ionosphere

Structure of the earth's atmosphere (left) and position of the ionosphere (right)
Ionospheric layers: electron density and ion composition
Detailed view of the atmosphere and ionosphere with the distribution of temperature, pressure, density and electron concentration

The ionosphere is that part of the atmosphere of a celestial body that contains large quantities of ions and free electrons . For the planets of the solar system , the ionosphere makes up the majority of the high atmosphere . The ionization of the gas molecules is carried out by high-energy components of solar radiation (hard ultraviolet and X-rays ). The range of the radiation determines the transition to the neutrosphere .

The earth's ionosphere influences radio traffic by reflecting short waves and thus enabling worldwide connections. It also dampens the propagation of radio waves with long wavelengths. It begins above the mesosphere at an altitude of around 80 km, reaches its greatest electron density at an altitude of around 300 km and ultimately passes into interplanetary space. The transition height between O + and H + at an altitude of 1000 km can be regarded as the boundary between the ionosphere and the plasma sphere . There the scale height increases with which the particle density decreases exponentially. The ionosphere is therefore largely within the thermosphere defined with regard to neutral particles .

Formation of the ionosphere

The tangential view of the aurora illustrates the height dependence of the energy input into the atmosphere by corpuscular radiation.

The ionosphere is created through the absorption of ionizing solar radiation, primarily through high-energy electromagnetic waves (ultraviolet and X-ray radiation) but also through particle radiation ( corpuscular radiation ) mainly electrons and protons . However, the cosmic background radiation and streams of meteorites , which continuously burn up in the earth's atmosphere, also make a certain contribution to ionization. The solar radiation detaches valence electrons from the atoms: positive ions and free electrons are created and thus an electrically conductive area of ​​the atmosphere. An (at least partially) ionized gas is also referred to as a plasma .

The electron content (
TEC ) of the ionosphere on February 12, 2007 at 09:00  UT = 10:00  CET

On its way down, the solar ultraviolet and X-rays are absorbed more and more. Radiation is most energetic at high altitudes ( exosphere ), but only hits a few ionizable gas molecules. The denser the atmosphere becomes, the greater the local ionization . However, due to the absorption, the radiation intensity decreases. The increase in atmospheric density also reduces the mean free path of the gas particles, which leads to an accelerated reunion of electrons and positive ions ( recombination ). The balance between ionization and recombination determines the local electron density. That describes Sydney Chapman's theory in its simplest form . However, because the molecular composition depends on the altitude and both the energy required for ionization and the possible recombination processes depend on the type of neutral gas, three maxima of ionization usually form between the exosphere and the lower ionosphere during the day (D-, E- and F- Region).

The height of these layers depends on the density distribution of the (predominant) neutrals, but also on the height-dependent occurrence of the various types of atoms and molecules. The intensity of the solar radiation only influences the local density of the electrons, not the height of the maxima of the electron density.

The degree of ionization depends primarily on the solar radiation intensity, but also on the recombination and attachment processes. Hence there is a diurnal (daily), a seasonal (seasonal) and a geographical (local) dependency. In the F region, the situation is more complicated, which is why empirical ionization maps are used. Solar activity also plays an important role in the eleven-year sunspot cycle , and occasionally events such as solar storms .

The ionospheric layers

Structure of the ionosphere depending on the time of year and time of day.

There are three local ionization maxima within the ionosphere, which is why it is divided into three areas: the D, E and F layers.

Structure of the ionospheric layers
layer Height (approx.) comment
D. 070 ... 090 km available during the day, ionization according to the position of the sun
E. 110 ... 130 km available during the day, ionization according to the position of the sun
E s 000 ...110 km thin, often patchy, sporadic, especially in summer
F 1 000 ...200 km present during the day, goes together with F 2 shift at night
F 2 250 ... 400 km Present day and night

Ionization maxima are assigned to the energy absorption by certain types of gas particles. Above a height of 100 km, the mixing of the air is no longer sufficient to achieve an even distribution of the gases; a heterogeneous distribution occurs. This area is known as the heterosphere . The absorption of radiation that ionizes a certain gas takes place where it is highly concentrated.

 ionization ${\ displaystyle h \, \ nu + \ mathrm {O_ {2}} \ Rightarrow \ mathrm {O} _ {2} ^ {+} + e ^ {-}}$ ${\ displaystyle h \, \ nu + \ mathrm {O} \ Rightarrow \ mathrm {O} ^ {+} + e ^ {-}}$ ${\ displaystyle h \, \ nu + \ mathrm {N} _ {2} \ Rightarrow \ mathrm {N} _ {2} ^ {+} + e ^ {-}}$ ${\ displaystyle (\ mathrm {N} _ {2} ^ {+} + \ mathrm {O} \ Rightarrow {\ mathrm {NO}} ^ {+} + \ mathrm {N})}$ ${\ displaystyle e ^ {-} + \ mathrm {O} \ Rightarrow \ mathrm {O} ^ {+} + 2 {e ^ {-}}}$ Charge exchange ${\ displaystyle \ mathrm {H} + \ mathrm {O} ^ {+} \ Rightarrow \ mathrm {H} ^ {+} + \ mathrm {O}}$ ${\ displaystyle \ mathrm {O} _ {2} + \ mathrm {O} ^ {+} \ Rightarrow \ mathrm {O} _ {2} ^ {+} + \ mathrm {O}}$ Recombination ${\ displaystyle \ mathrm {O} _ {2} ^ {+} + e ^ {-} \ Rightarrow 2 \, {\ mathrm {O}}}$ ${\ displaystyle {\ mathrm {NO}} ^ {+} + e ^ {-} \ Rightarrow \ mathrm {N} + \ mathrm {O}}$ ${\ displaystyle \ mathrm {O} ^ {+} + e ^ {-} \ Rightarrow \ mathrm {O} + h \, \ nu}$
Electron density within the ionosphere on the day side of the earth with the ionization maxima of the D, E and F layers

The D layer

The D-layer is the layer closest to the earth and only exists during the day at an altitude range between 70 and 90 km. Ionization takes place through radiation from the Lyman - series at 121.6 nm, which is absorbed by nitrogen monoxide (NO). In times with a sufficiently high number of sunspots, hard X-rays (wavelength <1 nm) also ionize the air molecules (N 2 , O 2 ). During the night there is a slight residual ionization due to cosmic radiation. ${\ displaystyle \ alpha}$

Because of the high air density there, on the one hand the recombination is great, which is why the layer almost dissolves within a few minutes at sunset, on the other hand the collision frequency between electrons and other particles is very high during the day (approx. 10 million collisions per second). For radio waves this means strong attenuation, which increases with increasing wavelength. In long-distance traffic, this prevents the use of the sky wave on radio frequencies below about 10 MHz ( ionospheric waveguide ). VHF signals can be scattered on the D-layer ( ionoscatter ).

The E-Layer

The E-layer is the middle ionospheric layer, which forms at an altitude between 90 and 130 km. Ionization takes place on the basis of soft X-rays (wavelength 1–10 nm) and ultraviolet radiation (between 80 and 102.7 nm) on atomic oxygen (O) as well as nitrogen and oxygen molecules (N 2 , O 2 ). It has an average electron concentration of around 100,000 per cm³, so that only 0.1% of the atoms present are ionized.

The E-layer forms on the day side of the earth, reaches its ionization maximum at noon and disappears almost completely within an hour after sunset. In the sunspot maximum, the layer is higher than in the minimum. Within the E layer, strong local ionizations occur frequently, but not regularly, in a layer only a few kilometers thick, which is referred to as the sporadic E layer .

For shortwave, reflection at the E-layer is only of interest in local traffic, since its critical frequency is only between 2 and 4 MHz.

The E-Layer is also known as the Kennelly-Heaviside Layer , or for short the Heaviside Layer. The name goes back to Arthur Edwin Kennelly and Oliver Heaviside , who independently predicted their existence almost simultaneously in 1902. The E-layer was proven as the first of the ionospheric layers in 1924 by Edward Victor Appleton , who first referred to it as the E (electrical) layer in 1927. The later discovered, further layers were then referred to as D and F layers according to their relative altitude. (See also history ).

The F-layer

The F layer is the highest at 200 to 400 km and is the most strongly ionized layer. It is ionized by extreme ultraviolet radiation (EUV, wavelength 14 to 80 nm), which hits atomic oxygen or nitrogen molecules. It is a broad region with a maximum ionization of up to one million free electrons per cm³.

Comparison of the frequency of the two types of electron impact: elastic Coulomb collisions and inelastic neutral collisions

In the F-layer, electron collisions are mostly elastic (contactless) with positive ions, which is known as Coulomb collision. In contrast, inelastic collisions between electrons and the neutral gas predominate in the denser D and E layers. [The earth's ionosphere is thus an exception - in most astrophysical plasmas the Coulomb collisions predominate.]

The F-layer continues to exist at night because the free electrons recombine only very slowly because of the large mean free path. During the day there is often a deformation in the profile of the F-layer. the so-called F 1 layer, but the peak of the profile lies in the F 2 layer. The F 1 layer is the site of the greatest ion production, which decreases sharply without solar radiation. However, the strongest ion concentration is found in the F 2 layer because of the weaker recombination there. The F 1 layer, which only appears during the day, is in a photochemical equilibrium in which the losses occur through rapid recombination . In contrast, the predominant loss process in the F 2 layer is linked to the conversion of O + ions into NO + and O 2 + ions. This loss process is slower. The summit of the F 2 layer is higher in summer than in winter. It is the most important layer for short waves because radio traffic over 3500 km only comes about through repeated reflections on this layer.

The F layer is also known as the Appleton layer . The name goes back to Edward Victor Appleton , who was able to prove the existence of the Kennelly-Heaviside layer in 1924 (see also history ).

Use of the ionosphere

Ground wave and a sky wave reflected by the ionosphere (with multi-hop)

Higher layers of the ionosphere are partially ionized by solar radiation and therefore contain free electrons, which can be excited to vibrate by the electric field of radio waves of the frequency . The oscillating electrons in turn emit waves that are strongly out of phase in the vicinity of the plasma frequency (2 to 7 MHz) and are superimposed on the original wave . Since the ionosphere is penetrated by the earth's magnetic field, the free electrons can also be excited by the radio waves to make circular movements around the field lines. This cyclotron frequency is around 1.3 MHz over Central Europe. The direction of rotation of the circularly polarized radio wave may or may not match the movement of the electrons, which is why the ionosphere is circularly birefringent . Linearly polarized waves must therefore be interpreted as a superposition of two circular waves with opposite directions of rotation, for which different refractive indices apply. If the direction of propagation runs parallel to the magnetic field lines, the  following approximations apply for f > 1 MHz: ${\ displaystyle f}$ ${\ displaystyle f _ {\ text {P}}}$ ${\ displaystyle f _ {\ text {B}}}$ ${\ displaystyle n}$

${\ displaystyle n _ {\ text {left}} = {\ sqrt {1 - {\ frac {f _ {\ text {P}} ^ {2}} {f (f + f _ {\ text {B}})} }}}}$
${\ displaystyle n _ {\ text {right}} = {\ sqrt {1 - {\ frac {f _ {\ text {P}} ^ {2}} {f (f-f _ {\ text {B}})} }}}}$

The difference between the two formulas is negligible in the VHF range and disappears if the wave vector forms a right angle with the direction of the magnetic field, because then ( anisotropy ). The two circularly polarized radio waves move through the material at different phase speeds (a higher phase speed corresponds to a smaller refractive index) and can be attenuated differently. When receiving, both components overlap to form an elliptically polarized wave, the main direction of which is usually rotated ( Faraday effect ). ${\ displaystyle f _ {\ text {B}} = 0}$

Vertical propagation behavior of two radio signals with different frequencies. The lower of the right signal is reflected in the E-layer, the higher of the left penetrates the E-layer, but remains lower than the critical frequency of the F-layer , which is why it is reflected in this layer.
Waves radiated flatly hit the ground again at some distance

For the refractive index is purely imaginary. Therefore, all lower frequencies are fully reflected . As with exceeding the cut-off frequency of a waveguide - if the layer is thick enough - the waves cannot penetrate the ionosphere, but are not absorbed either. Long and medium wave signals always return to the ground, as do radio frequencies below the plasma frequency of the F2 layer, which is usually above 7 MHz. Radio signals above this critical frequency can penetrate the ionosphere at normal incidence. For an obliquely incident wave, the corresponding cut-off frequency, the Maximum Usable Frequency (MUF), is higher than the critical one, the more the shallower the incidence is. It can be approximately determined from the critical frequency as follows: ${\ displaystyle f

${\ displaystyle f _ {\ text {muf}} \ approx {\ frac {f _ {\ text {critical}}} {\ sin \ alpha}} = f _ {\ text {critical}} {\ sqrt {1+ \ left ({\ frac {d} {2h _ {\ text {virt}}}} \ right) ^ {2}}}}$

with the horizon, relatively = beam angle of the shaft = distance between the transmitting and the receiving, = virtual height of reflection. ${\ displaystyle \ alpha}$${\ displaystyle d}$${\ displaystyle h _ {\ text {virt}}}$

Waves radiated flatly hit the ground again at some distance after total reflection at an ionospheric layer. If the bump has a shorter range, a dead zone is created around the transmitter in which reception is not possible, but it is possible at a greater distance. The term “reach” loses its meaning here.

The lowest usable frequency (LUF) is the lower limit frequency in the shortwave range that can be used for the transmission of a signal between two points at a given point in time. It depends on the electron density and the frequency of the collisions in the cushioning lower ionospheric layers and is generally highest at noon. These considerations do not apply to the VHF range, the frequencies of which are above the plasma frequency.

Electrical and mechanical energy generation

Energy conversion with a propulsive tether system.

The Propulsive Small Expendable Deployer System (ProSEDS) is a cable-based system for generating electrical energy and exerting electrodynamic forces on spacecraft, which works on the principle of a space tether . Its start has been postponed several times and is currently uncertain. A previous system ( Tethered Satellite Systems (TSS) ) was successfully tested in 1996 during the space shuttle mission STS-75 .

Earthquake forecast

It is believed that effects in the ionosphere occur during and before earthquakes . Chemical, acoustic and electromagnetic mechanisms are discussed as possible causes. For example, the release of charge carriers from oxidic minerals due to tectonic stresses, but also effects such as the excitation of atmospheric gravity waves due to outgassing (Fig. 12 in). Even if the ionosphere has been monitored from the ground and with satellites for a long time, a coupling cannot currently be regarded as sustainably proven.

Satellites that investigate this phenomenon in more detail are Demeter (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) from the French space agency CNES from 2004 and the Russian Kompas 2, launched in 2006 .

Characteristics of the ionosphere

Parameters

The sizes presented below can be divided into local physical sizes and parameters of the layers. [The latter are directly accessible for measurement from the outside and are usually sufficient for applications.] The practical application of the definitions is explained in the URSI Handbook.

Plasma frequency

The plasma frequency is a key parameter for applications in connection with electromagnetic waves. It indicates the frequencies down to which the waves propagate in the plasma. The plasma frequency depends mainly on the particle density of the electrons, as these follow the alternating field more easily than the inert ions. Neglecting the ions, the plasma frequency is ${\ displaystyle n_ {e}}$

${\ displaystyle f _ {\ mathrm {p}} = {\ frac {1} {2 \ pi}} {\ sqrt {\ frac {e ^ {2} {n} _ {\ mathrm {e}}} {m_ {e} {\ varepsilon} _ {0}}}} \ ,.}$

This is where the charge and mass of an electron are . And is the electric field constant . If you insert these constants, the result is ${\ displaystyle e}$${\ displaystyle m _ {\ text {e}}}$${\ displaystyle {\ varepsilon} _ {0}}$

${\ displaystyle f _ {\ mathrm {p}} = 8 {,} 978 \, \ mathrm {Hz} \ cdot {\ sqrt {n _ {\ mathrm {e}} / \ mathrm {m} ^ {- 3}} }}$

Depending on the density of the free electrons, the plasma frequency increases from about 1.5 MHz at a height of 100 km to about 7 MHz at about 800 km.

A similar quantity is the gyration frequency , which is dependent on the magnetic field . Except for solar storms, the magnetic field is terrestrial and the gyro frequency is close to 1 MHz.

Schumann resonances

The space between the earth and the ionosphere can act as a cavity resonator . Schumann resonances are those frequencies at which the wavelength of an electromagnetic oscillation in the waveguide between the earth's surface and the ionosphere is an integral part of the earth's circumference . When excited with electromagnetic oscillations of such frequencies, standing waves , the so-called Schumann waves , are created . The energy for the low-frequency excitation comes from the worldwide thunderstorm activity. The fundamental wave of the Schumann resonance is 7.8 Hz, plus various harmonics between 14 and 45 Hz. Due to atmospheric turbulence, these values ​​fluctuate.

Measurement

Ionoprobes

The antenna system of the HAARP ion probe

An ion probe is a radar system for the active investigation of the ionosphere. Ion probes monitor the altitude and the critical frequency of the ionospheric layers. To do this, they send shortwave radar pulses of different frequencies against the ionosphere and mainly measure the transit time of the received echo, from which the height of the reflection can be determined.

With increasing frequency, the signal is backscattered less strongly and thus penetrates deeper into the ionosphere before it is reflected. The deeper penetration changes the measured, so-called virtual , height of the reflective layer. When the critical frequency is exceeded, the ionosphere is no longer able to reflect the signal. Only half of the ionosphere can be probed to the maximum electron density at a time. The measuring systems are usually located on the ground to examine the underside ("bottomside") or on satellites for the upper side ("topside").

The probes can be used to create recordings of the signal propagation time or the calculated reflection height versus frequency, the so-called ionograms .

Riometer

HAARP receiving systems, above those of the two riometers

A Relative Ionospheric Opacity Meter or riometer for short is a device for the passive observation of the ionospheric absorption capacity .

It measures the reception strength of the cosmic background radiation in the area of ​​radio waves, which are constantly emitted by stars or galaxies and reach the earth after crossing the ionosphere ( radio window ). Although the strength varies with the rotation of the earth, it is nevertheless sufficiently constant for earthly scales depending on the region of the sky and is therefore predictable. In particular, the absorption is measured at heights of up to 110 km, since the majority of the absorption takes place in the lower layers of the ionosphere such as the D-layer .

Missile probes

Rocket probes ( English sounding rockets ) are research rockets equipped with measuring instruments that are preferably used to create profiles of the ion distribution in the ionosphere. They are inexpensive and allow measurements at heights above the maximum height of balloons (≈ 40 km) and below the minimum height of satellites (~ 120 km). They also achieve a spatial resolution in the centimeter range that is not possible with other measuring methods.

Satellites

One of the first satellites commissioned by ionospheric research: Alouette 1

Satellites are used for two purposes of measuring the ionosphere. On the one hand, satellite-based ionograms (topside recordings) complete the measurement data from the ground stations (bottomside recordings), and on the other hand, the measured variables are not influenced by the atmosphere, as is the case with ground stations. For example, the solar X-ray flux is measured by GOES . The solar flux at 10.7 cm wavelength, on the other hand, is not changed by the atmosphere and is measured daily by ground stations.

The satellite measurement methods can be divided into passive (only receiving sensors) and active (signal transmission and reception). With the active method, the transmitter and receiver are usually located close to each other (in the same satellite) as with a radar, but this does not necessarily have to be the case. Examples of this are the radio occultation method or GPS- assisted ionospheric tomography, in which two-frequency measurements are used to determine the total electron content (TEC ) integrated along the signal path .

One of the first satellites to be used to study the ionosphere was the US Explorer 1, launched in 1958, and the Canadian Alouette 1 (French lark ) satellite launched in 1962 . After its ten-year mission, it was shut down as planned. It is still in orbit today (as of January 2006) and its engineers in charge even see a small chance that it could be reactivated. It was followed by other ionospheric satellites from the International Satellites for Ionospheric Studies (ISIS) program. The measurement program of the two German-American Eros satellites was created in connection with the international project International Reference Ionosphere and has made important contributions to it.

One of the youngest satellites for ionospheric research is Demeter ( Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions ) from 2004, which the French CNES sent, among other things, to investigate the possibilities for earthquake predictions .

The locations of all operational scatter radar stations

This designates a technology that sends earth-based radar waves against the ionosphere. This releases valence electrons there, the echo of which is evaluated. Information on electron density, ion and electron temperature, ion composition and plasma speed can be derived from the echo.

The word incoherent here means out of phase and refers to the fact that the medium to be examined is to be regarded as unstable in relation to the observation possibilities of the radar, i.e. H. the medium changes so quickly that these changes cannot be observed in detail with the radar.

There are currently nine such facilities worldwide.

Models

Precise knowledge of the parameters of the ionosphere, in particular the electron density, is essential for numerous applications such as radio communications, tracking satellites, and space observation of the earth. For this reason, models have been developed that are used to describe and analyze the ionosphere.

The most mature model in terms of its development time and the number of derivable quantities is the International Reference Ionosphere (IRI). The IRI is a joint project of the Committee of Space Research ( COSPAR ) and the International Union of Radio Science (URSI), which is further developed at annual workshops. This model has been the world standard for the terrestrial ionosphere since 1999.

Further models concentrate on certain ionospheric parameters such as electron density, maximum electron density in the F 2 layer, electron temperature and drift and strength of the electric field (see also web links ). In addition to global models, regional models are used to describe geographical details more precisely.

Ionospheric anomalies

Illustration of some of the processes that affect the state of the ionosphere.

A model of the ionosphere assumes a structurally homogeneous ionosphere due to its simplistic character. In reality, however, this is chaotic and does not show regular ionization structures. Ionospheric anomalies are deviations from the expected general behavior of the ionosphere. These irregularities are constantly observable and distinguish the anomalies from the spontaneously occurring, short-term ionospheric disturbances.

Some of the known anomalies are now presented.

Equatorial feature: Sun-generated, electrical ring currents on the day side of the ionosphere (equatorial electric jet)
Day anomaly
The maximum of the electron density does not coincide with the time of the highest sun position, but lies in the early afternoon hours.
Night anomaly
The ionization can increase even further during the night despite the lack of solar radiation.
Polar anomaly
An F-layer can be found over the areas of the polar night despite the long-term lack of solar radiation.
Seasonal anomaly
The electron density is higher in winter than in summer. Furthermore, the summer ionization maximum does not correlate with the highest position of the sun, but can be found at the equinoxes (equinoxes). Atmospheric processes are responsible for this, which lead to a decrease in the electron density in summer. In particular, the O / O 2 and O / N 2 ratio seems to be relevant, which controls the build-up and loss of ions in the F 2 layer. A summer excess of O 2 due to the global atmospheric circulation is seen as the cause of a decrease in electron density during this time of year.

The terrestrial magnetic anomaly

The fountain effect displaces electrons.

The maximum of the electron density does not lie above the equator . Rather, a strip with reduced ionization is formed there. The so-called fountain effect over the true magnetic equator occurs because the interaction of electrical and magnetic fields (ExB drift) pushes the free electrons of the F-layer to greater heights, from where they are then moved along the north-south magnetic field lines of the Earth's magnetic field to be shifted to the north or south. This increases the electron density on both sides of the magnetic equator. The terrestrial magnetic anomaly is also known as the equatorial anomaly.

The causal electric field is created by thermospheric tidal winds, which are directed westward during the day and which entrain the comparatively large ions through impact friction, but only a few electrons. Since field lines in the electric field point in the direction of the force that acts on a positive test charge, this is directed eastwards ( ionospheric dynamo layer ). In the magnetic field , the field lines in the outer area of ​​each magnet run from the magnetic north to the south pole, i.e. H. the geomagnetic north steep place. According to the three-finger rule, the Lorentz force acts on the free electrons of the ionosphere upwards at the equator.

The D-Layer Winter Anomaly

The D-layer winter anomaly was discovered in 1937 by Edward Victor Appleton and describes the phenomenon that above 35 ° latitude ( Berlin ≈ 52.5 °), on many winter days, the absorption capacity of the D-layer is significantly higher than that Angle of incidence of solar radiation would establish, often even higher than on summer days around noon. The anomaly typically extends several thousand kilometers, which is why a meteorological component is assumed to be the cause. However, the exact causes have not yet been established with certainty.

Furthermore, the day-to-day variance of the absorption capacity is much higher in winter than in summer and seems to increase with increasing geographical latitude, but this trend towards the poles is overlaid by other ionization influences. Although not influenced by special solar effects, the absorption can increase by a factor of 5 within two days, on average about 80% increase in attenuation is likely.

Ionospheric disturbances

Ionospheric disturbances are all spontaneously occurring irregularities in the structure of the ionosphere. The cause of an ionospheric disturbance can usually be found directly or indirectly in the solar radiation activity, but meteorites can also influence their ionization. The direct factors include increased solar ultraviolet, X-ray and / or particle radiation (corpuscular radiation) due to a disrupted increase in solar activity, the indirect ones include atmospheric-electromagnetic processes that can also occur in an undisturbed sun.

Ionospheric disturbances are of a short-term nature and can last from a few minutes to several days. The best known and most aesthetically valuable form of ionospheric disturbance is the aurora, the polar light , which is triggered by high-energy solar wind particles. On the other hand, the impairment of global shortwave radio traffic that it triggers is undesirable.

Ionospheric disturbances should not be confused with ionospheric anomalies. The latter do not occur spontaneously, but are subject to regularity and describe deviations from the expected general behavior of the ionosphere.

Coronal mass ejection from a flare (sun)
The propagation conditions of a flare ( red rays ) compared to those of a normal, calm ionosphere ( blue ray ): The electron density is increased in all layers. This leads to increased attenuation in the D-layer ( matt red ) up to total signal loss or unusual refraction at the E-layer.

The ionosphere is created by various types of radiation emitted by the sun , charged particles (also called corpuscles) or light waves, and have a direct effect on its state. A very intense short-term disturbance occurs as a result of an eruption on the sun's surface, which is known as flare (English: flare = bright, flickering light). On the sun, the outbreak of light affects only a very small area in the often particularly radiation-active peripheral areas of sunspots (so-called torch areas). This often leads to the ejection of charged particles, which is known as coronal mass ejection .

Outbreaks from charged ponds travel as a plasma cloud from the sun to the earth, where they are guided by the earth's magnetic field into the areas near the poles ( magnetospheric electric convection field ). There they change the ionosphere considerably, often for days, which leads to many failures in radio communications. While electromagnetic radiation travels to earth in around eight minutes, particle radiation takes up to 40 hours. The ionospheric disturbance it causes occurs at a different time than disturbances that can be traced back to electromagnetic radiation. Long-term disruptions are more serious for radio operations.

Characteristics of the ionospheric disturbances
event Arrival time after flare typical duration Type of radiation Effects
Sudden Ionospheric Disturbance (SID) 8.3 minutes
(travel at the speed of light )
10 to 60 minutes Ultraviolet and X-rays Increase in D-layer absorption on the day side
Polar Cap Absorption (PCA) 15 minutes to several hours ≈ 1 to 2 days, sometimes several days high energy protons and alpha particles Increase in D-layer absorption, especially in the polar regions
Ionospheric storm 20 to 40 hours 2 to 5 days weak energy protons and electrons Increase in D-layer absorption, decrease in F 2 MUF , Auroras , Sporadic-E

Electromagnetic radiation: Sudden Ionospheric Disturbance (SID)

Sudden Ionospheric Disturbances (SIDs) originate from increased X-ray and ultraviolet radiation from the sun. This is absorbed by the ionosphere and there leads to a strong increase in ionization, especially in the D-layer. SIDs are most frequently observed at the sunspot maximum and only occur on the day side of the earth.

The high plasma density increases the ability of the D-layer to absorb short waves up to their complete extinction, which is known as the Mögel-Dellinger effect . At the same time, an improvement in the propagation of long waves ( Very Low Frequency , VLF ) can be observed, since the D-layer can serve as a reflector for long waves. Increased ionization improves this reflective property. The sudden increase in signal strength from long-wave transmitters is used as an indicator of SIDs.

Particle radiation: polar cap absorption (PCA)

Entry of solar wind particles through the polar funnels
Polar cap absorption: change in the path of propagation in the polar regions

Connected to solar flares, high-energy protons (≈ 10 MeV) are ejected, which then penetrate into the atmosphere along the magnetic field lines of the earth near the magnetic poles and greatly increase the electron density in the lower ionosphere (D-layer, E-layer).

The additional charge carriers attenuate short waves so much that radio links, whose propagation path runs over the polar caps, can fail completely. Radio waves with a lower frequency, which would normally be reflected in the lower ionosphere, are now reflected at a much lower level, so that their propagation paths change significantly. This phenomenon is known as polar cap absorption (PCA).

PCA effects are usually short-lived in nature. While the Rothammel mentions the average duration of PCA effects 2–3 days, Kenneth Davies only mentions up to 5–6 hours.

More ionospheric disturbances

Paths of propagation during a Spread F event. The uneven distribution of the free electrons in the F-layer scatters short waves or causes unusual propagation paths.

As already mentioned, not all disturbances in the ionosphere can be traced back to solar radiation bursts. One such example is the so-called equatorial spread-F, an uneven distribution of the electron density of the F-layer in the equatorial region. The reason for this are electrical currents in the ionosphere as a result of rotational differences between free electrons and ions, since the latter are subject to mechanical friction, but the former are not. These non-sun-induced events are divided into two types, namely with regard to the spatial structure of the disturbances. According to these are transient phenomena ( Transient Phenomena ) and wandering ionospheric disturbances ( Traveling Ionic Disturbances , TIDs).

As their name suggests, the transient phenomena are short-lived, fleeting in nature. Furthermore, they occurred locally in cloud-shaped form and move horizontally, i.e. at the same height, through the ionosphere. This type includes, for example, sporadic e-events and equatorial spread-F .

In contrast, TIDs are wave-like fluctuations in electron density with a front width of up to several hundred kilometers. They can last from a few minutes to several hours and are expressed in strong fluctuations in the level of reflection and the MUF. These TID effects do not have a serious impact on shortwave propagation. The largest TIDs start in the northern lights area and spread out towards the equator.

Luminous phenomenon in the ionosphere: Elves

Thunderstorms can cause minor TID fronts that travel approximately 200 km before they disperse. Thunderstorms are also the cause of a luminous phenomenon known as Elves in the ionosphere, which however only lasts less than a thousandth of a second and is therefore not a TID. Another thunderstorm phenomenon are the low-frequency electromagnetic signals known as Whistler . wander through the ionosphere.

The sporadic E-layer (E S )

Paths of propagation during a sporadic E-event ( blue ) and without ( red )

The sporadic E-layer (English Sporadic-E ) lies in the area of ​​the E-layer and occurs only sporadically. It is strongly ionized and can cover all higher layers. Their structure is often cloud-like, but can also be homogeneous over a wide area. It can lead to unexpectedly long ranges.

Usually radio signals above the normal cut-off frequency penetrate the E-layer. During a sporadic E-event, the signals are reflected there, which worsens long-range connections, but leads to better reception within the first jump zone or dead zone .

There are several theories about the origin of the E S layer, but it has not yet been fully elucidated.

Ionospheric storms

Daily fluctuations in temperature and wind at an altitude of 100 km in September 2005.

In the course of ionospheric storms, both an abnormal increase and decrease in electron density can occur. The former case is known as a positive ionospheric storm , the latter as a negative ionospheric storm.

Ionospheric storms can have solar or terrestrial causes. For example, increased particle radiation from the sun can reduce the electron density: The solar plasma consisting of protons and electrons ejected by a flare influences the earth's magnetic field and penetrates the atmosphere. This results in a decrease in the critical frequency of the F 2 layer to half of its normal value and an increase in the D layer absorption. As a result, the frequency range that can be used for shortwave radio is narrowed on both sides. Intense ionospheric storms can cause complete blackouts for long-distance connections. This is known as a so-called short-wave fade (out) .

Ionospheric storms can also have atmospheric causes: Today it is assumed that increases in electron density are often due to thermospheric winds , while decreases are mainly caused by changes in the composition of the neutral gas, e.g. B. by a decrease in elemental oxygen and thus a reduced rate of ion production. Bubbles with a reduced plasma density are seen as the cause of the transequatorial propagation (TEP for short).

Scientific Research

The Arecibo Observatory was originally designed to study the ionosphere.
Arecibo observatory
The Arecibo Observatory in Puerto Rico , known from some movies ( GoldenEye , Contact ), was originally designed to study the ionosphere. It was the world's second largest radio telescope and served primarily astronomical purposes. Its use was open to all astronomers; an independent committee decided on the applications.
HAARP
The High Frequency Active Auroral Research Program (HAARP) is an American research project in which the ionosphere is irradiated with intense short waves through a network of transmitter systems .
Sura
A similar research facility to HAARP is the Russian Sura Research Facility .
EISCAT
The European Incoherent Scatter (EISCAT) is a research radar that investigates the ionosphere with microwave radiation according to the functional principle of the incoherent scatter radar.
SHARE
The Southern Hemisphere Auroral Radar Experiment (SHARE) is a research project in Antarctica that observes the electrical fields of the ionosphere and magnetosphere.
MARSIS
The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is one of seven instruments on board the Mars Express Mars probe launched by ESA in 2003 , which is used to explore the Mars ionosphere . MARSIS sends out radio waves in the range from 1.3 to 5.5 MHz and creates ionograms from the reflected echoes . The measurements have shown that the Mars ionosphere has a third layer in the range between 65 and 110 km in addition to the two known ionospheric layers at an altitude of 110 and 135 km. This layer is sporadic and localized.

History

• 1899: Nikola Tesla researches ways to wirelessly transmit energy over long distances. In his experiments he sends extremely low frequencies to the ionosphere, up to the Kennelly-Heaviside layer (Grotz 1997). From calculations based on the measurement results, Tesla can predict a resonance frequency of this layer that deviates only 15% from the value assumed today (Corum, 1986). In the 1950s, researchers confirmed that the resonance frequency is 6.8 Hz.
• Guglielmo Marconi, around 1907
1901: On December 12th, Guglielmo Marconi received the first transatlantic radio signal in St. John's (Newfoundland) . It uses a 400-foot receiving antenna stretched by a kite. The transmitter station on The Lizard peninsula in Poldhu , Cornwall , uses a spark inductor to generate the transmission frequency of approximately 500 kHz with a power 100 times stronger than any previously generated signal. The message received consists of three dots in Morse code, an S. To reach Newfoundland, the signal had to be reflected twice from the ionosphere.
• Oliver Heaviside
1902: Oliver Heaviside predicts the existence of the Kennelly-Heaviside layer that bears his name. His proposal included ideas on how radio signals could be transmitted along the curvature of the earth. In the same year Arthur Edwin Kennelly described some of the radio-electrical properties of the ionosphere.
• 1909: Guglielmo Marconi receives the Nobel Prize in Physics together with Karl Ferdinand Braun .
• 1912: The Congress of the United States of America passes the Radio Act , which limits radio amateurs' radio operations to frequencies above 1.5 MHz (wavelengths less than 200 m). These frequencies were considered useless by the government. This decision led to the discovery of the ionospheric RF radio wave propagation in 1923 ( Léon Deloy ).
• 1924: Edward Victor Appleton proves the existence of the Heaviside layer and receives the Nobel Prize for it in 1947 .
• 1926: The British physicist Robert Watson-Watt coined the term "ionosphere".
• 1926: The American physicist Merle Antony Tuve develops a radar method with variable frequency for exploring the ionosphere.
• 1926 A. Hoyt Taylor and Edward Olson Hulburt develop a theory of the electron density distribution in the ionosphere, which is based on the observed jump distance of short-wave radio waves and thus also provide a theory for the propagation of short-wave radio waves in the earth's atmosphere.
• 1932: Sydney Chapman derives a distribution function for ionization in the ionosphere, assuming monochromatic ionizing radiation from the sun.
• 1932: Lloyd Viel Berkner was the first to measure the height and density of the ionosphere, which enabled the first complete model of shortwave propagation. In doing so, he discovers the F 1 layer.
• 1936: Maurice V. Wilkes does his doctorate on the propagation of longitudinal waves in the ionosphere.
• 1942: Vitaly Ginzburg investigates radio wave propagation in the ionosphere and develops a theory about the propagation of electromagnetic waves in the plasma of the ionosphere. In 2003 he received the Nobel Prize for his pioneering work in the field of superconductors .
• 1946: On January 10, John Hibbett DeWitt and his research group succeeded in proving that radio waves can penetrate the ionosphere as part of Project Diana . He uses the moon as a reflector and thus establishes the first earth-moon-earth connection.
• 1946: On November 23, Arthur Covington proves during a partial solar eclipse that the sunspot activity can be determined via the solar radio flux .
• 1955: The Schumann resonances are verified by the physicist WO Schumann at the Technical University of Munich .
• 1958: In August and September 1958, the US Navy conducts three secret atomic bomb tests in the ionosphere during Operation Argus to investigate the effect of electromagnetic pulse (EMP) on radio and radar.
• 1962: The Canadian Alouette 1 satellite is launched to explore the ionosphere. After its successful use, Alouette 2 and two satellites of the ISIS program (International Satellites for Ionospheric Studies) followed in 1969 and 1971, all in use for ionospheric research.
• 1970: Hannes Alfvén receives the Nobel Prize in Physics "for his fundamental achievements and discoveries in magnetohydrodynamics with fruitful applications in various parts of plasma physics ".
• 1992: The luminous phenomenon known as Elves is detected for the first time with the help of recordings from on board the space shuttle .
• 1999 The model "International Reference Ionosphere" (IRI) supported by the URSI and COSPAR unions becomes "international standard"

Trivia

The Kennelly Heaviside layer, named after Oliver Heaviside among others, was taken up by TS Eliot in his poem "The Journey To The Heaviside Layer", which was set to music in the musical Cats .

literature

The following web links are in English.

Further information
Basics of ionospheric wave propagation: Navy Postgraduate School: HF and Lower Frequency Radiation ( Memento from May 20, 2007 in the Internet Archive )
Introduction to Space Weather: Space Weather, A Research Perspective
Introduction to the Ionosphere: Space Environment Center, Dave Anderson and Tim Fuller-Rowell: The Ionosphere (1999) (PDF file; 128 kB)
Current data
Current Space Weather: Space Weather Enthusiasts Dashboard | NOAA / NWS Space Weather Prediction Center
Current ionospheric data: SEC's Radio User's Page
Current 2D map of electron density ( TEC ): NASA: Ionospheric and Atmospheric Remote Sensing
Current 3D view of electron density ( TEC ) via Google Earth: NASA: 4D Ionosphere
Current TEC maps (global / Europe) from DLR: SWACI (Space Weather Application Center - Ionosphere)
Ionospheric models
Overview of ionospheric models: NASA Space Physics Data Facility: Ionospheric Models index
International Reference Ionosphere
Ionospheric parameters
Overview of all ionospheric parameters: Space Physics Interactive Data Resource: Ionospheric Vertical Incidence Parameters ( Memento from June 19, 2008 in the Internet Archive )
Ionospheric measurement
Incoherent scatter radar tutorial: National Astronomy and Ionosphere Center: How does the Arecibo 430 MHz radar make measurements in the ionosphere?
List of ion probes: UMass Lowell Center for Atmospheric Research: Digisonde Station List
Millstone Hill incoherent scatter radar ( Memento June 6, 2010 in the Internet Archive )
Current diagrams of the ionospheric probe in Juliusruh
multimedia
Commons : Ionosphere  - collection of images, videos and audio files

Individual evidence

1. American Meteorological Society: Glossary of Meteorology ( Memento of February 2, 2007 in the Internet Archive )
2. a b c d Stefan Heise: The ionosphere and plasma sphere of the earth. urn : nbn: de: kobv: 188-2002002731 , Chapter 2
3. Karl Rothammel : Rothammels Antennenbuch . Newly edited and expanded by Alois Krischke. 12th updated and expanded edition. DARC-Verl., Baunatal 2001, ISBN 3-88692-033-X , 2. The propagation of electromagnetic waves ( online ).
4. W. Suttrop: Astrophysical Plasmas I (PDF file; 557 kB). P. 7.
5. Max Planck Institute for Aeronomy: Research Info (8/98) , p. 2. ( Memento from May 23, 2009 in the Internet Archive ) (PDF file; 1.1 MB)
6. E. Chvojková: properties of the ionospheric F-SchichtII
7. a b S.J. Bauer: Physics and Chemistry in Space 6 - Physics of Planetary Ionospheres - Chapter IX: Observed Properties of Planetary Ionospheres. Springer-Verlag (1973)
8. PROPAGATION IN HOMOGENEOUS PLASMAS ( Memento of February 17, 2013 in the Internet Archive ) (PDF; 2.2 MB)
9. IONOSPHERIC WAVE PROPAGATION ( Memento from January 23, 2013 in the Internet Archive ) (PDF; 1.4 MB)
10. Ionospheric effects (PDF; 4.1 MB)
11. ^ Dielectric constant of a plasma
12. Eckart Moltrecht (DARC e.V. online for the amateur radio examination): Amateur radio course for the amateur radio certificate class E. ( Memento from June 21, 2008 in the Internet Archive )
13. ^ Beer, Tom: The Aerospace Environment , p. 80.
14. ^ Leslie Curtis, Les Johnson: Propulsive Small Expendable Deployer System (ProSEDS). NASA, 2002, accessed July 1, 2019 .
15. Friedemann T. Freund: Rocks That Crackle and Sparkle and Glow: Strange Pre-Earthquake Phenomena ( Memento from July 6, 2010 in the Internet Archive ) (PDF; 556 KB)
16. a b O. Molchanov et al. Global diagnostics of the ionospheric perturbations related to the seismic activity using the VLF radio signals collected on the DEMETER satellite. In: Nat. Hazards Earth Syst. Sci. 6 (2006), pp. 745-753.
17. AJ Foppiano, EM Ovalle, K. Bataille and M. Stepanova: Ionospheric evidence of the May 1960 earthquake over Concepcion? ( Memento from September 21, 2008 in the Internet Archive )
18. a b Zhu Rong, Yang Dong-mei, Jing Feng, Yang Jun-ying and Ouyang Xin-yan: Ionospheric perturbations before Pu'er earthquake observed on DEMETER ; In: Acta Seismologica Sinica, January 2008, Volume 21, Issue 1, pp 77-81 doi: 10.1007 / s11589-008-0077-8
19. Hanns-Jochen Kaffsack, DPA: When the ionosphere coughs
20. W.Piggott, K.Rawer: URSI Handbook of Ionogram Interpretation and Reduction. Elsevier, Amsterdam 1961.
21. ^ Background to Ionospheric Sounding
22. Leibniz Institute for Atmospheric Physics, Radar / Rockets Department: General ( Memento from December 9, 2008 in the Internet Archive )
23. C. Stolle, S. Schlüter, N. Jakowski, Ch. Jacobi, S. Heise, A. Raabe: in the ionosphere with the integration of GPS occultations ( Memento from January 7, 2007 in the Internet Archive ) (PDF file; 371 kB). Retrieved March 5, 2010.
24. a b International Reference Ionosphere
25. a b Gerd W. Prölss: Physics of Near-Earth Space ( limited preview in the Google book search)
26. URSI Incoherent Scatter Working Group: Incoherent Scatter Radars
27. ^ IRI Workshops and Proceedings
28. D. Bilitza: Solar-Terrestrial Mpdels and Application software. National Space Science Data Center / WDC-A 1990.
29. a b Tadanori Ondoh, Katsuhide Marubashi: Science of Space Environment ( limited preview in Google book search)
30. ^ EV Appleton: The Bakerian Lecture. Regularities and Irregularities in the Ionosphere. I . In: Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences . tape 162 , no. 911 , 1937, pp. 451-479 .
31. WJG Beynon, ER Williams, F. Arnold, D. Krankowsky, WC Bain, PHG Dickinson: D-region rocket measurements in winter anomaly absorption conditions . In: Nature . tape 261 , no. 5556 , 1976, pp. 118-119 , doi : 10.1038 / 261118a0 .
32. a b R. W. Knecht: The Distribution of Electrons in the Lower and Middle Ionosphere. In: Progress in Radio Science, 1960–1963. Volume 3: The ionosphere. Review papers presented at commission III on ionospheric radio during the XIVth general assembly of URSI. 1965, pp. 14-45.
33. a b c Navy Postgraduate School: HF and Lower Frequency Radiation ( Memento from May 20, 2007 in the Internet Archive )
34. YES Adcock (VK3ACA): Propagation of long radio waves ( Memento of 22 February 2014 Internet Archive ).
35. ^ The American Association of Variable Star Observers: Sudden Ionospheric Disturbances ( Memento from May 2, 2009 in the Internet Archive ).
36. Windows to the Universe: Polar Cap Absorption Events - Massive Short Wave Communications Blackouts .
37. Kenneth Davies: Ionospheric radio propagation. 1965 (US Department of Commerce, National Bureau of Standards).
38. a b
39. ^ A b NOAA National Severe Storms Laboratory: Transient Luminous Events ( Memento from July 25, 2012 in the Internet Archive ).
40. ^ ESA: Results from Mars Express and Huygens: Mars Express radar reveals complex structure in ionosphere of Mars
41. ^ ESA: Results from Mars Express and Huygens: Mars Express discovers new layer in Martian ionosphere
42. ^ Text of 1912 Act, Fifteenth
43. ^ Niels Klussmann, Arnim Malik: Lexicon of aviation. P. 130 ( limited preview in Google Book search)
44. National Academy of Sciences: Biographical Memoirs Vol. 70
45. ^ A. Hoyt Taylor, EO Hulburt: The Propagation of Radio Waves Over the Earth . In: Physical Review . 27, No. 2, February 1926, pp. 189-215. doi : 10.1103 / PhysRev.27.189 .
46. ^ National Academy of Sciences: Biographical Memoirs Vol. 61
47. ^ Kertz, Walter: Biographical Lexicon for the History of Geophysics ( Memento from June 19, 2008 in the Internet Archive )
48. ^ Virginia Tech, Department of Computer Science: The History of Computing: Maurice Vincent Wilkes
49. ^ PN Lebedev Physical Institute, IETamm Theory Department: V. Ginzburg - Selected Scientific Papers
50. International Reference Ionosphere.