Ionosphere

General information

History

1901 : Marconi establishes a transatlantic connection by radio.
1902 : The electromagnetic waves are propagated only in straight line, at least in a homogeneous medium. To explain how the radiotelegraphic signals emitted by Marconi could circumvent the rotundity of the Earth, Heaviside in England and Kennelly in America very imagine the existence with high-altitude of reflective layers for the radio waves: the layers of Kennelly-Heaviside
1925: The English physicist Appleton highlights by the experiment the presence of the layers imagined by Heaviside and Kennelly. These layers take the name of layer of Appleton
1925: shortly after Appleton, the American physicists Gregory Breit and Merle Antony Tuve measure the height of the layers of the ionosphere using a transmitter of radioelectric impulses.
1929 : The word ionosphere , proposed by Robert Watson-Watt, replaces that of layer of Appleton
1931: Sydney Chapman works out its theory of formation of the layers of the ionosphere by the action of the solar radiation UV.

Principle

The density of the air which constitutes the atmosphere decreases as one moves away from the surface of the ground. To 60 km of altitude the pressure of the air is not any more but of 2 Pa. Beyond 60 km of altitude, the atmosphere hardly any more acts like filters solar and cosmic radiation, the rays Ultraviolet S and X is increasingly aggressive and causes an ionization of the Molécule S of gas (Diazote, Dioxygène…) air by tearing off electron S with the Atom S the component. Among molecules of air find thus ions positive (molecules or atoms with which it misses one or more electrons) and of the orphan electrons. A very localized ionization and during very a short duration can be caused by the falls of Météorite S.

In the low part of the ionosphere, the density of molecules of air is still high, promiscuity between electrons and ions is large and an electron can find a positive ion quickly: the recombination is fast. In the highest layers, the recombination is slower and ionization decreases only slowly after the solar radiation stops with laying down it Sun.

Study of the ionosphere

The vertical sounder is a kind of Radar whose frequency is variable between 1 and 30 MHz. The transmitter sends very short impulses which are thought of an altitude depending on the frequency and the electronic density in the ionosphere. The measurement of time separating the emitted impulse and the reception from the echo makes it possible to calculate the altitude to which the reflection was carried out. The layout of this altitude according to the frequency is a ionogramme .

Since the years 1960, the artificial satellites and space probes allowed a better in situ comprehension of the ionospheric phenomena and the interactions with the magnetosphere.

Moreover, during these same years a novel method of study of the ionosphere developed since the ground: the incoherent Scattering. In this technique a wave UHF (400 MHz with 1 GHz following the installations) of very strong power (several hundred kw) is emitted towards the ionosphere where it is diffused in all the directions by the ionospheric electrons. The power received on the ground in return is very low, and requires large antennas and a treatment of the signal to extract information. This technique makes it possible to have access to the composition of the ionosphere, the temperature of the ions, and the rates of travel of these ions (" winds ionosphériques"). Sounders were installed in France with Saint-Santin-with-Maurs with 3 receivers of which the Radiotélescope of Nançay; Malvern (Great Britain); Millstone Hill (the United States); Arecibo (Porto-Rico); Jicamarca (Peru), like in Russia. The last three like EISCAT, the European sounder established in the Scandinavian Far North (Finland, Sweden, Norway), are always in activity.

Layers

One generally distinguishes 3 layers with the particular properties with respect to the Propagation from the waves.

Layer D : altitude from 60 to 90 km, pressure 2 Pa, temperature -76°C electronic density 10⁴. Composed of polyatomic ions. Absorbing for the waves of frequency lower than a few MHz, it appears with the rising of the Sun and disappears immediately after laying down it this one.
Layer E : altitude from 90 to 120 km, pressure 0,01 Pa, temperature -50°C electronic density 10⁵. Composed of oxygen and ionized molecular oxyde nitric and meteoritic ions. Diurnal and presents throughout the solar cycle. It reflects the waves of a few MHz until a limiting frequency which depends on the angle of incidence of the wave on the layer and the density of this one. During the summer, in average latitudes, appear sometimes during a few tens of minutes, even a few hours, of the “clouds” strongly ionized in the layer E (one speaks about sporadic E or Es )
Couche F : altitude from 120 to 800 km, pressure 1.10^-4 Pa, temperature 1000°C electronic density 10⁶. Composed of hydrogen and nitrogen, oxygen atoms. Very dependant on the solar activity, it has a very important level of ionization during the maximum ones of the solar Cycle. Its altitude fluctuates according to the solar radiation; the layer F breaks up during the day into two underlayers F1 and F2. These two underlayers recombine the night several hours after laying down it Sun but it happens that they persist all the night at the time of the maximum ones of solar activity. As for the layer E, the role of the layer F is essential for the propagation of the short waves.

Solar radiations and mechanism of creation of the layers

The ionosphere is anything else only one ionized gas, i.e. a plasma. As we saw with the college a gas uniformly fills all the space it has, then why there is it layers in the ionosphere?

To include/understand the mechanism of formation of the layers one is obliged to complicate the things a little, thus the ionosphere is not a gas but a succession of layers of gas whose composition depends on the atmospheric pressure. Moreover, there is not a solar radiation but of the radiations from which the spectrum extends from the infra-red to x-ray. let us not forget the jets of particles by the sun which sometimes manage to penetrate in the layer D…

The layers are thus the fruit of the interaction between gases, radiations nonionizing, ionizing rays and particles.

Gases of the ionosphere

Solar radiations

Solar particles

Concept of creation and absorption of electron-ion pair

With balance: layers of the ionosphere

Radio ionosphere and waves

The existence of the ionosphere was highlighted with the first intercontinental experiments of radio transmission. The propagation of the waves radio of frequencies ranging between a few hundred kilocycles and a few tens of megahertz is closely related to the state of the ionosphere. It can be favoured or disturbed according to the frequency of the radio wave, the geographical position of the transmitter and the receiver as well as the moment when the communication is tried. The moment of the day, the season and the solar Cycle are very important parameters in certain cases.

Thus decametric waves (so called " waves courtes") make it possible they to very establish connections with long distances while being reflected on certain layers of the ionosphere. For other frequencies, like the hectometric waves (still called " waves moyennes"), the propagation strongly depends on the absorption caused by the layer D (see higher) which prevents in the course of the day the waves from being reflected on the layers E and F higher located in altitude. The waves of very high frequencies (VHF, UHF and ultra high frequencies) used for the communications via satellites can be also deviated or absorptive by the ionosphere but that generally a great disturbance does not constitute.

See: Propagation waves radio

Ionosphere and GPS

Although GPS works in band VHF, it undergoes the modifications of the ionosphere. The variations of the ionosphere involve a deviation of signal GPS as well as a modification of the run time of the wave. All that obliged the originators of satellites GPS to implement techniques of correction.

Relation between time and the distance

The position

Use of two frequencies

Differential GPS

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