1,219 research outputs found
Detection of emission in the Si i 1082.7 nm line core in sunspot umbrae
We analyze spectropolarimetric sunspot umbra observations taken in the
near-infrared Si i 1082.7 nm line taking NLTE effects into account. The data
were obtained with the GRIS instrument installed at the German GREGOR
telescope. A point spread function (PSF) was constructed using prior Mercury
observations with GRIS and the information provided by the adaptive optics
system of the GREGOR telescope. The data were then deconvolved from the PSF
using a principal component analysis deconvolution method and were analyzed via
the NICOLE inversion code. The Si i 1082.7 nm line seems to be in emission in
the umbra of the observed sunspot after the effects of scattered light are
removed. We show how the spectral line shape of umbral profiles changes
dramatically with the amount of scattered light. Indeed, the continuum levels
range, on average, from 44% of the quiet Sun continuum intensity to about 20%.
The inferred levels are in line with current model predictions and empirical
umbral models. Current umbral empirical models are not able to reproduce the
emission in the deconvolved umbral Stokes profiles. The results of the NLTE
inversions suggests that to obtain the emission in the Si i 1082.7 nm line, the
temperature stratification should first have a hump located at about log tau -2
and start rising at lower heights when moving into the transition region. This
is, to our knowledge, the first time the Si i 1082.7 nm line is seen in
emission in sunspot umbrae. The results show that the temperature
stratification of current umbral models may be more complex than expected with
the transition region located at lower heights above sunspot umbrae. Our
finding might provide insights into understanding why the sunspot umbra
emission in the millimeter spectral range is less than that predicted by
current empirical umbral models
Solar polarimetry through the K I lines at 770 nm
We characterize the K I D1 & D2 lines in order to determine whether they
could complement the 850 nm window, containing the Ca II infrared triplet lines
and several Zeeman sensitive photospheric lines, that was studied previously.
We investigate the effect of partial redistribution on the intensity profiles,
their sensitivity to changes in different atmospheric parameters, and the
spatial distribution of Zeeman polarization signals employing a realistic
magnetohydrodynamic simulation. The results show that these lines form in the
upper photosphere at around 500 km and that they are sensitive to the line of
sight velocity and magnetic field strength at heights where neither the
photospheric lines nor the Ca II infrared lines are. However, at the same time,
we found that their sensitivity to the temperature essentially comes from the
photosphere. Then, we conclude that the K I lines provide a complement to the
lines in the 850 nm window for the determination of atmospheric parameters in
the upper photosphere, especially for the line of sight velocity and the
magnetic field.Comment: 10 pages, 9 figures, main journal publicatio
Study of the polarization produced by the Zeeman effect in the solar Mg I b lines
The next generation of solar observatories aim to understand the magnetism of
the solar chromosphere. Therefore, it is crucial to understand the polarimetric
signatures of chromospheric spectral lines. For this purpose, we here examine
the suitability of the three Fraunhofer Mg I b1, b2, and b4 lines at 5183.6,
5172.7, and 5167.3 A, respectively. We start by describing a simplified atomic
model of only 6 levels and 3 line transitions for computing the atomic
populations of the 3p-4s (multiplet number 2) levels involved in the Mg I b
line transitions assuming non-local thermodynamic conditions and considering
only the Zeeman effect using the field-free approximation. We test this
simplified atom against more complex ones finding that, although there are
differences in the computed profiles, they are small compared with the
advantages provided by the simple atom in terms of speed and robustness. After
comparing the three Mg I lines, we conclude that the most capable one is the b2
line as b1 forms at similar heights and always show weaker polarization signals
while b4 is severely blended with photospheric lines. We also compare Mg I b2
with the K I D1 and Ca II 8542 A lines finding that the former is sensitive to
the atmospheric parameters at heights that are in between those covered by the
latter two lines. This makes Mg I b2 an excellent candidate for future
multi-line observations that aim to seamlessly infer the thermal and magnetic
properties of different features in the lower solar atmosphere.Comment: 14 pages, 11 figures, and 5 table
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
Un estudio experimental de la estimación de la correlación a partir de diferentes representaciones
Correlation is a relevant topic at secondary school and university level. However, most didactical research in this field comes from psychology and only refers to 2x2 contingency tables. On the other hand, representation has fundamental influence on understanding mathematical concepts. In this paper, we take into account the different representations of the correlation: verbal description, numerical table, scatter plot, and correlation coefficient, and we study the different translations between representations. Taking into account the most important task variables in this type of problems, we have studied the accuracy in estimating correlation coefficients, and the students' strategies to solve these tasks. We have identified two factors relating the tasks and strategies. We conclude with some implications for the teaching of the topic
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
Probing the Solar Atmosphere Using Oscillations of Infrared CO Spectral Lines
Oscillations were observed across the whole solar disk using the Doppler
shift and line depth of spectral lines from the CO molecule near 4666~nm with
the National Solar Observatory's McMath/Pierce solar telescope. Power,
coherence, and phase spectra were examined, and diagnostic diagrams reveal
power ridges at the solar global mode frequencies to show that these
oscillations are solar p-modes. The phase was used to determine the height of
formation of the CO lines by comparison with the IR continuum intensity phase
shifts as measured in Kopp et al., 1992; we find the CO line formation height
varies from 425 < z < 560 km as we move from disk center towards the solar limb
1.0 > mu > 0.5. The velocity power spectra show that while the sum of the
background and p-mode power increases with height in the solar atmosphere as
seen in previous work, the power in the p-modes only (background subtracted)
decreases with height, consistent with evanescent waves. The CO line depth
weakens in regions of stronger magnetic fields, as does the p-mode oscillation
power. Across most of the solar surface the phase shift is larger than the
expected value of 90 degrees for an adiabatic atmosphere. We fit the phase
spectra at different disk positions with a simple atmospheric model to
determine that the acoustic cutoff frequency is about 4.5 mHz with only small
variations, but that the thermal relaxation frequency drops significantly from
2.7 to 0 mHz at these heights in the solar atmosphere
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