Figure 2: Formation of butterfly distribution of relativistic electrons.

Data from the EMFISIS instrument for magnetic and electric spectral intensity (in unit of log₁₀) of chorus and MS waves (a,b), wave normal angle (c), the angle between Earth’s magnetic field and the normal to the plane of the wave, and wave ellipticity (d), the degree of elliptical polarization. Chorus wave occurs above 0.1f_ce (the white dashed line). MS wave occur as a series of narrow tones, spaced at multiples of the proton gyrofrequency f_cp up to f_lhr (the white solid line). (c,d) The observed MS or chorus wave has a high (θ≈90°) or low (θ≈0°) normal angle and a high (ellipticity≈0) or low (ellipticity≈1) degree of elliptical polarization. (e–h) Pitch angle distribution for different indicated energies (2–3.6 MeV) over ∼40 min duration from the REPT instrument. Fluxes (in the same unit as that in Fig. 1) of relativistic electrons have minima around pitch angle 90°, and peaks around the pitch angle range of 30°–60° or 120°–150°.