Ionosphere

The ionosphere is a special part of Earth's upper atmosphere. It stretches from about 48 kilometers (30 miles) to 965 kilometers (600 miles) above sea level. This area includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere gets charged with electricity because of energy from the Sun.

This layer is very important for our planet. It helps control electrical activity in the atmosphere and forms the inner part of Earth's magnetic shield, called the magnetosphere. One of the most useful jobs of the ionosphere is that it helps radio waves travel far distances across the Earth. It also affects GPS signals, bending their paths slightly and causing small delays in when they reach us.

History of discovery

In 1839, a scientist named Carl Friedrich Gauss thought a special layer in Earth’s upper atmosphere might explain changes in Earth’s magnetic field.

In 1901, Guglielmo Marconi sent the first radio signal across the Atlantic Ocean to St. John's, Newfoundland, now part of Canada. This helped scientists learn how radio waves travel through the air.

More discoveries came after. In 1902, two scientists suggested a layer in the sky that helps radio waves bend around Earth. Experiments and satellites, like Alouette 1 from Canada, helped us learn more about this part of our atmosphere, called the ionosphere.

Geophysics

The ionosphere is a layer of charged particles around Earth. It starts about 50 km (30 mi) above us and goes up to over 1,000 km (600 mi). This layer forms when sunlight, especially ultraviolet radiation, breaks apart atoms and molecules in the air, giving them a charge.

Scientists learned that radio waves can travel far by bouncing off this charged layer. They found that the ionosphere has different layers, like the "E layer" about 80 to 90 km up, and a higher "F layer." These layers change with how much sunlight there is and where the Sun is in the sky. The ionosphere helps radio signals travel around the world.

Layers of ionization

At night, only the F layer has a lot of ionization. The E and D layers have very little. During the day, the D and E layers become more ionized, and the F layer gets an extra, weaker part called the F₁ layer. The F₂ layer stays all day and night and is the main place where radio waves bend and reflect.

The D layer is the innermost layer, about 60 to 90 km above Earth. The E layer is in the middle, about 105 to 160 km up. The F layer is the top layer, starting around 160 km up and going much higher. It has the most free electrons and is very important for radio communication.

Main article: Kennelly–Heaviside layer

D layer

The D layer is the innermost layer, 60 to 90 km above the Earth with an electron density of 10² to 10⁴ per cm³. Low frequency radio waves are weakened by the D layer. This stops low frequency radio signals from reaching the upper atmosphere, except when the D layer is not present, such as at night.

E layer

The E layer is the middle layer, 105 to 160 km above the Earth with an electron density several times 10⁵ per cm³. Radio signals going upward meet more electrons, bending them back toward Earth. This region is also called the Kennelly–Heaviside layer or the Heaviside layer.

E_s layer

The E_s layer has more electrons than normal in the E layer, allowing it to reflect radio waves up to 100 MHz. These events can last from a few minutes to many hours. They are common during the day near the equator and more often happen in summer at mid-latitudes.

Sporadic E propagation makes VHF communication very exciting for radio amateurs when long-distance paths open up for two-way communication.

F layer

Main article: F region

The F layer includes the F1 from 160 km to 180 km. The F2 layer extends from 200 km to over 800 km. 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. It looks at where you are, how high up you are, the time of year, and how active the sun is.

The model uses four main things to describe the ionosphere: how many tiny particles called electrons are there, how hot the electrons and ions are, and what kinds of ions are present.

These models are usually made as computer programs. They can be based on the science of how ions and electrons act with the air and sunlight. Or, they can use many observations to make a description. One popular model is the International Reference Ionosphere (IRI). This model is updated every year and helps us understand how the ionosphere changes. It is especially good at showing how the number of electrons changes from the bottom of the ionosphere up to where there are the most electrons.

Persistent anomalies to the idealized model

Special tools called ionograms help us learn about the real shape of the layers in the ionosphere. Sometimes, the tiny charged particles look rough in these tools, especially at night, far from the equator, and during busy space weather.

Winter anomaly

In places around the middle of the Earth, we might expect more charged particles in the ionosphere during summer because the Sun shines more strongly. But because of changes in the air, these particles are lost even faster in summer. This means there are fewer charged particles in summer than in winter. This surprising pattern is called the winter anomaly. It always happens in the northern part of the Earth but is usually not seen in the southern part when the Sun is less active.

Equatorial anomaly

Close to the Earth’s magnetic equator, within about ± 20 degrees, there is a special pattern called the equatorial anomaly. In this area, there is less ionization right at the equator but more ionization about 17 degrees away from it in either direction. The Earth’s magnetic field is flat at the magnetic equator. Heat from the Sun and movements in the lower ionosphere push charged particles upward and sideways along these magnetic field lines. This creates a thin layer of electric current in a lower part of the ionosphere, which, together with the flat magnetic field, pushes ionization higher up, focusing it about ± 20 degrees from the magnetic equator. This movement is known as the equatorial fountain.

Equatorial electrojet

All around the world, winds driven by the Sun create a current system in the lower part of the ionosphere. This results in an electric field that goes from west to east during the day near the equator. At the exact magnetic equator, where the magnetic field is flat, this electric field causes an extra strong current to flow eastwards within about ± 3 degrees of the magnetic equator, known as the equatorial electrojet.

Ephemeral ionospheric perturbations

X-rays: sudden ionospheric disturbances (SID)

Main article: Sudden ionospheric disturbance

When the Sun is very active, powerful solar flares can send strong X-rays to Earth. These X-rays reach the lower part of the ionosphere and can block some radio signals for many hours. During this time, certain low-frequency radio waves will bounce off a different layer of the atmosphere. Once the X-rays stop, the ionosphere slowly returns to normal.

Protons: polar cap absorption (PCA)

Further information: Solar particle event § Polar cap absorption events

Solar flares can also release high-energy particles called protons. These protons can reach Earth within minutes to a couple of hours and affect the atmosphere near the magnetic poles. These effects can last from an hour to several days. Coronal mass ejections can also release protons that affect the polar regions.

Storms

Main articles: Geomagnetic storm and Ionospheric storm

Geomagnetic storms and ionospheric storms are strong, temporary disturbances in Earth's magnetosphere and ionosphere.

During a geomagnetic storm, a layer of the ionosphere can become unstable or even vanish. In the northern and southern parts of Earth, aurorae can be seen in the night sky.

Lightning

Further information: Lightning

Lightning can affect the ionosphere in two ways. One way is through very low frequency radio waves that can cause more particles to enter the ionosphere. Another way is through the direct energy from lightning strikes, which can also add ionization. Scientists have studied how lightning can enhance certain layers of the ionosphere.

Applications

Radio communication

The ionosphere can bend radio waves back to Earth, allowing communication over long distances. This happens because the ionosphere has charged particles that interact with radio waves. By sending radio waves into the sky at an angle, they can curve back to Earth beyond what we can normally see. This is called "skip" or "skywave" propagation. This method has been used since the 1920s for international communication, like telephone and telegraph services across oceans. It can be unreliable due to changes in time of day, seasons, and solar activity, but it is still useful for areas where satellites are not available. Hobbyists, emergency services, and armed forces use it.

Mechanism of refraction

When radio waves reach the ionosphere, the electric field in the wave makes electrons vibrate. These vibrating electrons can either lose energy or re-radiate the wave. The bending of the wave depends on the electron density and the wave's frequency. If the wave's frequency is too high, the electrons cannot respond quickly enough, and the wave passes through instead of bending back.

GPS/GNSS ionospheric correction

The ionosphere can affect GPS signals, so models are used to correct for these effects. The Klobuchar model is used for GPS, developed by John Klobuchar in the 1970s. The Galileo system uses the NeQuick model, which receives coefficients from the system to adjust for ionization levels and calculate delays in the signals.

Other applications

Researchers are studying electrodynamic tethers, which use the ionosphere and Earth's magnetic field to generate energy through electromagnetic induction. These tethers use plasma contactors and the ionosphere as part of a circuit.

Measurements

Scientists study the ionosphere in many ways. They watch light and radio waves from the ionosphere. They also send radio waves to bounce off it. Special radars like EISCAT, Millstone Hill, and Jicamarca help them learn more.

Experiments such as HAARP (High Frequency Active Auroral Research Program) use strong radio transmitters. This helps scientists understand how to improve communications. The SuperDARN radar project studies areas near the poles. Scientists also look at how radio waves from satellites and stars change when they pass through the ionosphere. The Arecibo Telescope in Puerto Rico was built to study Earth's ionosphere.

Indices of the ionosphere

In models that study the ionosphere, scientists use special numbers to understand it better.

Solar intensity

Two important numbers, F10.7 and R12, help scientists learn about the Sun. F10.7 measures the Sun’s radio waves using a ground radio telescope. R12 looks at the average number of dark spots on the Sun, called sunspot numbers, over 12 months. These numbers have been collected for many years.

Geomagnetic disturbances

Scientists also watch Earth’s magnetic field to see how it moves. The A and K-indices measure this movement. The K-index uses numbers from 0 to 9 to show how strong the changes are. These measurements come from places like the Boulder Geomagnetic Observatory. The Earth’s magnetic field is measured in units called teslas or gauss. Special numbers, like the A_p-index, help show the overall activity of Earth’s magnetic field.

Ionospheres of other planets and natural satellites

Many objects in our Solar System, like big planets and some larger moons, have atmospheres that create ionospheres. Planets with known ionospheres include Venus, Mars, Jupiter, Saturn, Uranus, and Neptune.

The moon Titan, which orbits Saturn, has an ionosphere that stretches from about 880 to 1,300 kilometers above its surface and contains carbon compounds. We have also found ionospheres on moons like Io, Europa, Ganymede, Triton, and even Pluto.