4,509 research outputs found
Coronal heating in multiple magnetic threads
Context. Heating the solar corona to several million degrees requires the
conversion of magnetic energy into thermal energy. In this paper, we
investigate whether an unstable magnetic thread within a coronal loop can
destabilise a neighbouring magnetic thread. Aims. By running a series of
simulations, we aim to understand under what conditions the destabilisation of
a single magnetic thread can also trigger a release of energy in a nearby
thread. Methods. The 3D magnetohydrodynamics code, Lare3d, is used to simulate
the temporal evolution of coronal magnetic fields during a kink instability and
the subsequent relaxation process. We assume that a coronal magnetic loop
consists of non-potential magnetic threads that are initially in an equilibrium
state. Results. The non-linear kink instability in one magnetic thread forms a
helical current sheet and initiates magnetic reconnection. The current sheet
fragments, and magnetic energy is released throughout that thread. We find
that, under certain conditions, this event can destabilise a nearby thread,
which is a necessary requirement for starting an avalanche of energy release in
magnetic threads. Conclusions. It is possible to initiate an energy release in
a nearby, non-potential magnetic thread, because the energy released from one
unstable magnetic thread can trigger energy release in nearby threads, provided
that the nearby structures are close to marginal stability
Thermal and non-thermal emission from reconnecting twisted coronal loops
Twisted magnetic fields should be ubiquitous in flare-producing active
regions where the magnetic fields are strongly non-potential. It has been shown
that reconnection in helical magnetic coronal loops results in plasma heating
and particle acceleration distributed within a large volume, including the
lower coronal and chromospheric sections of the loops. This scenario can be an
alternative to the standard flare model, where particles are accelerated only
in a small volume located in the upper corona. We use a combination of MHD
simulations and test-particle methods, which describe the development of kink
instability and magnetic reconnection in twisted coronal loops using resistive
compressible MHD, and incorporate atmospheric stratification and large-scale
loop curvature. The resulting distributions of hot plasma let us estimate
thermal X-ray emission intensities. The electric and magnetic fields obtained
are used to calculate electron trajectories using the guiding-centre
approximation. These trajectories combined with the MHD plasma density
distributions let us deduce synthetic HXR bremsstrahlung intensities. Our
simulations emphasise that the geometry of the emission patterns produced by
hot plasma in flaring twisted coronal loops can differ from the actual geometry
of the underlying magnetic fields. The twist angles revealed by the emission
threads (SXR) are consistently lower than the field-line twist present at the
onset of the kink-instability. HXR emission due to the interaction of energetic
electrons with the stratified background are concentrated at the loop
foot-points in these simulations, even though the electrons are accelerated
everywhere within the coronal volume of the loop. The maximum of HXR emission
consistently precedes that of SXR emission, with the HXR light-curve being
approximately proportional to the temporal derivative of the SXR light-curve.Comment: (accepted for publication on A&A
Theoretical limits on magnetic field strengths in low-mass stars
Observations have suggested that some low-mass stars have larger radii than
predicted by 1-D structure models. Some theoretical models have invoked very
strong interior magnetic fields (of order 1 MG or more) as a possible cause of
such large radii. Whether fields of that strength could in principle by
generated by dynamo action in these objects is unclear, and we do not address
the matter directly. Instead, we examine whether such fields could remain in
the interior of a low mass object for a significant time, and whether they
would have any other obvious signatures. First, we estimate timescales for the
loss of strong fields by magnetic buoyancy instabilities. We consider a range
of field strengths and simple morphologies, including both idealized flux tubes
and smooth layers of field. We confirm some of our analytical estimates using
thin flux tube magnetohydrodynamic (MHD) simulations of the rise of buoyant
fields in a fully-convective M-dwarf. Separately, we consider the Ohmic
dissipation of such fields. We find that dissipation provides a complementary
constraint to buoyancy: while small-scale, fibril fields might be regenerated
faster than they rise, the dissipative heating associated with such fields
would in some cases greatly exceed the luminosity of the star. We show how
these constraints combine to yield limits on the internal field strength and
morphology in low-mass stars. In particular, we find that for stars of 0.3
solar masses, no fields in flux tubes stronger than about 800 kG are
simultaneously consistent with both constraints.Comment: 19 pages, 10 figures, accepted to Ap
Three-Dimensional Simulations of Solar and Stellar Dynamos: The Influence of a Tachocline
We review recent advances in modeling global-scale convection and dynamo
processes with the Anelastic Spherical Harmonic (ASH) code. In particular, we
have recently achieved the first global-scale solar convection simulations that
exhibit turbulent pumping of magnetic flux into a simulated tachocline and the
subsequent organization and amplification of toroidal field structures by
rotational shear. The presence of a tachocline not only promotes the generation
of mean toroidal flux, but it also enhances and stabilizes the mean poloidal
field throughout the convection zone, promoting dipolar structure with less
frequent polarity reversals. The magnetic field generated by a convective
dynamo with a tachocline and overshoot region is also more helical overall,
with a sign reversal in the northern and southern hemispheres. Toroidal
tachocline fields exhibit little indication of magnetic buoyancy instabilities
but may be undergoing magneto-shear instabilities.Comment: 14 pages, 5 color figures, to appear in Proc. GONG 2008/SOHO XXI
Meeting on Solar-Stellar Dynamos as Revealed by Helio and Asteroseismology,
held August 15-18, 2008, Boulder, CO, Astronomical Soc. Pac. Conf. Series,
volume TB
Two-fluid and magnetohydrodynamic modelling of magnetic reconnection in the MAST spherical tokamak and the solar corona
Twisted magnetic flux ropes are ubiquitous in space and laboratory plasmas,
and the merging of such flux ropes through magnetic reconnection is an
important mechanism for restructuring magnetic fields and releasing free
magnetic energy. The merging-compression scenario is one possible start up
scheme for spherical tokamaks, which has been used on the Mega Amp Spherical
Tokamak MAST. Two current-carrying plasma rings, or flux ropes, approach each
other through the mutual attraction of their like currents, and merge, through
magnetic reconnection, into a single plasma torus, with substantial plasma
heating. 2D resistive MHD and Hall MHD simulations of this process are
reported, and new results for the temperature distribution of ions and
electrons are presented. A model of the based on relaxation theory is also
described, which is now extended to tight aspect ratio geometry. This model
allows prediction of the final merged state and the heating. The implications
of the relaxation model for heating of the solar corona are also discussed, and
a model of the merger of two or more twisted coronal flux ropes is presented,
allowing for different senses of twist
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