Butterfly distribution of outer zone relativistic electrons and their potential connection to the solar wind dynamic pressure

Butterfly distributions, characterized by flux minima at pitch angles around 90º, are broadly observed in the Earth's magnetosphere. While butterfly distributions have been well recognized for radiation belt electrons below ~ 1 MeV, there is lack of investigation of butterfly distributions for relativistic (> MeV) electrons. We conduct a comprehensive analysis of outer zone (L >= 3) relativistic electron butterfly distribution based upon a survey of over-two-year Van Allen Probes REPT measurements. The global profile of butterfly distribution is investigated in detail for relativistic electrons at REPT energies, along with its dependence on L-shell, MLT, and the level of geomagnetic activity. Furthermore, the occurrence pattern of outer zone relativistic electron butterfly distribution is explored with respect to the solar wind dynamic pressure, which suggests that there exist a good correlation between these two phenomena especially at high L-shells (e.g., L ~ 6) and that some other mechanism(s), besides losses through the magnetopause, should play to contribute to the occurrence of outer zone relativistic electron butterfly distribution at lower L-shells.