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Nascent Flux Rope Observations at Earth's Dayside Magnetopause

Two THEMIS satellites, separated by only 0.38 Earth radii (RE), traversed two consecutive dayside magnetopause (MP) current sheets at 3.5 < ZGSM < 3.8 RE on 15 November 2010. An early‐stage crater‐like flux transfer event (FTE) with weakly enhanced total pressure is sampled at the first, complete outbound MP crossing. A mature FTE flux rope is observed just 70 s later across the second, inbound crossing of the MP current sheet. Two counterstreaming magnetosheath ion beams at the interface of two converging reconnection exhausts earthward of the MP current sheet provide direct evidence of two X‐lines across the early‐stage crater FTE. A D‐shaped ion beam observed along the southward magnetic field in the magnetosheath boundary layer and northward exhausts observed earthward of the MP provide evidence of two X‐lines across the mature FTE flux rope. Tripolar out‐of‐plane magnetic field perturbations (ΔBM) of substantial magnitudes were sampled across the MP current sheet in both stages of the FTE flux rope evolution. These dayside observations suggest an association between a tripolar ΔBM perturbation with two magnetic reconnection X‐lines and FTE flux rope formation. A dedicated particle‐in‐cell numerical simulation of magnetic reconnection reproduces similar ion velocity distributions and out‐of‐plane magnetic field perturbations at an asymmetric current sheet as those observed across a nascent flux rope between two X‐lines at the dayside asymmetric MP.