228 research outputs found
On the nature of reconnection at a solar coronal null point above a separatrix dome
Three-dimensional magnetic null points are ubiquitous in the solar corona,
and in any generic mixed-polarity magnetic field. We consider magnetic
reconnection at an isolated coronal null point, whose fan field lines form a
dome structure. We demonstrate using analytical and computational models
several features of spine-fan reconnection at such a null, including the fact
that substantial magnetic flux transfer from one region of field line
connectivity to another can occur. The flux transfer occurs across the current
sheet that forms around the null point during spine-fan reconnection, and there
is no separator present. Also, flipping of magnetic field lines takes place in
a manner similar to that observed in quasi-separatrix layer or slip-running
reconnection.Comment: Accepted for publication in the Astrophysical Journa
Resistive magnetohydrodynamic reconnection : resolving long-term, chaotic dynamics
We acknowledge financial support from the EC FP7/2007-2013 Grant Agreement SWIFF (No. 263340) and from project GOA/2009/009 (KU Leuven). This research has been funded by the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy Office (IAP P7/08 CHARM). Part of the simulations used the infrastructure of the VSC-Flemish Supercomputer Center, funded by the Hercules Foundation and the Flemish Government-Department EWI. Another part of the simulations was done at the former Danish Center for Scientific Computing at Copenhagen University which is now part of DeIC Danish e-Infrastructure Cooperation.In this paper, we address the long-term evolution of an idealised double current system entering reconnection regimes where chaotic behavior plays a prominent role. Our aim is to quantify the energetics in high magnetic Reynolds number evolutions, enriched by secondary tearing events, multiple magnetic island coalescence, and compressive versus resistive heating scenarios. Our study will pay particular attention to the required numerical resolutions achievable by modern (grid-adaptive) computations, and comment on the challenge associated with resolving chaotic island formation and interaction. We will use shock-capturing, conservative, grid-adaptive simulations for investigating trends dominated by both physical (resistivity) and numerical (resolution) parameters, and confront them with (visco-)resistive magnetohydrodynamic simulations performed with very different, but equally widely used discretization schemes. This will allow us to comment on the obtained evolutions in a manner irrespective of the adopted discretization strategy. Our findings demonstrate that all schemes used (finite volume based shock-capturing, high order finite differences, and particle in cell-like methods) qualitatively agree on the various evolutionary stages, and that resistivity values of order 0.001 already can lead to chaotic island appearance. However, none of the methods exploited demonstrates convergence in the strong sense in these chaotic regimes. At the same time, nonperturbed tests for showing convergence over long time scales in ideal to resistive regimes are provided as well, where all methods are shown to agree. Both the advantages and disadvantages of specific discretizations as applied to this challenging problem are discussed.Publisher PDFPeer reviewe
Generalised models for torsional spine and fan magnetic reconnection
Three-dimensional null points are present in abundance in the solar corona,
and the same is likely to be true in other astrophysical environments. Recent
studies suggest that reconnection at such 3D nulls may play an important role
in the coronal dynamics. In this paper the properties of the torsional spine
and torsional fan modes of magnetic reconnection at 3D nulls are investigated.
New analytical models are developed, which for the first time include a current
layer that is spatially localised around the null, extending along either the
spine or the fan of the null. These are complemented with numerical
simulations. The principal aim is to investigate the effect of varying the
degree of asymmetry of the null point magnetic field on the resulting
reconnection process - where previous studies always considered a non-generic
radially symmetric null. The geometry of the current layers within which
torsional spine and torsional fan reconnection occur is found to be strongly
dependent on the symmetry of the magnetic field. Torsional spine reconnection
still occurs in a narrow tube around the spine, but with elliptical
cross-section when the fan eigenvalues are different, and with the short axis
of the ellipse being along the strong field direction. The spatiotemporal peak
current, and the peak reconnection rate attained, are found not to depend
strongly on the degree of asymmetry. For torsional fan reconnection, the
reconnection occurs in a planar disk in the fan surface, which is again
elliptical when the symmetry of the magnetic field is broken. The short axis of
the ellipse is along the weak field direction, with the current being peaked in
these weak field regions. The peak current and peak reconnection rate in this
case are clearly dependent on the asymmetry, with the peak current increasing
but the reconnection rate decreasing as the degree of asymmetry is increased
A Contemporary View of Coronal Heating
Determining the heating mechanism (or mechanisms) that causes the outer
atmosphere of the Sun, and many other stars, to reach temperatures orders of
magnitude higher than their surface temperatures has long been a key problem.
For decades the problem has been known as the coronal heating problem, but it
is now clear that `coronal heating' cannot be treated or explained in isolation
and that the heating of the whole solar atmosphere must be studied as a highly
coupled system. The magnetic field of the star is known to play a key role,
but, despite significant advancements in solar telescopes, computing power and
much greater understanding of theoretical mechanisms, the question of which
mechanism or mechanisms are the dominant supplier of energy to the chromosphere
and corona is still open. Following substantial recent progress, we consider
the most likely contenders and discuss the key factors that have made, and
still make, determining the actual (coronal) heating mechanism (or mechanisms)
so difficult
The effect of the relative orientation between the coronal field and new emerging flux: I Global Properties
The emergence of magnetic flux from the convection zone into the corona is an
important process for the dynamical evolution of the coronal magnetic field. In
this paper we extend our previous numerical investigations, by looking at the
process of flux interaction as an initially twisted flux tube emerges into a
plane parallel, coronal magnetic field. Significant differences are found in
the dynamical appearance and evolution of the emergence process depending on
the relative orientation between the rising flux system and any preexisting
coronal field. When the flux systems are nearly anti-parallel, the experiments
show substantial reconnection and demonstrate clear signatures of a high
temperature plasma located in the high velocity outflow regions extending from
the reconnection region. However, the cases that have a more parallel
orientation of the flux systems show very limited reconnection and none of the
associated features. Despite the very different amount of reconnection between
the two flux systems, it is found that the emerging flux that is still
connected to the original tube, reaches the same height as a function of time.
As a compensation for the loss of tube flux, a clear difference is found in the
extent of the emerging loop in the direction perpendicular to the main axis of
the initial flux tube. Increasing amounts of magnetic reconnection decrease the
volume, which confines the remaining tube flux.Comment: 21 pages, 16 figures Accepted for Ap
Current sheets at three-dimensional magnetic nulls:effect of compressibility
The nature of current sheet formation in the vicinity of three-dimensional
(3D) magnetic null points is investigated. The particular focus is upon the
effect of the compressibility of the plasma on the qualitative and quantitative
properties of the current sheet. An initially potential 3D null is subjected to
shearing perturbations, as in a previous paper [Pontin et al., Phys. Plasmas,
in press (2007)]. It is found that as the incompressible limit is approached,
the collapse of the null point is suppressed, and an approximately planar
current sheet aligned to the fan plane is present instead. This is the case
regardless of whether the spine or fan of the null is sheared. Both the peak
current and peak reconnection rate are reduced. The results have a bearing on
previous analytical solutions for steady-state reconnection in incompressible
plasmas, implying that fan current sheet solutions are dynamically accessible,
while spine current sheet solutions are not.Comment: to appear in Physics of Plasmas. This version contains updated
figures and references, additional discussion, and typos are fixed. This is
the second in a series of papers - the first of which (by the same authors)
is located at astro-ph/0701462. A version with higher quality figures can be
found at http://www.maths.dundee.ac.uk/~dpontin
Magnetohydrodynamic evolution of magnetic skeletons
The heating of the solar corona is likely to be due to reconnection of the
highly complex magnetic field that threads throughout its volume. We have run a
numerical experiment of an elementary interaction between the magnetic field of
two photospheric sources in an overlying field that represents a fundamental
building block of the coronal heating process. The key to explaining where, how
and how much energy is released during such an interaction is to calculate the
resulting evolution of the magnetic skeleton. A skeleton is essentially the web
of magnetic flux surfaces (called separatrix surfaces) that separate the
coronal volume into topologically distinct parts. For the first time the
skeleton of the magnetic field in a 3D numerical MHD experiment is calculated
and carefully analysed, as are the ways in which it bifurcates into different
topologies. A change in topology normally changes the number of magnetic
reconnection sites.
In our experiment, the magnetic field evolves through a total of six distinct
topologies. Initially, no magnetic flux joins the two sources. Then a new type
of bifurcation, called a global double-separator bifurcation, takes place: this
bifurcation is likely to be one of the main ways in which new separators are
created in the corona (separators are field lines at which 3D reconnection
takes place). This is the first of five bifurcations in which the skeleton
becomes progressively more complex before simplifying. Surprisingly, for such a
simple initial state, at the peak of complexity there are five separators and
eight flux domains present.Comment: 18 pages, 5 figure
Elementary Heating Events - Magnetic Interactions Between Two Flux Sources. III Energy Considerations
The magnetic field plays a crucial role in heating the solar corona, but the
exact energy release mechanism(s) is(are) still unknown. Here, we investigate
in detail, the process of magnetic energy release in a situation where two
initially independent flux systems are forced into each other. Work done by the
foot point motions goes in to building a current sheet in which magnetic
reconnection takes place. The scaling relations of the energy input and output
are determined as functions of the driving velocity and the strength of fluxes
in the independent flux systems. In particular, it is found that the energy
injected into the system is proportional to the distance travelled not the rate
of travel. Similarly, the rate of Joule dissipation is related to the distance
travelled. Hence, rapidly driven foot points lead to bright, intense, but
short-lived events, whilst slowly driven foot points produce weaker, but
longer-lived brightenings. Integrated over the lifetime of the events both
would produce the same heating if all other factors were the same. A strong
overlying field has the affect of creating compact flux lobes from the sources.
These appear to lead to a more rapid injection of energy, as well as a more
rapid release of energy. Thus, the stronger the overlying field the more
compact and more intense the heating. This means observers must know the rate
of movement of the magnetic fragments involved in an events, as well as
determine the strength and orientation of the surrounding field to be able to
predict anything about the energy dissipated.Comment: A&A accepted, 24 pages, 11 figure
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