A search for parallel electric fields by observing secondary electrons and photoelectrons in the low‐altitude auroral zone

Abstract

The high sensitivities of the auroral secondary electron and atmospheric photoelectron spectra (<100 eV) to weak parallel dc electric fields make the particle distributions ideal for locating the lower boundary to the auroral electrostatic potential structures, which are often regarded as providing the mechanism responsible for the acceleration of auroral particles. The effects of weak parallel electric fields on the differential spectra of the low‐energy electrons observed in inverted‐V events are demonstrated with a set of heuristic model calculations. The spectral variations as a function of altitude of these electron populations in inverted‐V events observed by the Dynamics Explorer 2 spacecraft are examined separately for events connected to dark and sunlit local atmospheres. By comparing the altitude dependence of the observed spectra with the model calculations, we show that there can be no more than a 2‐volt potential drop on average between 400 and 900 km altitudes, corresponding to a distributed parallel dc electric field of less than 4 µV/m under the inverted‐V electron precipitation regions. Isotropy of the photoelectrons in inverted‐V events is also found to extend beyond the topside ionosphere, in accordance with the existence of a potential structure above the satellite. Statistical results on the spectral dependence of the secondary electrons on the inverted‐V primary beam parameters are also presented.