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

Abstract

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 (|D st | < 20 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.

Occurrence characteristics of outer zone relativistic electron butterfly distribution: A survey of Van Allen Probes REPT measurements

“…While so far we do not have a good explanation for such a small dependence on P dyn , this feature reasonably agrees with the results of Selesnick and Blake [, their Figures 7–9], which demonstrated that with increasing geomagnetic activity the relativistic electron anisotropies do not change as much as is predicted, particularly on the nightside where the butterfly distributions often occur. Another possible explanation of relativistic electron butterfly PADs suggests that they 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 [ Artemyev et al ., ].…”

Relativistic electron's butterfly pitch angle distribution modulated by localized background magnetic field perturbation driven by hot ring current ions

“…During magnetic activities, the observation of butterfly PAD can be located within L = 6 at all magnetic local time (MLT) [Lyons and Williams, 1975;Sibeck et al, 1987]. Currently, seven mechanisms have been proposed to explain the formation of the butterfly PAD during magnetic activities: (a) wave-particle interaction with magnetosonic waves and chorus waves [Horne et al, 2005a;Xiao et al, 2015;Li et al, 2016bLi et al, , 2016aChen et al, 2015;Maldonado et al, 2016], (b) nonlinear resonance with oblique electromagnetic ion cyclotron waves [Wang et al, 2016], (c) wave-particle interaction with hiss waves, lightening-generated whistlers and ground VLF transmitters [Albert et al, 2016], (d) outward adiabatic transports [Su et al, 2010], (e) magnetopause shadowing effect [e.g., Wilken et al, 1986], (f ) substorm injection combined with drift shell splitting [Sibeck et al, 1987], and (g) nonadiabatic scattering due to the field line curvature scattering [Artemyev et al, 2015] and adiabatic effect caused by ring current through the conservation of electron's magnetic moment [Lyons, 1977].…”

Ultrarelativistic electron butterfly distributions created by parallel acceleration due to magnetosonic waves

“…The mechanisms controlling ultrarelativistic electron acceleration and loss have long been a fundamental topic of radiation belt studies [ Reeves et al ., , ; Thorne et al ., ; Turner et al ., ]. More recently, because of the unprecedented high angular resolution of particle measurements made by the Van Allen Probes, the pitch angle distributions have become the focus of quantifying various kinds of wave‐particle interactions [ Ni et al ., ; Artemyev et al ., ; Vasko et al ., ]. Three basic kinds of pitch angle distributions are typically identified: 90°‐peaked, flat‐top (or pancake), and butterfly distributions [ West et al ., ; Baker et al ., ; Fritz et al ., ; Horne et al ., ; Meredith et al ., ; Gannon et al ., ; Zhao et al ., ; Fennell et al ., ].…”