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Implosive formation of current sheets is a fundamental plasma process.Previous studies focused on the early time evolution, while here our primaryaim is to explore the longer-term evolution, which may be critical fordetermining the efficiency of energy release. To address this problem weinvestigate two closely-related problems, namely: (i) 1D, pinched anti-parallelmagnetic fields and (ii) 2D, null point containing fields which are locallyimbalanced ('null-collapse' or 'X-point collapse'). Within the framework ofresistive MHD, we simulate the full nonlinear evolution through three distinctphases: the initial implosion, its eventual halting mechanism, and subsequentevolution post-halting. In a parameter study, we find the scaling withresistivity of current sheet properties at the halting time is in goodagreement - in both geometries - with that inferred from a known 1D similaritysolution. We find that the halting of the implosions occurs rapidly afterreaching the diffusion scale by sudden Ohmic heating of the dense plasma withinthe current sheet, which provides a pressure gradient sufficient to opposefurther collapse and decelerate the converging flow. This back-pressure growsto exceed that required for force balance and so the post-implosion evolutionis characterised by the consequences of the current sheet `bouncing' outwards.These are: (i) the launching of propagating fast MHD waves (shocks) outwardsand (ii) the width-wise expansion of the current sheet itself. The expansion isonly observed to stall in the 2D case, where the pressurisation is relieved byoutflow in the reconnection jets. In the 2D case, we quantify the maximumamount of current sheet expansion as it scales with resistivity, and analysethe structure of the reconnection region which forms post-expansion, repletewith Petschek-type slow shocks and fast termination shocks.

Resistively-limited current sheet implosions in planar anti-parallel (1D) and null-point containing (2D) magnetic field geometries