5,470 research outputs found
Modeling the Rise of Fibril Magnetic Fields in Fully Convective Stars
Many fully convective stars exhibit a wide variety of surface magnetism,
including starspots and chromospheric activity. The manner by which bundles of
magnetic field traverse portions of the convection zone to emerge at the
stellar surface is not especially well understood. In the Solar context, some
insight into this process has been gleaned by regarding the magnetism as
consisting partly of idealized thin flux tubes (TFT). Here, we present the
results of a large set of TFT simulations in a rotating spherical domain of
convective flows representative of a 0.3 solar-mass, main-sequence star. This
is the first study to investigate how individual flux tubes in such a star
might rise under the combined influence of buoyancy, convection, and
differential rotation. A time-dependent hydrodynamic convective flow field,
taken from separate 3D simulations calculated with the anelastic equations,
impacts the flux tube as it rises. Convective motions modulate the shape of the
initially buoyant flux ring, promoting localized rising loops. Flux tubes in
fully convective stars have a tendency to rise nearly parallel to the rotation
axis. However, the presence of strong differential rotation allows some
initially low latitude flux tubes of moderate strength to develop rising loops
that emerge in the near-equatorial region. Magnetic pumping suppresses the
global rise of the flux tube most efficiently in the deeper interior and at
lower latitudes. The results of these simulations aim to provide a link between
dynamo-generated magnetic fields, fluid motions, and observations of starspots
for fully convective stars.Comment: 20 pages, 15 figures, accepted to Astrophysical Journa
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
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
Inferring physical conditions in interstellar clouds of H_2
We have developed a code that models the formation, destruction, radiative
transfer, and vibrational/rotational excitation of H_2 in a detailed fashion.
We discuss how such codes, together with FUSE observations of H_2 in diffuse
and translucent lines of sight, may be used to infer various physical
parameters. We illustrate the effects of changes in the major physical
parameters (UV radiation field, gas density, metallicity), and we point out the
extent to which changes in one parameter may be mirrored by changes in another.
We provide an analytic formula for the molecular fraction, f_H2, as a function
of cloud column density, radiation field, and grain formation rate of H_2. Some
diffuse and translucent lines of sight may be concatenations of multiple
distinct clouds viewed together. Such situations can give rise to observables
that agree with the data, complicating the problem of uniquely identifying one
set of physical parameters with a line of sight. Finally, we illustrate the
application of our code to an ensemble of data, such as the FUSE survey of H_2
in the Large and Small Magellanic Clouds (LMC/SMC), in order to constrain the
elevated UV radiation field intensity and reduced grain formation rate of H_2
in those low- metallicity environments.Comment: 33 pages (aastex, manuscript), 9 figures (3 color). 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
Volcanotectonic interactions between inclined sheets, dykes, and faults at the Santorini Volcano, Greece
Dykes and inclined sheets are known occasionally to exploit faults as parts of their paths, but the conditions that allow this to happen are still not fully understood. In this paper, we report field observations from a swarm composed of 91 segments of dykes and inclined sheets, the swarm being particularly well-exposed in the mechanically layered caldera walls of the Santorini volcano, Greece. Here the focus is on dykes and sheets in the swarm that are seen deflected into faults and the mechanical conditions that encourage such deflections. In particular, we present new analytical and numerical models to explain the mechanical principles of dyke/sheet deflections into faults. The numerical models are applied to a normal-fault dipping 65° with a damage zone composed of parallel layers or zones of progressively stiffer rocks with increasing distance from the fault rupture plane. We model a sheet-intrusion, dipping from 0° to 90° and with an overpressure of alternatively 1 MPa and 5 MPa, approaching the fault. We further tested the effects of changing (1) the thickness of the sheet-intrusion, (2) the fault-zone thickness, (3) the fault-zone dip-dimension (height), and (4) the loading by, alternatively, regional tension and compression. We find that the stiffness of the fault core, where a compliant core characterises recently active fault zones, has pronounced effects on the orientation and magnitudes of the local stresses and, thereby, on the likelihood of dyke/sheet deflection into the fault zone. Similarly, the analytical models, focusing on the fault-zone tensile strength and energy conditions for dyke/sheet deflection, indicate that dykes/sheets are most likely to be deflected into and use steeply dipping recently active (zero tensile-strength) normal faults as parts of their paths
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