637 research outputs found
On Fabry P\'erot Etalon based Instruments. I. The Isotropic Case
Here we assess the spectral and imaging properties of Fabry P\'erot etalons
when located in solar magnetographs. We discuss the chosen configuration
(collimated or telecentric) for both ideal and real cases. For the real cases,
we focus on the effects caused by the polychromatic illumination of the filter
by the irregularities in the optical thickness of the etalon and by deviations
from the ideal illumination in both setups. We first review the general
properties of Fabry P\'erots and we then address the different sources of
degradation of the spectral transmission profile. We review and extend the
general treatment of defects followed by different authors. We discuss the
differences between the point spread functions (PSFs) of the collimated and
telecentric configurations for both monochromatic and (real)
quasi-monochromatic illumination of the etalon. The PSF corresponding to
collimated mounts is shown to have a better performance, although it varies
from point to point due to an apodization of the image inherent to this
configuration. This is in contrast to the (perfect) telecentric case, where the
PSF remains constant but produces artificial velocities and magnetic field
signals because of its strong spectral dependence. We find that the unavoidable
presence of imperfections in the telecentrism produces a decrease of flux of
photons and a shift, a broadening and a loss of symmetrization of both the
spectral and PSF profiles over the field of view, thus compromising their
advantages over the collimated configuration. We evaluate these effects for
different apertures of the incident beam.Comment: 20 pages 22 figures 2 Appendice
Spatial deconvolution of spectropolarimetric data: an application to quiet Sun magnetic elements
Observations of the Sun from the Earth are always limited by the presence of
the atmosphere, which strongly disturbs the images. A solution to this problem
is to place the telescopes in space satellites, which produce observations
without any (or limited) atmospheric aberrations. However, even though the
images from space are not affected by atmospheric seeing, the optical
properties of the instruments still limit the observations. In the case of
diffraction limited observations, the PSF establishes the maximum allowed
spatial resolution, defined as the distance between two nearby structures that
can be properly distinguished. In addition, the shape of the PSF induce a
dispersion of the light from different parts of the image, leading to what is
commonly termed as stray light or dispersed light. This effect produces that
light observed in a spatial location at the focal plane is a combination of the
light emitted in the object at relatively distant spatial locations. We aim to
correct the effect produced by the telescope's PSF using a deconvolution
method, and we decided to apply the code on Hinode/SP quiet Sun observations.
We analyze the validity of the deconvolution process with noisy data and we
infer the physical properties of quiet Sun magnetic elements after the
deconvolution process.Comment: 14 pages, 9 figure
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
Milne-Eddington inversion of the Fe I line pair at 630~nm
The iron lines at 630.15 and 630.25 nm are often used to determine the
physical conditions of the solar photosphere. A common approach is to invert
them simultaneously under the Milne-Eddington approximation. The same
thermodynamic parameters are employed for the two lines, except for their
opacities, which are assumed to have a constant ratio. We aim at investigating
the validity of this assumption, since the two lines are not exactly the same.
We use magnetohydrodynamic simulations of the quiet Sun to examine the behavior
of the ME thermodynamic parameters and their influence on the retrieval of
vector magnetic fields and flow velocities. Our analysis shows that the two
lines can be coupled and inverted simultaneously using the same thermodynamic
parameters and a constant opacity ratio. The inversion of two lines is
significantly more accurate than single-line inversions because of the larger
number of observables.Comment: Accepted for publication in Astronomy and Astrophysics (Research
Note
Applicability of Milne-Eddington inversions to high spatial resolution observations of the quiet Sun
The physical conditions of the solar photosphere change on very small spatial
scales both horizontally and vertically. Such a complexity may pose a serious
obstacle to the accurate determination of solar magnetic fields. We examine the
applicability of Milne-Eddington (ME) inversions to high spatial resolution
observations of the quiet Sun. Our aim is to understand the connection between
the ME inferences and the actual stratifications of the atmospheric parameters.
We use magnetoconvection simulations of the solar surface to synthesize
asymmetric Stokes profiles such as those observed in the quiet Sun. We then
invert the profiles with the ME approximation. We perform an empirical analysis
of the heights of formation of ME measurements and analyze the uncertainties
brought about by the ME approximation. We also investigate the quality of the
fits and their relationship with the model stratifications. The atmospheric
parameters derived from ME inversions of high-spatial resolution profiles are
reasonably accurate and can be used for statistical analyses of solar magnetic
fields, even if the fit is not always good. We also show that the ME inferences
cannot be assigned to a specific atmospheric layer: different parameters sample
different ranges of optical depths, and even the same parameter may trace
different layers depending on the physical conditions of the atmosphere.
Despite this variability, ME inversions tend to probe deeper layers in granules
as compared with intergranular lanes.Comment: Accepted for publication in Astronomy and Astrophysic
Quiet Sun magnetic fields from space-borne observations: simulating Hinode's case
We examine whether or not it is possible to derive the field strength
distribution of quiet Sun internetwork regions from very high spatial
resolution polarimetric observations in the visible. In particular, we consider
the case of the spectropolarimeter attached to the Solar Optical Telescope
aboard Hinode. Radiative magneto-convection simulations are used to synthesize
the four Stokes profiles of the \ion{Fe}{1} 630.2 nm lines. Once the profiles
are degraded to a spatial resolution of 0\farcs32 and added noise, we infer the
atmospheric parameters by means of Milne-Eddington inversions. The comparison
of the derived values with the real ones indicates that the visible lines yield
correct internetwork field strengths and magnetic fluxes, with uncertainties
smaller than 150 G, when a stray light contamination factor is included
in the inversion. Contrary to the results of ground-based observations at
1\arcsec, weak fields are retrieved wherever the field is weak in the
simulation.Comment: Accepted for publication in ApJ Letter
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
Granular Scale Magnetic Flux Cancellations in the Photosphere
We investigate the evolution of 5 granular-scale magnetic flux cancellations
just outside the moat region of a sunspot by using accurate spectropolarimetric
measurements and G-band images with the Solar Optical Telescope aboard Hinode.
The opposite polarity magnetic elements approach a junction of the
intergranular lanes and then they collide with each other there. The
intergranular junction has strong red shifts, darker intensities than the
regular intergranular lanes, and surface converging flows. This clearly
confirms that the converging and downward convective motions are essential for
the approaching process of the opposite-polarity magnetic elements. However,
motion of the approaching magnetic elements does not always match with their
surrounding surface flow patterns in our observations. This suggests that, in
addition to the surface flows, subsurface downward convective motions and
subsurface magnetic connectivities are important for understanding the approach
and collision of the opposite polarity elements observed in the photosphere. We
find that the horizontal magnetic field appears between the canceling opposite
polarity elements in only one event. The horizontal fields are observed along
the intergranular lanes with Doppler red shifts. This cancellation is most
probably a result of the submergence (retraction) of low-lying photospheric
magnetic flux. In the other 4 events, the horizontal field is not observed
between the opposite polarity elements at any time when they approach and
cancel each other. These approaching magnetic elements are more concentrated
rather than gradually diffused, and they have nearly vertical fields even while
they are in contact each other. We thus infer that the actual flux cancellation
is highly time dependent events at scales less than a pixel of Hinode SOT
(about 200 km) near the solar surface.Comment: Accepted for publication in the Astrophysical Journa
- …