Earth’s global magnetic field envelops us all, protecting us from cosmic rays, aiding our navigation, and shielding our oxygen-rich atmosphere. Yet details about its origin, operation, and future remain unknown. Recent space-based magnetic observatories let us observe the field more precisely than ever before, and we can use these measurements to study the deep interior of the Earth and illuminate a region of our planet previously observed mainly by listening to the echoes of earthquakes. In this thesis, I use observations of changes in Earth’s magnetic field to study the structure and processes occurring near the top of Earth’s core. In particular, I examine the long-debated question of whether a stratified layer of fluid, termed by some as the stratified ocean of the core or SOC (Braginsky, 1998), lies at the top of Earth’s outer core. I first implement a model to simulate fluid motions in the SOC, which I describe in chapter two. Then, I derive the properties of a class of eastward-propagating equatorially trapped magnetic waves I term eMAC waves in chapter three and develop a statistical threshold to detect these waves in chapter four. In chapter five, I apply my statistical test to observations of Earth’s geomagnetic field and find evidence for these waves in Earth’s core. Finally, in chapter six, I propose a 20-40 km thick SOC with buoyancy strength of N ≥ 10 Ω to support the observed eMAC wave modes. I then discuss the implications of this layer, propose a double-layer SOC as a way to reconcile eMAC signals with previous observations, and enumerate possible future avenues of investigation.