893 research outputs found
The Effects of Spatio-temporal Resolution on Deduced Spicule Properties
Spicules have been observed on the sun for more than a century, typically in
chromospheric lines such as H-alpha and Ca II H. Recent work has shown that
so-called 'type II' spicules may have a role in providing mass to the corona
and the solar wind. In chromospheric filtergrams these spicules are not seen to
fall back down, and they are shorter-lived and more dynamic than the spicules
that have been classically reported in ground-based observations. Observations
of type II spicules with Hinode show fundamentally different properties from
what was previously measured. In earlier work we showed that these dynamic type
II spicules are the most common type, a view that was not properly identified
by early observations.The aim of this work is to investigate the effects of
spatio-temporal resolution in the classical spicule measurements. Making use of
Hinode data degraded to match the observing conditions of older ground-based
studies, we measure the properties of spicules with a semi-automated algorithm.
These results are then compared to measurements using the original Hinode data.
We find that degrading the data has a significant effect on the measured
properties of spicules. Most importantly, the results from the degraded data
agree well with older studies (e.g. mean spicule duration more than 5 minutes,
and upward apparent velocities of about 25 km/s). These results illustrate how
the combination of spicule superposition, low spatial resolution and cadence
affect the measured properties of spicules, and that previous measurements can
be misleading.Comment: Accepted for publication in ApJ. 5 pages, 3 figures. Movies of
figures 1 and 3 available via Data Conservanc
The Formation of IRIS diagnostics. IV. The Mg II triplet lines as a new diagnostic for lower chromospheric heating
A triplet of subordinate lines of Mg II exists in the region around the h&k
lines. In solar spectra these lines are seen mostly in absorption, but in some
cases can become emission lines. The aim of this work is to study the formation
of this triplet, and investigate any diagnostic value they can bring. Using 3D
radiative magnetohydrodynamic simulations of quiet Sun and flaring flux
emergence, we synthesize spectra and investigate how spectral features respond
to the underlying atmosphere. We find that emission in the lines is rare and is
typically caused by a steep temperature increase in the lower chromosphere
(above 1500 K, with electron densities above 10 m). In both
simulations the lines are sensitive to temperature increases taking place at
column masses >= 5e-4 g cm. Additional information can also be inferred
from the peak-to-wing ratio and shape of the line profiles. Using observations
from NASA's Interface Region Imaging Spectrograph we find both absorption and
emission line profiles with similar shapes to the synthetic spectra, which
suggests that these lines represent a useful diagnostic that complements the
MgII h&k lines.Comment: 8 pages, 7 figures. Accepted for publication in Ap
RH 1.5D: a massively parallel code for multi-level radiative transfer with partial frequency redistribution and Zeeman polarisation
The emergence of three-dimensional magneto-hydrodynamic (MHD) simulations of
stellar atmospheres has sparked a need for efficient radiative transfer codes
to calculate detailed synthetic spectra. We present RH 1.5D, a massively
parallel code based on the RH code and capable of performing Zeeman polarised
multi-level non-local thermodynamical equilibrium (NLTE) calculations with
partial frequency redistribution for an arbitrary amount of chemical species.
The code calculates spectra from 3D, 2D or 1D atmospheric models on a
column-by-column basis (or 1.5D). While the 1.5D approximation breaks down in
the cores of very strong lines in an inhomogeneous environment, it is
nevertheless suitable for a large range of scenarios and allows for faster
convergence with finer control over the iteration of each simulation column.
The code scales well to at least tens of thousands of CPU cores, and is
publicly available. In the present work we briefly describe its inner workings,
strategies for convergence optimisation, its parallelism, and some possible
applications.Comment: 6 pages, 3 figures. A&A in press. Updated version reflects changes in
latest proof
The formation of IRIS diagnostics. III. Near-ultraviolet Spectra and Images
The Mg II h&k lines are the prime chromospheric diagnostics of NASA's
Interface Region Imaging Spectrograph (IRIS). In the previous papers of this
series we used a realistic three-dimensional radiative magnetohydrodynamics
model to calculate the h&k lines in detail and investigated how their spectral
features relate to the underlying atmosphere. In this work, we employ the same
approach to investigate how the h&k diagnostics fare when taking into account
the finite resolution of IRIS and different noise levels. In addition, we
investigate the diagnostic potential of several other photospheric lines and
near-continuum regions present in the near-ultraviolet (NUV) window of IRIS and
study the formation of the NUV slit-jaw images. We find that the instrumental
resolution of IRIS has a small effect on the quality of the h&k diagnostics;
the relations between the spectral features and atmospheric properties are
mostly unchanged. The peak separation is the most affected diagnostic, but
mainly due to limitations of the simulation. The effects of noise start to be
noticeable at a signal-to-noise ratio (S/N) of 20, but we show that with noise
filtering one can obtain reliable diagnostics at least down to a S/N of 5. The
many photospheric lines present in the NUV window provide velocity information
for at least eight distinct photospheric heights. Using line-free regions in
the h&k far wings we derive good estimates of photospheric temperature for at
least three heights. Both of these diagnostics, in particular the latter, can
be obtained even at S/Ns as low as 5.Comment: 16 pages, 13 figures. Accepted for publication in ApJ. Updated
version with fixed typos in line list and language edit
How realistic are solar model atmospheres?
Recently, new solar model atmospheres have been developed to replace
classical 1D LTE hydrostatic models and used to for example derive the solar
chemical composition. We aim to test various models against key observational
constraints. In particular, a 3D model used to derive the solar abundances, a
3D MHD model (with an imposed 10 mT vertical magnetic field), 1D models from
the PHOENIX project, the 1D MARCS model, and the 1D semi-empirical model of
Holweger & M\"uller. We confront the models with observational diagnostics of
the temperature profile: continuum centre-to-limb variations (CLV), absolute
continuum fluxes, and the wings of hydrogen lines. We also test the 3D models
for the intensity distribution of the granulation and spectral line shapes. The
predictions from the 3D model are in excellent agreement with the continuum CLV
observations, performing even better than the Holweger & M\"uller model
(constructed largely to fulfil such observations). The predictions of the 1D
theoretical models are worse, given their steeper temperature gradients. For
the continuum fluxes, predictions for most models agree well with the
observations. No model fits all hydrogen lines perfectly, but again the 3D
model comes ahead. The 3D model also reproduces the observed continuum
intensity fluctuations and spectral line shapes very well. The excellent
agreement of the 3D model with the observables reinforces the view that its
temperature structure is realistic. It outperforms the MHD simulation in all
diagnostics, implying that recent claims for revised abundances based on MHD
modelling are premature. Several weaknesses in the 1D models are exposed. The
differences between the PHOENIX LTE and NLTE models are small. We conclude that
the 3D hydrodynamical model is superior to any of the tested 1D models, which
gives further confidence in the solar abundance analyses based on it.Comment: 17 pages, 15 figures. Accepted for publication in A&
Irregular grids for 3D NLTE radiative transfer in stellar atmospheres
Context. Three-dimensional non-local thermodynamical equilibrium (NLTE)
radiative transfer calculations are a fundamental tool for a detailed spectral
analysis in stellar atmospheres, but require vast amounts of computer power.
This prevents their broader application. Aims. We undertake a first exploration
of the use of 3D irregular grids in stellar atmospheres. In particular, we aim
to test whether irregular grids can be used to speed up the 3D NLTE problem, in
the same way as depth optimisation can lead to faster running times in 1D.
Methods. We created irregular grids based on 3D Voronoi diagrams, sampling
different distributions from a 3D radiation-magnetohydrodynamic Bifrost
simulation. We developed a method for solving radiation on the 3D irregular
grid and implemented a simple NLTE solver using -iteration and
statistical equilibrium. We applied this to a simplified hydrogen-like atom and
studied the convergence properties and accuracy of the irregular grid methods.
For reference, we compared them to a standard short-characteristics solver on a
regular grid. Results. We find that our method for radiation in irregular grids
gives similar results to those from regular grids, and that it is possible to
obtain nearly the same results with about ten times fewer points in the
irregular grid for the continuum intensity in local thermodynamical
equilibrium. We find that the irregular grid can give good results for the NLTE
problem, but it takes four times longer per iteration than the regular grid,
and it converges in about the same number of iterations. This makes it
particularly inefficient. Our formulation therefore does not lead to an
improvement. We also find that the design of the irregular grid is crucial for
accurate results, and find it non-trivial to design an irregular grid that can
work well across a wide range of heights.Comment: 10 pages, 9 figures, accepted for publication in A&
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