Identifying Typical Relativistic Electron Pitch Angle Distributions:
Evolution During Geomagnetic Storms
Abstract
Van Allen radiation belt electron dynamics are governed by a wide range
of physical processes that can simultaneously drive acceleration,
transport and loss. However, each individual process can be linked to a
specific energy-dependent pitch angle distribution (PAD). We employ a
new, unsupervised machine learning technique on 7-years of Van Allen
Probe Relativistic Electron-Proton Telescope data and discover that six
PADs, two each of: pancake, butterfly, and flattop, successfully
describe >70% of classified relativistic PADs. We
investigate the occurrence and storm-time evolution of each PAD through
45 geomagnetic storms. We find new populations of PADs, including:
“shadowing-like” and wave-particle interaction signatures at low-L,
and radial diffusion and substorm injections at higher-L, as well as
determining that wave-particle interaction dominated PADs are swamped by
radial diffusion processes through geomagnetic storms. Our results
clearly demonstrate that PAD characterisation is a key component of
understanding Van Allen radiation belt electron dynamics