O⁺ Transport from the Ionospheric Cusp to the Inner Magnetosphere: A Global MHD and Test Particle Study

  Speaker: Yiwen Zhu Doctoral Candidate Thesis Defense Department: Physics & Astronomy Location: Herman Brown Hall HBH 254 During geomagnetic storms, ionospheric oxygen (O⁺) can become a significant constituent of Earth's ring current, the current system whose buildup drives the main-phase depression of the surface magnetic field. The polar cusp is a principal source, feeding O⁺ outflow into the magnetotail lobe, but how much of this oxygen is delivered inward to the inner magnetosphere, and how it is energized along the way, remain poorly quantified, in part because the transport spans scales that no single model resolves.   In this thesis, we follow ionospheric oxygen from the cusp to the ring current using a global magnetohydrodynamic simulation (GAMERA/MAGE) coupled one-way to a test-particle tracer (CHIMP). Because MAGE includes an inner-magnetosphere model, it produces and resolves the mesoscale bursty bulk flows (BBFs) of the plasma sheet that drive earthward transport. With it, we identify a "drop-down" channel that controls the delivery to the inner magnetosphere: the dipolarization field carried by these flows pulls cusp oxygen from the lobe down to the equatorial plasma sheet, and the ions that experience it form a more efficient ring-current injection channel than those that convect down the tail unaided. Within an adiabatic (guiding-center) framework, we decompose the energy gain along the transport into its Fermi, betatron, and E×B contributions and quantify each, and we characterize where the oxygen departs from adiabatic behavior. To put these results on a quantitative footing, we weight the test particles by an O⁺ outflow rate computed directly from the simulation's MHD variables. For a real storm, this lets us quantify the absolute amount of cusp oxygen delivered to the inner magnetosphere and follow its contribution to the ring current through the storm, showing that a portion of the cusp oxygen is accelerated to ring-current energies and adds to the ring-current population. As a separate diagnostic, we characterize the energy and pitch-angle phase-space distributions, f(K, α), of the cusp oxygen at several locations along the transport and at ring-current entry, mapping how the population is energized and reshaped as it moves inward. Finally, we step back to the cusp itself. Forecasting where the cusp lies from the upstream solar wind is operationally important, yet no data-driven model has provided the cusp position directly. We develop a machine-learning model, gradient-boosted decision trees trained on DMSP spacecraft cusp crossings, that predicts the cusp location from solar-wind and interplanetary-magnetic-field conditions, offering a fast alternative to a full simulation. Together, these results advance two distinct fronts. The simulation and test-particle study closes a missing link in the cusp-to-ring-current pathway: it resolves how cusp oxygen reaches the inner magnetosphere, decomposes how t