The ionosphere is that region of the upper atmosphere of a planet where charged particles (electrons and ions) of thermal energy are present, which are the result of ionization of the neutral atmospheric constituents by electromagnetic and corpuscular radiation. The lower boundary of the ionosphere (which is by no means sharp) coincides with the region where the most penetrating radiation (generally, cosmic rays) produce free electron and ion pairs in numbers sufficient to affect the propagation of radio waves (D-region). The upper boundary of the ionosphere is directly or indirectly the result of the interaction of the solar wind with the planet. For weakly or essentially non-magnetic planets (e.g.; Venus), the interaction region between the solar wind and the ionospheric plasma represents the termination of the ionosphere on the sunward side; it may be called the ionopause. On the night-side the ionosphere can extend to greater distances in a tail-like formation, representing the solar wind shadow. In the tail the extent of the ionosphere is limited by the condition for ion escape. 

(from S. J. Bauer, Physics of Planetary Ionospheres, 1973)

For magnetic planets (Earth, Jupiter), the ionosphere terminates within the magnetosphere which comprises all charged particles of low (thermal) and high energies (radiation belts). In this case the solar wind interacts with the intrinsic planetary magnetic field terminating at the magnetopause. The termination of the ionosphere is then the indirect result of the solar wind interaction;  e. g., in the case of Earth, represented by the boundary between solar-wind induced convective motions inside the magnetosphere and the co-rotating ionospheric plasma called the plasmapause. (The region inside the plasmapause is also called the plasmasphere; however, according to our definition this is simply part of the ionosphere.)



The discovery of the Earth's ionospherehas an interesting history. It came from the observation of reflected radiowaves, whose properties could only be explained by the presence of a reflectinglayer in the Earth's atmosphere composed of electrons and positive ions. TheEarth's ionosphere is divided into several regions designated by the letters D,E, and F (see Figure 1), the latter being subdivided into F1 and F2.Historically, the division arose from the successive plateaus of electrondensity (Ne) observed on records of the time delay (i.e., virtual height) ofradio reflections as the transmitted signal was swept through frequency. (Thecritical frequency at which reflection occurs varies as Ne^‡. Thus higherfrequencies penetrate farther into the ionosphere and are reflected by higher Ne)The E "layer" was the first to be detected and was so labeled as beingthe atmospheric layer reflecting the E vector of the radio signal. Later thelower D and higher F layers were discovered. Distinct ionospheric regionsdevelop because (a) the solar spectrum deposits its energy at various heightsdepending on the absorption characteristics of the atmosphere, (b) the physicsof recombination depends on the density, and (c) the composition of theatmosphere changes with height. Thus the four main ionospheric regions can beassociated with different governing physical processes, and this physics (ratherthan simple height differentiation) is the basis for labeling an ionosphericregion on another planet as a D, E, F1, or F2 region.


Ionospheric Densities: The ionosphere is made up of a number of different layers due to ionization at a number of different layers. These layers are known as the D, E, and F layers. The location of these layers vary by day and night and is shown in the figure above. The lower most region of the ionosphere extending from about 50 km to 90 km is the D-region, which principally absorbs radio waves. Above the D layer is the E-region extending from 90 km to 150 km. The peak in the E region during day time is seen near 110 km. Above the E region is the F region consisting of two parts: the lower F1 region between 150 km and 180 km and the F2 region from 180 km and above. Note, the daytime densities are much larger than the nighttime densities. At night, recombination can result in the loss of the D region.

Ionospheric Temperatures: The D region characterized by small ion densities and low collision frequencies of electrons and ions with neutral particles is also characterized by  low values of neutral temperature (Tn), ion temperature (Ti), and electron temperature (Te). In the E region, Tn and Ti are nearly the same while Te starts to deviate to higher values during the daytime.  Plasma temperature variations in the F region indicate that up to about 250 km, the thermal coupling between the ions and neutrals is good enough to ensure that Ti = Tn. At greater heights this coupling becomes worse while coupling between ions and electrons improves. Hence Ti > Tn as Ti increases with increasing altitude, while Tn becomes almost constant with altitude and is called the exospheric neutral temperature. At very high altitudes, daytime Ti and Te values are nearly equal. 


Table 1 gives a thumbnail sketch of theEarth's ionospheric regions with some characteristic values (From J. W.Chamberlain, Theory of Planetary Atmospheres, 1987)


Table 1. Ionospheric Processes (After Chamberlain, 1987)
Region Nominal Height of layer peak (km) max Ne (1/cm„) Effective Recombination Coefficient (cm„/sec) Ion Production Recombination
D 90
(Lower following solar flare)
1.5 × 10^4 (noon): absent at night 3 × 10^-8 Ionization by solar x-rays, or Lyman alpha ionization of NO. Enhanced ionization following solar flares due to x-ray ionization of all species. Elec tron attachment to O and O2 forms negative ions; ratio of negative ions to electrons increases with depth and at night. Electrons form negative ions which are destroyed by photodetachment (daytime only), associative detachment (O + O- -> O2 + e), and mutual neutralization (O- + A+ -> O + A)
E 110 1.5 × 10^5 (noon): <1 × 10^4 (night) 10^-8 Ionization of O2 may occur directly by absorption in the first ionization continuum (hv > 12.0 eV). Coronal x-rays also contribute, ionizing O O2 and N2 Nighttime E and sporadic E (thin patches of extra ionization) are due to electron and meteor bombardment. Some sporadic E radio reflections may be due to turbulence in normal E layer. Dissociative recombination (O2+ + e -> O + O and NO+ + e -> N + O.
F1 200 2.5 × 10^5 (noon): absent at night 7 × 10^-9 Ionization of O by Lyman "continuum" or by emission lines of He. This ionization probably accompanied by N2 ionization, which disappears rapidly after sunset. O+ ions readily transfer charge to NO and perhaps to O2. Most of the ionization is thus in molecular form and disappears by dissociative recombination.
F2 300
Height and electron density highly variable. Large daily, seasonal, and sunspot-cycle variations are combined with general erratic behavior
10^6 (noon); 10^5 (midnight) 10^-10 - 10^-9 Variable; probably decreases with increasing height Ionization of O by same process producing F1; F2 formed because the effective recombination coefficient decreases with in creasing height; F2 region produces little attenuation of radiation. Additional ionization processes may contribute in F2 that are attenuated in Fl. Recombination of molecular ions as in Fl; but limiting process here is charge transfer, giving an attachment-like recombination law.

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