FAST observations of the solar illumination dependence of downgoing auroral electron beams: Relationship to electron energy flux

The dependence of the occurrence frequency of downgoing auroral electron beams on solar illumination, as a function of energy flux, has been examined utilizing data from the FAST satellite. Solar illumination has almost no effect on the occurrence frequency of electron beams with energy flux less than or equal to a few erg/cm²s; however, the ratio of the occurrence frequency in darkness to that in sunlight increases with the energy flux carried by the beam. For an energy flux >5 ergs/cm²s, the ratio is ∼2 to 2.5, consistent with the results of Newell et al. (1996b). The characteristic energy of dayside beams is less than that on the nightside by a factor of ∼2. Both on the dayside and nightside, the characteristic energy of the electron beams increases with the energy flux carried by the beam and the energy is somewhat dependent on solar illumination, ∼50% (20%) higher in darkness for dusk (morning) beams. Although there are fewer very intense aurora in sunlight, the very intense aurora that occur have characteristic energies comparable to those in darkness. These results are consistent with a density and scale height dependent mechanism for parallel potential drops in the auroral acceleration region.