The Hot Electrons in and Above the Auroral Ionosphere: Observations and Physical Implications

Abstract

One of the great attractions of the aurora, rivalling even its dramatic appearance, is its continuing ability to mystify over the nature of its production. The mystery is nowhere more intriguing than where it surrounds the electrons that are the prime cause of the excitation. There is now widespread agreement that photon emission is controlled directly by the energy flux carried by electrons into the atmosphere (Omholt, 1959; Bryant et al., 1970; Lepine et al., 1979; McEwen et al., 1981) and recent measurements from a DMSP satellite (Fennell et al., 1981) give further reassurance on this point. Even so there is reason to believe that a straightforward correspondence between energy flux and emission may not hold in all circumstances (Stenbaek-Nielsen and Hallinan, 1979). The major uncertainty, though, is over the processes responsible for the electron streams that produce discrete forms such as auroral arcs and the more irregular and variable structures that occur during auroral breakup. In view of its overriding importance, the present discussion will concentrate on this issue, even though other important areas such as the electrons that produce pulsating aurora will remain untouched.