IonosphereEarth's atmosphereAn ionized region of.The ionosphere is exposed to solar high-energy radiation andcosmic rayThe excitation of ionized upper atmosphere.The whole earth over 60 kilometersatmosphereBoth are in the state of partial ionization or complete ionization. The ionosphere is a partially ionized atmospheric area, and the fully ionized atmospheric area is calledMagnetosphere。Others call the whole ionized atmosphere the ionosphere, so the magnetosphere is regarded as a part of the ionosphere.In addition to Earth, Venus, Mars and Jupiter all have ionosphere.The ionosphere extends from about 50km above the ground to about 1000km above the earth's upper atmosphere. There are quite a few free electrons and ions in the ionosphere, which can make radio waves change their propagation speed, refract, reflect and scatter, produce rotation of the polarization surface and be absorbed to varying degrees.
Due to the ionization of neutral, atoms and air molecules by radiation outside the earth (mainly solar radiation), the entire earth's atmosphere more than 60 kilometers from the earth's surface is in a state of partial or complete ionization. The ionosphere is a partially ionized atmospheric region, and the fully ionized atmospheric region is called the magnetosphere.Others call the whole ionized atmosphere the ionosphere, so the magnetosphere is regarded as a part of the ionosphere.In addition to Earth, Venus, Mars and Jupiter also have ionosphere.
While ionization produces free electrons, collision recombination between electrons and positive ions, as well as electron attachment to neutral molecules and atoms, will cause the disappearance of free electrons.The movement of various wind systems in the atmosphere, the existence of polarized electric fields, the frequent invasion of foreign charged particles, and the diffusion of the gas itself cause the migration of free electrons.In the area below 55km height, the atmosphere is relatively dense, collision is frequent, free electrons disappear quickly, and the gas remains non-conductive.At the top of the ionosphere, the atmosphere is unusually thin, and the migration of ionization is mainly controlled by the earth's magnetic field, which is called the magnetosphere.
The main characteristics of the ionosphere are represented by the basic parameters of spatial distribution, such as electron density, electron temperature, collision frequency, ion density, ion temperature and ion composition.However, the ionosphere is mainly concerned with the distribution of electron density with height.Electron density (or electron concentration) refers to the number of free electrons per unit volume. The change with height is related to atmospheric composition, atmospheric density, solar radiation flux and other factors at each height.The electron density at any point in the ionosphere is determined by the three effects of the generation, disappearance and migration of free electrons mentioned above.In different regions, the relative role of the three and their specific ways of action are also very different.
The discovery of the ionosphere not only makes people have a deeper understanding of the various mechanisms of radio wave propagation, but also has a clearer understanding of the structure and formation mechanism of the Earth's atmosphere.
Research history
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Figure 2 Trends of Ionospheric Beijing in China
In 1899, Nikola Tesla tried to use the ionosphere for long-distance wireless energy transmission.He sends extremely low frequency waves between the ground and the ionosphere, the so-called Konor Lihai Weisai layer.Based on his experiment, he carried out mathematical calculation, and his calculation of the resonance frequency in this area is less than 15% different from today's test results.In the 1950s, scholars confirmed that the resonance frequency was 6.8 Hz.
On December 12, 1901, Guglielmo Marconi harvested the transatlantic signal transmission for the first time.Marconi used a 400 foot long antenna erected by a kite.The frequency used by the transmitting station in the UK is about 500kHz, and its power is 100 times that of all transmitters up to that time.The received signal is S (three points) in Morse code.To cross the Atlantic, this signal must be reflected twice by the ionosphere.Some people doubt Marconi's results by continuing theoretical calculations and today's experiments, but Marconi undoubtedly achieved trans Atlantic propagation in 1902.
In 1902, Oliver Hewieser put forward the theory of Konor Lihai Weisai layer in the ionosphere.This theory shows that radio waves can bypass the earth's sphere.This theory, together with Planck's black body radiation theory, may have hindered the development of radio astronomy.In fact, it was not until 1932 that human beings detected radio waves from celestial bodies.In 1902, Arthur Kennelly also discovered some radio electronic properties of the ionosphere.
In 1912, the United States Congress passed the 1912 Broadcasting Act, which ordered amateur radio stations to work only at 1.5 MHz or more.At that time, the government thought that the above frequency was useless.This led to the discovery of the use of the ionosphere to propagate high-frequency radio waves in 1923.
In 1947, Edward Appleton won the Nobel Prize in Physics for confirming the existence of the ionosphere in 1927.Morris Wilkes and John Laclif studied the propagation of extremely long wavelength radio waves in the ionosphere.Vitaly Ginzburg proposed the theory of electromagnetic wave propagation in plasma such as the ionosphere.
In 1962, the Canadian satellite Alouette 1 was launched to study the ionosphere.Its success drove the launch of Alouette 2 satellite in 1965 and the launch of ISIS1 and ISIS2 in 1969 and 1971.These satellites are all used to study the ionosphere.
Formation mechanism
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The ionization of the atmosphere is mainly caused by ultraviolet and X-ray in solar radiation.In addition, solar high-energy charged particles and galactic cosmic rays also play an important role.Molecules and atoms in the earth's upper atmosphere are ionized under the action of solar ultraviolet rays, x-rays and high-energy particles to produce free electrons, positive and negative ions, and form the plasma region, namely the ionosphere.The ionosphere is macroscopically neutral.The change of the ionosphere is mainly manifested in the change of electron density with time.The condition for the electron density to reach equilibrium mainly depends on the rate of electron formation and the rate of electron disappearance.
Fig. 3 Ionospheric knowledge broadening
The electron generation rate refers to the number of electrons produced per second per unit volume by neutral gas that absorbs solar radiation and ionizes.The rate of electron disappearance refers to the number of electrons disappeared per second per unit volume when the drift motion of electrons is not considered.Charged particles recombine through collision and other processes, reducing the number of electrons and ions;Drift and other movements of charged particles can also change the density of electrons or ions.
Internal tiering
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4.1 Overview
Ionospheric morphology is the spatial structure (height, longitude and latitude distribution) of basic parameters such as electron density in the ionosphere and its change with time (day and night, season and solar activity cycle).The ionosphere can be divided into D layer, E layer and F layer from low to high, among which F layer can also be divided into F1 layer and F2 layer.In E layer and F1 layer, the effect of electron migration is small, which has the main characteristics of Chapman layer.The critical frequency П of the layer (its square is proportional to the peak electron density) and the solar zenith angle ě approximately satisfy the relationship П=ě cos ě (MHz) derived from the simple layer theory, where ě and b are constants.This relation reflects the basic trend of the ionospheric electron density over time and region.In the higher F2 layer, ionization transport plays an important role;In the magnetic pole of the earth, there is bombardment of foreign charged particles, and the shape is more complex.The peak shape of layer D and layer F1 is generally not very prominent.Figure 1 shows the typical height distribution of the ionospheric electron density.
Figure 4 Current distribution of ionospheric TEC in China
4.2D layer
It is about 50~90 km from the ground.In daytime, the typical values of peak density NmD and corresponding height hmD are about 10 cm and 85 km respectively.The short wave in the radio wave is greatly absorbed in this layer.The absorption of solar activity in the highest year is almost twice that in the lowest year.In a year, the summer value of NmD is greater than the winter value, but in the mid latitude region, abnormal absorption sometimes occurs in winter.At night, ionization basically disappears.
Fig. 5 Typical height distribution of ionospheric electron density
4.3E floor
It is about 90-130km above the ground.In daytime, the typical values of peak density NmE and its corresponding height hmE are 10 cm and 115 km respectively.The three variations of NmE, day and night, season and solar activity cycle, roughly conform to the formula of simple layer theory, and reach the maximum at noon, summer and high activity year respectively;At this time, the constant or ≈ 0.9 (1801.44R) in the formula, b ≈ 0.25, R is the average value of sunspot number flow in 12 months.At night, NmE decreased and hmE increased;NmE ≈ 5 × 10 cm, and the change range of hmE generally does not exceed 20 km.
4.4F floor
About 130 km above the ground, it can be divided into F1 and F2 layers.
① F1 layer (about 130-210km from the ground): in daytime, the typical values of peak density NmF1 and its corresponding height hmF1 are 2 × 10cm and 180km respectively.The peak shape of F1 layer disappears at night, and F1 layer in mid latitude only appears in summer. F1 layer becomes obvious during high solar activity years and ionospheric storms.The change of NmF1 and hmF1 is similar to that of layer E, which roughly conforms to the theoretical formula of simple layer. At this time, ∨≈ 4.30.01R, b ≈ 0.2.
Figure 6 Peak density of ionospheric layers Nm and corresponding height hm
② F2 layer (about 210km above the ground): the upper boundary of the main area reflecting radio signals or affecting radio wave propagation conditions is connected with the magnetosphere.In the daytime, the typical values of peak density NmF2 and its corresponding height hmF2 are respectively 10 cm;At night, NmF2 generally still reaches 5 × 10 cm.In any season, the noon value of NmF2 is positively correlated with solar activity.Generally, there is also a positive correlation between hmF2 and solar activity. Except for the equatorial region, the nighttime value is higher than the daytime value.In F2 layer, various wind systems, diffusion and other dynamic factors of the earth's magnetic atmosphere play an important role, and their morphological changes cannot be described by Chapman's simple layer theory, so F2 layer has various "anomalies" compared with E layer and F1 layer.The so-called daily variation anomaly means that the maximum value of the F2 layer electron density does not appear at noon (usually between 13:00 and 15:00 local time). At the same time, NmF2 also has a half day variation component, and its maximum values are respectively at 10~11:00 local time and 22~23:00 local time.Seasonal anomaly means that the electron density of F2 layer at noon is higher in winter than in summer.The equatorial anomaly means that the electron density of F2 layer is not the maximum over the equator, it is obviously controlled by the geomagnetic field, and its geographical change shows a "double peak" phenomenon, reaching the maximum value near ± 20 degrees of magnetic latitude.In high latitudes, many abnormal phenomena related to the deposition of charged particles can be observed.Among them, the most important is the "trough" of the F layer, which is the zone of low electron density on the earth's sunward side from the aurora to the low latitude with a width of about 5~10 degrees.
The temporal variation of the electron density and the total ionospheric electron content at a fixed height above the peak is similar to that of NmF2.Figure 2 shows the average diurnal variation of the peak density (Nm) and corresponding height (hm) of each layer of the ionosphere in the mid latitude region.
In addition to the above uniform thick layers, there are two more common inhomogeneous structures in the ionosphere: the Es layer, namely the occasional E layer (see the radio wave propagation in the Es layer) and the extended F layer (see the ionospheric inhomogeneous body).
structure
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Solar radiation ionizes some neutral molecules and atoms into free electrons and positive ions. The deeper it penetrates the atmosphere, the weaker its strength (the ability to generate ionization) becomes, and the density of the atmosphere gradually increases, so the maximum value of ionization appears at a certain height.Different components of the atmosphere, such as molecular oxygen, atomic oxygen and molecular nitrogen, are distributed unevenly in space.They are ionized by radiation of different wave bands, forming their own extreme regions, thus leading to the layered structure of the ionosphere.The ionosphere is stratified vertically, generally divided into layer D, layer E and layer F, and layer F is divided into layer F1 and layer F2.The maximum electron density is about 10 cm, about 300 km high.In addition to the normal layer, there are also inhomogeneous structures in the ionosphere, such as the occasional E layer (Es) and extended F.Occasional E-layer is common, which is an uneven structure in E-layer area.The thickness ranges from several hundred meters to twelve thousand meters, the horizontal extension is generally 0.1 to 10 kilometers, the height is about 110 kilometers, and the maximum electron density can reach 10 centimeters.Extended F is an inhomogeneous structure that occurs in the F layer. In the equatorial region, it often extends along the geomagnetic direction and is distributed in the ionospheric region of 250~1000 km or more.
Figure 7 Route of radio wave propagation
The layered structure of the ionosphere is only an ideal description of the state of the ionosphere. In fact, the ionosphere always presents complex spatial changes with latitude and longitude, and has diurnal, seasonal, annual, sunspot cycle and other changes.Because the chemical structure and thermal structure of each layer of the ionosphere are different, the morphological changes of each layer are also different.
pattern
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The ionospheric model is a mathematical description of the variation of ionospheric parameters with height.This change is related to geographical location, season, local time, and solar and geomagnetic activity.The complex ionospheric morphology brings great difficulties to practical applications. Therefore, based on a large number of measured data, people use simpler mathematical models to describe the ionospheric morphology and structure, so as to apply them in engineering design such as radio communication and aerospace.The most studied is the electron density mode which has a direct impact on radio wave propagation.
Where N (h) is the electron density at the height h above the ground;H0 is the starting height;α is a constant;≮ is the half thickness of the layer.These models can only describe a part of the ionospheric electron density profile.In order to describe the profile completely, different mathematical expressions must be used in different parts.
For the electron density profile below the peak value of F layer, different combination modes can be used according to different practical applications.The Bradley Dudley model recommended by the International Radio Consultative Committee for short wave field strength calculation is a combination of parabolic model (F2 layer), linear model (F1 layer) and parabolic model (E layer).The model parameters can be calculated from the characteristic parameters obtained from the ionospheric observation stations.Generally, the height difference between the obtained electron density distribution and the actual distribution is less than 20 km.Other modes include: cosine mode (F2 layer) - secant mode (E-F layer) - parabolic mode (E-layer), which can be used for ray tracing calculation with high accuracy requirements;Parabolic mode (F2) layer and polynomial combination mode are convenient to calculate the peak height, peak elevation and equivalent plate thickness under the peak of F2 layer from the frequency height diagram of ionospheric altimeter.
Electron density, electron temperature and ion temperature profiles given in the International Reference Ionosphere (IRI, 1979).
In the electron density profile including the peak area of F layer, Bent model and Pennsylvania 1 ionospheric model are more typical.The altitude range of Bent mode is about 150km to 2000km.Below the peak height is parabolic square mode, above the peak height is parabolic mode;At a higher altitude, there are three connected exponential modes.Bent model ignores the details of the profile (especially the F region) and focuses on accurately expressing the ionospheric electron content.It is suitable for calculating the time delay and direction change caused by radio wave refraction.The Pennsylvania 1 ionospheric model (120-1250km) simulates the physical and chemical processes of the ionosphere in an empirical altitude range, and calculates the electron density by adjusting the ionization reaction rate and vertical electron flow.This model is mainly used to study the transport process and wind attenuation.
The International Reference Ionosphere is compiled by the International Union of Radio Sciences and the American Space Research Council based on the measured data of the ionosphere. It is a special computer program, and the input data are geographic longitude and latitude, month, local time, and sunspot number.The output data is the vertical distribution of ionospheric parameters.Figure 3 is an example of an output profile.
Due to the excitation of various disturbance sources from outer space, the sun and the earth's atmosphere, the ionosphere will also produce corresponding disturbance changes and irregular structures, showing various forms (see ionospheric disturbance, ionospheric inhomogeneity, ionospheric modulation)
abnormal
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7.1 Overview
In fact, the ionosphere is not composed of regular and smooth layers as described above.The actual ionosphere consists of massive, cloud like, and irregular ionized masses or layers.
7.2 Winter abnormalities
Figure 8 Current in the ionosphere on the Chaoyang surface
In summer, the ionization degree of F2 layer in the mid latitude region is higher in the daytime due to direct sunlight, but the ratio of molecules to single atoms is also higher in summer due to the influence of seasonal airflow, resulting in an increase in ion capture rate.The increase of the capture rate is even stronger than the increase of the ionization degree.Therefore, F2 layer in summer is lower than that in winter.This phenomenon is called winter anomaly.Winter anomalies occur every year in the Northern Hemisphere, and there is no winter anomaly in the Southern Hemisphere in the year of low solar activity.
7.3 Equatorial anomaly
The current in the ionosphere of the sunrise surface forms a trench with high ionization degree in F2 layer between about ± 20 degrees around the earth's magnetic equator, which is called equatorial anomaly.The causes are as follows: the earth's magnetic field near the equator is almost horizontal.The plasma in the lower ionosphere moves upward and crosses the earth's magnetic field line due to the heating of sunlight and the tidal effect.This forms a current in layer E, which interacts with the horizontal magnetic field line to strengthen the ionization degree of layer F between ± 20 degrees near the magnetic equator.
disturb
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8.1 X-ray
During the active period of the sun, when a strong flare occurs, hard X-rays will shoot at the earth.These rays can penetrate to layer D, where they quickly lead to a large number of free electrons, which absorb high-frequency (3-30MHz) radio waves and cause radio interruption.At the same time, low frequency (3-30kHz) will be reflected by layer D (not by layer E) (generally, layer D absorbs these signals).After the end of the X-ray, the D-layer electrons are quickly captured, the radio interruption will end soon, and the signal will recover.
8.2 Protons
The flare also releases high-energy protons.These protons arrived at Earth 15 minutes to 2 hours after the flare burst.These protons spiral along the Earth's magnetic field line and hit the Earth's atmosphere near the magnetic pole, increasing the ionization of the D and E layers.Polar absorption can last for an hour to several days, with an average duration of 24 to 36 hours.
8.3 Geomagnetic storm
The geomagnetic storm is a temporary and violent disturbance of the earth's magnetic field.
During geomagnetic storms, F2 layer is very unstable and will split or even disappear completely.There will be auroras near the poles.
measure
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9.1 Ionospheric map
The ionospheric chart shows the height of the ionospheric layer and its critical frequency measured by the ionosonde.The ionosonde sends a series of frequencies (generally from 0.1 to 30MHz) vertically to the ionosphere.As the frequency increases, the signal can penetrate higher layers before being reflected.Finally, the frequency is high enough that it is no longer reflected.
9.2 Solar flow
The solar current is the intensity of solar radiation at 2800MHz measured by a radio telescope in Ottawa, Canada.The measurement results show that this intensity is commensurate with sunspot activity.However, the ionization of the upper atmosphere of the earth is mainly caused by the ultraviolet and X-ray of the sun.The geostationary operational environment satellite can measure the X-ray flow of the sun.This data is more corresponding to the ionization degree of the ionosphere.
9.3 Research projects
Figure 9 Ionospheric Monitoring Diagram
Scientists use different means to study the structure of the ionosphere, including passive observation of optical and radio signals generated by the ionosphere, study of signals reflected by different radio telescopes, and the difference between the reflected signal and the original signal.
The 20-year high-frequency active aurora research program and similar projects started in 1993 studied the use of high-energy radio transmitters to change the characteristics of the ionosphere.These studies focus on the characteristics of the ionospheric plasma to better understand the ionosphere, and use it to improve civil and military communication and telemetry systems.
The super twin auroral radar network studies the coherent scattering of 8 to 20 MHz frequencies at high and medium altitudes.Coherent scattering, similar to Bragg scattering of crystals, is a kind of phase increasing diffraction scattering caused by the difference of ionospheric density.This project includes multiple radars in 11 different countries around the world.
Scientists also measure changes in the ionosphere caused by radio waves from satellites and other stars.The Arecibo Observatory in Puerto Rico was originally intended to study the Earth's ionosphere.
And radio wave propagation
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10.1 Overview
The influence of the ionosphere on radio wave propagation is closely related to human activities, such as radio communication, broadcasting, radio navigation, radar positioning, etc.The bands affected by the ionosphere range from extremely low frequency (ELF) to very high frequency (VHF), but the medium wave and short wave bands have the greatest impact.The ionosphere, as a kind of propagation medium, makes radio waves refracted, reflected, scattered and absorbed, thus losing part of energy in the propagation medium.3~30 MHz is a short band, which is the most appropriate band for ionospheric long-distance communication and broadcasting. Under normal ionospheric conditions, it corresponds to the lowest available frequency and the highest available frequency.However, due to multi-path effect, the signal fading is large;Ionospheric storms and sudden ionospheric disturbances may seriously affect ionospheric communication and broadcasting, or even interrupt signals.300 kHz to 3 MHz are medium band, widely used for short distance communication and broadcasting.
10.2 Radio
A hundred years ago, three short and weak signals announced the birth of radio to the world.In 1901, Marconi, an Italian scientist who camped at Signal Hill (located in the southeast corner of Canada), finally received the transatlantic radio signal sent from England. This experiment proved that radio is no longer just a novelty in the laboratory, but a practical communication medium.Since then, shortwave as a global international communication medium has begun to develop.
According to the journal Science《Geophysical Yearbook》(Annales Geophysicae),the Second World WarThe shock wave generated by the large-scale Allied bombing attack during the period was enough to reduce the electron concentration in the Earth's ionosphere.This weakening occurs at bombing sites up to 1000 km away.This effect is temporary. Although not dangerous, it weakens the ionosphere, which may interfere with low-frequency radio transmission during the war.[1-2]
And earthquake prediction
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11.1 Overview
In earthquake prone areas, the ionosphere above it is often abnormal. This is the conclusion reached by a research group composed of Russian and Japanese scholars through years of observation of ionospheric electron concentration, which will help mankind to study earthquake formation and earthquake prediction in the early stage.They analyzed and compared the relevant data obtained from ionospheric observation by a satellite launched by the former Soviet Union in five and a half years and the earthquake occurrence records around the world.Ionospheric disturbances are like a basin of water on the ground. Even if there is no wind blowing, bubbles in the ionosphere will cause the water surface to be unstable. Therefore, tracking the changes of the atmospheric ionospheric electron concentration can predict the occurrence of earthquakes, and can minimize the casualties and property losses caused by earthquakes.There are two widely recognized theories of earthquake affecting the ionosphere: one is the impact of internal gravity waves generated in seismic areas on the ionosphere, and the other is the abnormal vertical electric field in seismic areas entering the ionosphere, which causes ionospheric disturbances.
11.2 Research findings
The Japanese Space Development Agency and the Aerospace Monitoring Science Center of the Russian Academy of Sciences, who participated in the joint research work, found that the electron concentration in the atmosphere ionosphere had changed dramatically during the earthquake.In the past, some scientists pointed out that there was a connection between earthquakes and changes in the ionosphere, and there were also relevant records of the existence of geomagnetic waves and changes in the ionosphere observed before and after earthquakes. However, people generally doubt that "whether the electromagnetic waves on the ground will affect the ionosphere".This time, scientists analyzed the recorded data from 1977 to 1979, and found that more than 150 large earthquakes with magnitude above 5 on the Richter scale occurred in the western Pacific earthquake prone areas, including Japan, during this period, and the electron density of the ionosphere above these areas was also much higher than the usual density.In areas where earthquakes are rare, the ionospheric electrons are relatively low.
11.3 Monitoring methods
The variation of electron concentration in the ionosphere is relatively complex. Japanese expert Eryuzhezai who participated in the research pointed out that if the number of satellites observing the ionosphere is increased, accurate earthquake prediction will no longer be empty talk.However, with the help of the GPS of the United States and the GLONASS global satellite system of Russia, changes in the ionospheric state can be monitored.This method is very valuable for predicting short-term earthquakes, provided that the variation of the ionospheric electron concentration should be measured periodically.In order to periodically observe the state of the atmospheric ionosphere, Russian researchers used radio signals, and the dual frequency radio signals released by satellites can be received by ground stations.Based on the dual frequency signal of satellite positioning system, researchers have developed an algorithm to calculate the changes of signal parameters, and compiled a computer program.
In March 2009, the first detection test station in China to monitor earthquakes based on changes in the atmospheric ionosphere was built in Liaocheng Seismic Hydration Test Station.
11.4 Validated
The researchers pointed out that the method of tracking the change of the electron concentration in the atmosphere ionosphere to predict earthquakes was verified in the earthquake event in Kaliningrad, Russia, from September 16 to 22, 2004.The earthquake occurred in the same place at an interval of 2.5 hours, and the distance between the ground satellite signal receiving station and the epicenter was between 260 km and 320 km.The observational data show that the ionospheric electron concentration increased during the three to five days and nights before the earthquake, while the maximum value of the electron concentration decreased significantly during the two days and nights before the earthquake.Therefore, it can be considered that the sharp decline of the ionospheric electron concentration is caused by the earthquake effect, and this state of the ionosphere is a sign of an earthquake.Previous research results show that for earthquakes with magnitude greater than 5, ionospheric disturbances generally occur in areas near earthquakes, with a probability of about 74.1%.
11.4 Anomaly before earthquake
From May 5 to 15, 2008, the ionosphere east of Wenchuan to Okinawa, Japan, and south to southern Hainan showed significant disturbance, and the ionospheric TEC showed a significant increase, which is rarely seen in peacetime.The disturbance on May 9 was "throwing a stone into the water", and a large range of abnormal increase in ionospheric parameters occurred near the location of the large earthquake later.
Plasma state
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Due to the action of thermal movement and electromagnetic force, the electrons escaping from one molecule may collide with another cation that has lost electrons and compound, or temporarily combine with neutral molecules to form anions.In the ionosphere, ionization and recombination are always ongoing, but in a region, the concentration of free electrons and anions is basically the same as that of cations, so it is generally neutral.This is the fourth state of matter, called plasma state.The highest temperature of the ionosphere does not exceed 1000K, which belongs to cold and weak plasma.
Various components of solar radiation have different effects on the atmosphere. Short ultraviolet and X-ray ionize the atmosphere, longer ultraviolet light decomposes atmospheric molecules into single atoms, and longer ultraviolet light changes O2 into O3.Particle flow can cause atmospheric ionization, temperature rise and other effects.When solar radiation passes through the atmosphere, it is absorbed and attenuated.Through the same gas layer, the shorter the wavelength of radiation, the more attenuation.Therefore, only ultraviolet rays with longer wavelengths can reach the ground, and the composition of the atmosphere also changes with height due to the absorption of ultraviolet rays.
Ion distribution in the ionosphere
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The research and rocket measurement show that the atmospheric molecular weight has no obvious change below the altitude of about 90km, but the percentage of O3 content is large within the altitude of 10~50km, and the maximum value is about 20~35km.NO appears above 35~40km.Above 90km, O2 starts to decompose into oxygen atoms, and at a higher altitude, N2 also starts to decompose. Above about 100km, the main components of the atmosphere are O, N2 and N.Above about 500km, N2 and O2 will not exist, while the percentage of He and H content will gradually increase, and above 2000km, there will be only these two atoms.
Atmospheric molecules tend to escape outward.As a result of this trend against the gravity of the earth, the atmospheric pressure decreases exponentially with height.The number of ions contained in various components may reach the maximum at a certain height, but due to the simultaneous action of various factors (including the geomagnetic field) on the ionosphere and the migration and dissipation of charged particles, the actual ion concentration variation with height is not the superposition of the theoretical distribution of several components.In general, anions only exist below 70km (daytime) or 90km (nighttime), on which there are mainly cations and free electrons with basically the same concentration.The concentration distribution curve with height lingers at certain heights, which play an important role in the reflection of electromagnetic waves.Each area is named from bottom to top as Layer D (about 40~90km above the ground), Layer E (about 90~160km) and Layer F (extending thousands of kilometers away).
The distribution of electron concentration with height is greatly affected by time, season and solar activity, and the concentration value and the range of each region are not fixed.At night, due to the lack of solar radiation, the density of the lower atmosphere is larger and the recombination is stronger, the D layer will disappear, and the electron concentration in E and F layers will also decrease by one or two orders of magnitude.Occasionally, the Es layer will appear in the E layer. The electron concentration range is high, and it can even reflect about 50MHz electromagnetic waves. Its life is only a few hours or less.When there are many sunspots on the solar surface and a large number of particle streams are ejected, the concentration of F layer may be greatly reduced due to thermal expansion, resulting in the interruption of short wave communication for several hours or even dozens of hours.This situation is more serious in high latitude areas.
The ionosphere is a dispersive medium.When the refractive index becomes an imaginary number, the electromagnetic wave is cut off and cannot propagate.
The motion of free electrons in the ionosphere
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Under the action of electric field, the free electrons in the ionosphere move in the way of superposition of random thermal motion and regular vibration.When it collides with other heavier particles, its vibration kinetic energy is absorbed by the particle being collided, and this kinetic energy is converted from the electromagnetic energy flow exerting force on the electron, so the electromagnetic wave is absorbed and attenuated by the collision.In layer D, due to the high atmospheric density, the collision frequency is about 8 × 107 times/second.In the F layer, the collision is almost negligible except when the sun bursts (thermal turbulence).The movement of free electrons in the ionosphere is also affected by the geomagnetic field.The trajectory of electron thermal motion is not a straight polyline.When there is an external electromagnetic field in the ionosphere, due to the weak degree of ionization, the interaction between charges and the effect of the magnetic field in the electromagnetic wave on the electron are relatively weak, and the force determining the regular movement of the electron comes from the electric field of the electromagnetic wave and the geomagnetic field.The direction of the geomagnetic force is orthogonal to the plane shared by the geomagnetic field and the electron velocity, so that the electron can get transverse acceleration at any time, so the regular vibration of the electron is not in line with the electric field, so the equivalent polarization intensity vector is not parallel to the electric field intensity vector.Under the influence of the geomagnetic field, the ionosphere becomes a magnetorotationally anisotropic medium.The equivalent refractive index of the ionosphere has double values n1 and n2, and is related to the angle between the wave propagation direction and the geomagnetic direction. When n1 and n2 are both real numbers, n1<n2. When the wave propagation direction is perpendicular to the geomagnetic direction, n2 is independent of the geomagnetic field, so it is called ordinary refractive index and n1 is called abnormal refractive index.
The ionosphere is not static as a whole, and there are also random flows.The distribution of charged particles is superimposed with random fluctuations on its average value. In some areas, there may be clumps with high concentration, and fluctuations and clumps change with time.The detection and mechanism analysis of the ionospheric fine structure are attracting many people's attention.
