  We report for the first time the intrinsically three-dimensional (3D)
geometry of the magnetic reconnection process induced by ballooning instability
in a generalized Harris sheet. The spatial distribution and structure of the
quasi-separatrix layers, as well as their temporal emergence and evolution,
indicate that the associated magnetic reconnection can only occur in a 3D
geometry, which is irreducible to that of any two-dimensional reconnection
process. Such a finding provides a new perspective to the long-standing
controversy over the substorm onset problem, and elucidates the combined roles
of reconnection and ballooning instabilities. It also connects to the universal
presence of 3D reconnection processes previously discovered in various natural
and laboratory plasmas.
