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Magnetic reconnection is the reconfiguration of the topology of the magnetic field in a plasma. It is typically associated with the efficient conversion of magnetic energy, resulting, for example, in solar flares, and the generation of supra-thermal particle populations. Reconnection is widely present throughout the magnetized universe: from laboratory plasmas and the Earth’s magnetosphere to magnetar flares and gamma-ray bursts. It is also thought to be a key ingredient of plasma turbulence, where it may determine much of the energy dissipation. The importance of reconnection across multiple areas has meant that it has been the subject of intense investigation over the last 70 years. There are three main questions that we seek answers to: what triggers reconnection (the onset problem); how fast it can proceed (the rate problem); and how is the magnetic energy divided amongst the different possible channels (the energy partition problem). While much progress has been made, investigations are hampered by the fundamental nonlinear character of the problem and its intrinsic multi-scale nature; both conspire to make analytical and numerical calculations extremely challenging, and prompt researchers to seek to simplify the problem as much as possible. One common simplification is to eschew the onset stage: studies of reconnection often focus on assumed postâ€onset configurations and plasma parameters, bypassing not only the onset transition, but also the question of how such configurations came to be and, perhaps more pertinently, of whether they are even physically realizable. In this talk I will argue that, in general, the physics of the onset stage and the instabilities that arise then pose strong constraints on the parameters characterizing reconnection events. This implies that the onset cannot be trivially decoupled from the reconnection stage proper, and that physically realizable reconnection configurations are determined by it. I will illustrate these issues both in the MHD and the kinetic regimes. Host: Bill Daughton |