Tracing Martian Crustal Magnetic Connectivity Using Ion Composition During the 2018 Global Dust Storm

Crustal magnetic fields exert a fundamental control on the structure and dynamics of the Martian ionosphere. In this study, we use in situ ion composition measurements from the MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS) to investigate how crustal magnetic fields modulated the Martian upper atmosphere during the June 2018 global dust storm. By restricting the analysis to a narrow range of solar zenith angles and altitudes, we isolate magnetic effects from variations driven by solar illumination and vertical structure. We find that the densities of O₂⁺, O⁺, and CO₂⁺ differ systematically between regions of strong and weak crustal magnetic fields, with strong-field regions exhibiting reduced variability consistent with magnetic confinement. Importantly, a substantial fraction of observations located outside traditional geographic masks display ion composition signatures that closely resemble those observed in strong-field regions. Spatial analysis shows that these “strong-like” undetermined observations preferentially occur near known crustal magnetic anomalies, indicating that magnetic influence extends beyond fixed geographic boundaries. These results demonstrate that ion composition provides a sensitive diagnostic of magnetic topology at Mars and reveal the importance of magnetic connectivity in regulating ionospheric structure under extreme atmospheric conditions. Our findings suggest that static geographic classifications may underestimate the true spatial reach of crustal magnetic control during periods of enhanced atmospheric disturbance.