687 research outputs found
Proposal for a Light Universal Detector for the Study of Correlations between Photons and Charged Particles:addendum 2 to Proposal P252
Ca II 8542 \AA\ brightenings induced by a solar microflare
We study small-scale brightenings in Ca II 8542 \AA\ line-core images to
determine their nature and effect on localized heating and mass transfer in
active regions. High-resolution 2D spectroscopic observations of an active
region in the Ca II 8542 \AA\ line were acquired with the GFPI attached to the
1.5-meter GREGOR telescope. Inversions of the spectra were carried out using
NICOLE. We identified three brightenings of sizes up to 2"x2". We found
evidence that the brightenings belonged to the footpoints of a microflare (MF).
The properties of the observed brightenings disqualified the scenarios of
Ellerman bombs or IRIS bombs. However, this MF shared some common properties
with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs
and MFs are both apparent in chromospheric and coronal layers according to the
AIA channels, and (2) both show flaring arches with lifetimes of about 3.0-3.5
min and lengths of about 20". The inversions revealed heating by 600 K at the
footpoint location in the ambient chromosphere during the impulsive phase.
Connecting the footpoints, a dark filamentary structure appeared in the Ca II
line-core images. Before the start of the MF, the spectra of this structure
already indicated average blueshifts, meaning upward motions of the plasma
along the LOS. During the impulsive phase, these velocities increased up to
-2.2 km/s. Downflows dominated at the footpoints. However, in the upper
photosphere, slight upflows occurred during the impulsive phase. Hence,
bidirectional flows are present in the footpoints of the MF. Conclusions: We
detected Ca II brightenings that coincided with the footpoint location of an
MF. The MF event led to a rise of plasma in the upper photosphere, both before
and during the impulsive phase. Excess mass, previously raised to at most
chromospheric layers, slowly drained downward along arches toward the
footpoints of the MF.Comment: Accepted for publication in Astronomy & Astrophysics, 13 pages, 6
figures, 1 online movi
Proposal for a Light Universal Detector for the Study of Correlations between Photons and Charged Particles:addendum 2 to Proposal P252
Proposal for a Light Universal Detector for the Study of Correlations between Photons and Charged Particles:addendum 2 to Proposal P252
On the Origin of Asymmetries in Bilateral Supernova Remnants
AIMS: We investigate whether the morphology of bilateral supernova remnants
(BSNRs) observed in the radio band is determined mainly either by a non-uniform
interstellar medium (ISM) or by a non-uniform ambient magnetic field.
METHODS: We perform 3-D MHD simulations of a spherical SNR shock propagating
through a magnetized ISM. Two cases of shock propagation are considered: 1)
through a gradient of ambient density with a uniform ambient magnetic field; 2)
through a homogeneous medium with a gradient of ambient magnetic field
strength. From the simulations, we synthesize the synchrotron radio emission,
making different assumptions about the details of acceleration and injection of
relativistic electrons.
RESULTS: We find that asymmetric BSNRs are produced if the line-of-sight is
not aligned with the gradient of ambient plasma density or with the gradient of
ambient magnetic field strength. We derive useful parameters to quantify the
degree of asymmetry of the remnants that may provide a powerful diagnostic of
the microphysics of strong shock waves through the comparison between models
and observations.
CONCLUSIONS: BSNRs with two radio limbs of different brightness can be
explained if a gradient of ambient density or, most likely, of ambient magnetic
field strength is perpendicular to the radio limbs. BSNRs with converging
similar radio arcs can be explained if the gradient runs between the two arcs.Comment: 14 pages, 8 Figures; paper accepted for publication in A&A; the paper
with high-resolution figures can be downloaded at
http://www.astropa.unipa.it/~orlando/PAPERS/sorlando_6045.pd
Three-dimensional CFD simulations with large displacement of the geometries using a connectivity-change moving mesh approach
This paper deals with three-dimensional (3D) numerical simulations involving 3D moving geometries with large displacements on unstructured meshes. Such simulations are of great value to industry, but remain very time-consuming. A robust moving mesh algorithm coupling an elasticity-like mesh deformation solution and mesh optimizations was proposed in previous works, which removes the need for global remeshing when performing large displacements. The optimizations, and in particular generalized edge/face swapping, preserve the initial quality of the mesh throughout the simulation. We propose to integrate an Arbitrary Lagrangian Eulerian compressible flow solver into this process to demonstrate its capabilities in a full CFD computation context. This solver relies on a local enforcement of the discrete geometric conservation law to preserve the order of accuracy of the time integration. The displacement of the geometries is either imposed, or driven by fluid–structure interaction (FSI). In the latter case, the six degrees of freedom approach for rigid bodies is considered. Finally, several 3D imposed-motion and FSI examples are given to validate the proposed approach, both in academic and industrial configurations
A Space-time Smooth Artificial Viscosity Method For Nonlinear Conservation Laws
We introduce a new methodology for adding localized, space-time smooth,
artificial viscosity to nonlinear systems of conservation laws which propagate
shock waves, rarefactions, and contact discontinuities, which we call the
-method. We shall focus our attention on the compressible Euler equations in
one space dimension. The novel feature of our approach involves the coupling of
a linear scalar reaction-diffusion equation to our system of conservation laws,
whose solution is the coefficient to an additional (and artificial)
term added to the flux, which determines the location, localization, and
strength of the artificial viscosity. Near shock discontinuities, is
large and localized, and transitions smoothly in space-time to zero away from
discontinuities. Our approach is a provably convergent, spacetime-regularized
variant of the original idea of Richtmeyer and Von Neumann, and is provided at
the level of the PDE, thus allowing a host of numerical discretization schemes
to be employed. We demonstrate the effectiveness of the -method with three
different numerical implementations and apply these to a collection of
classical problems: the Sod shock-tube, the Osher-Shu shock-tube, the
Woodward-Colella blast wave and the Leblanc shock-tube. First, we use a
classical continuous finite-element implementation using second-order
discretization in both space and time, FEM-C. Second, we use a simplified WENO
scheme within our -method framework, WENO-C. Third, we use WENO with the
Lax-Friedrichs flux together with the -equation, and call this WENO-LF-C.
All three schemes yield higher-order discretization strategies, which provide
sharp shock resolution with minimal overshoot and noise, and compare well with
higher-order WENO schemes that employ approximate Riemann solvers,
outperforming them for the difficult Leblanc shock tube experiment.Comment: 34 pages, 27 figure
Three-dimensional modeling of the asymmetric blast wave from the 2006 outburst of RS Ophiuchi: Early X-ray emission
Chandra/HETG observations of the recurrent nova RS Ophiuchi at day 13.9 of
its 2006 outburst reveal a spectrum covering a large range in plasma
temperature and characterized by asymmetric and blue-shifted emission lines. We
investigate the origin of these asymmetries and broadening of emission lines.
We perform 3-D hydrodynamic simulations of the blast wave from the 2006
outburst, propagating through the inhomogeneous CSM. The model takes into
account the thermal conduction (including the effects of heat flux saturation)
and the radiative cooling. From the simulations, we synthesize the X-ray
emission and derive the spectra as they would be observed with Chandra/HETG.
Our model reproduces the observed X-ray emission in a natural way if the CSM in
which the outburst occurred is characterized by an equatorial density
enhancement. Our ``best-fit'' model predicts that most of the early X-ray
emission originates from a small region propagating in the direction
perpendicular to the line-of-sight and localized just behind the interaction
front between the blast wave and the equatorial density enhancement. The model
predicts asymmetric and blue-shifted line profiles remarkably similar to those
observed. These asymmetries are due to substantial X-ray absorption of
red-shifted emission by ejecta material. The comparison of high quality data of
Chandra/HETG with detailed hydrodynamic modeling has allowed us to unveil, for
the first time, the details of the structure emitting in the X-ray band in
early phases of the outburst evolution, contributing to a better understanding
of the physics of interactions between nova blasts and CSM in recurrent novae.
This may have implications for whether or not RS Ophiuchi is a Type Ia SN
progenitor system.Comment: 12 pages, 7 Figures; paper accepted for publication in A&A; the paper
with high-resolution figures can be downloaded at
http://www.astropa.unipa.it/~orlando/PAPERS/rs_oph_nova.pd
Effects of non-uniform interstellar magnetic field on synchrotron X-ray and inverse-Compton gamma-ray morphology of SNRs
Observations of SNRs in X-ray and gamma-ray bands promise to contribute with
important information in our understanding on the nature of galactic cosmic
rays. The analysis of SNRs images collected in different energy bands requires
the support of theoretical modeling of synchrotron and inverse Compton (IC)
emission. We develop a numerical code (REMLIGHT) to synthesize, from MHD
simulations, the synchrotron radio, X-ray and IC gamma-ray emission from SNRs
expanding in non-uniform interstellar medium (ISM) and/or non-uniform
interstellar magnetic field (ISMF). As a first application, the code is used to
investigate the effects of non-uniform ISMF on the SNR morphology in the
non-thermal X-ray and gamma-ray bands. We perform 3D MHD simulations of a
spherical SNR shock expanding through a magnetized ISM with a gradient of
ambient magnetic field strength. The model includes an approximate treatment of
upstream magnetic field amplification and the effect of shock modification due
to back reaction of accelerated cosmic rays. From the simulations, we
synthesize the synchrotron radio, X-ray and IC gamma-ray emission with
REMLIGHT, making different assumptions about the details of acceleration and
injection of relativistic electrons. A gradient of the ambient magnetic field
strength induces asymmetric morphologies in radio, X-ray and gamma-ray bands
independently from the model of electron injection if the gradient has a
component perpendicular to the line-of-sight. The degree of asymmetry of the
remnant morphology depends on the details of the electron injection and
acceleration and is different in the radio, X-ray, and gamma-ray bands. The
non-thermal X-ray morphology is the most sensitive to the gradient, showing the
highest degree of asymmetry. The IC gamma-ray emission is weakly sensitive to
the non-uniform ISMF, the degree of asymmetry of the SNR morphology being the
lowest in this band.Comment: 16 pages, 13 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_15505.pd
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