The thermal structure of the ionosphere

From a large number of rocket, satellite, and ground-based experiments since 1959 it is known that the electron and ion gases of the middle and upper ionosphere are substantially hotter than the neutral atmosphere. At low and midgeomagnetic latitudes the principal heating agent for the ionospheric plasma lies in the excess kinetic energy given to photoelectrons arising from the ionization of the atmospheric gases by solar ultraviolet radiation. Although the photoelectrons lose most of their kinetic energy in the excitation of atomic and molecular gases, a significant amount of energy is given to the ambient Maxwellian electron gas, increasing its temperature above that of the neutral gases. The ion gases, in contrast, appear to be heated almost entirely through the elastic collisions with ambient electrons so that the ion temperature generally lies between the electron and neutral temperatures. The calculation of theoretical temperature profiles has developed into a moderately sophisticated process with a fair degree of correspondence between predicted and observed values for undisturbed geophysical conditions. Current research emphasizes the global aspects of plasma temperatures and the connection between ionospheric and magnetospheric phenomena. However, many of the observed diurnal and seasonal variations in both electron and ion temperatures depend upon the couplings between the neutral and ionized atmospheres, and a complete understanding of all aspects of the ionospheric thermal balance is not possible at the present time.