A plasmoid is a coherent structure of plasma and magnetic fields. Plasmoids have been proposed to explain natural phenomena such as ball lightning, magnetic bubbles in the magnetosphere, and objects in cometary tails, in the solar wind, in the solar atmosphere, and in the heliospheric current sheet. Plasmoids produced in the laboratory include field-reversed configurations, spheromaks, and in dense plasma focuses.
The word plasmoid was coined in 1956 by Winston H. Bostick (1916-1991) to mean a “plasma-magnetic entity”. Bostick went on to apply his theory of plasmoids to astrophysics phenomena.
Active regions on the solar surface are often the site of eruptions. These are associated with magnetic fields from the solar interior rising to the surface and gradually expanding into the Sun’s outer atmosphere, the corona, in a process known as magnetic flux emergence.
A group of scientists from the University of St Andrews developed 3D computer models of these phenomena, revealing that the emergence of magnetic flux naturally leads to the formation and expulsion of plasmoids that adopt a twisted tube configuration.
The formation of the plasmoids is due to the motion of plasma in the lower atmosphere of the Sun. These motions bring magnetic fieldlines closer together to reconnect and build a new magnetic flux system (i.e. the plasmoid). Whether the plasmoids are ‘failed’ or ‘successful’ (i.e. they erupt into space) depends on the level of interaction between the new emerging field and the old, pre-existing magnetic field in the solar corona.