Flux transfer events formed in the reconnection exhaust region between two X-lines and in the velocity shear layer of Kelvin-Helmholtz vortices

We present two new generation mechanisms for flux transfer events (FTEs) observed in (a) the reconnection outflow region and (b) the velocity shear layer associated with Kelvin-Helmholtz vortices, during magnetopause crossings. In the two cases, the four MMS spacecraft detected isolated regions with the enhanced magnetic field and bipolar Bn that indicate FTEs. In the first event, observed on the duskside, FTEs were initially embedded within the exhaust region northward of an X-line, but were located southward/downstream of a subsequent X-line. The presence of an X-line in the exhaust region southward of the second X-line results from the southward drift of the initial X-line and the formation of a new X-line northward of the spacecraft. The north-south-elongated current layer between the two X-lines is unstable to the tearing instability, generating multiple ion-scale flux-rope-type secondary islands. In the second case, detailed analyses of the field variations and plasma moments during the isolated dawnside event suggest that it developed along the Kelvin-Helmholtz wave boundary between two coalescing/compressing flow vortices. We use these two events to demonstrate a path to determination of parameters enabling us to distinguish between the different generation mechanisms and evolutionary phases of FTEs.