In Earth's atmosphere above an altitude of about 60 kilometers (35 miles), there is a region known as the ionosphere, where there are significant numbers of atoms or molecules with a positive electrical charge—called positive ions—and electrons, with negative charges. In the daytime, four more-or-less distinct regions, at different altitudes, can be identified in the ionosphere, and these are designated by the symbols D, E, F1} and F2. The electron and ion densities in these regions and the dependence on altitude vary with the season and with the activity of the Sun, as well as with the time of day. For comparison with the results for Mars obtained from Mariner IV, the curves in figure 5.10 show the day and night electron-number densities (electrons per cubic centimeter) as a function of altitude in Earth's ionosphere at middle latitudes at about the period of sunspot minimum. The lowest, or D, region is not well defined so it has not been included; it is not of immediate interest because it can have no counterpart on Mars.
The E region of the terrestrial ionosphere arises mainly from ionization (the formation of electrons and positive ions) of atmospheric molecules (and some atoms) by X-rays from the Sun. In this region, at an altitude of abou,t 90 to 150 kilometers (55 to 90 miles), the production of ions and electrons by solar X-rays reaches a maximum. In the F1 region, between roughly 150 and 250 kilometers
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