Photoelectron Butterfly Pitch-angle Distributions in the Martian Ionosphere Based on MAVEN Observations

Using pitch-angle-resolved electron fluxes recorded by the Mars Atmosphere and Volatile Evolution spacecraft over 5 yr, we present a detailed analysis of the occurrence patterns of photoelectron butterfly pitch-angle distributions (PADs) in the Martian ionosphere. Statistical analysis indicates that Martian photoelectron butterfly PADs favorably occur near the moderate crustal magnetic fields with a strength of 10-30 nT on the dayside and 10-15 nT on the nightside. The nightside occurrence rates are much higher. Furthermore, dayside butterfly PADs prefer to occur near the vertical magnetic field lines in the ionosphere, and the significant day-to-night transport of photoelectrons evades the nightside strongest magnetic anomaly regions. These features strongly support the idea that Martian photoelectron butterfly PADs are more likely to occur in eclipse or near the terminator and that they mainly form due to the adiabatic evolution of photoelectrons that transport along the closed cross-terminator magnetic field lines. Despite the negligible energy dependence in the darkness, the occurrence rate of dayside butterfly PADs observed at higher altitudes and near the subsolar region increases with energy, presumably related to the increased proportion of electrons from the solar wind when measured at relatively higher electron energies, which, however, is limitedly understood and deserves future investigation. Our comprehensive observations suggest the diverse influence of Martian magnetic topology on the ionospheric plasma in different spatial regions, and, in turn, analysis of their influence allows us a better understanding of the intricate Martian global magnetic system.