2,374 research outputs found
Reconnection of a kinking flux rope triggering the ejection of a microwave and hard X-ray source. II. Numerical Modeling
Numerical simulations of the helical () kink instability of an
arched, line-tied flux rope demonstrate that the helical deformation enforces
reconnection between the legs of the rope if modes with two helical turns are
dominant as a result of high initial twist in the range . Such
reconnection is complex, involving also the ambient field. In addition to
breaking up the original rope, it can form a new, low-lying, less twisted flux
rope. The new flux rope is pushed downward by the reconnection outflow, which
typically forces it to break as well by reconnecting with the ambient field.
The top part of the original rope, largely rooted in the sources of the ambient
flux after the break-up, can fully erupt or be halted at low heights, producing
a "failed eruption." The helical current sheet associated with the instability
is squeezed between the approaching legs, temporarily forming a double current
sheet. The leg-leg reconnection proceeds at a high rate, producing sufficiently
strong electric fields that it would be able to accelerate particles. It may
also form plasmoids, or plasmoid-like structures, which trap energetic
particles and propagate out of the reconnection region up to the top of the
erupting flux rope along the helical current sheet. The kinking of a highly
twisted flux rope involving leg-leg reconnection can explain key features of an
eruptive but partially occulted solar flare on 18 April 2001, which ejected a
relatively compact hard X-ray and microwave source and was associated with a
fast coronal mass ejection.Comment: Solar Physics, in pres
On Flux Rope Stability and Atmospheric Stratification in Models of Coronal Mass Ejections Triggered by Flux Emergence
Flux emergence is widely recognized to play an important role in the
initiation of coronal mass ejections. The Chen-Shibata (2000) model, which
addresses the connection between emerging flux and flux rope eruptions, can be
implemented numerically to study how emerging flux through the photosphere can
impact the eruption of a pre-existing coronal flux rope. The model's
sensitivity to the initial conditions and reconnection micro-physics is
investigated with a parameter study. In particular, we aim to understand the
stability of the coronal flux rope in the context of X-point collapse and the
effects of boundary driving in both unstratified and stratified atmospheres. In
the absence of driving, we assess the behavior of waves in the vicinity of the
X-point. With boundary driving applied, we study the effects of reconnection
micro-physics and atmospheric stratification on the eruption. We find that the
Chen-Shibata equilibrium can be unstable to an X-point collapse even in the
absence of driving due to wave accumulation at the X-point. However, the
equilibrium can be stabilized by reducing the compressibility of the plasma,
which allows small-amplitude waves to pass through the X-point without
accumulation. Simulations with the photospheric boundary driving evaluate the
impact of reconnection micro-physics and atmospheric stratification on the
resulting dynamics: we show the evolution of the system to be determined
primarily by the structure of the global magnetic fields with little
sensitivity to the micro-physics of magnetic reconnection; and in a stratified
atmosphere, we identify a novel mechanism for producing quasi-periodic behavior
at the reconnection site behind a rising flux rope as a possible explanation of
similar phenomena observed in solar and stellar flares.Comment: Submitted Feb 28, 2014 to, accepted Aug 14, 2014 by Astronomy &
Astrophysics. 13 pages, 10 figures, 2 table
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On Unit Free Assessment of The Extent of Multilateral Distributional Variation
Multilateral comparison of outcomes drawn from multiple groups pervade the social sciences and measurement of their variability, usually involving functions of respective group location and scale parameters, is of intrinsic interest. However, such approaches frequently mask more fundamental differences that more comprehensive examination of relative group distributional structures reveal. Indeed, in categorical data contexts, location and scale based techniques are no longer feasible without artificial and questionable cardinalization of categories. Here, Ginis' Transvariation measure is extended and employed in providing quantitative and visual multilateral comparison tools in discrete, continuous, categorical, univariate or multivariate settings which are particularly useful in paradigms where cardinal measure is absent. Two applications, one analyzing Eurozone cohesion in terms of the convergence or divergence of constituent nations income distributions, the other, drawn from a study of aging, health and income inequality in China, exemplify their use in a continuous and categorical data environment
Requirement for a Uroplakin 3a-like protein in the development of zebrafish pronephric tubule epithelial cell function, morphogenesis, and polarity
Uroplakin (UP)3a is critical for urinary tract development and function; however, its role in these processes is unknown. We examined the function of the UP3a-like protein Upk3l, which was expressed at the apical surfaces of the epithelial cells that line the pronephric tubules (PTs) of the zebrafish pronephros. Embryos treated with upk3l-targeted morpholinos showed decreased pronephros function, which was attributed to defects in PT epithelial cell morphogenesis and polarization including: loss of an apical brush border and associated phospho-ERM proteins, apical redistribution of the basolateral Na+/K+-ATPase, and altered or diminished expression of the apical polarity complex proteins Prkcz (atypical protein kinase C zeta) and Pard3 (Par3). Upk3l missing its C-terminal cytoplasmic domain or containing mutations in conserved tyrosine or proline residues did not rescue, or only partially rescued the effects of Upk3l depletion. Our studies indicate that Upk3l promotes epithelial polarization and morphogenesis, likely by forming or stimulating interactions with cytoplasmic signaling or polarity proteins, and that defects in this process may underlie the pathology observed in UP3a knockout mice or patients with renal abnormalities that result from altered UP3a expression. © 2012 Mitra et al
A Model for Patchy Reconnection in Three Dimensions
We show, theoretically and via MHD simulations, how a short burst of
reconnection localized in three dimensions on a one-dimensional current sheet
creates a pair of reconnected flux tubes. We focus on the post-reconnection
evolution of these flux tubes, studying their velocities and shapes. We find
that slow-mode shocks propagate along these reconnected flux tubes, releasing
magnetic energy as in steady-state Petschek reconnection. The geometry of these
three-dimensional shocks, however, differs dramatically from the classical
two-dimensional geometry. They propagate along the flux tube legs in four
isolated fronts, whereas in the two-dimensional Petschek model, they form a
continuous, stationary pair of V-shaped fronts.
We find that the cross sections of these reconnected flux tubes appear as
teardrop shaped bundles of flux propagating away from the reconnection site.
Based on this, we argue that the descending coronal voids seen by Yohkoh SXT,
LASCO, and TRACE are reconnected flux tubes descending from a flare site in the
high corona, for example after a coronal mass ejection. In this model, these
flux tubes would then settle into equilibrium in the low corona, forming an
arcade of post-flare coronal loops.Comment: 27 pages plus 16 figure
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