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This study analyzes 25 ion-scale flux ropes in the Magnetospheric Multiscale (MMS) observations to determine their structures. The high temporal and spatial resolution MMS measurements enable the application of multispacecraft techniques to ion-scale flux ropes. Flux ropes are identified as quasi-one-dimensional (quasi-1-D) when they retain the features of reconnecting current sheets; that is, the magnetic field gradient is predominantly northward or southward, and quasi-2-D when they exhibit circular cross sections; that is, the magnetic field gradients in the plane transverse to the flux rope axis are comparable. The analysis shows that the quasi-2-D events have larger core fields and smaller pressure variations than the quasi-1-D events. These two types of flux ropes could be the result of different processes, including magnetic reconnection with different dawn-dusk magnetic field components, temporal transformation of flattened structure to circular, or interactions with external environments.

Plain Language Summary Magnetic flux ropes are fundamental magnetic structures in space plasma physics and are commonly seen in the universe, such as, astrophysical jets, coronal mass ejections, and planetary magnetospheres. Flux ropes are important in mass and energy transport across plasma and magnetic boundaries, and they are found in a wide range of spatial sizes, from several tens of kilometers, that is, ion-scale flux ropes, to tens of millions of kilometers, that is, coronal mass ejections, in the solar system. The ion-scale flux ropes can be formed during magnetic reconnection and are hypothesized to energize electrons and influence the reconnection rate. Previous examinations of the structure of ion-scale flux ropes were greatly limited by measurement resolution. The unprecedented Magnetospheric Multiscale (MMS) mission high temporal and spatial resolution measurements provide a unique opportunity to investigate flux rope structures. By employing multispacecraft techniques, this study has provided new insights into the magnetic field variations and dimensionality of ion-scale flux ropes in the Earth's magnetotail. The results are consistent with the evolution of ion-scale flux ropes from initially flattened current sheet-like flux ropes near the time of formation into lower energy state with circular cross section predicted by theory and termed as the "Taylor" state.