8,305 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
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