6,639 research outputs found
The magnitude of viscous dissipation in strongly stratified two-dimensional convection
Convection in astrophysical systems must be maintained against dissipation.
Although the effects of dissipation are often assumed to be negligible, theory
suggests that in strongly stratified convecting fluids, the dissipative heating
rate can exceed the luminosity carried by convection. Here we explore this
possibility using a series of numerical simulations. We consider
two-dimensional numerical models of hydrodynamic convection in a Cartesian
layer under the anelastic approximation and demonstrate that the dissipative
heating rate can indeed exceed the imposed luminosity. We establish a
theoretical expression for the ratio of the dissipative heating rate to the
luminosity emerging at the upper boundary, in terms only of the depth of the
layer and the thermal scale height. In particular, we show that this ratio is
independent of the diffusivities and confirm this with a series of numerical
simulations. Our results suggest that dissipative heating may significantly
alter the internal dynamics of stars and planets.Comment: 8 pages, 5 figures, accepted for publication in ApJ Letter
Differential Rotation and Magnetism in Simulations of Fully Convective Stars
Stars of sufficiently low mass are convective throughout their interiors, and
so do not possess an internal boundary layer akin to the solar tachocline.
Because that interface figures so prominently in many theories of the solar
magnetic dynamo, a widespread expectation had been that fully convective stars
would exhibit surface magnetic behavior very different from that realized in
more massive stars. Here I describe how recent observations and theoretical
models of dynamo action in low-mass stars are partly confirming, and partly
confounding, this basic expectation. In particular, I present the results of
3--D MHD simulations of dynamo action by convection in rotating spherical
shells that approximate the interiors of 0.3 solar-mass stars at a range of
rotation rates. The simulated stars can establish latitudinal differential
rotation at their surfaces which is solar-like at ``rapid'' rotation rates
(defined within) and anti-solar at slower rotation rates; the differential
rotation is greatly reduced by feedback from strong dynamo-generated magnetic
fields in some parameter regimes. I argue that this ``flip'' in the sense of
differential rotation may be observable in the near future. I also briefly
describe how the strength and morphology of the magnetic fields varies with the
rotation rate of the simulated star, and show that the maximum magnetic
energies attained are compatible with simple scaling arguments.Comment: 9 pages, 2 color figures, to appear in Proc. IAU Symposium 271,
"Astrophysical Dynamics: from Stars to Galaxies
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
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
Magnetic processes in astrophysics: theory, simulations, experiments
Copyright © 2014 Taylor & Francis. This is an Accepted Manuscript of an book review published by Taylor & Francis in Geophysical & Astrophysical Fluid Dynamics on 21 October 2014, available online: http://www.tandfonline.com/10.1080/03091929.2014.964919Book Review
Magnetic processes in astrophysics: theory, simulations, experiments, by Gunther Rudiger, Rainer Hollerbach, and Leonid L. Kitchatinov, Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany, 2013, 356 pp., hardcover (E-book also available) (ISBN 978-3-527-41034-7
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