Identifying Typical Relativistic Electron Pitch Angle Distributions: Evolution During Geomagnetic Storms

Van Allen radiation belt electron dynamics are governed by a multitude 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-year of Van Allen Probe Relativistic Electron-Proton Telescope data and discover that six PADs successfully describe 93% of outer belt relativistic electrons, two each of: pancake, butterfly, and flattop. 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 characterization is a key component of understanding Van Allen radiation belt electron dynamics. Plain Language Summary The population of relativistic electrons that occupy the Van Allen radiation belts are influenced by processes that occur within the near-Earth environment. These processes can result in the acceleration, transport and loss of electrons at different energies and pitch angles. Using a new, unsupervised machine learning technique on electron measurements from the entire Van Allen Probe mission, we classify similar populations of relativistic electron pitch angle distributions (PADs). Six different PAD shapes were identified, each with fundamentally different characteristics as a result of different physical processes. We investigate the occurrence and evolution of each PAD type through 45 geomagnetic storms and find clear evidence that PAD characterization is a key component of understanding Van Allen radiation belt behavior.