Non-monotonic oxygen ion escape during the transition from an induced to an intrinsic Martian magnetosphere

Understanding the evolution of the global dipole magnetic field on ancient Mars is critical for reconstructing its paleo-environment. However, the effectiveness of this intrinsic dipole field in protecting the atmosphere and the relationship between magnetic shielding and ion escape rates remain subjects of debate. In this study, we utilize a three-dimensional multi-fluid magnetohydrodynamic (MHD) model incorporating the Hall effect, with equatorial dipole field strengths ranging from 0 to 120 nT, to simulate the transition from an induced to an intrinsic magnetosphere. To understand the underlying ion escape mechanisms, we conduct an in-depth analysis of the variations in the driving electric fields (convection, Hall, and ambipolar) and evaluate the trans-terminator and radial O^+ fluxes across different altitudes and solar zenith angle ranges. Our results reveal that as the dipole field strengthens, the increased magnetic pressure significantly extends the magnetopause standoff distance. This effectively displaces the driving electric fields away from heavy-ion-rich regions, which significantly suppresses and spatially deflects the dayside plume escape channel at lower altitudes. Notably, despite this localized shielding, the total O^+ escape rate exhibits a distinct non-monotonic trend, reaching a pronounced global peak at an intermediate magnetization state of approximately 45-60 nT. By evaluating the trans-terminator and radial fluxes, we suggest that this peak reflects a dynamic competition between the progressive suppression of the dayside plume channel and the enhanced redistribution of O^+ toward the nightside and tailward escape channel under the reconfigured magnetic topology. This finding demonstrates that the ancient Martian magnetic field does not strictly follow a "stronger field equals better protection" paradigm, offering new insights into how planetary magnetic field evolution controls heavy-ion loss.