Resolving Fine-Scale Undulations on the Low-to-Midlatitude Magnetopause

The magnetopause is a key boundary formed by the interaction between Earth's magnetic field and the solar wind. While empirical models simplify the magnetopause as a smooth surface, observations reveal its complex fine-scale structure. Using data from the Cluster satellites, we identified 78 magnetopause crossing events and analyzed their geometric features through principal and Gaussian curvatures. The results show that the magnetopause commonly exhibits ripple-like structures, with approximately 72% of the events characterized by saddle-shaped geometries, with no significant correlation to solar wind parameters. In the flank region and under quasi-perpendicular shock conditions, the occurrence of ellipsoidal structures increases significantly, likely due to the impacts of Kelvin-Helmholtz instability and isolated high-speed jet collisions. Additionally, we estimated the effective phase velocity of the ripple-like boundary structures relative to the local plasma and found an overall preference for sunward propagation along the magnetopause, with magnitudes generally smaller than the bulk plasma speed.