The Spherical Tearing Mode: A new Model for 3D Reconnection in the Magnetosphere

Magnetic reconnection poleward of the cusp is widely believed to provide a mechanism for the formation of the low-latitude boundary layer and plasma sheet during northward IMF conditions. Theoretical frameworks for the understanding of such processes are often grounded in the 2D slab tearing model of Quest and Coroniti, who demonstrated that tearing mode growth rates are largest when the reconnecting magnetic fields are anti-parallel. We present an alternative theoretical model for understanding such processes which begin from a genuinely 3D (but idealized) model of the magnetosphere with a northward IMF. We demonstrate that this equilibrium is unstable to a new tearing instability, called the spherical tearing mode, which grows faster than its resistive slab counterpart. The tearing eigenfunction has global support along a 3D separatrix surface composed of null-null lines that thread the cusps, but is not localized near the cusps themselves. We show the geometrical structure of the 3D separatrices which determine the sites of reconnection. While the field lines may appear to be anti-parallel in certain local regions, they are not so globally. This model requires us to revisit recent observations and global simulations of reconnection in the magnetosphere, and sheds new light on the controversy pertaining to "anti-parallel" and "component" reconnection.