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

Abstract

AbstractWe 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.