On the three‐dimensional magnetic structure of the plasmoid created in the magnetotail at substorm onset

Reconnection of magnetic field lines at a neutral line that forms in the near‐earth region of the plasma sheet at substorm onset creates, from some longitudinal sector of the plasma sheet, a plasmoid that is then free to flow out of the magnetotail into the magnetosphere's wake. The magnetic field in the departing plasmoid frequently displays a strong positive or negative Y SM component that has been difficult to reconcile with the reconnection geometry as it is traditionally depicted in two dimensions. We show that this deviation of the magnetic field is a manifestation of a draping of the newly reconnected field line loops toward the tail's central (midnight) meridian and that the draping is a consequence of the three‐dimensional plasma flow associated with the reconnection process.