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The diurnal ionosphere

The Earth has a first important characteristic : it has an atmosphere, whose main components are molecular oxygen and the nitrogen (i.e. two atoms of oxygens or nitrogen dependant between them chemically) between 80 and 105 km. Above 200 km and until around 600 km, atomic oxygen becomes the component majority one, the minority ones being hydrogen, helium, argon and the atomic nitrogen. The typical total concentrations are a million billion particles per cubic meter to 400 km and hundred billion billion particles per cubic meter to 80 km. Is this much? Not, it is space! By comparison, the air that we breathe contains a few billion times more particles in same volume. The temperature is approximately 200 K to 90 km and increases up to 1000 K approximately to 400 km. This is why one qualifies this part of neutral atmosphere of thermosphère.

The photons in the E.U.V described above have energies higher than the thresholds of ionization of the various atmospheric components (these thresholds all are about 10 eV). I.e. that they are likely to break atoms and molecules, and to tear off one (sometimes two) outer-shell electron : it is an ionization. The excitations are also a process of absorption of the ultraviolet rays, so that our atmosphere constitutes a genuine filter for these rays, mortals for the men.
The ionized part of the atmosphere is called ionosphere. One amongst other things distinguishes there two areas, called E and F. In the first, between 80 and 140 km roughly, the ions are mainly molecular and the temperatures of the electrons and the ions are close one to the other. The concentration of the electrons reaches a few hundred billion per cubic meter to approximately 110 km (hundred million times smaller than that of neutral gas).

The area F as for it is divided into 3 parts: between 140 and 200 km (F1 area), there are transition between the molecular ions and the atomic ions. The F2 area, above 250 km, is that of the second maximum of concentration of electrons. There is only one abundant ion, atomic oxygen ionized once (i.e. having lost an electron). Above the F2 area is the higher area F, with a transition between ionized oxygen and the lighter helium or hydrogen ions. Above 300 km, the concentration becomes so weak that the collisions between particles are of negligible number to describe the dynamics of this part of the atmosphere. The temperature of the electrons is about 1400 K to 400 km whereas the temperature of the ions reaches “only” 1100 K with the same altitude.

Institut de Planétologie et d'Astrophysique de Grenoble (IPAG)