2,189 research outputs found
Magnetar Giant Flares in Multipolar Magnetic Fields --- II. Flux Rope Eruptions With Current Sheets
We propose a physical mechanism to explain giant flares and radio afterglows
in terms of a magnetospheric model containing both a helically twisted flux
rope and a current sheet (CS). With the appearance of CS, we solve a mixed
boundary value problem to get the magnetospheric field based on a domain
decomposition method. We investigate properties of the equilibrium curve of the
flux rope when the CS is present in background multipolar fields. In response
to the variations at the magnetar surface, it quasi-statically evolves in
stable equilibrium states. The loss of equilibrium occurs at a critical point
and, beyond that point, it erupts catastrophically. New features show up when
the CS is considered. Especially, we find two kinds of physical behaviors,
i.e., catastrophic state transition and catastrophic escape. Magnetic energy
would be released during state transitions. The released magnetic energy is
sufficient to drive giant flares. The flux rope would go away from the magnetar
quasi-statically, which is inconsistent with the radio afterglow. Fortunately,
in the latter case, i.e., the catastrophic escape, the flux rope could escape
the magnetar and go to infinity in a dynamical way. This is more consistent
with radio afterglow observations of giant flares. We find that the minor
radius of flux rope has important implications for its eruption. Flux ropes
with larger minor radius are more prone to erupt. We stress that the CS
provides an ideal place for magnetic reconnection, which would further enhance
the energy release during eruptions.Comment: 31 pages, 7 figures, accepted by Ap
Magnetar Giant Flares in Multipolar Magnetic Fields --- I. Fully and Partially Open Eruptions of Flux Ropes
We propose a catastrophic eruption model for magnetar's enormous energy
release during giant flares, in which a toroidal and helically twisted flux
rope is embedded within a force-free magnetosphere. The flux rope stays in
stable equilibrium states initially and evolves quasi-statically. Upon the loss
of equilibrium point is reached, the flux rope cannot sustain the stable
equilibrium states and erupts catastrophically. During the process, the
magnetic energy stored in the magnetosphere is rapidly released as the result
of destabilization of global magnetic topology. The magnetospheric energy that
could be accumulated is of vital importance for the outbursts of magnetars. We
carefully establish the fully open fields and partially open fields for various
boundary conditions at the magnetar surface and study the relevant energy
thresholds. By investigating the magnetic energy accumulated at the critical
catastrophic point, we find that it is possible to drive fully open eruptions
for dipole dominated background fields. Nevertheless, it is hard to generate
fully open magnetic eruptions for multipolar background fields. Given the
observational importance of the multipolar magnetic fields in the vicinity of
the magnetar surface, it would be worthwhile to explore the possibility of the
alternative eruption approach in multipolar background fields. Fortunately, we
find that flux ropes may give rise to partially open eruptions in the
multipolar fields, which involve only partial opening up of background fields.
The energy release fractions are greater for cases with central-arcaded
multipoles than those with central-caved multipoles emerged in background
fields. Eruptions would fail only when the centrally-caved multipoles become
extremely strong.Comment: 30 pages, 6 figures, accepted by Ap
Analytic properties of force-free jets in the Kerr spacetime -- III: uniform field solution
The structure of steady axisymmetric force-free magnetosphere of a Kerr black
hole (BH) is governed by a second-order partial differential equation of
depending on two "free" functions and ,
where is the component of the vector potential of the
electromagnetic field, is the angular velocity of the magnetic field
lines and is the poloidal electric current. In this paper, we investigate
the solution uniqueness. Taking asymptotically uniform field as an example,
analytic studies imply that there are infinitely many solutions approaching
uniform field at infinity, while only a unique one is found in general
relativistic magnetohydrodynamic simulations. To settle down the disagreement,
we reinvestigate the structure of the governing equation and numerically solve
it with given constraint condition and boundary condition. We find that the
constraint condition (field lines smoothly crossing the light surface (LS)) and
boundary conditions at horizon and at infinity are connected via radiation
conditions at horizon and at infinity, rather than being independent. With
appropriate constraint condition and boundary condition, we numerically solve
the governing equation and find a unique solution. Contrary to naive
expectation, our numerical solution yields a discontinuity in the angular
velocity of the field lines and a current sheet along the last field line
crossing the event horizon. We also briefly discuss the applicability of the
perturbation approach to solving the governing equation
Convergence on Gauss-Seidel iterative methods for linear systems with general H-matrices
It is well known that as a famous type of iterative methods in numerical
linear algebra, Gauss-Seidel iterative methods are convergent for linear
systems with strictly or irreducibly diagonally dominant matrices, invertible
matrices (generalized strictly diagonally dominant matrices) and Hermitian
positive definite matrices. But, the same is not necessarily true for linear
systems with nonstrictly diagonally dominant matrices and general matrices.
This paper firstly proposes some necessary and sufficient conditions for
convergence on Gauss-Seidel iterative methods to establish several new
theoretical results on linear systems with nonstrictly diagonally dominant
matrices and general matrices. Then, the convergence results on
preconditioned Gauss-Seidel (PGS) iterative methods for general matrices
are presented. Finally, some numerical examples are given to demonstrate the
results obtained in this paper
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