617 research outputs found
Toward understanding the early stages of an impulsively accelerated coronal mass ejection
The expanding magnetic flux in coronal mass ejections (CMEs) often forms a
cavity. A spherical model is simultaneously fit to STEREO EUVI and COR1 data of
an impulsively accelerated CME on 25 March 2008, which displays a well-defined
extreme ultraviolet (EUV) and white-light cavity of nearly circular shape
already at low heights ~ 0.2 Rs. The center height h(t) and radial expansion
r(t) of the cavity are obtained in the whole height range of the main
acceleration. We interpret them as the axis height and as a quantity
proportional to the minor radius of a flux rope, respectively. The
three-dimensional expansion of the CME exhibits two phases in the course of its
main upward acceleration. From the first h and r data points, taken shortly
after the onset of the main acceleration, the erupting flux shows an
overexpansion compared to its rise, as expressed by the decrease of the aspect
ratio from k=h/r ~ 3 to k ~ (1.5-2.0). This phase is approximately coincident
with the impulsive rise of the acceleration and is followed by a phase of very
gradual change of the aspect ratio (a nearly self-similar expansion) toward k ~
1.5 at h ~ 10 Rs. The initial overexpansion of the CME cavity can be caused by
flux conservation around a rising flux rope of decreasing axial current and by
the addition of flux to a growing, or even newly forming,flux rope by magnetic
reconnection. Further analysis will be required to decide which of these
contributions is dominant. The data also suggest that the horizontal component
of the impulsive cavity expansion (parallel to the solar surface) triggers the
associated EUV wave, which subsequently detaches from the CME volume.Comment: in press, A&A, 201
Ideal kink instability of a magnetic loop equilibrium
The force-free coronal loop model by Titov & D\'emoulin (1999} is found to be
unstable with respect to the ideal kink mode, which suggests this instability
as a mechanism for the initiation of flares. The long-wavelength () mode
grows for average twists \Phi\ga3.5\pi (at a loop aspect ratio of
5). The threshold of instability increases with increasing major loop radius,
primarily because the aspect ratio then also increases. Numerically obtained
equilibria at subcritical twist are very close to the approximate analytical
equilibrium; they do not show indications of sigmoidal shape. The growth of
kink perturbations is eventually slowed down by the surrounding potential
field, which varies only slowly with radius in the model. With this field a
global eruption is not obtained in the ideal MHD limit. Kink perturbations with
a rising loop apex lead to the formation of a vertical current sheet below the
apex, which does not occur in the cylindrical approximation.Comment: Astron. Astrophys. Lett., accepte
Cluster tails for critical power-law inhomogeneous random graphs
Recently, the scaling limit of cluster sizes for critical inhomogeneous
random graphs of rank-1 type having finite variance but infinite third moment
degrees was obtained (see previous work by Bhamidi, van der Hofstad and van
Leeuwaarden). It was proved that when the degrees obey a power law with
exponent in the interval (3,4), the sequence of clusters ordered in decreasing
size and scaled appropriately converges as n goes to infinity to a sequence of
decreasing non-degenerate random variables.
Here, we study the tails of the limit of the rescaled largest cluster, i.e.,
the probability that the scaling limit of the largest cluster takes a large
value u, as a function of u. This extends a related result of Pittel for the
Erd\H{o}s-R\'enyi random graph to the setting of rank-1 inhomogeneous random
graphs with infinite third moment degrees. We make use of delicate large
deviations and weak convergence arguments.Comment: corrected and updated first referenc
Characterisation of HTSC ceramics from their resistive transition
The resistivity vs. temperature relation in bulk ceramic HTSC under
self-field conditions as well as in weak external magnetic fields is modelled
by local Lorentz force induced fluxon motion with temperature dependent
pinning. A pinning force density and two viscous drag coefficients in
intergrain and intragrain regions, respectively, can be used as characteristic
parameters describing the temperature, current, and external field dependences
of the sample resistance.Comment: 12 pages, LaTeX2e, 6 figures (epsfig), to be published in Supercond.
Sci. and Techno
Observations and modeling of the early acceleration phase of erupting filaments involved in coronal mass ejections
We examine the early phases of two near-limb filament destabilization
involved in coronal mass ejections on 16 June and 27 July 2005, using
high-resolution, high-cadence observations made with the Transition Region and
Coronal Explorer (TRACE), complemented by coronagraphic observations by Mauna
Loa and the SOlar and Heliospheric Observatory (SOHO). The filaments' heights
above the solar limb in their rapid-acceleration phases are best characterized
by a height dependence h(t) ~ t^m with m near, or slightly above, 3 for both
events. Such profiles are incompatible with published results for breakout,
MHD-instability, and catastrophe models. We show numerical simulations of the
torus instability that approximate this height evolution in case a substantial
initial velocity perturbation is applied to the developing instability. We
argue that the sensitivity of magnetic instabilities to initial and boundary
conditions requires higher fidelity modeling of all proposed mechanisms if
observations of rise profiles are to be used to differentiate between them. The
observations show no significant delays between the motions of the filament and
of overlying loops: the filaments seem to move as part of the overall coronal
field until several minutes after the onset of the rapid-acceleration phase.Comment: ApJ (2007, in press
Slow Rise and Partial Eruption of a Double-Decker Filament. I Observations and Interpretation
We study an active-region dextral filament which was composed of two branches
separated in height by about 13 Mm. This "double-decker" configuration
sustained for days before the upper branch erupted with a GOES-class M1.0 flare
on 2010 August 7. Analyzing this evolution, we obtain the following main
results. 1) During hours before the eruption, filament threads within the lower
branch were observed to intermittently brighten up, lift upward, and then merge
with the upper branch. The merging process contributed magnetic flux and
current to the upper branch, resulting in its quasi-static ascent. 2) This
transfer might serve as the key mechanism for the upper branch to lose
equilibrium by reaching the limiting flux that can be stably held down by the
overlying field or by reaching the threshold of the torus instability. 3) The
erupting branch first straightened from a reverse S shape that followed the
polarity inversion line and then writhed into a forward S shape. This shows a
transfer of left-handed helicity in a sequence of writhe-twist-writhe. The fact
that the initial writhe is converted into the twist of the flux rope excludes
the helical kink instability as the trigger process of the eruption, but
supports the occurrence of the instability in the main phase, which is indeed
indicated by the very strong writhing motion. 4) A hard X-ray sigmoid, likely
of coronal origin, formed in the gap between the two original filament branches
in the impulsive phase of the associated flare. This supports a model of
transient sigmoids forming in the vertical flare current sheet. 5) Left-handed
magnetic helicity is inferred for both branches of the dextral filament. 6) Two
types of force-free magnetic configurations are compatible with the data, a
double flux rope equilibrium and a single flux rope situated above a loop
arcade
Eruption of a Kink-Unstable Filament in Active Region NOAA 10696
We present rapid-cadence Transition Region And Coronal Explorer (TRACE)
observations which show evidence of a filament eruption from active region NOAA
10696, accompanied by an X2.5 flare, on 2004 November 10. The eruptive
filament, which manifests as a fast coronal mass ejection some minutes later,
rises as a kinking structure with an apparently exponential growth of height
within TRACE's field of view. We compare the characteristics of this filament
eruption with MHD numerical simulations of a kink-unstable magnetic flux rope,
finding excellent qualitative agreement. We suggest that, while tether
weakening by breakout-like quadrupolar reconnection may be the release
mechanism for the previously confined flux rope, the driver of the expansion is
most likely the MHD helical kink instability.Comment: Accepted by ApJ Letters. 4 figures (Fig. 3 in two parts). For MPEG
files associated with Figure 1, see:
http://www.mssl.ucl.ac.uk/~drw/papers/kink/ktrace.mpg
http://www.mssl.ucl.ac.uk/~drw/papers/kink/kmdi.mpg
http://www.mssl.ucl.ac.uk/~drw/papers/kink/ksimu.mp
Confined and ejective eruptions of kink-unstable flux ropes
The ideal helical kink instability of a force-free coronal magnetic flux
rope, anchored in the photosphere, is studied as a model for solar eruptions.
Using the flux rope model of Titov & Demoulin (1999} as the initial condition
in MHD simulations, both the development of helical shape and the rise profile
of a confined (or failed) filament eruption (on 2002 May 27) are reproduced in
very good agreement with the observations. By modifying the model such that the
magnetic field decreases more rapidly with height above the flux rope, a full
(or ejective) eruption of the rope is obtained in very good agreement with the
developing helical shape and the exponential-to-linear rise profile of a fast
coronal mass ejection (CME) (on 2001 May 15). This confirms that the helical
kink instability of a twisted magnetic flux rope can be the mechanism of the
initiation and the initial driver of solar eruptions. The agreement of the
simulations with properties that are characteristic of many eruptions suggests
that they are often triggered by the kink instability. The decrease of the
overlying field with height is a main factor in deciding whether the
instability leads to a confined event or to a CME.Comment: minor update to conform to printed version; typo in table correcte
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Hybrid microscopic depletion model in nodal code DYN3D
The paper presents a general hybrid method that combines the micro-depletion technique with correction of micro- and macro- diffusion parameters to account for the spectral history effects. The fuel in a core is subjected to time- and space-dependent operational conditions (e.g. coolant density), which cannot be predicted in advance. However, lattice codes assume some average conditions to generate cross sections (XS) for nodal diffusion codes such as DYN3D. Deviation of local operational history from average conditions leads to accumulation of errors in XS, which is referred as spectral history effects. Various methods to account for the spectral history effects, such as spectral index, burnup-averaged operational parameters and micro-depletion, were implemented in some nodal codes. Recently, an alternative method, which characterizes fuel depletion state by burnup and ÂČÂłâčPu concentration (denoted as Pu-correction) was proposed, implemented in nodal code DYN3D and verified for a wide range of history effects. The method is computationally efficient, however, it has applicability limitations.
The current study seeks to improve the accuracy and applicability range of Pu-correction method. The proposed hybrid method combines the micro-depletion method with a XS characterization technique similar to the Pu-correction method.
The method was implemented in DYN3D and verified on multiple test cases. The results obtained with DYN3D were compared to those obtained with Monte Carlo code Serpent, which was also used to generate the XS. The observed differences are within the statistical uncertainties.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.anucene.2016.02.01
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