Structure of magnetic separators and separator reconnection

Magnetic separators are important locations of three-dimensional magnetic reconnection. They are field lines that lie along the edges of four flux domains and represent the intersection of two separatrix surfaces. Since the intersection of two surfaces produces an X-type structure, when viewed along the line of intersection, the global three-dimensional topology of the magnetic field around a separator is hyperbolic. It is therefore usually assumed that the projection of the magnetic field lines themselves onto a two-dimensional plane perpendicular to a separator is also hyperbolic in nature. In this paper, we use the results of a three-dimensional MHD experiment of separator reconnection to show that, in fact, the projection of the magnetic field lines in a cut perpendicular to a separator may be either hyperbolic or elliptic and that the structure of the magnetic field projection may change in space, along the separator, as well as in time, during the life of the separator. Furthermore, in our experiment, we find that there are both spatial and temporal variations in the parallel component of current (and electric field) along the separator, with all high parallel current regions (which are associated with reconnection) occurring between counterrotating flow regions. Importantly, reconnection occurs not only at locations where the structure of the projected perpendicular magnetic field is hyperbolic but also where it is elliptic.