Statistical Study of Energetic Electron Butterfly Pitch Angle Distributions during Magnetic Dip Events

The magnetic dips can produce the electron butterfly pitch angle distribution (PAD) in the inner magnetosphere. However, its universality is still unconfirmed. We statistically investigate the global distribution of the occurrence rate of the butterfly PAD for 466 keV and 2.1 MeV electrons during magnetic dips using the measurements of Van Allen Probes spanning from September 2012 to November 2018. The statistical results show that the butterfly PADs are confined to 4<L<6 and MLT=14-04 with a peak occurrence rate 15% for 466 KeV and 40% for 2.1 MeV at dusk-side around L=5. The results of the parametric study suggest the butterfly PADs preferably occur for high energy electrons at high L-shell with large bz-dips, and once the negative flux gradient vanishes, an isotropic initial PAD is needed to produce butterfly PADs. This work confirms the vital importance of the magnetic dips on the generation of the electron butterfly PADs.