17 research outputs found
Is null-point reconnection important for solar flux emergence?
The role of null-point reconnection in a 3D numerical MHD model of solar
emerging flux is investigated. The model consists of a twisted magnetic flux
tube rising through a stratified convection zone and atmosphere to interact and
reconnect with a horizontal overlying magnetic field in the atmosphere. Null
points appear as the reconnection begins and persist throughout the rest of the
emergence, where they can be found mostly in the model photosphere and
transition region, forming two loose clusters on either side of the emerging
flux tube. Up to 26 nulls are present at any one time, and tracking in time
shows that there is a total of 305 overall, despite the initial simplicity of
the magnetic field configuration. We find evidence for the reality of the nulls
in terms of their methods of creation and destruction, their balance of signs,
their long lifetimes, and their geometrical stability. We then show that due to
the low parallel electric fields associated with the nulls, null-point
reconnection is not the main type of magnetic reconnection involved in the
interaction of the newly emerged flux with the overlying field. However, the
large number of nulls implies that the topological structure of the magnetic
field must be very complex and the importance of reconnection along separators
or separatrix surfaces for flux emergence cannot be ruled out.Comment: 26 pages, 12 figures. Added one referenc
Topological constraints on magnetic relaxation
The final state of turbulent magnetic relaxation in a reversed field pinch is
well explained by Taylor's hypothesis. However, recent
resistive-magnetohydrodynamic simulations of the relaxation of braided solar
coronal loops have led to relaxed fields far from the Taylor state, despite the
conservation of helicity. We point out the existence of an additional
topological invariant in any flux tube with non-zero field: the topological
degree of the field line mapping. We conjecture that this constrains the
relaxation, explaining why only one of three example simulations reaches the
Taylor state.Comment: 8 pages, 4 figures, accepted for publication in Physical Review
Letter
Heating of Braided Coronal Loops
Aims. We investigate the relaxation of braided magnetic loops in order to find out how the type of braiding via footpoint motions affects resultant heating of the loop. Methods. Two magnetic loops, braided in different ways, are used as initial conditions in resistive MHD simulations and their subsequent evolution is studied. Results. The fields both undergo a resistive relaxation in which current sheets form and fragment and the system evolves towards a state of lower energy. In one case this relaxation is very efficient with current sheets filling the volume and homogeneous heating of the loop occurring. In the other case fewer current sheets develop, less magnetic energy is released in the process and a patchy heating of the loop results. The two cases, although very similar in their setup, can be distinguished by the mixing properties of the photospheric driver. The mixing can be measured by the topological entropy of the plasma flow, an observable quantity
Dynamic non-null magnetic reconnection in three dimensions II: composite solutions
In this series of papers we examine magnetic reconnection in a domain where
the magnetic field does not vanish and the non-ideal region is localised in
space. In a previous paper we presented a technique for obtaining analytical
solutions to the stationary resistive MHD equations in such a situation and
examined specific examples of non-ideal reconnective solutions. Here we further
develop the model, noting that certain ideal solutions may be superimposed onto
the fundamental non-ideal solutions and examining the effect of imposing
various such flows. Significant implications are found for the evolution of
magnetic flux in the reconnection process. It is shown that, in contrast to the
two-dimensional case, in three-dimensions there is a very wide variety of
physically different steady reconnection solutions.Comment: 20 pages, 8 figure
Solar magnetic carpet III : coronal modelling of synthetic magnetograms
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