We present clear evidence of the formation of three-dimensional plasmoids in
the current sheet at the interface between two different magnetic flux
systems in a 3D numerical experiment of flux emergence into the solar
atmosphere and study their properties and time evolution. Plasmoids are
formed at the transition between the upcoming and pre-existing atmospheric
flux systems and are most likely the result of resistive tearing-mode
instabilities. They adopt the shape of a solenoid contained within the
current sheet: the solenoid is tightly wound when the field in the two flux
systems is close to antiparallel.
The plasmoids are expelled to the sides of the sheet
as a result of a reconnection imbalance between the two X-lines on their
sides. We show the complex, three-dimensional fieldline geometry in various
plasmoids: individual plasmoid field lines have external linkages to the flux
system on either side of the current sheet; we also find field lines that
go through a few plasmoids in succession, probably indicating that the field
line has resulted from multiple reconnection events.