Collisionless Magnetic Reconnection in a Stressed X-point Collapse
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Magnetic X-point collapse is investigated using a 2.5D fully relativistic particle-in-cell simulation, with varying strengths of guide-field as well as open and closed boundary conditions. In the zero guide-field case we discover a new signature of Hall-reconnection in the out-of-plane magnetic field, namely an octupolar pattern, as opposed to the wellstudied quadrupolar out-of-plane field of reconnection. The emergence of the octupolar components was found to be caused by ion currents and is a general feature of X-point collapse. The effect was shown to be independent of system size and ion mass and confined to a few ion inertial lengths from the reconnection current sheet. In a comparative study of tearing-mode reconnection, signatures of octupolar components are found only in the out-flow region. It is argued that space-craft observations of magnetic fields at reconnection sites may be used accordingly to identify the type of reconnection. Further, initial oscillatory reconnection is observed, prior to reconnection onset, generating electromagnetic waves at the upper-hybrid frequency, matching solar flare progenitor emission. When applying a guide-field, in both open and closed boundary conditions, thinner dissipation regions are obtained and the onset of reconnection is increasingly delayed. Investigations with open boundary conditions show that, for guide-fields close to the strength of the in-plane field, shear flows emerge, leading to the formation of electron flow vortices and magnetic islands. Asymmetries in the components of the generalised Ohm’s law across the dissipation region are observed and inertial components are shown to play a role at the X-point. Extended in 3D geometry, it is shown that locations of magnetic islands and vortices are not constant along the height of the current-sheet. Vortices formed on opposite sides of the current-sheet travel in opposite directions along it, leading to a criss-cross vortex pattern. Similarly to oblique current sheets previously observed in 3D guide-field reconnection studies, vortex-tubes are inclined at the same angle as the magnetic field.
Authorsvon der Pahlen, Jan Graf
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