Butterfly pitch-angle distribution of relativistic electrons in the outer radiation belt: Evidence of nonadiabatic scattering

In this paper we investigate the scattering of relativistic electrons in the nightside outer radiation belt (around the geostationary orbit). We consider the particular case of low geomagnetic activity (|Dst|\u3c20 nT), quiet conditions in the solar wind, and absence of whistler wave emissions. For such conditions we find several events of Van Allen probe observations of butterfly pitch angle distributions of relativistic electrons (energies about 1–3 MeV). Many previous publications have described such pitch angle distributions over a wide energy range as due to the combined effect of outward radial diffusion and magnetopause shadowing. In this paper we discuss another mechanism that produces butterfly distributions over a limited range of electron energies. We suggest that such distributions can be shaped due to relativistic electron scattering in the equatorial plane of magnetic field lines that are locally deformed by currents of hot ions injected into the inner magnetosphere. Analytical estimates, test particle simulations, and observations of the AE index support this scenario. We conclude that even in the rather quiet magnetosphere, small scale (magnetic local time (MLT)-localized) injection of hot ions from the magnetotail can likely influence the relativistic electron scattering. Thus, observations of butterfly pitch angle distributions can serve as an indicator of magnetic field deformations in the nightside inner magnetosphere. We briefly discuss possible theoretical approaches and problems for modeling such nonadiabatic electron scattering