Comparison between pitch angle–resolved electron fluxes before (T₁) and during (T₂) the electron flux dropout on 2014 September 12. Panels (a)–(d) present a 2 hr interval (17:00–19:00 UT) of REPT’s B electron fluxes (blue circles) at a 90° pitch angle for several energy channels ranging from ∼1.80 MeV to ∼3.4 MeV. A Gaussian-like fit (see text for details) was also superposed (black curve) onto these plots. The time T₁ corresponds to the maximum fitted flux value, whereas T₂ corresponds to when the flux drops to 80% (roughly 1/e²) of this maximum value. The corresponding electron pitch angle distributions (PADs) at these two time instants are shown as red and green curves, respectively, in panels (e)–(h). At T₁ (T₂), the electron PADs have a 90°-peaked (butterfly) shape, as evidenced by the r parameter, which provides the ratio between the flux at the 90° pitch angle bin to the average flux at both 47.6° and 132.4° pitch angle bins. It is defined that whenever r > 1.1 (r < 0.9), the analyzed electron PAD has a 90°-peaked-like (butterfly-like) shape (see text for details).