Investigating the Link Between Outer Radiation Belt Losses and Energetic Electron Escape at the Magnetopause: A Case Study Using Multi-Mission Observations and Simulations.

Radiation belt flux dropout events are sudden and often significant reductions in high-energy electrons from Earth's outer radiation belts. These losses are theorized to be due to interactions with the dayside magnetopause and possibly connected to observations of escaping magnetospheric particles. This study focuses on radiation belt losses during a moderate-strength, nonstorm dropout event on November 21, 2016. The potential loss mechanisms and the linkage to dayside escape are investigated using combined energetic electron observations throughout the dayside magnetosphere from the Magnetospheric Multiscale and Van Allen Probes spacecraft along with global magnetohydrodynamic and test particle simulations. In particular, this nonstorm-time event simplifies the magnetospheric conditions and removes ambiguity in the interpretation of results, allowing focus on subsequent losses from enhanced outward radial transport that can occur after initial compression and relaxation of the magnetopause boundary. The evolution of measured phase space density profiles suggest a total loss of approximately 60% of the initial radiation belt content during the event. Together the in situ observations and high-resolution simulations help to characterize the loss by bounding the following parameters: (a) the duration of the loss, (b) the relative distribution of losses and surface area of the magnetopause over which loss occurs, and (c) the escaping flux (i.e., loss) rate across the magnetopause. In particular, this study is able to estimate the surface area of loss to less than 2.9 × 10 R and the duration of loss to greater than 6 h, while also demonstrating the magnetic local time-dependence of the escaping flux and energy spectrum.