - Mr. John A. Klobuchar|website=www.ion.org|access-date=8 May 2018|url-status=live|archive-url=https://web.archive.org/web/20171004140058/https://www.ion.org/awards/2003-ionfellow-Klobuchar.cfm|archive-date=4 October 2017}}</ref>  The [[Galileo (satellite navigation)|Galileo]] navigation system uses the '''NeQuick model'''.<ref name="Galileo2016">{{cite web|title=Ionospheric Correction Algorithm for Galileo Single Frequency Users|url=https://www.gsc-europa.eu/system/files/galileo_documents/Galileo_Ionospheric_Model.pdf|publisher=Galileo Open Service|access-date=9 February 2018|url-status=live|archive-url=https://web.archive.org/web/20180210120957/https://www.gsc-europa.eu/system/files/galileo_documents/Galileo_Ionospheric_Model.pdf|archive-date=10 February 2018}}</ref> '''GALILEO''' broadcasts 3 coefficients to compute the effective ionization level, which is then used by the '''NeQuick''' model to compute a range delay along the line-of-sight.<ref>{{Cite web |title=Ionospheric Corrections for GNSS |url=https://www.icao.int/sam/documents/2008/ionosferaseminar/ionospheric%20corrections%20for%20gnss.pdf}}</ref>

===Other applications===
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]].

==Measurements==
[[File:469649main earths-atmosphere-orig full.webp|thumb|upright=1.6|Properties of Earth's Upper Atmosphere]]

===Overview===
Scientists explore the structure of the ionosphere by a wide variety of methods. They include:
* 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 Telescope|Arecibo]], Advanced Modular Incoherent Scatter Radar (AMISR) and [[Jicamarca Radio Observatory|Jicamarca]] radars
* coherent scatter radars such as the [[Super Dual Auroral Radar Network|Super Dual Auroral Radar Network (SuperDARN)]] radars
* special receivers to detect how the reflected waves have changed from the transmitted waves.

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.

The SuperDARN radar project researches the high- and mid-latitudes using coherent backscatter of radio waves in the 8 to 20&nbsp;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 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 Telescope]] located in [[Puerto Rico]], was originally intended to study Earth's ionosphere.

===Ionograms===
{{main|Ionogram}}
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&nbsp;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).

===Incoherent scatter radars===
[[Incoherent scatter]] radars operate above the critical frequencies. Therefore, the technique allows probing 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. [[Incoherent scatter]] radars can also measure neutral atmosphere movements, such as [[atmospheric tides]], after making assumptions about ion-neutral [[collision frequency]] across the [[ionospheric dynamo region]].<ref>{{cite journal |last1=Günzkofer|first1=F.|last2=Pokhotelov |first2=D.|last3=Stober|first3=G.|display-authors=etal |title= Determining the Origin of Tidal Oscillations in the Ionospheric Transition Region With EISCAT Radar and Global Simulation Data| journal=Journal of Geophysical Research: Space Physics|date=2022-09-25|volume=127|issue=10|doi=10.1029/2022JA030861}}</ref>

=== GNSS radio occultation ===
[[Radio occultation]] is a [[remote sensing]] technique where a [[GNSS]] signal tangentially scrapes the Earth, passing through the atmosphere, and is received by a Low Earth Orbit (LEO) satellite. As the signal passes through the atmosphere, it is refracted, curved and delayed. An LEO satellite samples the total electron content and bending angle of many such signal paths as it watches the GNSS satellite rise or set behind the Earth. Using an Inverse [[Abel transform|Abel's transform]], a [[radial profile]] of refractivity at that tangent point on earth can be reconstructed.

Major GNSS radio occultation missions include the [[Gravity Recovery and Climate Experiment|GRACE]], [[CHAMP (satellite)|CHAMP]], and [[Constellation Observing System for Meteorology, Ionosphere, and Climate|COSMIC]].

== Indices of the ionosphere ==
In empirical models of the ionosphere such as Nequick, the following indices are used as indirect indicators of the state of the ionosphere.

===Solar intensity===
F10.7 and R12 are two indices commonly used in ionospheric modelling. Both are valuable for their long historical records covering multiple solar cycles. F10.7 is a measurement of the intensity of solar radio emissions at a frequency of 2800&nbsp;MHz made using a ground [[radio telescope]]. R12 is a 12 months average of daily sunspot numbers. The two indices have been shown to be correlated with each other.

However, both indices are only indirect indicators of solar ultraviolet and X-ray emissions, which are 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.

=== Geomagnetic disturbances ===
* 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 semi-logarithmic scale from 0 to 9 to measure the strength of the horizontal component of the geomagnetic field. The Boulder ''K''-index is measured at the [[Boulder Geomagnetic Observatory]].
* 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 ''A''<sub>p</sub>-index, called the ''planetary A-index'' (PAI).

==Ionospheres of other planets and natural satellites==
Objects in the Solar System that have appreciable atmospheres (i.e., all of the major planets and many of the larger [[natural satellite]]s) generally produce ionospheres.<ref>{{cite web |url=https://www.physics.uu.se/research/astronomy-and-space-physics/research/planets/ionospheres/#:~:text=Since%20the%20main%20ionization%20source,Saturn's%20moons%20Titan%20and%20Enceladus. |title=Planetary ionospheres |website=Department of Physics and Astronomy. Uppsala University |access-date=2023-06-04}}</ref> Planets known to have ionospheres include [[Atmosphere of Venus#Upper atmosphere and ionosphere|Venus]], [[Mars]],<ref>{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Mars_Express/Mars_Express_First_global_map_of_martian_ionosphere |title=Mars Express: First global map of martian ionosphere |access-date=2015-10-31 |url-status=live |archive-url=https://web.archive.org/web/20150910095730/http://www.esa.int/Our_Activities/Space_Science/Mars_Express/Mars_Express_First_global_map_of_martian_ionosphere |archive-date=2015-09-10 }}</ref> [[Magnetosphere of Jupiter#Interaction with rings and moons|Jupiter]], [[Saturn]], [[Atmosphere of Uranus#Thermosphere and ionosphere|Uranus]], and [[Neptune]].

The [[atmosphere of Titan]] includes an ionosphere that ranges from about {{convert|880|to|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] {{webarchive|url=https://web.archive.org/web/20110511085002/http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=1498 |date=2011-05-11 }} Accessed 2010-08-25</ref> Ionospheres have also been observed at [[Io (moon)|Io]], [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], [[Triton (moon)|Triton]], and [[Pluto]].

==See also==

{{col-float}}
* [[Aeronomy]]
* [[Geospace]]
* [[Space physics]]
* '''[[Geophysics]]'''
** [[International Reference Ionosphere]]
** [[Ionospheric dynamo region]]
** [[Magnetospheric electric convection field]]
** [[Protonosphere]]
** [[Schumann resonances]]
** [[Van Allen radiation belt]]
* '''[[Radio]]'''
** [[Earth–ionosphere waveguide]]
** [[Fading]]
** [[Ionospheric absorption]]
** [[Ionospheric scintillation]]
** [[Line-of-sight propagation]]
** [[Sferics]]
{{col-float-break}}
* '''Related'''
** [[Canadian Geospace Monitoring]]
** [[High Frequency Active Auroral Research Program]]
** [[Ionospheric heater]]
** [[S4 Index]]
** [[Soft gamma repeater]]
** [[Sprite halo|Upper-atmospheric lightning]]
** [[Sura Ionospheric Heating Facility]]
** [[TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)]]

{{col-float-end}}

==Notes==
{{reflist}}

==References==
* {{cite book |last=Davies |first=Kenneth |title=Ionospheric Radio |isbn=978-0-86341-186-1 |series=IEE Electromagnetic Waves Series #31 |date=1990 |publisher=Peter Peregrinus Ltd/The Institution of Electrical Engineers |location=London, UK}}
* {{cite book |first=J. K. |last=Hargreaves |title=The Upper Atmosphere and Solar-Terrestrial Relations |publisher=Cambridge University Press |date=1992}}
* {{cite book |last=Kelley |first=M. C. |title=The Earth's Ionosphere: Plasma Physics and Electrodynamics |date=2009 |publisher=Academic Press |isbn=9780120884254 |edition=2nd}}
* {{cite book |first=Leo F. |last=McNamara |date=1994 |isbn=978-0-89464-804-5 |title=Radio Amateurs Guide to the Ionosphere}}
* {{cite book |first=K. |last=Rawer |title=Wave Propagation in the Ionosphere |publisher=Kluwer Academic Publ. |location=Dordrecht |date=1993 |isbn=978-0-7923-0775-4}}
* {{cite journal |url=http://iri.gsfc.nasa.gov/docs/iri_2000_rs.pdf |first=Dieter |last=Bilitza |title=International Reference Ionosphere 2000 |journal=Radio Science |volume=36 |issue=2 |pages=261–275 |date=2001|bibcode = 2001RaSc...36..261B |doi = 10.1029/2000RS002432 |hdl=2060/19910021307 |s2cid=116976314 |hdl-access=free }}
* J. Lilensten, 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}}.
* P.-L. Blelly, D. Alcaydé: ''Ionosphere'', in: Y. Kamide, A. Chian, ''Handbook of the Solar-Terrestrial Environment'', Springer-Verlag Berlin Heidelberg, pp.&nbsp;189–220, 2007. {{doi|10.1007/11367758_8}}
* {{cite book |first=H. |last=Volland |title=Atmospheric Electrodynamics |url=https://archive.org/details/trent_0116300004144 |url-access=registration |publisher=Springer Verlag |location=Berlin |date=1984}}
* {{Cite journal |last1=Schunk |first1=R. W. |last2=Nagy |first2=A. F. |title=Ionospheres: Physics, Plasma Physics, and Chemistry |journal=Eos Transactions |volume=82 |issue=46 |page=556 |date=2009 |isbn=9780521877060 |edition=2nd|bibcode=2001EOSTr..82..556K |doi=10.1029/01EO00328 |doi-access=free }}

==External links==
{{Commons category|Ionosphere}}
{{Wiktionary|ionosphere}}
* Gehred, Paul, and Norm Cohen, ''[https://web.archive.org/web/20031203000823/http://sec.noaa.gov/radio/ SWPC's Radio User's Page]''.
*[https://web.archive.org/web/20080719143702/http://www.esa-spaceweather.net/sda/ionosfera/ Amsat-Italia project on Ionospheric propagation (ESA SWENET website)]
*[http://www.wcflunatall.com/nz4o1.htm NZ4O Solar Space Weather & Geomagnetic Data Archive]
*[https://web.archive.org/web/20090923133049/http://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
*[http://geomag.usgs.gov USGS Geomagnetism Program]
*[https://www.britannica.com/eb/article-9042708/ionosphere-and-magnetosphere Encyclopædia Britannica, Ionosphere and magnetosphere]
*[https://web.archive.org/web/20051219164541/http://www.sec.noaa.gov/SWN/ Current Space Weather Conditions]
*[https://web.archive.org/web/20050405102819/http://www.sec.noaa.gov/rt_plots/xray_1m.html Current Solar X-Ray Flux]
*[http://superdarn.jhuapl.edu/ Super Dual Auroral Radar Network]
*[http://www.eiscat.se/ European Incoherent Scatter radar system]

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