Ionosphere

The ionosphere is a part of Earth's upper atmosphere. It stretches from about 48 km (30 mi) to 965 km (600 mi) above sea level. This layer includes the thermosphere and parts of the mesosphere and exosphere.

The ionosphere is ionized by solar radiation. This is energy that comes from the Sun. The energy gives the layer its name and its special properties. The ionosphere also plays a part in atmospheric electricity. It forms the inner edge of the magnetosphere.

The ionosphere matters in everyday life. It affects radio propagation, which is how radio signals travel to faraway places on Earth. Travel through this layer also affects GPS signals. It bends their paths and slows how fast they arrive.

History of discovery

In 1839, German mathematician and physicist Carl Friedrich Gauss suggested that a layer of the air around Earth that conducts electricity could explain changes in Earth's magnetic field. In 1901, Guglielmo Marconi received the first radio signal across the Atlantic Ocean in St. John's, Newfoundland (now in Canada). The signal was sent from Poldhu, Cornwall, using a spark-gap transmitter at about 500 kHz frequency. The message was three dots, the Morse code for the letter S. To reach Newfoundland, the signal would have had to bounce off the ionosphere twice, but Dr. Jack Belrose has disputed this. One year later, Marconi sent wireless messages across the Atlantic from Glace Bay, Nova Scotia. In 1902, Oliver Heaviside proposed a layer of the ionosphere, now called the Kennelly–Heaviside layer after him, and Arthur Edwin Kennelly discovered some of its radio properties. In 1912, the U.S. Congress passed the Radio Act of 1912, which dealt with amateur radio operators. This led to the discovery in 1923 that radio signals can travel through the ionosphere. In 1925, Dr. Alfred N. Goldsmith and his team watched a solar eclipse in New York and saw how sunlight affects radio waves.

In 1926, Scottish physicist Robert Watson-Watt introduced the word ionosphere in a letter. The letter was not published until 1969, in Nature. In the early 1930s, test broadcasts from Radio Luxembourg accidentally gave the first evidence that radio can change the ionosphere. In 2017, HAARP ran experiments using the Luxembourg Effect. Edward V. Appleton won a Nobel Prize in 1947 for confirming that the ionosphere exists, which he had done in 1927. Lloyd Berkner was the first to measure its height and density, which made possible the first complete theory of how short-wave radio travels. Maurice V. Wilkes and J. A. Ratcliffe studied very long radio waves in the ionosphere. Vitaly Ginzburg developed a theory of how electromagnetic waves travel through plasmas, such as the ionosphere. In 1962, Canada launched the satellite Alouette 1 to study the ionosphere. It was followed by Alouette 2 and the ISIS satellites. In 1963, Syncom 2 became the first working geosynchronous satellite. Its radio beacons allowed scientists to measure total electron content for the first time. From 1969, Australian geophysicist Elizabeth Essex-Cohen used this method to monitor the atmosphere above Australia and Antarctica.

Geophysics

The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth. It stretches from about 50 km (30 mi) to more than 1,000 km (600 mi) high. It exists mainly because of ultraviolet light from the Sun. Early scientists studied how it helps radio waves travel long distances. Heaviside and Kennelly proposed that the ground and an upper layer of the atmosphere act like a guide for radio waves. Heaviside used his Telegrapher's equations, and Kennelly gave a physical cause for the idea. In 1919, G. N. Watson used mathematics to reproduce the Austin-Cohen formula. In 1924, Joseph Larmor showed how ionized air bends radio waves. Between 1923 and 1925, Appleton, Miles Barnett, Reginald Smith-Rose, and R.H. Barfield studied skywaves. They found that the ionized layer sat about 80 to 90 km high. Appleton and Barnett also measured the fading of radio signals. In 1925, Gregory Breit and Merle Tuve used short pulses to measure the height of the ionosphere. Their results varied from 88 km to 211 km.

In the lowest part of Earth's atmosphere, the troposphere, the temperature drops as you go higher. The troposphere reaches up to the tropopause, about 10 to 12 km high. Above it is the stratosphere, where the temperature rises to a peak at the stratopause, about 50 km high. This happens because the ozone layer absorbs ultraviolet light. Above that, the temperature falls in the mesosphere to a low at the mesopause. In the thermosphere, the temperature rises until it levels off. Above 100 km is the turbopause, where gases separate by gaseous diffusion into the heliosphere and protonosphere. Above 600 km, at the exosphere, atoms can escape Earth's gravity. The ionosphere is the ionized part of the atmosphere, which is important for radio propagation. The magnetosphere is where the geomagnetic field controls how particles move. Above about 125 km, there is more atomic oxygen because of the dissociation of molecular oxygen by ultraviolet radiation. Ionization is caused by extreme ultraviolet and X-ray solar radiation. The number of sunspots affects the ionosphere, as does the varying activity of the Sun over a period of about 11 years. This includes how often flares occur and the strength of the radio flux. The ionosphere also changes with the time of day (diurnal variation) and with the seasons. Sydney Chapman proposed calling the region below the ionosphere the neutrosphere. Radio waves between 30 MHz and 10 GHz use line of sight propagation. Troposcatter and meteor scatter also carry signals at some frequencies, while skywaves and groundwaves carry signals at lower frequencies.

Layers of ionization

At night, the F layer is the only layer with much ionization. The E and D layers have very little. During the day, the D and E layers become much more ionized. The F layer also gets an extra, weaker part called the F₁ layer. The F₂ layer stays both day and night. It is the main part that bends and reflects radio waves. Molecular ions recombine about 1000 times faster than atomic ions, so there are fewer of them at night. Meteors can also add sharp layers of ionization at low altitudes.

The D layer is the lowest layer, about 60 to 90 km above Earth. It is ionized by Lyman series-alpha hydrogen light, which has a wavelength of 121.6 nanometre (nm), and this light ionizes nitric oxide. Solar flares can add more ionization. Low frequency radio waves lose much of their energy here, so they usually cannot pass into the upper atmosphere. The exception is at night, when the D layer fades. This makes low frequency signals weaker during peak sunspot cycles. During solar proton events, the D layer can become very ionized over the poles. This can cause polar radio blackouts that last for days.

The middle E layer is about 105 to 160 km up. It is also called the Kennelly–Heaviside layer or Heaviside layer. Its existence was predicted in 1902 by Arthur Edwin Kennelly and Oliver Heaviside. It was detected in 1924 by Edward V. Appleton and Miles Barnett. Sometimes the E layer becomes even more ionized in patches. This is called sporadic E propagation, and it can let radio amateurs talk over long distances.

The F layer's F₁ part sits at about 160 to 180 km. The F₂ part reaches from about 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

For broader coverage of this topic, see Upper-atmospheric models.

An ionospheric model is a math description of the ionosphere. It shows how the ionosphere changes depending on location, height, time of year, the sunspot cycle, and activity in Earth's magnetic field. Scientists describe the plasma of the ionosphere with four measurements: electron density, electron and ion temperature, and ionic composition, since several kinds of ions are present. Radio propagation depends only on electron density. Models are usually written as computer programs. They may be based on how ions, electrons, air, and sunlight interact, on many observations, or on both. One of the most widely used is the International Reference Ionosphere (IRI), which is based on data and describes these four measurements. It is an international project supported by the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). Its main data come from a worldwide network of ionosondes, powerful incoherent scatter radars at several sites, including Arecibo, and instruments on satellites and rockets. The IRI is updated every year. It is more accurate for electron density from the bottom of the ionosphere up to the height where density is highest than for total electron content (TEC). Since 1999, this model has been the "International Standard" for the Earth's ionosphere (standard TS16457).

Persistent anomalies to the idealized model

Ionograms help scientists work out the shape of the different layers. Electrons and ions mixed up in plasma make the echo traces look rough. This happens mostly at night, at higher latitudes, and when conditions are disturbed. At middle latitudes, the F₂ layer gets more ions in summer, when the Sun shines more directly on Earth. But seasonal changes in the air cause even more ions to be lost in summer. So the total ionization of the F₂ layer is lower during local summer months. This is called the winter anomaly. It is always present in the northern hemisphere but is usually absent in the southern hemisphere when solar activity is low.

Within about 20 degrees of the magnetic equator is the equatorial anomaly. Here, the ionization in the F₂ layer dips at the equator and peaks about 17 degrees away in magnetic latitude. The Earth's magnetic field lines run flat across the magnetic equator. Heat from the Sun and tidal movements in the lower ionosphere push plasma up and across these field lines. This creates a sheet of electric current. Together with the horizontal magnetic field, it pushes ionization up into the F layer, where it bunches up about 20 degrees from the magnetic equator. This is called the equatorial fountain. The solar-driven wind also creates the Sq (solar quiet) current system in the E region of the ionosphere (ionospheric dynamo region), at about 100–130 km (60–80 mi) altitude. This current produces an electric field in the daytime equatorial ionosphere. Near the magnetic equator, it causes a stronger eastward current flow within about 3 degrees of the equator, known as the equatorial electrojet.

Ephemeral ionospheric perturbations

Main article: Sudden ionospheric disturbance

Further information: Solar particle event § Polar cap absorption events

Main articles: Geomagnetic storm and Ionospheric storm

Further information: Lightning

When the Sun is active, strong solar flares can send X-rays toward the side of Earth facing the Sun. These X-rays add electrons to the upper air, which can block high frequency radio signals. This can cause a radio blackout that lasts for many hours. When the X-rays stop, the disturbance slowly fades over minutes to hours. Solar flares can also release fast-moving particles called protons. These reach Earth within 15 minutes to 2 hours. They enter the atmosphere near the magnetic poles, where they add to the electric charge there. This usually lasts from about an hour to several days. Coronal mass ejections can also release fast protons. Geomagnetic storms and ionospheric storms are short, strong disturbances of the Earth's magnetosphere and ionosphere. During a geomagnetic storm, the F₂ layer can become unsteady, break apart, or even disappear. Aurorae can be seen in the night sky near the North and South poles.

Lightning can also disturb the ionosphere. Radio waves from lightning can travel into the magnetosphere and push particles down into the ionosphere. This adds charge to the D-region. These events are called "lightning-induced electron precipitation." Lightning strikes can also add charge directly, through large movements of electricity.

Applications

Measurements

Main article: Ionogram

Scientists study the ionosphere in many ways. They watch light and radio signals that it gives off, and they bounce radio waves off it. They use incoherent scatter radars, such as the EISCAT, Sondre Stromfjord, Millstone Hill, Arecibo, Advanced Modular Incoherent Scatter Radar (AMISR) and Jicamarca radars. They also use coherent scatter radars like the Super Dual Auroral Radar Network (SuperDARN) radars. Special receivers compare the reflected waves with the waves that were sent. Some experiments, such as HAARP (High Frequency Active Auroral Research Program), use powerful radio transmitters to change the ionosphere. HAARP started in 1993 as a planned twenty-year experiment. It is still active near Gakona, Alaska. Scientists also study how radio waves from satellites and stars change as they pass through the ionosphere. The Arecibo Telescope, in Puerto Rico, was first meant to study Earth's ionosphere.

Ionograms are graphs that show the height of radio frequency reflections at each frequency. An ionosonde sends a range of frequencies, from 0.5 to 25 MHz, as pulses or continuous wave-FM. The returning signal gives information such as Doppler shift. Line of sight velocity can also be worked out. Higher frequencies go further into the ionosphere before they are reflected. At a certain frequency, the signal passes through the layer without being reflected. Rules for reading these graphs are given in the "URSI Handbook of Ionogram Interpretation and Reduction" by William Roy Piggott and Karl Rawer. Incoherent scatter radars work above these critical frequencies, so they can study the ionosphere above the electron density peaks. Their signals lack coherence, which gave the technique its name. These radars can also measure movements of the neutral atmosphere, such as atmospheric tides. To do this, they make assumptions about collision frequency in the ionospheric dynamo region. In radio occultation, a GNSS signal passes through the atmosphere and is received by a Low Earth Orbit (LEO) satellite. The satellite measures how the signal bends and how many electrons it passes through. An inverse Abel's transform then rebuilds a profile of the atmosphere. This method is used in remote sensing. Major missions that use this method include GRACE, CHAMP, and COSMIC.

Indices of the ionosphere

Scientists use certain numbers, called indices, to learn about the ionosphere. They put these numbers into computer models such as Nequick. Two solar indices, F10.7 and R12, are often used because they have long records that cover many solar cycles. F10.7 measures the strength of radio waves from the Sun at 2800 MHz, using a ground radio telescope. R12 is the 12-month average of daily sunspot numbers. The two indices are linked to each other. However, they only indirectly show the ultraviolet and X-ray light from the Sun. This light mainly causes ionization in the upper atmosphere. Data from the GOES spacecraft now measures the background X-ray light from the Sun. This is more closely tied to ionization levels.

Other indices track geomagnetic disturbances, which are changes in Earth's magnetic field. The A and K indices show how the horizontal part of the geomagnetic field behaves. The K index uses a scale from 0 to 9. The Boulder K-index is measured at the Boulder Geomagnetic Observatory. Earth's magnetic activity is measured by how the field changes. This is measured in SI units called teslas, or in gauss in older books. Many observatories around the world measure the field, and their data is turned into indices. Daily values for the whole planet come from an estimate called the planetary A-index (PAI). This estimate is based on the A_p index.

Ionospheres of other planets and natural satellites

Many objects in the Solar System have thick atmospheres, and most of them have ionospheres. This includes all of the major planets and many of the larger natural satellites. Planets known to have ionospheres include Venus, Mars, Jupiter, Saturn, Uranus, and Neptune.

The atmosphere of Titan has an ionosphere. It is about 880 to 1,300 km (550 to 810 mi) high and contains carbon compounds. Scientists have also seen ionospheres around Io, Europa, Ganymede, Callisto, Triton, and Pluto.