9 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
Role of electron inertia and reconnection dynamics in a stressed X-point collapse with a guide-field
In previous simulations of collisionless 2D magnetic reconnection it was
consistently found that the term in the generalised Ohm's law that breaks the
frozen-in condition is the divergence of the electron pressure tensor's
non-gyrotropic components. A fully relativistic particle-in-cell (PIC) code was
used to model -point collapse with a guide-field in two and three spatial
dimensions. We show that in a 2D -point collapse with a guide-field close to
the strength of the in-plane field, the increased induced shear flows along the
diffusion region lead to a new reconnection regime in which electron inertial
terms play a dominant role at the -point. This transition is marked by the
emergence of a magnetic island - and hence a second reconnection site - as well
as electron flow vortices moving along the current sheet. The reconnection
electric field at the -point is shown to exceed all lower guide-field cases
for a brief period, indicating a strong burst in reconnection. By extending the
simulation to three spatial dimensions it is shown that the locations of
vortices along the current sheet (visualised by their -value) vary in the
out-of-plane direction, producing tilted vortex tubes. The vortex tubes on
opposite sides of the diffusion region are tilted in opposite directions,
similarly to bifurcated current sheets in oblique tearing-mode reconnection.
The tilt angles of vortex tubes were compared to a theoretical estimation and
were found to be a good match. Particle velocity distribution functions for
different guide-field runs, for 2.5D and 3D simulations, are analysed and
compared.Comment: A&A, Forthcoming articl
Interference of Higgs boson resonances in mu^+ mu^- -> neutralino_i neutralino_j with longitudinal beam polarization
We study the interference of resonant Higgs boson exchange in neutralino
production in \mu^+ \mu^- annihilation with longitudinally polarized beams. We
use the energy distribution of the decay lepton in the process \neutralino_j
\to \ell^\pm \slepton^\mp to determine the polarization of the neutralinos. In
the CP conserving Minimal Supersymmetric Standard Model a non-vanishing
asymmetry in the lepton energy spectrum is caused by the interference of Higgs
boson exchange channels with different CP eigenvalues. The contribution of this
interference is large if the heavy neutral bosons H and A are nearly
degenerate. We show that the asymmetry can be used to determine the couplings
of the neutral Higgs bosons to the neutralinos. In particular, the asymmetry
allows to determine the relative phase of the couplings. We find large
asymmetries and cross sections for a set of reference scenarios with nearly
degenerate neutral Higgs bosons.Comment: 20 pages, 6 figures, minor typos corrected, to appear in Eur. Phys.
J.