A new generation mechanism of a butterfly pitch angle distribution (PAD) of energetic ions is demonstrated by a test particle simulation in the stretched magnetic field. The Polar satellite detected some events of the butterfly PADs of the energetic protons (≳80 keV) in the outer ring current region around midnight near the equatorial plane. They were observed at relatively disturbed time in the inner magnetosphere and the ring current appeared to be developed; that is, magnitude of the magnetic field at the Polar satellite was highly depressed and the adiabaticity of the protons were expected to be violated. To reproduce the butterfly PAD, we perform a test particle simulation in which the first adiabatic invariant μ can be changed. When the radius of the field line becomes almost comparable to the Larmor radius of a proton, it suffers significant scattering of pitch angle (α) due to change of μ. This μ‐scattering process causes reform of the PADs. Owing to cumulative μ‐scattering, the flux of the protons ends to have a peak at α ∼ 40° (or 140°) and collapses at α ∼ 0° (or 180°) and 90°, which is a butterfly PAD. The reproduced PAD resembles the butterfly PADs observed by the Polar satellite. We suggest that the nonadiabatic effect is important for not only the generation mechanisms of the butterfly PAD but also the development of the storm‐time ring current.
