Evolution of Pitch Angle Distributions of Relativistic Electrons During Geomagnetic Storms: Van Allen Probes Observations

Abstract

We present a study analyzing relativistic and ultra relativistic electron energization and the evolution of pitch angle distributions using data from the Van Allen Probes. We study the connection between energization and isotropization to determine if there is a coherence across storms and across energies. Pitch angle distributions are fit with a J0sinnθ function, and the variable “n” is characterized as the pitch angle index and tracked over time. Our results show that consistently across all storms with ultra relativistic electron energization, electron distributions are most anisotropic within around a day of Dstmin and become more isotropic in the following week. Also, each consecutively higher energy channel is associated with higher anisotropy after storm main phase. Changes in the pitch angle index are reflected in each energy channel; when 1.8 MeV electron pitch angle distributions increase (or decrease) in pitch angle index, so do the other energy channels. We show that the peak anisotropies differ between coronal mass ejection (CME)‐ and corotating interaction region (CIR)‐driven storms and measure the relaxation rate as the anisotropy falls after the storm. The isotropization rate in pitch angle index for CME‐driven storms is −0.15 ± 0.02 day−1 at 1.8 MeV, −0.30 ± 0.01 day−1 at 3.4 MeV, and −0.39 ± 0.02 day−1 at 5.2 MeV. For CIR‐driven storms, the isotropization rates are −0.10 ± 0.01 day−1 for 1.8 MeV, −0.13 ± 0.02 day−1 for 3.4 MeV, and −0.11 ± 0.02 day−1 for 5.2 MeV. This study shows that there is a global coherence across energies and that storm type may play a role in the evolution of electron pitch angle distributions.

Persistent Pitch Angle Anisotropies of Relativistic Electrons in the Outer Radiation Belts

“…The waves are in a different MLT sector, and last for many electron drift periods. Previous studies (Greeley et al., 2021) show slow isotropization rates of energetic electrons on the scale of days during recovery phase, which suggests that electrons may remain anisotropic for a period of time after interactions with waves.…”

“…These distributions are typically described by characteristic shapes such as butterfly, flattop, and pancake (West et al, 1973;Zhao et al, 2018). To characterize these PADs, studies have used the J 0 sin n α function to fit pancake and flattop PADs (A. D. Gannon et al, 2007;Greeley et al, 2021;. Recently, A. D. Greeley et al (2021) temporally averaged electron PADs from the Van Allen Probes over several spins.…”

“…To characterize these PADs, studies have used the J 0 sin n α function to fit pancake and flattop PADs (A. D. Gannon et al, 2007;Greeley et al, 2021;. Recently, A. D. Greeley et al (2021) temporally averaged electron PADs from the Van Allen Probes over several spins. Then these averaged PADs were fit to J 0 sin n α.…”

“…Recently, A. D. Greeley et al. (2021) temporally averaged electron PADs from the Van Allen Probes over several spins. Then these averaged PADs were fit to J 0 sin n α .…”

Differentiating Between the Leading Processes for Electron Radiation Belt Acceleration

“…The equatorial pitch angle distribution of MeV electron flux enhancements at the center of the outer belt is most anisotropic (i.e., 90 °peaked) within a day of the start of the recovery phase, and the degree of anisotropy increases with energy (e.g., Ozeke et al, 2022). The pitch-angle distribution of MeV electron flux becomes more isotropic in the week following the start of the recovery phase (Greeley et al, 2021).…”