1,258 research outputs found
Second-harmonic generation in vortex-induced waveguides
We study the second-harmonic generation and localization of light in a
reconfigurable waveguide induced by an optical vortex soliton in a defocusing
Kerr medium. We show that the vortex-induced waveguide greatly improves
conversion efficiency from the fundamental to the second harmonic field.Comment: 3 pages, 4 figures, submitted to Optics Letter
Three-dimensional modeling of the Ca II H&K lines in the solar atmosphere
CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar Telescope (SST),
can observe the chromosphere in the H and K lines of Ca II at high spatial and
spectral resolution. Accurate modeling as well as an understanding of the
formation of these lines are needed to interpret the SST/CHROMIS observations.
Such modeling is computationally challenging because these lines are influenced
by strong departures from local thermodynamic equilibrium, three-dimensional
radiative transfer, and partially coherent resonance scattering of photons. We
aim to model the CaII H&K lines in 3D model atmospheres to understand their
formation and to investigate their diagnostic potential for probing the
chromosphere. We model the synthetic spectrum of Ca II using the radiative
transfer code Multi3D in three different radiation-magnetohydrodynamic model
atmospheres computed with the Bifrost code. We classify synthetic intensity
profiles according to their shapes and study how their features are related to
the physical properties in the model atmospheres. We investigate whether the
synthetic data reproduce the observed spatially-averaged line shapes,
center-to-limb variation and compare with SST/CHROMIS images. The
spatially-averaged synthetic line profiles show too low central emission peaks,
and too small separation between the peaks. The trends of the observed
center-to-limb variation of the profiles properties are reproduced by the
models. The Ca II H&K line profiles provide a temperature diagnostic of the
temperature minimum and the temperature at the formation height of the emission
peaks. The Doppler shift of the central depression is an excellent probe of the
velocity in the upper chromosphere.Comment: 19 pages, 20 figures, accepted for publication by A&
Dissecting bombs and bursts: non-LTE inversions of low-atmosphere reconnection in SST and IRIS observations
Ellerman bombs and UV bursts are transient brightenings that are ubiquitously
observed in the lower atmospheres of active and emerging flux regions. Here we
present inversion results of SST/CRISP and CHROMIS, as well as IRIS data of
such transient events. Combining information from the Mg II h & k, Si IV and Ca
II 8542A and Ca II H & K lines, we aim to characterise their temperature and
velocity stratification, as well as their magnetic field configuration. We find
average temperature enhancements of a few thousand kelvin close to the
classical temperature minimum, but localised peak temperatures of up to
10,000-15,000 K from Ca II inversions. Including Mg II generally dampens these
temperature enhancements to below 8000 K, while Si IV requires temperatures in
excess of 10,000 K at low heights, but may also be reproduced with secondary
temperature enhancements of 35,000-60,000 K higher up. However, reproducing Si
IV comes at the expense of overestimating the Mg II emission. The line-of-sight
velocity maps show clear bi-directional jet signatures and strong correlation
with substructure in the intensity images, with slightly larger velocities
towards the observer than away. The magnetic field parameters show an
enhancement of the horizontal field co-located with the brightenings at similar
heights as the temperature increase. We are thus able to largely reproduce the
observational properties of Ellerman bombs with UV burst signature with
temperature stratifications peaking close to the classical temperature minimum.
Correctly modelling the Si IV emission in agreement with all other diagnostics
is, however, an outstanding issue. Accounting for resolution differences,
fitting localised temperature enhancements and/or performing spatially-coupled
inversions is likely necessary to obtain better agreement between all
considered diagnostics.Comment: Accepted for publication in Astronomy & Astrophysics. 24 pages, 17
figure
Chromospheric polarimetry through multi-line observations of the 850 nm spectral region II: A magnetic flux tube scenario
In this publication we continue the work started in Quintero Noda et al.
(2017) examining this time a numerical simulation of a magnetic flux tube
concentration. Our goal is to study if the physical phenomena that take place
in it, in particular, the magnetic pumping, leaves a specific imprint on the
examined spectral lines. We find that the profiles from the interior of the
flux tube are periodically dopplershifted following an oscillation pattern that
is also reflected in the amplitude of the circular polarization signals. In
addition, we analyse the properties of the Stokes profiles at the edges of the
flux tube discovering the presence of linear polarization signals for the Ca II
lines, although they are weak with an amplitude around 0.5% of the continuum
intensity. Finally, we compute the response functions to perturbations in the
longitudinal field and we estimate the field strength using the weak field
approximation. Our results indicate that the height of formation of the
spectral lines changes during the magnetic pumping process which makes the
interpretation of the inferred magnetic field strength and its evolution more
difficult. These results complement those from previous works demonstrating the
capabilities and limitations of the 850 nm spectrum for chromospheric Zeeman
polarimetry in a very dynamic and complex atmosphere.Comment: 12 pages, 12 figures, 0 tables, MNRAS main journal publicatio
Chromospheric polarimetry through multi-line observations of the 850 nm spectral region III: Chromospheric jets driven by twisted magnetic fields
We investigate the diagnostic potential of the spectral lines at 850 nm for
understanding the magnetism of the lower atmosphere. For that purpose, we use a
newly developed 3D simulation of a chromospheric jet to check the sensitivity
of the spectral lines to this phenomenon as well as our ability to infer the
atmospheric information through spectropolarimetric inversions of noisy
synthetic data. We start comparing the benefits of inverting the entire
spectrum at 850 nm versus only the Ca II 8542 A spectral line. We found a
better match of the input atmosphere for the former case, mainly at lower
heights. However, the results at higher layers were not accurate. After several
tests, we determined that we need to weight more the chromospheric lines than
the photospheric ones in the computation of the goodness of the fit. The new
inversion configuration allows us to obtain better fits and consequently more
accurate physical parameters. Therefore, to extract the most from multi-line
inversions, a proper set of weights needs to be estimated. Besides that, we
conclude again that the lines at 850 nm, or a similar arrangement with Ca II
8542 A plus Zeeman sensitive photospheric lines, poses the best observing
configuration for examining the thermal and magnetic properties of the lower
solar atmosphere.Comment: 14 pages, 11 figure
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