Cross‐Terminator Variations of the Photoelectron Energy Distribution in the Martian Ionosphere

Photoelectrons are produced on the dayside of Mars via solar Extreme Ultraviolet (EUV) and X-ray ionization and are also frequently observed on the nightside as a result of field-aligned transport. Based on the Solar Wind Electron Analyzer (SWEA) measurements made on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we show that the shape of the photoelectron energy distribution exhibits interesting cross-terminator variations, in that the He II peak becomes less pronounced and the 10 - 50 eV smooth background becomes hardened, both more evident at lower altitudes. These observations could be understood as the outcome of "atmospheric absorption" via photoelectron collisions with ambient neutrals during day-to-night transport. Such a scenario could successfully interpret the diversity of the MAVEN SWEA observations such as the dawn-dusk and north-south asymmetries, the impacts of crustal magnetization, upstream solar wind conditions, and solar EUV and X-ray radiation, all using the 10-50 eV spectral hardness as a diagnostic. In particular, the magnetic control of the cross-terminator spectral reshaping is proposed as the outcome of varying day-to-night magnetic connectivity which naturally implies varying field-aligned atmospheric column for "absorption." Crude estimates suggest that an "absorbing" atmospheric column of approximate to 1.5 x 10(15) cm(-2) is required for a significant photoelectron spectral reshaping beyond the EUV terminator. Therefore the observations reported here may shed light on the extension of the cross-terminator magnetic field lines, as complementary to the day-to-night magnetic connectivity inferred from the occurrence of nightside photoelectrons.