The ionosphere is the charged part of Earth's upper atmosphere, located between about 48 km (30 mi) and 965 km (600 mi) above sea level. This region includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere becomes charged due to solar radiation. It is important for atmospheric electricity and is the inner part of the magnetosphere. It has real-world importance because it helps radio waves travel long distances across Earth. GPS signals are also affected as they pass through this layer, causing their paths to change and their arrival to be delayed.

History of discovery

In 1839, the German mathematician and physicist Carl Friedrich Gauss suggested that an electrically conducting part of the atmosphere might explain changes in Earth's magnetic field. Sixty years later, on December 12, 1901, Guglielmo Marconi received the first trans-Atlantic radio signal in St. John's, Newfoundland (now in Canada) using a 152.4 m (500 ft) kite-supported antenna. The signal was sent from a station in Poldhu, Cornwall, which used a spark-gap transmitter to create a signal with a frequency of about 500 kHz and a power 100 times greater than any previous radio signal. The message received was three dits, the Morse code for the letter "S." For the signal to reach Newfoundland, it had to reflect off the ionosphere twice. However, Dr. Jack Belrose later questioned this, based on his research. Marconi did successfully send transatlantic wireless communications in Glace Bay, Nova Scotia, one year later.

In 1902, Oliver Heaviside proposed the existence of the Kennelly–Heaviside layer of the ionosphere, which is named after him. He also described how radio signals could travel around Earth's curvature. Around the same time, Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties.

In 1912, the U.S. Congress passed the Radio Act of 1912, which limited amateur radio operators to frequencies above 1.5 MHz (wavelengths shorter than 200 meters). The government believed these frequencies were not useful. This restriction led to the discovery of HF radio propagation through the ionosphere in 1923.

In 1925, Dr. Alfred N. Goldsmith and his team observed a solar eclipse in New York. Their findings showed that sunlight affects radio wave propagation. During the eclipse, short waves became weak or inaudible, while long waves remained strong, helping scientists understand the ionosphere's role in radio transmission.

In 1926, Scottish physicist Robert Watson-Watt introduced the term "ionosphere" in a letter published in 1969 in the journal Nature.

In the early 1930s, test transmissions from Radio Luxembourg accidentally provided evidence of the first radio modification of the ionosphere. In 2017, HAARP conducted experiments using the Luxembourg Effect.

Edward V. Appleton was awarded a Nobel Prize in 1947 for proving the ionosphere's existence in 1927. Lloyd Berkner first measured the ionosphere's height and density, enabling the first complete theory of short-wave radio propagation. Maurice V. Wilkes and J. A. Ratcliffe studied how very long radio waves travel through the ionosphere. Vitaly Ginzburg developed a theory of how electromagnetic waves move through plasmas like the ionosphere.

In 1962, the Canadian satellite Alouette 1 was launched to study the ionosphere. Later, Alouette 2 (1965) and the ISIS satellites (1969 and 1971) were launched, followed by AEROS-A and -B (1972 and 1975), all to measure the ionosphere.

On July 26, 1963, the first operational geosynchronous satellite, Syncom 2, was launched. Radio beacons on Syncom 2 and later satellites allowed scientists to measure total electron content (TEC) along a radio beam from geostationary orbit to Earth. Australian geophysicist Elizabeth Essex-Cohen used this method from 1969 onward to monitor the atmosphere above Australia and Antarctica.

Geophysics

The ionosphere is a layer of electrons and electrically charged atoms and molecules that surrounds Earth, extending from about 50 km (30 mi) to more than 1,000 km (600 mi). It forms mainly because of ultraviolet radiation from the Sun.

Heaviside and Kennelly proposed that the Earth’s conductive surface and an upper atmospheric layer could act as a waveguide for long-range radio waves. Heaviside used his Telegrapher’s equations and the conductivity of seawater to suggest that a conductive layer in the upper air might guide radio waves. Kennelly calculated that air at 80 km had 20 times more conductivity than seawater, allowing radio signals to travel as cylindrical waves, which could be detected over long distances. In 1919, G. N. Watson used mathematical models of Earth and a reflective atmospheric layer to reproduce long-distance radio experiments. In 1924, Joseph Larmor developed a theory explaining how ionized air bends the direction of radio waves.

Between 1923 and 1925, Appleton, Miles Barnett, Reginald Smith-Rose, and R.H. Barfield tested radio signals to study skywaves. They identified an ionized layer in the upper atmosphere, called the "E layer," located between 80 and 90 km. Appleton and Barnett adjusted the frequency of transmitted waves and measured interference from ground waves. They found that skywaves were weaker during the day when solar radiation made the ionized layer thicker and closer to Earth. Skywaves traveled farther than ground waves and did not weaken as quickly. Smith-Rose and Barfield studied the polarization, angles, and height of skywave deflections.

In 1925, Gregory Breit and Merle Tuve used pulse-echo techniques to measure the ionosphere’s height. Their results, ranging from 88 km to 211 km, showed the ionosphere was not a flat reflective surface but behaved as Larmor’s theory predicted. In 1927, Appleton’s experiments identified a higher layer called the "F layer," which explained that Breit and Tuve’s measurements corresponded to the bottom of the E layer and the top of the F layer. The E layer had fewer electrons than the F layer, so it refracted longer radio waves more. Appleton noted that solar radiation created more ions around noon, lowering the F layer, while lower ion production at midnight raised it higher.

The lowest part of Earth’s atmosphere, the troposphere, is where temperature decreases by 10 degrees K per km. It extends from Earth’s surface to about 10 km (6 mi) to 12 km (7 mi) at the tropopause. Above the troposphere is the stratosphere, where temperature increases with height until reaching a maximum at 50 km due to ozone absorbing ultraviolet radiation (stratopause). In the mesosphere, temperature decreases until reaching a minimum of about 180 K at 80 km (50 mi) to 85 km (50 mi) (mesopause). In the thermosphere, temperature rises to about 1000 K. At 100 km is the turbopause, above which gases separate into the heliosphere and protonosphere. Above 600 km, in the exosphere, atoms escape Earth’s gravity. The ionosphere is the ionized part of the atmosphere, important for radio signals. The magnetosphere is the region where Earth’s magnetic field controls particle movement.

Molecular and atomic oxygen are equally present at 125 km. Above this height, atomic oxygen increases due to molecular oxygen breaking apart from ultraviolet radiation (102.7 nm to 175.9 nm). Ionization of nitrogen (N₂), oxygen (O₂), and oxygen (O) in the ionosphere occurs from extreme ultraviolet (17 nm to 175 nm) and X-ray (0.1 to 17 nm) solar radiation. Radiative and dissociative recombination occur when electrons absorb energy. Dissociative recombination causes electrons and ions to lose energy faster.

According to Hunsucker and Hargreaves, the height of the E, F1, and F2 layers depends on the number of sunspots (R). Solar activity, measured by sunspot numbers, flare frequency, or 10-cm radio flux, affects the ionosphere because of changes in X-ray and EUV radiation intensity. The F2 layer also shows changes due to daily and seasonal variations and does not disappear at night in mid-latitudes.

Sydney Chapman suggested the region below the ionosphere be called the neutrosphere (neutral atmosphere).

Frequencies between 30 MHz to 10 GHz use line-of-sight propagation, though ionoscatter (30–150 MHz), troposcatter (200 MHz–19 GHz), and meteor scatter (40–150 MHz) can also be used. Frequencies between 2–30 MHz travel via skywaves day or night. Frequencies between 300 kHz to 3 MHz travel as groundwaves day or night and as skywaves at night.

Layers of ionization

At night, only the F layer has significant ionization, while the E and D layers have very little ionization. During the day, the D and E layers become much more ionized, and the F layer forms a weaker region called the F1 layer. The F2 layer exists both day and night and is the main area that bends and reflects radio waves.

Molecular ions recombine much faster than atomic ions, which reduces their numbers at night. When meteor particles deposit iron (Fe) and magnesium (Mg) in the atmosphere, they create layers of increased plasma density at low altitudes. Above 100 km, ion and electron densities increase because there is less mixing in the air and slower recombination.

The D layer is the innermost layer, located 60 to 90 km above Earth’s surface. It has an electron density of 10 to 10 per cm³. Ionization here is caused by hydrogen radiation at 121.6 nm, which ionizes nitric oxide (NO). Solar flares can also produce hard X-rays (wavelengths less than 1 nm) that ionize nitrogen (N₂) and oxygen (O₂).

Low-frequency radio waves lose strength quickly in the D layer because free electrons absorb their energy. This stops low-frequency signals from reaching the upper atmosphere, except at night when the D layer weakens. Signals below 50 MHz are less effective during periods of high solar activity because increased ultraviolet radiation raises ionization levels.

During solar proton events, the D layer can become extremely ionized over high and polar latitudes. These rare events, called Polar Cap Absorption (PCA) events, increase radio signal absorption in the region. They can block radio signals over large areas and may last for several days.

The E and F layers have lower air density, which means fewer electron collisions. As radio signals travel upward, they encounter higher electron density, which bends the signals back toward Earth, reflecting them. The amount of bending depends on the signal’s angle and frequency, with higher frequencies bending less. High-frequency signals may pass through the D layer and reflect off the E or F layers, but very high frequencies may escape into space.

The E layer is located 105 to 160 km above Earth’s surface and has an electron density several times 10 per cm³. It is ionized by ultraviolet radiation (80 to 103 nm) from the sun and X-rays from solar flares. Ionization is lowest just before sunrise and peaks at noon.

This layer is also called the Kennelly–Heaviside layer. Its existence was predicted in 1902 by Arthur Kennelly and Oliver Heaviside. In 1924, Edward Appleton and Miles Barnett confirmed its presence.

The E s layer (sporadic E layer) has higher-than-normal electron density and can reflect radio waves up to 100 MHz. These layers can last from a few minutes to many hours and may be 2 km thick and hundreds of kilometers long. They are common in equatorial regions during the day and more frequent in mid-latitudes during summer. They are often linked to metallic ions from meteor debris.

Sporadic E layers allow radio amateurs using VHF frequencies to communicate over long distances. Signals can skip about 1,640 km (1,020 mi) in one hop, with distances ranging from 900 to 2,500 km (560 to 1,550 mi). Multi-hop signals can travel over 3,500 km (2,200 mi), sometimes reaching 15,000 km (9,300 mi) or more.

The F layer includes the F1 region, from 160 to 180 km, with an electron density several times 10 to 10 per cm³. It is ionized by extreme ultraviolet radiation (20–900 nm) that affects atomic oxygen. The F1 layer merges with the F2 layer at night or during winter. The F2 layer extends from 200 km to over 800 km. The highest electron density occurs at 300 km during the day. Above 700 km, ionized hydrogen becomes more common than ionized oxygen.

From 1972 to 1975, NASA launched the AEROS and AEROS B satellites to study the F region.

Ionospheric model

An ionospheric model is a way to describe the ionosphere using math, based on factors like where it is, how high up it is, the time of year, the sunspot cycle, and how active the Earth's magnetic field is. In terms of Earth's physical processes, the state of ionospheric plasma can be described by four things: the number of electrons, the temperature of electrons and ions, and the types of ions present. How radio waves travel depends mostly on the number of electrons.

These models are often written as computer programs. A model may use basic physics to explain how ions and electrons interact with the atmosphere and sunlight, or it may use a statistical description based on many observations, or a mix of both. One of the most commonly used models is the International Reference Ionosphere (IRI), which uses data to describe the four parameters mentioned earlier. The IRI is an international project supported by the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). The main sources of data include a global network of ionosondes, powerful incoherent scatter radars (such as Jicamarca, Arecibo, Millstone Hill, Malvern, and St Santin), the ISIS and Alouette topside sounders, and instruments on satellites and rockets. The IRI is updated every year. It is more accurate at showing how electron density changes from the bottom of the ionosphere to the height where density is highest than it is at showing the total electron content (TEC). Since 1999, this model has been recognized as an international standard for the Earth's ionosphere (standard TS16457).

Persistent anomalies to the idealized model

Ionograms help scientists calculate the actual shape of different ion layers in the atmosphere. Uneven structures in the electron and ion plasma create rough echo patterns, which are most visible at night, near the poles, and during times of atmospheric disturbance.

At mid-latitudes, the F2 layer produces more ions during the day in summer because sunlight hits Earth more directly. However, changes in the balance of molecules and atoms in the neutral atmosphere cause ions to be lost more quickly during summer. This means that the increased loss of ions during summer outweighs the increased production, leading to lower total ionization in the F2 layer during summer months. This effect is called the winter anomaly. It occurs consistently in the northern hemisphere but is usually not observed in the southern hemisphere during periods of low solar activity.

Near the magnetic equator, within about ±20 degrees, a phenomenon called the equatorial anomaly occurs. This is marked by a dip in ionization in the F2 layer at the equator and higher ionization at about 17 degrees in magnetic latitude. The Earth's magnetic field lines are horizontal at the magnetic equator. Solar heating and wave-like movements in the lower ionosphere push plasma upward and across magnetic field lines. This movement creates an electric current in the E region of the atmosphere. Combined with the horizontal magnetic field, this current pushes ionization upward into the F layer, concentrating it at ±20 degrees from the magnetic equator. This process is called the equatorial fountain.

Global winds driven by the Sun create the Sq (solar quiet) current system in the E region of Earth's ionosphere (ionospheric dynamo region) at altitudes between 100–130 km (60–80 mi). This current system generates an electric field that flows west to east (dawn to dusk) on the day side of the ionosphere near the magnetic dip equator. At the magnetic dip equator, where the Earth's magnetic field is horizontal, this electric field causes a stronger eastward current flow within ±3 degrees of the magnetic equator. This current is known as the equatorial electrojet.

Ephemeral ionospheric perturbations

When the Sun is active, strong solar flares can occur. These flares send hard X-rays toward the sunlit side of Earth. The X-rays enter the D-region of the atmosphere, releasing electrons that increase absorption. This causes a radio blackout at high frequencies (3–30 MHz), which may last for many hours after a strong flare. During this time, very low frequency (3–30 kHz) signals are reflected by the D-layer instead of the E-layer. The E-layer usually absorbs these signals more strongly, reducing their strength. When the X-rays stop, the sudden ionospheric disturbance (SID) or radio blackout gradually decreases as electrons in the D-region recombine quickly. Radio signal conditions return to normal over minutes to hours, depending on the strength and frequency of the solar flare.

Solar flares also release high-energy protons. These protons can reach Earth within 15 minutes to 2 hours. The protons follow Earth’s magnetic field lines and enter the atmosphere near the magnetic poles, increasing ionization in the D and E layers. Proton-caused auroras (PCA) typically last about 1 to several days, averaging 24 to 36 hours. Coronal mass ejections can also release protons that increase absorption in the D-region near the poles.

Geomagnetic storms and ionospheric storms are short, intense changes in Earth’s magnetosphere and ionosphere. During a geomagnetic storm, the F₂ layer becomes unstable, breaks apart, or may disappear. In Earth’s polar regions, auroras can be seen in the night sky.

Lightning can cause changes in the D-region of the ionosphere in two ways. First, very low frequency (VLF) radio waves from lightning enter the magnetosphere. These "whistler" mode waves interact with particles in the radiation belts, causing them to fall into the ionosphere and increase ionization in the D-region. These events are called "lightning-induced electron precipitation" (LEP).

Ionization can also occur directly from the movement of electric charge in lightning strikes. These events are called "early/fast."

In 1925, C. T. R. Wilson proposed how electrical discharge from lightning storms could travel upward from clouds to the ionosphere. Around the same time, Robert Watson-Watt, working at the Radio Research Station in Slough, UK, suggested that lightning might increase the ionospheric sporadic E layer (E s ), but more research was needed. In 2005, C. Davis and C. Johnson, working at the Rutherford Appleton Laboratory in Oxfordshire, UK, showed that lightning activity does enhance the E s layer. Their later research focused on how this process occurs.

Applications

Ionized gases in the atmosphere can bend high-frequency (HF or shortwave) radio waves. This allows the ionosphere to reflect radio waves sent into the sky back toward Earth. When radio waves are directed at an angle into the sky, they can travel beyond the horizon and return to Earth. This method, called "skip" or "skywave" propagation, has been used since the 1920s to send messages across long distances. Radio waves that return to Earth can reflect off the surface again, allowing signals to travel even farther with multiple reflections. This communication method is not always reliable, as reception depends on factors like time of day, season, weather, and the 11-year sunspot cycle. In the first half of the 20th century, it was widely used for long-distance telephone and telegraph services, as well as for business and diplomatic communication. Due to its unpredictability, shortwave radio is no longer commonly used by modern telecommunications companies, though it remains important in high-latitude regions where satellite communication may be limited. Shortwave broadcasting is effective for reaching large areas across international borders at low cost. Automated services and radio hobbyists still use shortwave frequencies for private communication and emergency support during natural disasters. Military forces also use shortwave to avoid reliance on vulnerable infrastructure like satellites, and its quick transmission time makes it useful for stock traders.

When a radio wave reaches the ionosphere, the electric field in the wave causes electrons in the ionosphere to vibrate at the same frequency as the wave. Some of the wave’s energy is absorbed by these vibrating electrons. The electrons then either combine with other particles or re-emit the wave’s energy. If the ionosphere’s electrons move slowly compared to the radio wave’s frequency and if the electron density is high enough, the wave can be completely bent.

To understand how radio waves travel through the ionosphere, imagine how light bends through different materials. The ionosphere is a type of plasma, and its ability to bend waves depends on the wave’s frequency. Because the ionosphere’s refractive index is less than one, radio waves bend away from the normal direction instead of toward it. This bending also depends on the wave’s frequency.

The critical frequency is the highest frequency that can be reflected by an ionospheric layer when the radio wave hits it straight on. If the radio wave’s frequency is higher than the ionosphere’s plasma frequency, the electrons cannot respond quickly enough to re-emit the signal. The critical frequency is calculated using the electron density in the ionosphere.

The Maximum Usable Frequency (MUF) is the highest frequency that can be used to send a signal between two points at a specific time. This depends on the angle at which the wave reaches the ionosphere and the sine of that angle.

The cutoff frequency is the lowest frequency that can pass through a layer of the ionosphere at the angle needed for a signal to be reflected between two points.

Scientists use models to study how the ionosphere affects GPS signals. The Klobuchar model, developed by John Klobuchar in the 1970s, is used to correct for ionospheric effects in GPS systems. The Galileo navigation system uses the NeQuick model, which calculates the ionosphere’s effect on signal delays using three coefficients.

Researchers are studying the open system electrodynamic tether, which uses the ionosphere. This system uses plasma contactors and the ionosphere as part of a circuit to generate energy from Earth’s magnetic field through electromagnetic induction.

Measurements

Scientists study the ionosphere using many different methods. These include:

• Observing light and radio waves naturally produced in the ionosphere
• Sending radio waves of different frequencies to bounce off the ionosphere
• Using incoherent scatter radars such as EISCAT, Sondre Stromfjord, Millstone Hill, Arecibo, AMISR, and Jicamarca
• Using coherent scatter radars such as SuperDARN
• Using special receivers to detect changes in radio waves after they reflect off the ionosphere

Some experiments, like HAARP (High Frequency Active Auroral Research Program), use powerful radio transmitters to study how the ionosphere changes. These studies focus on understanding the ionosphere’s plasma to improve communication and surveillance systems for both civilian and military use. HAARP began in 1993 and is currently active near Gakona, Alaska.

The SuperDARN radar project studies the high- and mid-latitudes by using coherent backscatter of radio waves between 8 to 20 MHz. Coherent backscatter is similar to how light reflects off a crystal and involves waves combining to create stronger signals from ionospheric density changes. The project involves over 11 countries and multiple radars in both hemispheres.

Scientists also study the ionosphere by observing changes in radio waves from satellites and stars as they pass through it. The Arecibo Telescope in Puerto Rico was originally built to study Earth’s ionosphere.

Ionograms are graphs that show how high radio waves reflect in the ionosphere based on their frequency. An ionosonde sends radio waves between 0.5 and 25 MHz as pulses or continuous signals. The received signals provide information about transit time, frequency, amplitude, phase, polarization, Doppler shift, and spectrum shape. These measurements can also reveal the true height of ionospheric layers and line-of-sight velocity. A group of receiving antennas can detect irregularities in the ionosphere’s layers.

As radio wave frequency increases, the waves are refracted less by ionization and can travel deeper before reflecting. When the frequency is high enough, the wave passes through the layer without reflecting. For ordinary mode waves, this happens when the frequency exceeds the layer’s critical frequency. Detailed rules about this are described in URSI Handbook of Ionogram Interpretation and Reduction, edited by William Roy Piggott and Karl Rawer, published by Elsevier in 1961 (translations are available in Chinese, French, Japanese, and Russian).

Incoherent scatter radars operate above the critical frequencies, allowing scientists to study the ionosphere beyond the densest areas of electrons. These radars use scattered signals that lack a clear pattern, which is why they are called "incoherent." Their measurements provide information about electron density, ion and electron temperatures, ion masses, and drift velocities. These radars can also measure movements in the neutral atmosphere, such as atmospheric tides, by estimating how often ions and neutral particles collide.

Radio occultation is a method used to study Earth’s atmosphere. A GNSS signal passes through the atmosphere and is received by a Low Earth Orbit (LEO) satellite. As the signal travels through the atmosphere, it bends and delays. A LEO satellite measures the total electron content and bending angle of many signals as it observes a GNSS satellite rising or setting behind Earth. Using a mathematical method called the Inverse Abel’s transform, scientists can create a detailed profile of atmospheric refractivity at the point where the signal touched Earth.

Major GNSS radio occultation missions include GRACE, CHAMP, and COSMIC.

Indices of the ionosphere

In models that study the ionosphere, such as Nequick, certain indices are used to show the ionosphere's condition indirectly. These indices are F10.7 and R12. Both are useful because they have long records of data covering many solar cycles. F10.7 measures the strength of solar radio waves at 2800 MHz, recorded by a radio telescope on Earth. R12 is the average of daily sunspot numbers over 12 months. These two indices are related to each other.

However, neither F10.7 nor R12 directly measure solar ultraviolet and X-ray emissions, which are the main causes of ionization in Earth's upper atmosphere. Now, data from the GOES spacecraft provides measurements of the Sun's X-ray light, a value more closely connected to ionization levels in the ionosphere.

• The A- and K-indices measure how the horizontal part of Earth's magnetic field changes. The K-index uses a scale from 0 to 9 to show the strength of the horizontal magnetic field. The Boulder K-index is measured at the Boulder Geomagnetic Observatory.
• Earth's geomagnetic activity is measured by changes in Earth's magnetic field, recorded in units called teslas (or gauss in older texts). Observatories around the world measure Earth's magnetic field. The collected data is analyzed to create measurement indices. Daily measurements for the entire planet are estimated using the A p -index, known as the planetary A-index (PAI).

Ionospheres of other planets and natural satellites

Objects in the Solar System with noticeable atmospheres, such as the major planets and many larger moons, usually have ionospheres. Planets that are known to have ionospheres are Venus, Mars, Jupiter, Saturn, Uranus, and Neptune.

Titan's atmosphere has an ionosphere that extends from about 880 to 1,300 kilometers (550 to 810 miles) above its surface and includes carbon compounds. Ionospheres have also been found on Io, Europa, Ganymede, Triton, and Pluto.