The radio propagation
Discovery of the ionosphere (II)
A sentence resumes all the problem of radio communications in the atmosphere : without propagation no traffic ! The discovery of the ionosphere goes back to the first suggestion made by the physicit Carl F. Gauss in 1839 that speculed that the dialy variation of the geomagnetic field might have its origin in atmospheric electric currents. In 1860, Lord Kelvin suggested the existence of a conducting layer.
Then in 1901 Guglielmo Marconi established the first transatlantic communication. Taking into account the Earth curvature, this feat was hard to explain. Physicists O.Heaviside, A.Kennelly and K.Nagaoka explained this achievement in suggesting the presence of a permanent electrically conducting layer high in the atmosphere.
This is at that time that Kennelly also suggested to make "accurate measurements [...] to compute the electrical conditions of the upper atmosphere". The first steps to a future ionospheric physics.
In 1903 and 1906 J.E. Taylor and J.A. Fleming, among others, suggested that the conducting properties of this layer were produced by the ionization action of ultraviolet light from the sun on the upper atmosphere. That meant that the sun controlled radio propagation, which was confirmed as soon as commercial communications were established across the Atlantic ocean. Experimentations were conduct on this subject between 1910 and 1925.
In 1910, G.W. Pierce proposed that waves reflected from the ionized layers might interfere with the direct or ground wave, canceling or amplifying signals, what Lee De Forest confirmed in 1912 during a radio experiment between Los Angeles, San Francisco and Phoenix.
Then in 1921, the ARRL staff succeeded the first CW transmission over the Atlantic and in 1923 the first two-way contact was established between the U.S.A and France. Amateurs and professional discovered HF and DX communications but had not a clear idea yet of what the matter above their head.
The final experimental proof of the existence of a reflecting layer came in 1924 with the "frequency change" experiments of E.V.Appleton and M.Barnett from Cavendish Laboratories using the BBC transmitter at Bournemouth. They calculated the signal strength of signals and fade variation according the frequency following two different routes and deduced that the reflection occured at a height of about 100 km.
At last in 1925 G.Breit and M.A.Tuve made the first direct measurements of the reflecting layer using a pulse-sounding technique, the forerunner of modern radar. Meanwhile Appleton focus on his experiment and on properties of the upper atmosphere, exploring "as busy as a bee" as he told himself, the young ionospheric physics. If you are interested in these discoveries, you will find a complete review of the early history of the ionosphere in the December 1974 issue of the U.S. "Journal of Atmospheric and Terrestrial Physics".
Formation and production of the ionosphere
Properties of the ionosphere are due to the existence of the atmosphere and the effect of the sun. Elements present in the air that are affected by the sun are N2, O, O2 and NO, that constitute over 99% of the air, 75% of its weigth being represented by Nitrogen and 21% by Oxygen.
Since Niels Bohr's works, we know that in its fundamental state, an electron placed on the first orbit of an atom has an energy of 13.6 eV, no more no less. We as seen that the EUV radiation released by the sun shows an energy level over 18 eV. That means that this radiation is able to strip electrons from neutral atoms and molecules present in the air. Although these ions are electrically charged, they are too heavy to respond quickly to oscillations of radio waves. These are electrons free in the photoionization process and up to 20,000 times lighter that reflect radio waves.
From the ground to the upper limit of the ionosphere (thermosphere) we find four ionospheric regions : Region D, E, F1 and F2.
Thanks to physicist J.W. Chamberlain, specialist of planetary atmospheres, let's see in detail specifications of each ionospheric region.
The nominal height of the D-layer peak is 90 km but during period of intense solar activity it can go down to 50 km only. At noon its electron density is about 10 electrons/cm3 at an altitude of 40 km, 100 electrons/cm3 at 60 km, 1000 electrons/cm3 at 80 km and up to 15,000 electrons/cm3 at an altitude of 90 km. At night conversely, the electronic density drops of a 10-factor, and these figures become 10 electrons electrons/cm3 at 85 km, 100 electrons/cm3 at 88 km and 1000 electrons/cm3 at 95 km, and then remains somewhat the same up to at least 140 km aloft. As it doesn't affect the night propagation one often considers its concentration as equals to 0 and that the layer completely vanishes at night.
Its effective recombination rate is the highest with about 3x10-8 cm3/sec. How occurs the absorption by the D-layer ? Ions are produced by solar X-rays, or Lyman-alpha (line at 1215 Å) ionization of NO molecules. An enhanced ionization always follows solar flares due to X-ray ionization of all elements present in the air. This is the electron attachment to O and O2 that forms negatives ions; the ratio of negative ions to electrons increases with depth and at night.
The recombination that mainly occurs at night is performed by electrons that form negative ions which are destroyed by photodetachment (at daytime only), associative detachment (O + O- → O2 + e-), and mutual neutralization (O- + A+ → O + A).
The nominal height of the E-layer peak is 110 km where it shows a electron density of 1.5x105 electrons/cm3 at noon, and more than 10 times less at night.
Its effective recombination rate is about 10-8 cm3/sec. Its production is achieved by ionization of O2 that may occur directly by absorption in the first ionization continuum (> 12 eV). Coronal X-rays also contribue, ionizing O, O2, and N2. At nightime the E-layer and sporadic-E appear due to electron and meteor bombardment. Some sporadic-E radio reflections may also be due to turbulence in the normal E-layer.
The recombination is dissociative : O2+ + e- → O + O and NO+ + e- → N + O.
The nominal height of the F1-layer peak is 200 km. Its density is estimated at 2.5x105 electrons/cm3 at noon but it can be seven times higher (2 millions electrons/cm3) depending on the ultraviolet amount it receives from the sun. This layer vanishes at night. Its effective recombination rate is about 7x10-9 cm3/sec.
Its production is achieved by ionization of O by Lyman "continuum" or by emissions lines of He. This ionization is probably accompanied by N2 ionization, which disappears rapidly after sunset.
The recombination is ensured by O+ ions that readily transfer charge to NO and perhaps to O2. Most of the ionization is this in molecular form and disappear by dissociative recombination.
The nominal height of the F2-layer peak is 300 km. Its density is estimated at 106 electrons/cm3 at noon, and ten times less at midnight. It is thus the denser ionospheric layer. However, its height and electron density are highly variable due to large daily, seasonal, and sunspot-cycle variation that, combined, produce a general erratic behaviour.
Its effective recombination rate is between 10-10 - 10-9 cm3/sec; it is mainly variable and probably decreases with increasing height. Ionization of O is achieve by the same process producing F1. F2 is formed because the effective recombination rate decreases with increasing height. F2 region produces little attenuation of radiation. Additional ionization processes may contribute in F2 that are attenuated in F1.
The recombination is ensured by molecular ions as in F1. But the limiting process here is charge transfer, giving an attachment-like recombination law.
At last, recall that the F2 region is also the home of the South Atlantic Anomaly (SAA), the lower limit of Van Allen belts that goes down to 200 km of altitude between South America and South Atlantic ocean. Constituted of fast electrons and heavy protons released by the solar wind trapped by the geomagnetic field, during quiet days, the SAA doesn't affect radio propagation in this region of the world.
Air density and amount of EUV : two opposing phenomena
If we understand better now how are produced ionospheric layers from a purely physico-chemical point of view, remain to understand why they appear as these altitudes and nowhere else.
In fact two opposing phenomena are working together to create a layer at some altitude, with a density decreasing as we move away from that altitude, up or down the layer.
The density of the neutral atmosphere decreases as altitude increases, while the amount of EUV radiation increases as altitude increases. Thus, as we go to the lower altitudes, even if there is more elements which could potentially be photoionized by the EUV, most of them were filtered by the atmosphere EUV and less can do the job because it has been progressively absorbed penetrating deeper in the atmosphere.