The comparison of flux rope and crustal field observations on Mars

The crustal magnetic field on Mars often exhibits smooth rotations that can mimic magnetic flux ropes, complicating their identification. To address this challenge, we analyze two nightside events observed by Mars Atmosphere and Volatile EvolutioN: a true flux rope at an altitude of approximately 1400 km and a crustal field with flux-rope-like characteristics at approximately 250 km. We calculate magnetic tension from single-spacecraft measurements by using a local discontinuity system derived via minimum variance analysis of the magnetic field and by deriving field gradients along the satellite trajectory. In the flux rope event, magnetic tension peaks at the structure's center, coinciding with the maximum field strength, and decays outward in a manner consistent with a coherent helical configuration. In the crustal field event, tension is weaker, spatially irregular, and uncorrelated with field maxima. Total pressure gradient and magnetic tension co-peak only in the flux rope, while enhanced CO2+ density in the crustal field suggests vertical ion transport from lower atmospheric layers. In regions where remnant crustal magnetization is uncertain, calculating magnetic tension provides an effective means to distinguish flux ropes from crustal fields.