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

Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope

Arch filament systems occur in active sunspot groups, where a fibril structure connects areas of opposite magnetic polarity, in contrast to active region filaments that follow the polarity inversion line. We used the GREGOR Infrared Spectrograph (GRIS) to obtain the full Stokes vector in the spectral lines Si I 1082.7 nm, He I 1083.0 nm, and Ca I 1083.9 nm. We focus on the near-infrared calcium line to investigate the photospheric magnetic field and velocities, and use the line core intensities and velocities of the helium line to study the chromospheric plasma. The individual fibrils of the arch filament system connect the sunspot with patches of magnetic polarity opposite to that of the spot. These patches do not necessarily coincide with pores, where the magnetic field is strongest. Instead, areas are preferred not far from the polarity inversion line. These areas exhibit photospheric downflows of moderate velocity, but significantly higher downflows of up to 30 km/s in the chromospheric helium line. Our findings can be explained with new emerging flux where the matter flows downward along the fieldlines of rising flux tubes, in agreement with earlier results.Comment: Proceedings 12th Potsdam Thinkshop to appear in Astronomische Nachrichte

Venus transit 2004: Illustrating the capability of exoplanet transmission spectroscopy

The transit of Venus in 2004 offered the rare possibility to remotely sense a well-known planetary atmosphere using ground-based observations for absorption spectroscopy. Transmission spectra of Venus' atmosphere were obtained in the near infrared using the Vacuum Tower Telescope (VTT) in Tenerife. Since the instrument was designed to measure the very bright photosphere of the Sun, extracting Venus' atmosphere was challenging. CO_2 absorption lines could be identified in the upper Venus atmosphere. Moreover, the relative abundance of the three most abundant CO_2 isotopologues could be determined. The observations resolved Venus' limb, showing Doppler-shifted absorption lines that are probably caused by high-altitude winds. This paper illustrates the ability of ground-based measurements to examine atmospheric constituents of a terrestrial planet atmosphere which might be applied in future to terrestrial extrasolar planets.Comment: 7 pages, 5 figures, 1 tabl

Spectro-polarimetric analysis of a short lived solar active region

The physical processes related to the formation, evolution and disappearance of solar active regions are not completely clear. High-resolution solar spectro-polarimetric data are needed to investigate these processes with unprecedented details. Here we present the analysis of the short-lived NOAA 12549 active region using high-resolution spectro-polarimetric data acquired with the GREGOR solar telescope and the GRIS instrument, inverted using the SIR code

Detection of emission in the Si i 1082.7 nm line core in sunspot umbrae

Context. Determining empirical atmospheric models for the solar chromosphere is difficult since it requires the observation and analysis of spectral lines that are affected by non-local thermodynamic equilibrium (NLTE) effects. This task is especially difficult in sunspot umbrae because of lower continuum intensity values in these regions with respect to the surrounding brighter granulation. Umbral data is therefore more strongly affected by the noise and by the so-called scattered light, among other effects. Aims. The purpose of this study is to analyze spectropolarimetric sunspot umbra observations taken in the near-infrared Si I 1082.7 nm line taking NLTE effects into account. Interestingly, we detected emission features at the line core of the Si I 1082.7 nm line in the sunspot umbra. Here we analyze the data in detail and offer a possible explanation for the Si I 1082.7 nm line emission. Methods. Full Stokes measurements of a sunspot near disk center in the near-infrared spectral range were obtained with the GRIS instrument installed at the German GREGOR telescope. A point spread function (PSF) including the effects of the telescope, the Earth's atmospheric seeing, and the scattered light was constructed using prior Mercury observations with GRIS and the information provided by the adaptive optics system of the GREGOR telescope during the observations. The data were then deconvolved from the PSF using a principal component analysis deconvolution method and were analyzed via the NICOLE inversion code, which accounts for NLTE effects in the Si I 1082.7 nm line. The information of the vector magnetic field was included in the inversion process. Results. 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 (stray light coming from wide angles) 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%. Although very low, the inferred levels are in line with current model predictions and empirical umbral models. The Si I 1082.7 nm line is in emission after adding more that 30% of scattered light so that it is very sensitive to a proper determination of the PSF. Additionally, we have thoroughly investigated whether the emission is a byproduct of the particular deconvolution technique but have not found any evidence to the contrary. Only the circular polarization signals seem to be more sensitive to the deconvolution strategy because of the larger amount of noise in the umbra. Interestingly, 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 τ = -2 and start rising at lower heights when moving into the transition region. Conclusions. 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.© 2017 ESO.This work was partly supported by the BK21 plus program through the National Research Foundation (NRF) funded by the Ministry of Education of Korea. This study is supported by the European Commissions FP7 Capacities Programme under the Grant Agreement number 312495. The GRIS instrument was developed thanks to the support by the Spanish Ministry of Economy and Competitiveness through the project AYA2010-18029 (Solar Magnetism and Astrophysical Spectropolarimetry) and AYA2014-60476-P. This work has also been supported by Spanish Ministry of Economy and Competitiveness through projects ESP2014-56169-C6-1-R and ESP-2016-77548-C5-1-R.Peer Reviewe

Remote sensing of the solar photosphere: A tale of two methods

Solar spectro-polarimetry is a powerful tool to investigate the physical processes occurring in the solar atmosphere. The different states of polarization and wavelengths have in fact encoded the information about the thermodynamic state of the solar plasma and the interacting magnetic field. In particular, the radiative transfer theory allows us to invert the spectro-polarimetric data to obtain the physical parameters of the different atmospheric layers and, in particular, of the photosphere. In this work, we present a comparison between two methods used to analyze spectro-polarimetric data: The classical Center of Gravity method in the weak field approximation and an inversion code that solves numerically the radiative transfer equation. The Center of Gravity method returns reliable values for the magnetic field and for the line-of-sight velocity in those regions where the weak field approximation is valid (field strength below 400 G), while the inversion code is able to return the stratification of many physical parameters in the layers where the spectral line used for the inversion is formed

An approach using deep learning for tomographic reconstruction in solar observation

AO4ELT5 Adaptive Optics for Extremely Large Telescopes (5th. 2017. Tenerife