Sources of Planetary Ionospheres
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The sources of dayside and nightside ionospheres of the terrestrial and outer planets and their satellites are discussed. The sources of dayside ionospheres include photoionization of neutral thermospheric constituents by solar ultraviolet and x-ray photons. Lyman alpha photons are important in the ionospheres of earth, Venus and Mars, where they ionize atmospheric NO, and on the outer planets, and their satellites, where hydrocarbon radicals with ionization potentials lower than 10.2 eV, such as CH3 and C2H5, are present. On Jupiter, photons in the 854-1116 \AA\ range penetrate to below the homopause in the wings of the H2 absorption lines to ionize hydrocarbon radicals and stable molecules as well. Longer wavelength photons (in the near UV) may ionize metal atoms that arise from ablation of meteors in the terrestrial lower thermosphere, and other planets as well. Photoelectrons are important contributors near the F1 peaks, and may dominate at lower altitudes, in the region where soft x-rays are absorbed. On the nightsides, ionization by primary auroral particles and secondary electrons may dominate ion production in the auroral regions of planets with intrinsic magnetic fields, such as the earth and outer planets. On Venus, the nightside ionosphere is produced mainly by transport of atomic ions to the nightside and precipitation of electrons that are observed in the umbra. The different sources of ionization are reflected in the composition of ions. Not much is known about the strength or composition of the Martian nightside ionosphere, but there are a few radio occultation measurements near the terminators, and predictions about the composition have been made.
Fox, J. L.
(1994). Sources of Planetary Ionospheres. Eos, 75 (44 - Fall Meeting Supplement).