1,601 research outputs found
Kelvin-Helmholtz instability in coronal magnetic flux tubes due to azimuthal shear flows
Transverse oscillations of coronal loops are often observed and have been
theoretically interpreted as kink magnetohydrodynamic (MHD) modes. Numerical
simulations by Terradas et al. (2008, ApJ 687, L115) suggest that shear flows
generated at the loop boundary during kink oscillations could give rise to a
Kelvin-Helmholtz instability (KHI). Here, we investigate the linear stage of
the KHI in a cylindrical magnetic flux tube in the presence of azimuthal shear
motions. We consider the basic, linearized MHD equations in the beta = 0
approximation, and apply them to a straight and homogeneous cylindrical flux
tube model embedded in a coronal environment. Azimuthal shear flows with a
sharp jump of the velocity at the cylinder boundary are included in the model.
We obtain an analytical expression for the dispersion relation of the unstable
MHD modes supported by the configuration, and compute analytical approximations
of the critical velocity shear and the KHI growth rate in the thin tube limit.
A parametric study of the KHI growth rates is performed by numerically solving
the full dispersion relation. We find that fluting-like modes can develop a KHI
in time-scales comparable to the period of kink oscillations of the flux tube.
The KHI growth rates increase with the value of the azimuthal wavenumber and
decrease with the longitudinal wavenumber. However, the presence of a small
azimuthal component of the magnetic field can suppress the KHI. Azimuthal
motions related to kink oscillations of untwisted coronal loops may trigger a
KHI, but this phenomenon has not been observed to date. We propose that the
azimuthal component of the magnetic field is responsible for suppressing the
KHI in a stable coronal loop. The required twist is small enough to prevent the
development of the pinch instability.Comment: Submitted in Ap
Toward detailed prominence seismology - I. Computing accurate 2.5D magnetohydrodynamic equilibria
Context. Prominence seismology exploits our knowledge of the linear
eigenoscillations for representative magnetohydro- dynamic models of filaments.
To date, highly idealized models for prominences have been used, especially
with respect to the overall magnetic configurations.
Aims. We initiate a more systematic survey of filament wave modes, where we
consider full multi-dimensional models with twisted magnetic fields
representative of the surrounding magnetic flux rope. This requires the ability
to compute accurate 2.5 dimensional magnetohydrodynamic equilibria that balance
Lorentz forces, gravity, and pressure gradients, while containing density
enhancements (static or in motion).
Methods. The governing extended Grad-Shafranov equation is discussed, along
with an analytic prediction for circular flux ropes for the Shafranov shift of
the central magnetic axis due to gravity. Numerical equilibria are computed
with a finite element-based code, demonstrating fourth order accuracy on an
explicitly known, non-trivial test case.
Results. The code is then used to construct more realistic prominence
equilibria, for all three possible choices of a free flux-function. We quantify
the influence of gravity, and generate cool condensations in hot cavities, as
well as multi- layered prominences.
Conclusions. The internal flux rope equilibria computed here have the
prerequisite numerical accuracy to allow a yet more advanced analysis of the
complete spectrum of linear magnetohydrodynamic perturbations, as will be
demonstrated in the companion paper.Comment: Accepted by Astronomy & Astrophysics, 15 pages, 15 figure
4pi Models of CMEs and ICMEs
Coronal mass ejections (CMEs), which dynamically connect the solar surface to
the far reaches of interplanetary space, represent a major anifestation of
solar activity. They are not only of principal interest but also play a pivotal
role in the context of space weather predictions. The steady improvement of
both numerical methods and computational resources during recent years has
allowed for the creation of increasingly realistic models of interplanetary
CMEs (ICMEs), which can now be compared to high-quality observational data from
various space-bound missions. This review discusses existing models of CMEs,
characterizing them by scientific aim and scope, CME initiation method, and
physical effects included, thereby stressing the importance of fully 3-D
('4pi') spatial coverage.Comment: 14 pages plus references. Comments welcome. Accepted for publication
in Solar Physics (SUN-360 topical issue
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