Flux transfer events are transient magnetized plasma structures that are
self-balancing, rope-like phenomena that can appear at the Earth's
magnetopause when the interplanetary magnetic field is southward. The
formation of this type of flux rope has been unclear. Using measurements
of particles and magnetic fields on the MMS spacecraft, we find that
rope-like structures containing magnetospheric energetic electrons have
exactly the same occurrence rate as those without magnetospheric
electrons, independent of solar wind properties and location of
observation. This equality is consistent with a pair of flux ropes being
generated at the same time as an outcome of magnetic flux pileup
reconnection. One resulting flux rope has its two ends connected to the
magnetosphere, and the other is connected on both ends to the shocked
solar wind.