arXiv subjects

H. Socas-Navarro

Publications and source records attributed to H. Socas-Navarro.

At least 19 recordsLinked to original sources

arXiv2026

Operating the Fabry-Pérot systems of the European Solar Telescope in multi-aperture mode

We discuss how to optimise the science output of the European Solar Telescope (EST), when used without the wide-field compensation for high-altitude seeing that the EST multi conjugate adaptive optics (MCAO) will offer. This will be the mode of operating EST during its first year(s). Without MCAO, the spatial resolution of a much smaller telescope could surpass that of EST. We therefore propose to operate EST in multi-aperture mode, by optically segmenting the 4.2 m aperture into six 1.4 m subapertures, until MCAO is operational. Operating at smaller aperture diameter pushes down the root mean square wavefront errors from the high altitude seeing to levels that can more reliably be compensated for in restored images using post processing methods. This will significantly improve image quality. In particular, the multi-aperture mode will provide the sustained stable high image quality needed for obtaining time sequences of spectropolarimetric data. The multi-aperture mode is implemented with low-cost modifications of the camera lenses of the three Fabry-Perot systems that will be used to cover the wavelength range 380--860~nm. Switching between the full-aperture and multi-aperture modes can be done quickly and independently for the three FPI systems. This allows flexible optimisation of EST, taking into account that the seeing is much better at long wavelengths than at short wavelengths, without any impact on the EST primary or secondary optical systems or on the actual FPI systems. The multi-aperture addition to EST provides a powerful and flexible option that has the potential of significantly improving the quality and amount of its science data before MCAO is operational. In this publication, we perform simulations and image reconstructions of simulated data to demonstrate the benefits of the multi-aperture option, and provide a simple optical design to demonstrate its feasibility.

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arXiv2024

Observations of umbral flashes in the resonant sunspot chromosphere

In sunspot umbrae, the core of some chromospheric lines exhibits periodic brightness enhancements known as umbral flashes. The consensus is that they are produced by the upward propagation of shock waves. This view has recently been challenged by the detection of downflowing umbral flashes and the confirmation of the existence of a resonant cavity above sunspots. We aim to determine waves' propagating or standing nature in the low umbral chromosphere and confirm or refute the existence of downflowing umbral flashes. Spectroscopic temporal series of Ca II 8542 Å, Ca II H, and Halpha in a sunspot were acquired with the Swedish Solar Telescope. The Halpha velocity was inferred using bisectors. Simultaneous inversions of the Ca II 8542 Å line and the Ca II H core were performed using the NICOLE code. The nature of the oscillations and insights into the resonant oscillatory pattern were determined by analyzing the phase shift between the velocity signals and examining the temporal evolution. Propagating waves in the low chromosphere are more common in regions with frequent umbral flashes, where the transition region is shifted upward, making resonant cavity signatures less noticeable. In contrast, areas with fewer umbral flashes show velocity fluctuations that align with standing oscillations. Evidence suggests dynamic changes in the location of velocity resonant nodes due to variations in transition region height. Downflowing profiles appear at the onset of some umbral flashes, but upflowing motion dominates during most of the flash. These downflowing flashes are more common in standing umbral flashes. We confirm the existence of a chromospheric resonant cavity above sunspot umbrae produced by wave reflections at the transition region. The oscillatory pattern depends on the transition region height, which exhibits spatial and temporal variations due to the impact of the waves.

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arXiv2023

Impact of opacity effects on chromospheric oscillations inferred from NLTE inversions

Chromospheric inferences rely on the interpretation of spectral lines that are formed under Non-Local Thermodynamic Equilibrium (NLTE) conditions. In the presence of oscillations, changes in the opacity impact the response height of the spectral lines and hinder the determination of the real properties of the fluctuations. We aim to explore the relationship between the chromospheric oscillations inferred by NLTE inversion codes and the waves' intrinsic fluctuations in velocity and temperature. Numerical simulations of wave propagation in an umbra have been computed with the code MANCHA. The NLTE synthesis and inversion code NICOLE has been used to compute spectropolarimetric Ca II 8542 Å line profiles for the models obtained from the simulations. The synthetic profiles have been inverted and the inferences from the inversions have been compared with the known simulated atmospheres. NLTE inversions of the Ca II 8542 Å line capture low-frequency oscillations, including those in the main band of chromospheric oscillations around 6 mHz. In contrast, waves with frequencies above 9 mHz are poorly characterized. Velocity oscillations at those higher frequencies exhibit clear indications of opacity fluctuations. The main response of the line to velocity fluctuations comes from low chromospheric heights, whereas the response to temperature shows sudden jumps between the high photosphere and the low chromosphere. This strong variation in the heights where the line is sensitive to temperature is revealed as a strong oscillatory power in the inferred fluctuations. Our results validate the use of NLTE inversions to study chromospheric oscillations with frequencies below 9 mHz. The interpretation of higher frequency oscillations and the power of temperature oscillations must be addressed with care since they exhibit signatures of opacity oscillations.

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arXiv2021

Downflowing umbral flashes as an evidence of standing waves in sunspot umbrae

Umbral flashes are sudden brightenings commonly visible in the core of chromospheric lines. Theoretical and numerical modeling suggest that they are produced by the propagation of shock waves. According to these models and early observations, umbral flashes are associated with upflows. However, recent studies have reported umbral flashes in downflowing atmospheres. We aim to understand the origin of downflowing umbral flashes. We explore how the existence of standing waves in the umbral chromosphere impacts the generation of flashed profiles. We performed numerical simulations of wave propagation in a sunspot umbra with the code MANCHA. The Stokes profiles of the Ca II 8542 Å line were synthesized with NICOLE. For freely-propagating waves, the chromospheric temperature enhancements of the oscillations are in phase with velocity upflows. In this case, the intensity core of the Ca II 8542 Å atmosphere is heated during the upflowing stage of the oscillation. If we consider a different scenario with a resonant cavity, the wave reflections at the sharp temperature gradient of the transition region lead to standing oscillations. In this situation, temperature fluctuations are shifted backward and temperature enhancements partially coincide with the downflowing stage of the oscillation. In umbral flashes produced by standing oscillations, the reversal of the emission feature is produced when the oscillation is downflowing. The chromospheric temperature keeps increasing while the atmosphere is changing from a downflow to an upflow. During the appearance of flashed Ca II 8542 Å cores, the atmosphere is upflowing most of the time, and only 38\% of the flashed profiles are associated with downflows. We find a scenario that remarkably explains the recent empirical findings of downflowing umbral flashes as a natural consequence of the presence of standing oscillations above sunspot umbrae.

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arXiv2020

Semi-empirical models of spicule from inversion of Ca II 8542 Å line

We study a solar spicule observed off-limb using high-resolution imaging spectroscopy in the Ca II 8542 Å line obtained with the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1-m Solar Telescope. Using a new version of the non-LTE code NICOLE specifically developed for this problem we invert the spicule single- and double-peak line profiles. This new version considers off-limb geometry and computes atomic populations by solving the 1D radiative transfer assuming a vertical stratification. The inversion proceeds by fitting the observed spectral profiles at 14 different heights with synthetic profiles computed in the model by solving the radiative transfer problem along its length. Motivated by the appearance of double-peak Ca II 8542 Å spicule profiles, which exhibit two distinct emission features well separated in wavelength, we adopt a double-component scenario. We start from the ansatz that the spicule parameters are practically constant along the spicule axis for each component, except for a density drop. Our results support this ansatz by attaining very good fits to the entire set of 14$\times$4 profiles (14 heights and 4 times). We show that the double-component model with uniform temperature of 9560 K, exponential decrease of density with a height scale of $1000-2000$ km, and the counter-oriented line-of-sight velocities of components reproduce the double-peak line profiles at all spicule segments well. Analyses of the numerical response function reveals the necessity of the inversions of spectra at multiple height positions to obtain height-dependent, degeneracy-free reliable model with a limited number of free parameters.

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arXiv2020

Discovery of long-period magnetic field oscillations and motions in isolated sunspots

We analyse the temporal evolution of the inclination component of the magnetic field vector for the penumbral area of 25 isolated sunspots. Compared to previous works, the use of data from the HMI instrument aboard the SDO observatory facilitates the study of very long time series ($\approx$1 week), compared to previous works, with a good spatial and temporal resolution. We used the wavelet technique and we found some filamentary-shaped events with large wavelet power. Their distribution of periods is broad, ranging from the lower limit for this study of 48 minutes up to 63 hours. An interesting property of these events is that they do not appear homogeneously all around the penumbra but they seem to concentrate at particular locations. The cross-comparison of these wavelet maps with AIA data shows that the regions where these events appear are visually related to the coronal loops that connect the outer penumbra to one or more neighbouring opposite polarity flux patches.

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arXiv2018

Height dependence of the penumbral fine-scale structure in the inner solar atmosphere

We studied the physical parameters of the penumbra in a large and fully-developed sunspot, one of the largest over the last two solar cycles, by using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional Spectrometer. Inverting measurements with the NICOLE code, we obtained the three-dimensional structure of the magnetic field in the penumbra from the bottom of the photosphere up to the middle chromosphere. We analyzed the azimuthal and vertical gradient of the magnetic field strength and inclination. Our results provide new insights on the properties of the penumbral magnetic fields in the chromosphere at atmospheric heights unexplored in previous studies. We found signatures of the small-scale spine and intra-spine structure of both the magnetic field strength and inclination at all investigated atmospheric heights. In particular, we report typical peak-to-peak variations of the field strength and inclination of $\approx 300$ G and $\approx 20^{\circ}$, respectively, in the photosphere, and of $\approx 200$ G and $\approx 10^{\circ}$ in the chromosphere. Besides, we estimated the vertical gradient of the magnetic field strength in the studied penumbra: we find a value of $\approx 0.3$ G km$^{-1}$ between the photosphere and the middle chromosphere. Interestingly, the photospheric magnetic field gradient changes sign from negative in the inner to positive in the outer penumbra.

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arXiv2018

Alfvén Wave Dissipation in the Solar Chromosphere

Magneto-hydrodynamic (MHD) Alfvén waves have been a focus of laboratory plasma physics and astrophysics for over half a century. Their unique nature makes them ideal energy transporters, and while the solar atmosphere provides preferential conditions for their existence, direct detection has proved difficult as a result of their evolving and dynamic observational signatures. The viability of Alfvén waves as a heating mechanism relies upon the efficient dissipation and thermalization of the wave energy, with direct evidence remaining elusive until now. Here we provide the first observational evidence of Alfvén waves heating chromospheric plasma in a sunspot umbra through the formation of shock fronts. The magnetic field configuration of the shock environment, alongside the tangential velocity signatures, distinguish them from conventional umbral flashes. Observed local temperature enhancements of 5% are consistent with the dissipation of mode-converted Alfvén waves driven by upwardly propagating magneto-acoustic oscillations, providing an unprecedented insight into the behaviour of Alfvén waves in the solar atmosphere and beyond.

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arXiv2018

Inversions of synthetic umbral flashes: effects of the scanning time on the inferred atmospheres

The use of instruments that record narrow band images at selected wavelengths is a common approach in solar observations. They allow the scanning of a spectral line by sampling the Stokes profiles with 2D images at each line position, but require a compromise between spectral resolution and temporal cadence. We evaluate the impact of the time-dependent acquisition of different wavelengths on the inversion of spectropolarimetric profiles from chromospheric lines during umbral flashes. Simulations of non-linear wave propagation in a sunspot were performed with the code MANCHA. Synthetic Stokes parameters in the Ca II 8542 A line in NLTE were computed for an umbral flash using the code NICOLE. Artificial profiles with the same wavelength coverage and temporal cadence from reported observations were constructed and inverted. The inferred atmospheric stratifications were compared with the original models. The inferred atmospheres provide a reasonable characterization of the thermodynamic properties of the atmosphere during most of the phases of the umbral flash. Only at the early stages of the flash, when the shock wave reaches the formation height of the line, the Stokes profiles present apparent wavelength shifts and other spurious deformations. These features are misinterpreted by the inversion code, which can return unrealistic atmospheric models from a good fit of the Stokes profiles. The misguided results include flashed atmospheres with strong downflows, even though the simulation exhibits upflows during the umbral flash, and large variations in the magnetic field strength. Our analyses validate the inversion of Stokes profiles acquired by sequentially scanning certain selected wavelengths of a line profile, even in the case of rapidly-changing events such as umbral flashes. However, the inversions are unreliable during a short period at the development phase of the flash.

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arXiv2017

Ca II 8542 Å brightenings induced by a solar microflare

We study small-scale brightenings in Ca II 8542 Å line-core images to determine their nature and effect on localized heating and mass transfer in active regions. High-resolution 2D spectroscopic observations of an active region in the Ca II 8542 Å line were acquired with the GFPI attached to the 1.5-meter GREGOR telescope. Inversions of the spectra were carried out using NICOLE. We identified three brightenings of sizes up to 2"x2". We found evidence that the brightenings belonged to the footpoints of a microflare (MF). The properties of the observed brightenings disqualified the scenarios of Ellerman bombs or IRIS bombs. However, this MF shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and MFs are both apparent in chromospheric and coronal layers according to the AIA channels, and (2) both show flaring arches with lifetimes of about 3.0-3.5 min and lengths of about 20". The inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. Connecting the footpoints, a dark filamentary structure appeared in the Ca II line-core images. Before the start of the MF, the spectra of this structure already indicated average blueshifts, meaning upward motions of the plasma along the LOS. During the impulsive phase, these velocities increased up to -2.2 km/s. Downflows dominated at the footpoints. However, in the upper photosphere, slight upflows occurred during the impulsive phase. Hence, bidirectional flows are present in the footpoints of the MF. Conclusions: We detected Ca II brightenings that coincided with the footpoint location of an MF. The MF event led to a rise of plasma in the upper photosphere, both before and during the impulsive phase. Excess mass, previously raised to at most chromospheric layers, slowly drained downward along arches toward the footpoints of the MF.

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arXiv2017

First detection of sign-reversed linear polarization from the forbidden [O I] 630.03 nm line

We report on the detection of linear polarization of the forbidden [O i] 630.03 nm spectral line. The observations were carried out in the broader context of the determination of the solar oxygen abundance, an important problem in astrophysics that still remains unresolved. We obtained spectro-polarimetric data of the forbidden [O i] line at 630.03 nm as well as other neighboring permitted lines with the Solar Optical Telescope of the Hinode satellite. A novel averaging technique was used, yielding very high signal-to-noise ratios in excess of $10^5$. We confirm that the linear polarization is sign-reversed compared to permitted lines as a result of the line being dominated by a magnetic dipole transition. Our observations open a new window for solar oxygen abundance studies, offering an alternative method to disentangle the Ni i blend from the [O i] line at 630.03 nm that has the advantage of simple LTE formation physics.

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arXiv2017

Uncertainties in the solar photospheric oxygen abundance

The purpose of this work is to better understand the confidence limits of the photospheric solar oxygen abundance derived from three-dimensional models using the forbidden [OI] line at 6300 Å, including correlations with other parameters involved. We worked with a three-dimensional empirical model and two solar intensity atlases. We employed Bayesian inference as a tool to determine the most probable value for the solar oxygen abundance given the model chosen. We considered a number of error sources, such as uncertainties in the continuum derivation, in the wavelength calibration and in the abundance/strength of Ni. Our results shows correlations between the effects of several parameters employed in the derivation. The Bayesian analysis provides robust confidence limits taking into account all of these factors in a rigorous manner. We obtain that, given the empirical three-dimensional model and the atlas observations employed here, the most probable value for the solar oxygen abundance is $\log(ε_O) = 8.86\pm0.04$. However, we note that this uncertainty does not consider possible sources of systematic errors due to the model choice.

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arXiv2016

Inference of the chromospheric magnetic field orientation in the Ca II 8542 A line fibrils

Solar chromospheric fibrils, as observed in the core of strong chromospheric spectral lines, extend from photospheric field concentrations suggesting that they trace magnetic field lines. These images have been historically used as proxies of magnetic fields for many purposes. We use a Bayesian hierarchical model to analyze several tens of thousands of pixels in spectro-polarimetric chromospheric images of penumbrae and chromospheric fibrils. We compare the alignment between the field azimuth inferred from the linear polarization signals through the transverse Zeeman effect and the direction of the fibrils in the image. We conclude that, in the analyzed fields of view, fibrils are often well aligned with the magnetic field azimuth. Despite this alignment, the analysis also shows that there is a non-negligible dispersion. In penumbral filaments, we find a dispersion with a standard deviation of ~16 degrees, while this dispersion goes up to ~34 degrees in less magnetized regions.

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arXiv2014

The Solar Oxygen Abundance from an Empirical Three-Dimensional Model

The Oxygen abundance in the solar photosphere, and consequently the solar metallicity itself, is still a controversial issue with far-reaching implications in many areas of Astrophysics. This paper presents a new determination obtained by fitting the forbidden OI line at 6300 A with an observational 3D model. The approach presented here is novel because previous determinations were based either on 1D empirical stratifications or on 3D theoretical models. The resulting best-fit abundances are lg E(O)=8.90 and lg E(Ni)=6.15. Nevertheless, introducing minor tweaks in the model and the procedure, it is possible to retrieve very different values, even down to lg E(O)=8.70. This extreme sensitivity of the abundance to possible systematic effects is not something specific to this particular work but probably reflects the real uncertainty inherent to all abundance determinations based on a prescribed model atmosphere.

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arXiv2014

An Open Source, Massively Parallel Code for Non-LTE Synthesis and Inversion of Spectral Lines and Zeeman-induced Stokes Profiles

With the advent of a new generation of solar telescopes and instrumentation, the interpretation of chromospheric observations (in particular, spectro-polarimetry) requires new, suitable diagnostic tools. This paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, both for synthesis and inversion of spectral lines and Zeeman-induced polarization profiles, spanning a wide range of atmospheric heights, from the photosphere to the chromosphere. The code fosters a number of unique features and capabilities and has been built from scratch with a powerful parallelization scheme that makes it suitable for application on massive datasets using large supercomputers. The source code is being publicly released, with the idea of facilitating future branching by other groups to augment its capabilities.

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arXiv2014

Synthetic observations of wave propagation in a sunspot umbra

Spectropolarimetric temporal series from Fe I $λ$ 6301.5 Å and Ca II infrared triplet lines are obtained by applying the Stokes synthesis code NICOLE to a numerical simulation of wave propagation in a sunspot umbra from MANCHA code. The analysis of the phase difference between Doppler velocity and intensity core oscillations of the Fe I $λ$ 6301.5 Å line reveals that variations in the intensity are produced by opacity fluctuations rather than intrinsic temperature oscillations, except for frequencies between 5 and 6.5 mHz. On the other hand, the photospheric magnetic field retrieved from the weak field approximation provides the intrinsic magnetic field oscillations associated to wave propagation. Our results suggest that this is due to the low magnetic field gradient of our sunspot model. The Stokes parameters of the chromospheric Ca II infrared triplet lines show striking variations as shock waves travel through the formation height of the lines, including emission self-reversals in the line core and highly abnormal Stokes V profiles. Magnetic field oscillations inferred from the Ca II infrared lines using the weak field approximation appear to be related with the magnetic field strength variation between the photosphere and the chromosphere.

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arXiv2013

Physical Properties of a Sunspot Chromosphere with Umbral Flashes

We present new high-resolution spectro-polarimetric Ca II 8542 observations of umbral flashes in sunspots. At nearly 0.18", and spanning about one hour of continuous observation, this is the most detailed dataset published thus far. Our study involves both LTE and non-LTE inversions (but includes also a weak field analysis as a sanity check) to quantify temperatures, mass flows and the full magnetic field vector geometry. We confirm earlier reports that UFs have very fine structure with hot and cool material intermixed at sub-arcsecond scales. The shock front is roughly 1000 K hotter than the surrounding material. We do not observe significant fluctuations of the field in the umbra. In the penumbra, however, the passage of the running penumbral waves alter the magnetic field strength by some 200 G (peak-to-peak amplitude) but it does not change the field orientation (at least not significantly within our sensitivity of a few degrees). We find a trend of decreasing high-frequency modulation power for more inclined magnetic fields for the line-of-sight velocity and magnetic field strength. In the outer penumbra we find an absence of high frequency power while there is increasingly more power at high frequencies towards the umbra.

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arXiv2012

Unusual Stokes V profiles during flaring activity of a delta sunspot

We analyze a set of full Stokes profile observations of the flaring active region NOAA 10808 recorded with the Vector-Spectromagnetograph (VSM) of the SOLIS facility. We aim to quantify transient and permanent changes in the magnetic field and velocity field. The results are put in context with MDI magnetograms and reconstructed RHESSI X-ray images. We find signs of restructuring of the photospheric magnetic field during the flare close to the polarity inversion line (PIL) at the flaring site. At two locations in the umbra we encounter strong fields (approx. 3 kG), as inferred from the Stokes I profiles which, however, exhibit a low polarization signal. During the flare we observe in addition asymmetric Stokes V profiles at one of these sites. The asymmetric Stokes V profiles appear co-spatial and co-temporal with a strong apparent polarity reversal observed in MDI-magnetograms and a chromospheric hard X-ray source. The two-component atmosphere fits of the asymmetric Stokes profiles result in line-of-sight velocity differences in the range of approx. 12km/s to 14 km/s between the two components in the photosphere. Another possibility is that local atmospheric heating is causing the observed asymmetric Stokes V profile shape. In either case our analysis shows that a very localized patch of approx. 5 arcsec in the photospheric umbra, co-spatial with a flare footpoint, exhibits a sub-resolution fine structure.

astro-ph.SR↗