plasmoid

A plasmoid is a coherent structure of magnetized plasma separated from its surroundings by magnetic field lines, behaving as a self-contained region within a larger plasma system. In two-dimensional descriptions, plasmoids often appear as magnetic islands; in three dimensions they take the form of twisted flux ropes.

Plasmoids commonly arise during magnetic reconnection, when oppositely directed magnetic fields rearrange and plasma is accelerated. In high-Lundquist-number systems, current sheets can fragment and form multiple plasmoids that may coalesce or be ejected along reconnection outflows.

Physically, plasmoids carry magnetic flux and currents, can have enhanced density and pressure, and are characterized by specific magnetic-field topology. Their motion is governed by magnetic tension, pressure balance, and ambient plasma flow; they may interact by merging or bouncing apart.

Observationally, plasmoids have been identified in the solar corona during solar flares and coronal mass ejections, in the Earth's magnetotail during substorms, and in laboratory plasma experiments exploring reconnection and magnetic confinement. In simulations they typically appear as chains of magnetic islands or flux tubes.

Studying plasmoids helps understand fast magnetic reconnection and energy release in plasmas. They provide a mechanism for rapid current-sheet fragmentation and flux transport, influencing astrophysical and space plasma dynamics as well as laboratory confinement devices.