Explaining the “Missing Loss” Process in Earth’s Magnetosphere

Abstract The dynamics of the Earth’s electron ring current are governed by a complex balance between transport , acceleration, and loss. Despite decades of modeling, a persistent discrepancy remains: physics-based simulations consistently overestimate electron fluxes by up to two orders of magnitude. The current theories fail to explain this "missing loss" process. Here, we demonstrate that no additional loss process is required to explain this apparent mismatch between theory and observations; rather, the discrepancy arises from neglecting the variability of the background plasma density. By coupling a time-dependent plasma density model with a ring current simulation, we show that density depletions modulate the plasma wave dispersion relation, enhancing electron precipitation by a factor of 100. This increase in loss rates explains the difference between theoretical predictions and satellite observations. Our results reveal that background plasma density, rather than wave intensity alone, is a primary driver of magnetospheric particle loss, providing a vivid example of how cold plasma density can influence the loss of particles that are up to 10,000 times more energetic than the cold plasma itself.