Filamentary structure of a three‐dimensional plasmoid

Abstract

We have examined the changes of the magnetic field topology and the field line connections of a simple explicit magnetic field model of a plasmoid in different stages from its formation to its complete disconnection from the Earth. Particular emphasis was on the effects of a net cross‐tail magnetic field component that breaks the symmetry around the neutral sheet. We find, consistent with the qualitative conclusions of Hughes and Sibeck (1987), that the initial stage of the plasmoid formation is characterized by the formation of helical field lines which cross the neutral sheet typically more than once, but are still connected with the Earth, so that no topology change occurs. In that case a nontopological notion of reconnection is required (Schindler et al., 1988). When reconnection proceeds to lobe field lines, the central plasmoid flux rope becomes enveloped by a sheath of open field lines that pull the plasmoid flux rope back toward the tail. The period of gradual separation of the plasmoid is characterized by a conversion through magnetic reconnection, of helical field lines connected with the Earth to open ones going tailward into interplanetary space. The detailed tracing of magnetic field lines reveals that the structure of the plasmoid region is more complicated than was earlier envisioned. We find that plasmoid field lines no longer form a separate class of field lines as in the symmetric case. During the stage of gradual separation from the Earth the plasmoid is characterized by the central flux rope connected with the Earth which is wrapped by strands of open, partially open, and closed field lines in a filamentary way forming a layer of intermingled flux tubes of different types inside the earlier mentioned sheath of open field lines. For decreasing values of |By| the field connection varies on smaller and smaller scales such that the connection becomes stochastic. This process can possibly cause or increase dissipation.