GEM: Quantifying the Role of Radial Diffusion on the Energy-dependent Acceleration of Ultrarelativistic Electrons in the Center of Outer Radiation Belt

In the radiation belts surrounding the Earth, electrons are accelerated to ultrarelativistic speeds. How this process occurs is a fundamental unanswered question. The project addresses the relative importance of two main acceleration mechanisms, inward radial diffusion and local acceleration.

Using a novel approach to calculate the radial diffusion coefficients, this study will quantify the contribution from radial diffusion to ultrarelativisitic electron acceleration and its dependence on electron energy. The results of this project will contribute to updating space weather forecast models, which is a priority set out in the National Space Weather Action Plan. An early career researcher and an undergraduate student will receive support.

This project uses a novel approach to calculate event-specific radial diffusion coefficients and quantify the role of radial diffusion in ultrarelativistic electron acceleration in the center of the outer radiation belt (L~3 – 6). Using observations of electron flux oscillations and electric/magnetic fields, combined with particle-tracing simulations, the radial diffusion coefficients will be calculated from the radial displacement of the traced particles during selected events.

Combining with the radial diffusion model using the Fokker-Planck equation, the contribution of radial diffusion in the ultrarelativistic electron acceleration will be quantified, and its dependence on electron energy will be revealed. Specifically, the following science questions will be answered: (i.) How often do the ultrarelativistic electron periodic flux oscillations occur and what are the associated radial diffusion coefficients for ultrarelativistic electrons with different energies? (ii.) For selected electron flux enhancement events, quantitatively, what is the role of radial diffusion to the ultrarelativistic electron acceleration in the center of outer belt (L~3 – 6)? (iii.) How does the relative contribution of radial diffusion to the ultrarelativistic electron acceleration in the center of outer belt vary with electron energy?

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.