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

Temporal relation between quiet-Sun transverse fields and the strong flows detected by IMaX/SUNRISE

Localized strongly Doppler-shifted Stokes V signals were detected by IMaX/SUNRISE. These signals are related to newly emerged magnetic loops that are observed as linear polarization features. We aim to set constraints on the physical nature and causes of these highly Doppler-shifted signals. In particular, the temporal relation between the appearance of transverse fields and the strong Doppler shifts is analyzed in some detail. We calculated the time difference between the appearance of the strong flows and the linear polarization. We also obtained the distances from the center of various features to the nearest neutral lines and whether they overlap or not. These distances were compared with those obtained from randomly distributed points on observed magnetograms. Various cases of strong flows are described in some detail. The linear polarization signals precede the appearance of the strong flows by on average 84+-11 seconds. The strongly Doppler-shifted signals are closer (0.19") to magnetic neutral lines than randomly distributed points (0.5"). Eighty percent of the strongly Doppler-shifted signals are close to a neutral line that is located between the emerging field and pre-existing fields. That the remaining 20% do not show a close-by pre-existing field could be explained by a lack of sensitivity or an unfavorable geometry of the pre-existing field, for instance, a canopy-like structure. Transverse fields occurred before the observation of the strong Doppler shifts. The process is most naturally explained as the emergence of a granular-scale loop that first gives rise to the linear polarization signals, interacts with pre-existing fields (generating new neutral line configurations), and produces the observed strong flows. This explanation is indicative of frequent small-scale reconnection events in the quiet Sun.Comment: 11 pages, 8 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

The small-scale structure of photospheric convection retrieved by a deconvolution technique applied to Hinode/SP data

Solar granules are bright patterns surrounded by dark channels called intergranular lanes in the solar photosphere and are a manifestation of overshooting convection. Observational studies generally find stronger upflows in granules and weaker downflows in intergranular lanes. This trend is, however, inconsistent with the results of numerical simulations in which downflows are stronger than upflows through the joint action of gravitational acceleration/deceleration and pressure gradients. One cause of this discrepancy is the image degradation caused by optical distortion and light diffraction and scattering that takes place in an imaging instrument. We apply a deconvolution technique to Hinode/SP data in an attempt to recover the original solar scene. Our results show a significant enhancement in both, the convective upflows and downflows, but particularly for the latter. After deconvolution, the up- and downflows reach maximum amplitudes of -3.0 km/s and +3.0 km/s at an average geometrical height of roughly 50 km, respectively. We found that the velocity distributions after deconvolution match those derived from numerical simulations. After deconvolution the net LOS velocity averaged over the whole FOV lies close to zero as expected in a rough sense from mass balance.Comment: 32 pages, 13 figures, accepted for publication in Ap

Chromospheric plasma ejection above a pore

We present high spatial resolution observations of short lived transients, ribbon and jets like events above a pore in Ca II H images where fine structure like umbral dots, lightbridge and penumbral micro filaments are present in the underlying photosphere. We found that current layers are formed at the edges of convective fine structure due to the shear between their horizontal field and the ambient vertical field. High vertical electric current density patches are observed in the photosphere around these events which indicates the formation of a current sheet at the reconnection site. In the framework of past studies, low altitude reconnection could be the mechanism that produces such events. The reconnection is caused by an opposite polarity field produced by the bending of field lines by convective downflows at the edge of the pore fine structures.Comment: 10 pages, 5 figures, accepted for publication in MNRA

Capabilities of bisector analysis of the Si I 10827 A line for estimating line-of-sight velocities in the quiet Sun

We examine the capabilities of a fast and simple method to infer line-of-sight (LOS) velocities from observations of the photospheric Si I 10827 A line. This spectral line is routinely observed together with the chromospheric He I 10830 A triplet as it helps to constrain the atmospheric parameters. We study the accuracy of bisector analysis and a line core fit of Si I 10827 A. We employ synthetic profiles starting from the Bifrost enhanced network simulation. The profiles are computed solving the radiative transfer equation, including non-local thermodynamic equilibrium effects on the determination of the atomic level populations of Si I. We found a good correlation between the inferred velocities from bisectors taken at different line profile intensities and the original simulation velocity at given optical depths. This good correlation means that we can associate bisectors taken at different line-profile percentages with atmospheric layers that linearly increase as we scan lower spectral line intensities. We also determined that a fit to the line-core intensity is robust and reliable, providing information about atmospheric layers that are above those accessible through bisectors. Therefore, by combining both methods on the Si I 10827 A line, we can seamlessly trace the quiet-Sun LOS velocity stratification from the deep photosphere to higher layers until around $\log \tau = -3.5$ in a fast and straightforward way. This method is ideal for generating quick-look reference images for future missions like the Daniel K. Inoue Solar Telescope and the European Solar Telescope, for example.Comment: Accepted for publication in Astronomy & Astrophysics, 7 pages, 7 figure

Chromospheric polarimetry through multi-line observations of the 850 nm spectral region

Future solar missions and ground-based telescopes aim to understand the magnetism of the solar chromosphere. We performed a supporting study in Quintero Noda et al. (2016) focused on the infrared Ca II 8542 A line and we concluded that is one of the best candidates because it is sensitive to a large range of atmospheric heights, from the photosphere to the middle chromosphere. However, we believe that it is worth to try improving the results produced by this line observing additional spectral lines. In that regard, we examined the neighbour solar spectrum looking for spectral lines that could increase the sensitivity to the atmospheric parameters. Interestingly, we discovered several photospheric lines that greatly improve the photospheric sensitivity to the magnetic field vector. Moreover, they are located close to a second chromospheric line that also belongs to the Ca II infrared triplet, i.e. the Ca II 8498 A line, and enhances the sensitivity to the atmospheric parameters at chromospheric layers. We conclude that the lines in the vicinity of the Ca II 8542 A line not only increase its sensitivity to the atmospheric parameters at all layers, but also they constitute an excellent spectral window for chromospheric polarimetry.Comment: 11 pages, 8 figures, 1 tabl