2 research outputs found
Collisionless Magnetic Reconnection in a Stressed X-point Collapse
PhDMagnetic 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