Characterising electron butterfly pitch angle distributions in the magnetosphere through observations and simulations

Abstract

The Imaging Electron Spectrometer (IES) on the Polar satellite has measured the average characteristics of the equatorial electron pitch angle distributions (PADs) in the midnight sector as a function of radial distance out to the 9 <I>R</I><sub>E</sub> apogee of the Polar satellite. Depressions in the observed fluxes of electrons occur with pitch angles around 90° in the equatorial zone, while the more field-aligned electrons remain largely unchanged. The orbital precessions of the satellite have allowed much of the inner equatorial magnetosphere to be observed. Statistically, butterfly PADs with different shapes are observed selectively in different regions, which can provide insight to their source and possible history. Electron paths of varied pitch angles were modelled using Runge-Kutta approximations of the Lorentz force in a Tsyganenko (T96) simulated magnetosphere. The resulting drift paths suggest that the process of magnetopause shadowing plays a significant role in the loss of these electrons. Case studies of the drifting patterns of electrons with varied pitch angles were simulated from Polar's orbit when a butterfly PAD was observed on 3 October 2002 at an altitude near 9 <I>R</I><sub>E</sub> and on 12 September 2000 at an altitude near 6 <I>R</I><sub>E</sub>. These two locations represent regions on each side of the boundary of stable trapping. The modelling effort strongly suggests that magnetopause shadowing does play a significant role in the loss of equatorially drifting electrons from the outer regions of the inner magnetosphere

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

“…The minimum flux at α ∼ 90 ∘ is well seen. In contrast to some previous observations [e.g., Fritz et al , ; Klida and Fritz , , ], the butterfly pitch angle distribution is formed only for high energies, while for energies <1 MeV, the pancake pitch angle distribution is not modified during the entire time interval. A weak minima of fluxes of high‐energy equatorial (with α ∼ 90 ∘ ) electrons shown in Figure e is likely related to the beginning of the formation of the butterfly pitch angle distribution.…”

“…Further, the survey of ISEE‐1 observations supported the idea of the formation of butterfly pitch angle distributions over a wide energy range (20 keV–2 MeV) due to splitting of drift shells and magnetopause shadowing [ Fritz et al , ]. Similar results were obtained using the Polar observations of electron pitch angle distributions [ Klida and Fritz , , ]. However, Gannon et al [] demonstrated the dominance of electron butterfly distributions in the nightside and higher L shells for energies larger than 500 keV.…”

Drift Orbit Bifurcations and Cross‐Field Transport in the Outer Radiation Belt: Global MHD and Integrated Test‐Particle Simulations

“…This shows that the consequence of large jumps in J for particles with small initial values of J, is net transport away from = 90°. Butterfly PADs are observed throughout the night‐time sector (West et al., 1973) and have been identified as deriving from drift shell splitting combined with magnetopause shadowing (e.g., Klida & Fritz, 2013), interactions with ULF waves (e.g., Kamiya et al., 2018) and particle injections (e.g., Artemyev et al., 2015). Here, we identify DOBs as a mechanism that might also produce such a distribution.…”

Pitch angle distributions of electrons at dipolarization sites during geomagnetic activity: THEMIS observations

“…Butterfly distributions have been attributed to the effects of shell splitting or magnetopause shadowing as electrons drift [ West et al ., ; Klida and Fritz , ]. Considering the longer time scale of electrons drifting over one orbit, however, this effect should be less important during dipolarization.…”