Leaky Bucket Hypothesis in Energetic Electron Injections Associated with Substrom Dipolarization

Energetic particle injections – sudden flux enhancement at energies of tens to hundreds of eV- are critical for supplying particles and energy to the inner magnetosphere. Injection of these particles is often associated with substorm dipolarization, sudden reconfiguration of the nightside magnetosphere from a taillike configuration to a more dipolar-like configuration. These injected particles may also be accelerated by interactions with the earthward propagating magnetic bubbles. How electrons injected into the inner magnetosphere are accelerated to energies above 1 MeV is still under active investigation. However, it is generally believed that they are significant in providing the seed population for radiation belt particles, populating the ring current, and creating particle velocity space anisotropies that drives ion cyclotron or whistler-mode chorus waves that are eventually responsible for losses or further acceleration in the Earth's radiation belts. Simultaneous observation of injections with other plasma and field measurements provides unique opportunities to reveal the potential mechanisms for particle transport and energization. In this work, we present two case studies from NASA's Magnetospheric Multiscale (MMS) mission along with test particle simulations to develop better understanding of the nature and energy limits of energetic electron injections associated with substorm activity. The results show that lower-energy electrons can stay in an earthward propagating magnetic bubble longer than higher energy electrons, which favors them in outer radiation belt penetration. This "leaky bucket" hypothesis suggests that leaking out of the magnetic bubble occurs more slowly for lower-energy electrons, which thus allows them access to lower L-shells.