Thermal Structure of Planetary Ionospheres

Abstract

Electrons released in the photoionization process due to the absorption of solar XUV radiation in planetary atmospheres may have initially kinetic energies of one to 100 eV. The energy E of photoelectrons resulting from photons of energy h v interacting with an atmospheric constituent whose ionization potential is IP is given by

$$E = hv - IP$$

These photoelectrons loose their excess energy by both inelastic and elastic collisions [77]. As long as the energy of the primary photoelectron exceeds IP, secondary ionization may occur, while below IP energy loss will occur by excitation of atmospheric constituents. The rate of energy loss per unit pathlength due to an inelastic collision with a neutral constituent whose density is n _j , can be expressed by

$$\frac{1}{{{{n}_{j}}}}\frac{{dE}}{{dx}} = E\prime {{\sigma }_{j}}$$

(3-1)

where E′ is the excitation energy and σ _j is the appropriate cross section.