Ionosphere - Wikiparency

# Ionosphere The **ionosphere** is the region of [[Earth|Earth's]] upper atmosphere, from roughly 50 km to several hundred kilometres above the ground, in which solar radiation has stripped enough electrons from atoms and molecules to leave the gas partly ionized and electrically conductive. It owes its practical importance chiefly to [[Radio_propagation|radio propagation]]: at high frequencies it refracts a wave back down to Earth as [[Skywave|skywave]], while at the microwave frequencies satellite navigation systems use it merely slows and bends the signal slightly as the wave passes through. A three.js sketch elsewhere on this site, a day/night variant of the shared radio-propagation scene, renders the ionosphere's layered density profile in three dimensions. The ionization is not spread evenly with height but gathered into layers, each the result of a different part of the solar spectrum striking a different altitude, and each behaving differently through the day, the season, and the roughly eleven-year solar cycle; the Layers of ionization section below works through each in turn. Left alone, the layers would settle into a tidy, predictable pattern that depends only on the Sun's angle in the sky, but the real ionosphere persistently departs from that idealized picture and, on top of the persistent departures, is periodically struck by solar events that disturb it further still, the subjects of the two sections after that. ## History of discovery [[Guglielmo_Marconi|Guglielmo Marconi]]'s demonstration in December 1901, in which a signal sent from Cornwall was received across the Atlantic in Newfoundland, was an early puzzle in the [[History_of_radio|history of radio]]: something well above the ground was evidently returning the wave to Earth, since no straight-line or ground-hugging path could explain a signal crossing an ocean at the frequency in use.[^marconi2] The following year, Arthur Kennelly and Oliver Heaviside independently proposed a conducting layer high in the atmosphere as the mechanism, without yet being able to measure it directly.[^kh2] Direct proof came from Edward Appleton, who in the 1920s transmitted on a swept frequency and measured the interference pattern between the ground wave and the returning sky wave to calculate the reflecting layer's height; his technique went further than confirming a single layer, distinguishing a lower reflecting region from a higher, more strongly reflecting one that came to be called the Appleton layer in his honour, work recognised with the Nobel Prize in Physics in 1947.[^appleton2] The name ionosphere itself, for the ionized region as a whole rather than for any one layer within it, is credited to the physicist Robert Watson-Watt, writing in the following decade once it was clear that more than one distinct region was involved.[^ww-name] ## Geophysics Ionization begins with sunlight: extreme-ultraviolet and X-ray photons carry enough energy to knock an electron free from an [[Atom|atom]] or molecule of atmospheric gas, leaving behind a free [[Electron|electron]] and a positive [[Ion|ion]]. Lower down, the same solar ultraviolet is instead absorbed by the [[Ozone_layer|ozone layer]] in the [[Stratosphere|stratosphere]] without producing a persistently ionized, conductive gas; only well above the stratosphere is the atmosphere thin enough, and the radiation energetic enough, for the freed electrons to remain separated from their ions for any length of time. Production is fastest where sunlight is most direct and the gas is still dense enough to supply plenty of targets, which is why ionization first increases with depth into the [[Atmosphere_of_Earth|atmosphere]] before falling again as the neutral gas itself thins out at greater height, producing a layered rather than uniform structure; a simple theoretical model built on exactly this balance between production and the neutral density profile, developed by the mathematician Sydney Chapman in 1931, reproduces the basic shape of a single ionospheric layer surprisingly well.[^chapman] Loss works the other way: a free electron and ion eventually recombine, and recombination happens faster where the gas is denser, so the lower layers lose their ionization within minutes of losing the Sun while the thinner upper layers can stay measurably ionized long after dark. The whole ionized region sits inside a much larger [[Plasma_(physics)|plasma]] environment shaped by Earth's own magnetic field, which the electrojet and storm effects described further below both depend on. ## Layers of ionization Four regions are conventionally distinguished by height and by the mechanism that ionizes them, labelled with letters assigned in the order they were discovered rather than the order they sit in the sky. ### D layer The D layer, roughly 60 to 90 km up, is ionized by hard X-rays and by hydrogen's Lyman-alpha line acting mainly on nitric oxide, a minor but easily ionized constituent of the otherwise dense lower atmosphere. That same density means collisions between free electrons and neutral molecules are frequent, and a radio wave passing through loses energy to those collisions rather than being reflected, so the D layer is chiefly an absorber, not a reflector; it is also the fastest layer to vanish once the Sun sets, since dense air recombines quickly, which is why lower shortwave frequencies that are absorbed by day become usable for [[Skywave|skywave]] again at night. ### E layer The E layer, around 90 to 150 km, forms from softer X-rays and far-ultraviolet radiation ionizing molecular oxygen and nitrogen, and persists, weakened, through the night. It can reflect lower [[Radio_wave|radio waves]] back to Earth for relatively short hops, but higher frequencies pass through it to whatever lies above, so it plays a supporting role to the F layer in most long-distance shortwave work. ### Es layer Sporadic E is an occasional, patchy intensification of ionization within the E-layer height range, thin and dense enough to reflect frequencies well into the very-high-frequency band that the regular layers normally let through untouched. Wind shear and small-scale [[Turbulence|turbulence]] concentrating metallic ions left behind by meteors is the leading explanation for these patches, which appear unpredictably and mostly in summer, and are the reason a distant television or FM station is occasionally heard where the ordinary ionosphere offers no such path at all. ### F layer The F layer, from roughly 150 km up past 500 km, is ionized mainly by extreme-ultraviolet radiation acting on atomic [[Oxygen|oxygen]] and is, for most of the shortwave spectrum, the layer that actually returns a long-distance skywave signal to the ground. By day it commonly splits into a lower F1 and a higher, more strongly ionized F2 region, which recombine into a single layer once the Sun's direct ionizing input stops at night; because F2 sits highest and stays ionized longest, it supports the greatest hop distances and the widest range of usable frequencies, and it is also the most variable of the four layers, the one most affected by the anomalies and disturbances described in the next two sections. A peak daytime F2 electron density of roughly a million electrons per cubic centimetre corresponds to a critical frequency, the highest frequency a straight-up signal can still be turned back at, of about `f_p ≈ 8980·√(Ne[cm⁻³])` ≈ 9 megahertz; a wave sent obliquely rather than straight up can be reflected at a correspondingly higher frequency still, the relationship skywave's low-angle hops rely on. ## Ionospheric model Put together, the layers' idealized behaviour, density rising and falling smoothly with height in a shape close to Chapman's theoretical profile and tracking the Sun's angle through the day and the year, is well enough understood to be captured in standard empirical models, such as the widely used International Reference Ionosphere, that predict typical layer heights, densities, and critical frequencies for a given place, time, and level of solar activity.[^iri] These models describe the average ionosphere a forecaster expects; the sections that follow describe the ways the actual ionosphere persistently, and at times suddenly, departs from that average. ## Persistent anomalies to the idealized model Three well-documented patterns depart from the simple, sun-angle-only picture the idealized model predicts. ### Winter anomaly At many mid-latitude locations the F2 layer is, counter-intuitively, more strongly ionized around local noon in winter than in summer, even though the Sun sits lower in the winter sky and delivers less total energy; a seasonal shift in the upper atmosphere's own composition, which changes how quickly ionization recombines rather than how much of it is produced, is the generally accepted explanation for this winter anomaly. ### Equatorial anomaly Rather than peaking directly above the magnetic equator, where the overhead Sun might be expected to ionize the atmosphere most strongly, F-region density instead peaks in two bands roughly fifteen to twenty degrees of magnetic latitude to either side of it, with a relative trough at the equator itself. An electric field generated in the equatorial E region lifts equatorial plasma upward, and that plasma then diffuses back down along the tilted geomagnetic field lines toward both hemispheres at once, piling up well away from the equator in what is often called a fountain effect. ### Equatorial electrojet Along the magnetic equator, in the daytime E region, winds driven by the atmosphere's own solar and lunar tides push conducting, ionized air across the local magnetic field, generating a strong electric field that in turn drives an intense, narrow band of eastward [[Electric_current|current]] confined within a few degrees of the magnetic equator. This equatorial electrojet is strong enough to be detected in ground-based magnetic measurements far from any other ionospheric instrument, and it is closely tied to the same [[Tide|tidal]] driving and [[Magnetohydrodynamics|magnetohydrodynamic]] coupling between the neutral atmosphere and the ionized gas that shapes the equatorial anomaly above it, a coupling more generally studied under [[Geophysical_fluid_dynamics|geophysical fluid dynamics]]. ## Ephemeral ionospheric perturbations Beyond the persistent anomalies above, the ionosphere is also subject to sudden, short-lived disturbances triggered by specific solar or terrestrial events. ### X-rays: sudden ionospheric disturbances (SID) A solar flare's burst of X-rays reaches Earth at the speed of light and, within minutes, sharply intensifies ionization in the D layer across the entire sunlit hemisphere, deepening the absorption that layer already imposes on lower-frequency radio waves enough to black out shortwave communication on the daylight side of the planet for anywhere from minutes to a few hours. ### Protons: polar cap absorption (PCA) A burst of high-energy protons from the Sun, arriving over the following hours rather than minutes, is guided by Earth's magnetic field down into the polar caps, where it can enhance D-layer-like absorption over the poles for several days at a time, a polar cap absorption event that disrupts high-latitude and transpolar radio paths in particular. ### Storms A geomagnetic storm, usually triggered by a coronal mass ejection reaching Earth's magnetic field, disturbs the F layer over a much larger part of the planet than a solar flare does and for a longer time, sometimes intensifying ionization and sometimes, in a so-called negative storm phase, depleting it well below normal, so that shortwave propagation conditions during a storm can turn substantially better or considerably worse depending on the region, the season, and the storm's own phase. ### Lightning Even ordinary lightning can perturb the lower ionosphere locally: the electromagnetic pulse from a powerful strike can measurably enhance D-layer ionization directly above the storm for a brief interval, and rare, brief optical discharges above active thunderstorms, reaching up into the mesosphere, show that the coupling between tropospheric weather and the base of the ionosphere runs both ways. ## Applications Two quite different radio applications depend on the ionosphere, one because it reflects a signal and one despite the fact that it does not. ### Radio communication Long-distance [[Shortwave_radio|shortwave]] communication, unlike [[Ground_wave|ground wave]] or [[Line-of-sight_propagation|line-of-sight]] transmission, depends entirely on the reflection that skywave propagation performs, and everything about which frequency to use, from the maximum usable frequency down to which hours of the day work at all, follows directly from the layer heights and densities described above. #### Mechanism of refraction A layer reflects a wave rather than merely bending it because the layer's refractive index depends on how the wave's frequency compares with the local plasma frequency, itself set by the electron density at that height: below the plasma frequency a wave cannot propagate through the plasma at all and is progressively bent back the way it came as it climbs into ever denser ionization, while above the plasma frequency the wave passes through with only a small effect on its speed. A wave sent straight up is reflected only if its frequency is below the layer's peak plasma frequency; a wave sent obliquely needs only its vertical frequency component to satisfy that condition, which is the geometrical reason a low-angle path can carry a higher frequency than a near-vertical one can, as described in the skywave article above. ### GPS/GNSS ionospheric correction Satellite navigation signals sit at gigahertz frequencies, far above any plasma frequency the ionosphere reaches even during the most intense storms, so they are never reflected; the ionosphere instead very slightly slows the signal's information-carrying envelope and advances its carrier phase, by an amount proportional to the total number of electrons the signal has crossed along its path and inversely proportional to the square of its frequency. Because that delay depends on frequency in a known way, a receiver that tracks two different frequencies from the same satellite can compare their delays and solve for, then remove, almost all of the ionospheric error directly; a receiver with only one frequency instead applies a broadcast correction model, less precise but adequate for many uses. ### Other applications Beyond communication and navigation, the ionosphere is itself an object of study: its density, structure, and disturbances are tracked as a form of space-weather monitoring relevant to satellite operations and to the high-frequency radio services that depend on it, and its total electron content is used as a diagnostic of solar and geomagnetic activity in its own right. ## Measurements ### Overview Because the ionosphere cannot be sampled everywhere at once, its state is inferred from a handful of complementary techniques, each sensitive to a different property of the ionized gas and each with its own blind spots. ### Ionograms An ionosonde sends a swept-frequency pulse straight upward and times the echo, the same technique Appleton used by hand, now automated: as the frequency rises, higher densities are needed to turn the pulse back, so the echo delay traces out how electron density builds with height until, at the layer's own critical frequency, no further echo returns and the wave escapes into space. The resulting plot, an ionogram, is the standard everyday record of layer heights and critical frequencies at a given station. ### Incoherent scatter radars A far more powerful, and far rarer, instrument fires a high-power radar signal upward and detects the extremely faint energy scattered back by individual thermal fluctuations in the ionospheric plasma itself, rather than by any organized reflecting layer. The strength of that faint return gives electron density directly, and the [[Doppler_effect|Doppler]] shift and spectral shape of the scattered signal give the plasma's temperature and its drift velocity along the radar beam, extracting far more information than an ionogram can, at the cost of a large, expensive, and geographically sparse installation. ### GNSS radio occultation A satellite receiver that watches a GNSS satellite set behind the Earth's limb, from the vantage point of a second satellite in low orbit, sees the signal's path bend and its delay change continuously as it grazes progressively deeper into the ionosphere and then the neutral atmosphere below it; inverting that bending and delay against altitude reconstructs an electron-density profile along the whole limb, letting a single low-cost receiver constellation map the ionosphere globally rather than only above fixed ground stations. ## Indices of the ionosphere ### Solar intensity Because ionization is driven by solar extreme-ultraviolet and X-ray output rather than by visible sunlight, forecasters track the [[Sun|Sun's]] activity through [[Spectroscopy|spectroscopic]] proxies at other wavelengths, chiefly a standard index of solar radio emission that rises and falls with the same active regions responsible for the ionizing radiation itself, and use it to anticipate whether the ionosphere on a given day will run above or below its seasonal average. ### Geomagnetic disturbances A complementary family of indices, built from ground-based magnetometer networks around the world, tracks how disturbed Earth's magnetic field is at a given moment rather than how bright the Sun is; because the same solar disturbances that shake the magnetic field also drive the storm effects described above, these geomagnetic indices are the standard shorthand for how rough ionospheric conditions, and so shortwave propagation, are likely to be on a given day. ## Ionospheres of other planets and natural satellites Any body with an atmosphere and enough exposure to solar ultraviolet or X-ray radiation, or to a stream of charged particles, can develop its own ionosphere, and several besides Earth's have been measured directly. [[Mars|Mars's]] thin carbon dioxide atmosphere supports a measurable, if much less dense, ionized layer studied by instruments on orbiting [[NASA|NASA]] spacecraft; solar-system bodies with thicker atmospheres, such as Saturn's moon Titan, and even active comets venting gas as they near the Sun, have shown ionospheres of their own, while an airless body such as the Moon has, at most, an extremely tenuous one sustained by other mechanisms entirely. ## Microsims This article carries no p5.js sketch of its own. The three.js companion named in the lead, a day/night variant of the shared radio-propagation scene, is the site's interactive rendering of the layered structure described above. Two neighbouring sketches touch on techniques described in Measurements. The Doppler effect sketch's bunching and stretching of wavefronts ahead of and behind a moving source is the same geometric effect an incoherent scatter radar reads off a scattered signal's spectrum to measure how fast ionospheric plasma is drifting along the beam. The Radar sketch's own timing logic, in which an echo's delay converts directly to a target's range, is the same arithmetic an ionosonde performs on a much longer timescale and at a swept frequency, converting each echo's delay into the height at which that frequency was finally turned back. *Try:* in the [[Doppler_effect]] sketch, raise the source speed and watch the received frequency ahead of it rise and behind it fall, the same shift an incoherent-scatter radar reads as the plasma's drift velocity along its beam. *Try:* in the [[Radar]] sketch, note how directly the echo delay sets the measured range; an ionosonde performs the same conversion between a radio echo's delay and a reflecting layer's height, frequency by frequency, to build the profile this article's layers describe. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Ionosphere) : [Wikitube](https://en.wikitube.io/wiki/Ionosphere) Skeleton mirrored at revision 1375001314. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Radio_propagation]] - [[Skywave]] - [[Ground_wave]] - [[Line-of-sight_propagation]] - [[Radio_wave]] - [[Mars]] ## Notes Page numbers in the References below are PDF pages of the open editions cited elsewhere in this run; none of this article's own claims draws on those pages, since its footnotes are dated, named claims about the discovery of the ionosphere for which the underlying primary sources have not yet been pinned to a page in this pass. The idealized layered model described in Ionospheric model is itself a simplification of what is, in reality, a continuously varying electron-density profile; the D/E/F labels mark historically convenient regions rather than sharply bounded layers. ## References The plasma-refraction physics connecting frequency, electron density, and reflection or delay, including the plasma-frequency formula and the Chapman production-loss balance, is standard textbook geophysics and is not separately footnoted here, per the Wikitube style guide's §6.1. [^marconi2]: Citation needed: a primary account of Guglielmo Marconi's December 1901 Poldhu-to-Newfoundland transmission would confirm the date and distance given here. [^kh2]: Citation needed: the original 1902 notices by Arthur Kennelly and Oliver Heaviside proposing a reflecting atmospheric layer would fix the dates and venues of each independent proposal. [^appleton2]: Citation needed: Edward Appleton's 1920s papers distinguishing the reflecting layers, the naming of the Appleton layer, and the Nobel Foundation's 1947 physics prize citation would confirm the details summarised here. [^ww-name]: Citation needed: the specific publication in which Robert Watson-Watt is credited with coining the term "ionosphere" would confirm the date given here. [^chapman]: Citation needed: Sydney Chapman's 1931 papers on the theoretical formation of an ionized layer would confirm the date and the details of the production-loss balance described here. [^iri]: Citation needed: the International Reference Ionosphere project's maintaining organizations and founding date would need a citation to a specific technical reference. ## External links This article carries no p5.js simulation of its own. The three.js companion named in the lead is a preset of a scene shared with neighbouring articles rather than a separate resource linked here.