sphere, which are the immediate product of the photoionization of neutral atmospheric particles, and the ambient electron gas, which is the ensemble of electrons that have reached thermal equilibrium. Photoelectrons are the more energetic of the two populations, capable even of producing further ionization, whereas the thermal electrons in the present undisturbed ionosphere have energies only of the order of a few tenths of an electron volt. In the auroral region, much more intense radiations of primarily the same character are stimulated by precipitated auroral electrons and ions with typically 1-30 keV of energy; these are properly known as the aurorae. The energy deposition associated with the argon ion engines of the SPS stimulates artificial airglow in two ways. First, the beam stopping mechanism heats the ambient magnetosphere and ionosphere to a thermal energy level of a few electron volts, comparable to the energy of natural photoelectrons and therefore capable of stimulating similar airglow. This source of artificial airglow involves the natural atmospheric constituents and the well-known natural emission lines. The intensity is expected to be much greater than the natural airglow because the thermal electron flux is orders of magnitude higher than the photoelectron flux; however, it is not possible to assess at present exactly what the modified thermal energy level is, except that averaging consideration would put it in the 0.2 - 5 eV range. The most important consequence appears to be the enhancement of the well-known atomic oxygen lines at 5577 [[spi:math]] and 6300 [[spi:math]]. The 1S and 1D states of atomic oxygen, which lie at about 4 and 2 eV respectively above the ground state and give rise to the aforementioned lines, can easily be excited by thermal electrons of ~ 5 eV. Under natural conditions these lines originate from photoelectron impact and ion-chemical processes. In a similar context, thermalized argon ions may also modify oxygen and nitrogen airglow intensity because they react efficiently with N2 and O2 in charge exchanging processes: 34
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