Solar Spectral Lines Research Articles

Calibration and Release of Magnetograms/Dopplergrams Obtained at the Mt. Wilson 150-Foot Tower Telescope (MWO)

The Mt. Wilson Observatory archive of observations of solar disk magnetic fields, Doppler velocities, and spectral line intensities is a resource for studying the Sun’s state from 1967 to 2013. Instrument changes/upgrades during this time must be considered when interpreting this record. Portions of this record have been previously released. This publication documents the data record in order to allow its independent interpretation. The archive is available through two directory trees which can be accessed at http://sha.stanford.edu/mwo/msm.html. The calibration of the observations is impacted by the solar surface convective flows, which produce offsets for both differential rotation and meridional circulation functions. The effects of these offsets have been reduced in this and other publications by temporal averaging.

Ground-Based Monitoring of the Variability of Visible Solar Spectral Lines for Improved Understanding of Solar and Stellar Magnetism and Dynamics

Ground-Based Monitoring of the Variability of Visible Solar Spectral Lines for Improved Understanding of Solar and Stellar Magnetism and Dynamics

The Magnetic Sensitivity of the (250–278 nm) Fe ii Polarization Spectrum

This paper presents a theoretical investigation of the polarization and magnetic sensitivity of the near-ultraviolet (near-UV) solar spectral lines of Fe ii between 250 and 278 nm. In recent years, UV spectropolarimetry has become key to uncovering the magnetism of the upper layers of the solar chromosphere. The unprecedented data obtained by the CLASP2 suborbital space experiment across the Mg ii h and k lines around 280 nm are a clear example of the capabilities of near-UV spectropolarimetry for magnetic field diagnostics throughout the whole solar chromosphere. Recent works have pointed out the possible complementary diagnostic potential of the many Fe ii lines in the unexplored spectral region between 250 and 278 nm, but no quantitative analysis of the polarization and magnetic sensitivity of those spectral lines has been carried out yet. To study the polarization signals in these spectral lines, we create a comprehensive atomic model including all the atomic transitions resulting in strong spectral lines. We then study the magnetic sensitivity of the linear and circular polarization profiles in a semiempirical model representative of the quiet Sun. We present a selection of Fe ii spectral lines with significant linear and circular polarization signals and evaluate their diagnostic capabilities by studying their formation heights and magnetic sensitivity through the action of the Hanle and Zeeman effects. We conclude that when combined with the CLASP2 spectral region these Fe ii lines are of interest for the inference of magnetic fields throughout the solar chromosphere, up to near the base of the corona.

Center-to-limb variation of spectral lines and continua observed with SST/CRISP and SST/CHROMIS

Context.Observations of center-to-limb variations (CLVs) of spectral lines and continua provide a good test for the accuracy of models with a solar and stellar atmospheric structure and spectral line formation. They are also widely used to constrain elemental abundances, and are becoming increasingly more important in atmospheric studies of exoplanets. However, only a few such data sets exist for chromospheric lines.Aims.We aim to create a set of standard profiles by means of mosaics made with the CRISP and CHROMIS instruments of the Swedish 1-m Solar Telescope (SST), as well as to explore the robustness of said profiles obtained using this method.Methods.For each spectral line, we used a mosaic that ranges from the center to the limb. Each of these mosaics were averaged down to 50 individual spectral profiles and spaced by 0.02 in theμscale. These profiles were corrected for p-mode oscillations, and their line parameters (equivalent width, line shift, full-width at half-maximum, and line depth) were then compared against literature values whenever possible.Results.We present a set of 50 average profiles that are spaced equidistantly along the cosine of the heliocentric angle (μ) by steps of 0.02 for five continuum points between 4001 and 7772 Å, as well as ten of the most commonly observed spectral lines at the SST (Ca II H & K, Ηβ, Mg I 5173 Å, C I 5380 Å, Fe I 6173 Å, Fe I 6301 Å, Ha, O I 7772 Å, and Ca II 8542 Å).Conclusions.The CLV of line profiles and continua are shared in the CDS as machine readable tables, providing a quantitative constraint on theoretical models that aim to model stellar atmospheres.

New Light on an Old Problem of the Cores of Solar Resonance Lines

Abstract We reexamine a 50+ yr old problem of deep central reversals predicted for strong solar spectral lines, in contrast to the smaller reversals seen in observations. We examine data and calculations for the resonance lines of H i, Mg ii, and Ca ii, the self-reversed cores of which form in the upper chromosphere. Based on 3D simulations, as well as data for the Mg ii lines from the Interface Region Imaging Spectrograph (IRIS), we argue that the resolution lies not in velocity fields on scales in either of the micro- or macroturbulent limits. Macroturbulence is ruled out using observations of optically thin lines formed in the upper chromosphere, and by showing that it would need to have unreasonably special properties to account for critical observations of the Mg ii resonance lines from the IRIS mission. The power in “turbulence” in the upper chromosphere may therefore be substantially lower than earlier analyses have inferred. Instead, in 3D calculations horizontal radiative transfer produces smoother source functions, smoothing out intensity gradients in wavelength and in space. These effects increase in stronger lines. Our work will have consequences for understanding the onset of the transition region, for understanding the energy in motions available for heating the corona, and for the interpretation of polarization data in terms of the Hanle effect applied to resonance line profiles.

Convective blueshifts in the solar atmosphere

Context. Convective motions in the solar atmosphere cause spectral lines to become asymmetric and shifted in wavelength. For photospheric lines, this differential Doppler shift varies from the solar disk center to the limb. Aims. Precise and comprehensive observations of the convective blueshift and its center-to-limb variation improve our understanding of the atmospheric hydrodynamics and ensuing line formation, and provide the basis to refine 3D models of the solar atmosphere. Methods. We performed systematical spectroscopic measurements of the convective blueshift of the quiet Sun with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. The spatial scanning of the solar disk covered 11 heliocentric positions each along four radial (meridional and equatorial) axes. The high-resolution spectra of 26 photospheric to chromospheric lines in the visible range were calibrated with a laser frequency comb to absolute wavelengths at the 1 m s−1 accuracy. Applying ephemeris and reference corrections, the bisector analysis provided line asymmetries and Doppler shifts with an uncertainty of only few m s−1. To allow for a comparison with other observations, we convolved the results to lower spectral resolutions. Results. All spectral line bisectors exhibit a systematic center-to-limb variation. Typically, a blueshifted “C”-shaped curve at disk center transforms into a less blueshifted “\”-shape toward the solar limb. The comparison of all lines reveals the systematic dependence of the convective blueshift on the line depth. The blueshift of the line minima describe a linear decrease with increasing line depths. The slope of the center-to-limb variation develops a reversal point at heliocentric positions between μ = 0.7 and 0.85, seen as the effect of horizontal granular flows in the mid photosphere. Line minima formed in the upper photosphere to chromosphere exhibit hardly any blueshift or even a slight redshift. Synthetic models yield considerable deviations from the observed center-to-limb variation. Conclusions. The obtained Doppler shifts of the quiet Sun can serve as an absolute reference for other observations, the relative calibration of Dopplergrams, and the necessary refinement of atmospheric models. Based on this, the development of high-precision models of stellar surface convection will advance the detection of (potentially habitable) exoplanets by radial velocity measurements.

Disentangling flows in the solar transition region

Context. The measured average velocities in solar and stellar spectral lines formed at transition region temperatures have been difficult to interpret. The dominant redshifts observed in the lower transition region naturally leads to the question of how the upper layers of the solar (and stellar) atmosphere can be maintained. Likewise, no ready explanation has been made for the average blueshifts often found in upper transition region lines. However, realistic three-dimensional radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere are able to reproduce the observed dominant line shifts and may thus hold the key to resolve these issues. Aims. These new 3D rMHD simulations aim to shed light on how mass flows between the chromosphere and corona and on how the coronal mass is maintained. These simulations give new insights into the coupling of various atmospheric layers and the origin of Doppler shifts in the solar transition region and corona. Methods. The passive tracer particles, so-called corks, allow the tracking of parcels of plasma over time and thus the study of changes in plasma temperature and velocity not only locally, but also in a co-moving frame. By following the trajectories of the corks, we can investigate mass and energy flows and understand the composition of the observed velocities. Results. Our findings show that most of the transition region mass is cooling. The preponderance of transition region redshifts in the model can be explained by the higher percentage of downflowing mass in the lower and middle transition region. The average upflows in the upper transition region can be explained by a combination of both stronger upflows than downflows and a higher percentage of upflowing mass. The most common combination at lower and middle transition region temperatures are corks that are cooling and traveling downward. For these corks, a strong correlation between the pressure gradient along the magnetic field line and the velocity along the magnetic field line has been observed, indicating a formation mechanism that is related to downward propagating pressure disturbances. Corks at upper transition region temperatures are subject to a rather slow and highly variable but continuous heating process. Conclusions. Corks are shown to be an essential tool in 3D rMHD models in order to study mass and energy flows. We have shown that most transition region plasma is cooling after having been heated slowly to upper transition region temperatures several minutes before. Downward propagating pressure disturbances are identified as one of the main mechanisms responsible for the observed redshifts at transition region temperatures.

Kyiv monitoring program of spectral line variations with the 11-year cycle

AbstractKyiv program of monitoring of long-term variation of solar spectral lines at the horizontal solar telescope of the Main Astronomical Observatory of Ukraine is described. The aim of the program is to clarify the issue how the physical parameters of the quiet solar atmosphere change over the 11-year cycle of solar activity. The diagnostics of the atmospheric variation includes analysis of more than 40 spectral lines of neutral and ionized chemical elements observed at the solar disk and at the limb near north and south poles with high spectral resolution. The results of monitoring show that during 2012–2017 a line core depths and a line full widths at half maximum respond to the cycle modulation of the global unsigned magnetic field of the Sun. Such a correlation can be explained by assuming that temperature gradient of the solar photosphere is growing with solar activity.

Solar Spectral Lines with Special Polarization Properties for the Calibration of Instrument Polarization

Abstract We investigate atomic transitions that have previously been identified as having zero polarization from the Zeeman effect. Our goal is to identify spectral lines that can be used for the calibration of instrumental polarization of large astronomical and solar telescopes, such as the Daniel K. Inouye Solar Telescope, which is currently under construction on Haleakala. We use a numerical model that takes into account the generation of scattering polarization and its modification by the presence of a magnetic field of arbitrary strength. We adopt values for the Landé factors from spectroscopic measurements or semi-empirical results, thus relaxing the common assumption of LS-coupling previously used in the literature. The mechanisms dominating the polarization of particular transitions are identified, and we summarize groups of various spectral lines useful for the calibration of spectropolarimetric instruments, classified according to their polarization properties.

Atomic physics and solar polarimetry

Major outstanding problems in solar physics relate to solar magnetism. Spectropolarimetry offers the best, and sometimes only, method of obtaining accurate measurements of the Sun’s magnetic field. New 1.5–2 m class telescopes with adaptive optics have come on line, and the Daniel K. Inouye 4 m Solar Telescope (DKIST) will begin observing in 2019. The calibration of polarized light entering such a large and polarizing ground-based telescope represents difficult challenges. This paper explores how special polarization properties of particular atomic transitions may provide calibration data, augmenting or even avoiding time-consuming calibration observations, as well as science data. This initial study concludes that solar spectral lines exist with special polarization properties, allowing the telescope calibration to be determined. The Sun’s visible and infrared spectrum is dominated by lines of neutral atoms and singly charged ions of iron and other complex atoms. Both solar and atomic physics should jointly benefit from telescopic advances, as observers explore regimes of broader wavelength ranges, and higher spatial resolutions and polarimetric sensitivities, than they have reached in the past. Further work is in progress to identify particular transitions of practical use to aid in calibrations.

First evidence of non-Gaussian solar flare EUV spectral line profiles and accelerated non-thermal ion motion

The properties of solar flare plasma can be determined from the observation of optically thin lines. The emitting ion distribution determines the shape of the line, with an isothermal Maxwellian ion distribution producing a Gaussian profile. Non-Gaussian lines may indicate more complex ion distributions. We investigate the possibility of determining flare-accelerated non-thermal ion or plasma velocity distributions. We study EUV spectral lines during a flare SOL2013-05-15T01:45 using the Hinode EUV Imaging Spectrometer (EIS). The flare is located close to the eastern solar limb with an extended loop structure, allowing the different flare features: ribbons, hard X-ray (HXR) footpoints and the loop-top source to be clearly observed in UV, EUV and X-rays. EUV line spectroscopy is performed in seven different regions. We study the line profiles of isolated and unblended Fe XVI lines (262.976 A) formed at temperatures of 2-4 MK. Fe XVI line profiles at one time close to the peak soft X-ray emission and free of directed mass motions are examined using: 1. a higher moments analysis, 2. Gaussian fitting, and 3. by fitting a kappa distribution profile convolved with a Gaussian to account for the EIS instrumental profile. Fe XVI line profiles in the loop-top, HXR footpoint and ribbon regions can be confidently fitted with a kappa line profile, giving low, non-thermal kappa values between 2-3.3. A higher moments analysis also finds that many of the line kurtosis values are higher than the Gaussian value of 3, even with the presence of a broad Gaussian instrumental profile. A flare-accelerated non-thermal ion population could account for both the observed non-Gaussian line profiles, and for the Fe XVI excess broadening found from Gaussian fitting, if the emitting ions are interacting with a thermalised 4 MK electron population, and the instrumental profile is well-approximated by a Gaussian.

Effect of Opacity on Solar Coronal Soft X-Ray Oxygen VIII Spectral Line

To study the effect of opacity on the solar x-ray spectral line, using the method of escape factor, we analyze the effect of opacity on the solar oxygen VIII spectral line, and we estimate the effective emission thickness of the oxygen VIII emitting region in the solar atmosphere. Results show that the opacity has a greater effect on the solar oxygen VIII spectral line, and the effective emission thickness of the oxygen VIII ion is greater than that for some ions in the solar ultraviolet spectral line. This study has important reference significance in solar plasma diagnoses in the x-ray region.

The Mark-I helioseismic experiment – I. Measurements of the solar gravitational redshift (1976–2013)

The resonant scattering solar spectrophotometer 'Mark-I', designed and build at the University of Birmingham (UK) and located at the Observatorio del Teide (Spain), has been continuously in operation for the past 38 years. During this period of time, it has provided high precision measurements of the radial velocity of the Sun as a star, which has enabled the study of the small velocity fluctuations produced by the solar oscillations and the characterization of their spectrum. So far, it has been one of the pioneer experiments in the field of helioseismology and contributed to the development of that area. Moreover, because of its high sensitivity and long term instrumental stability, it also provides an accurate determination (to within a few parts in $10^3$) of the absolute daily velocity offset, which contains the so-called 'solar gravitational red-shift'. In the present paper, results of the analysis of the measurements of this parameter over the whole period 1976--2013 are presented. The result of this series of measurements is 600.4 $\pm$ 0.8 m s$^{-1}$ with an amplitude variation of $\pm$5 m s$^{-1}$, which is in anticorrelation with the phase of the solar activity cycle. The 5\% difference found respect to the value predicted by the equivalence principle is probably due to the asymmetry of the solar spectral line used.

In situ calibration of tunable filters: Lyot and Michelson.

Solar imaging optical filter technology has progressed significantly over the past 75 years, and the ability to tune narrowband filters is particularly valuable for solar atmosphere sensing. For example, imaging while tuning over a narrow solar spectral line (emission or absorption) provides two-dimensional measurements of Doppler shifts and magnetic fields. While tuning ability has improved significantly, tuning accuracy can be a challenge over time given system actuator drifts. For many cases, the ability to calibrate these actuators in situ is convenient and cost effective (e.g., ground-based observatories), and for other cases it is required (e.g., in a spacecraft). It is ideal to calibrate in situ without the need for additional hardware such as a spectrometer, and if that cannot be achieved, the next best thing is to do so with minimum additional hardware. Two examples of solar filters that need to be calibrated periodically are: (1) a liquid crystal variable retarder Lyot filter and (2) a tunable Michelson interferometer. For the first, the filter can have a number of stages back-to-back to achieve the desired finesse. Within each stage there is a liquid crystal variable retarder that adds some amount of retardance to the stage's fixed birefringent crystal; this provides wavelength bandpass tuning. For the second, there can be several Michelson interferometers in series each with an actuator to adjust the optical path length in one of its optical paths for tuning. The stacking of these filters implies there is a need to calibrate more than one actuator. An algorithm has been developed to calibrate these types of stacked and nonstacked filters in situ with minimal, if any, hardware additions.

Narrow-Band Imaging System for the Multi-application Solar Telescope at Udaipur Solar Observatory: Characterization of Lithium Niobate Etalons

The Multi-application Solar Telescope is a 50 cm off-axis Gregorian telescope that has been installed at the lake site of Udaipur Solar Observatory. For quasi-simultaneous photospheric and chromospheric observations, a narrow-band imager has been developed as one of the back-end instruments for this telescope. Narrow-band imaging is achieved using two lithium niobate Fabry–Perot etalons working in tandem as a filter. This filter can be tuned to different wavelengths by changing either voltage, tilt, or temperature of the etalons. To characterize the etalons, a Littrow spectrograph was set up in conjunction with a 15 cm Carl Zeiss Coudé solar telescope. The etalons were calibrated for the solar spectral lines Fe i 6173 Å, and Ca ii 8542 Å. In this work, we discuss the characterization of the Fabry–Perot etalons, specifically, the temperature and voltage tuning of the system for the spectral lines proposed for observations. We present the details of the calibration set-up and various tuning parameters. We also present solar images obtained using the system.

The Earth as an extrasolar transiting planet

The atmospheric composition of transiting exoplanets can be characterized during transit by spectroscopy. For the transit of an Earth twin, models predict that biogenic $O_2$ and $O_3$ should be detectable, as well as water vapour, a molecule linked to habitability as we know it on Earth. The aim is to measure the Earth radius versus wavelength $\lambda$ - or the atmosphere thickness $h(\lambda)$ - at the highest spectral resolution available to fully characterize the signature of Earth seen as a transiting exoplanet. We present observations of the Moon eclipse of 21-12-2010. Seen from the Moon, the Earth eclipses the Sun and opens access to the Earth atmosphere transmission spectrum. We used HARPS and UVES spectrographs to take penumbra and umbra high-resolution spectra from 3100 to 10400 Ang. A change of the quantity of water vapour above the telescope compromised the quality of the UVES data. We corrected for this effect in the data processing. We analyzed the data by 3 different methods. The 1st method is based on the analysis of pairs of penumbra spectra. The 2nd makes use of a single penumbra spectrum, and the 3rd of all penumbra and umbra spectra. Profiles $h(\lambda)$ are obtained with the three methods for both instruments. The 1st method gives the best result, in agreement with a model. The second method seems to be more sensitive to the Doppler shift of solar spectral lines with respect to the telluric lines. The 3rd method makes use of umbra spectra which bias the result, but it can be corrected for this a posteriori from results with the first method. The 3 methods clearly show the spectral signature of the Rayleigh scattering in the Earth atmosphere and the bands of H$_2$O, O$_2$, and O$_3$. Sodium is detected. Assuming no atmospheric perturbations, we show that the E-ELT is theoretically able to detect the $O_2$ A-band in 8~h of integration for an Earth twin at 10pc.

Polarimetric shapes of spectral lines in solar observations

A century has elapsed since the first observation of the polarimetric profile of a line of the solar spectrum. Since then, dramatic progress has been made in the instrumentation, which is now reaching unprecedented levels of sensitivity in the measurement of polarization signals in solar spectral lines. At the same time, the theoretical framework needed for the interpretation of polarimetric observations has steadily evolved from the pioneering methods, based on simple formulae, to the sophisticated structure that is nowadays used with success in the interpretation of solar observations. The present paper is intended to give a historical perspective of the evolution of this research field and of its major achievements, with particular emphasis on the role played by the magnetic field in determining the polarimetric shapes of spectral lines.

The first observation and data reduction of the Multi-wavelength Spectrometer on the New Vacuum Solar Telescope

The Multi-wavelength Spectrometer is a medium-dispersion (R ∼ 130000) grating spectrometer installed on the New Vacuum Solar Telescope at the Fuxian Solar Observatory, Yunnan Astronomical Observatory, Chinese Academy of Sciences. It is designed to accurately observe the velocity and magnetic fields of the Sun. The present configuration of this spectrometer allows us to simultaneously observe three different solar spectral lines. This work is dedicated to showing the first observations carried out in both the Hα and Ca II 8542 Å lines. We give a detailed description of the data reduction process, focusing on the retrieval of a flat field from the high-resolution spectral data. Two different methods are also performed and compared to eliminate the residual fringe in the reduced data. The real spectral resolution and instrumental profile are analyzed based on the final results, which indicate that this spectrometer presently satisfies the expected performance and it is ready for further scientific observations.

Calculation of the Effect of Opacity on the Solar Spectral Lines of CIV

In this paper, using the intensity ratio between two spectral lines arising from the common lower level, we determine the optical depth of the solar atmosphere of CIV emission lines. The optical depth at the line center of the (1/2–3/2) component is calculated to be 0.05 at the center of the solar disc. By introducing a measured abundance of carbon and the results of ionization balance calculations, we estimate the line-of-sight physical thickness of the regions emitting CIV lines as Heff = 27 km, and we estimate the population density of the lower level to be 1.8 × 106 cm−3.

MODELING THE QUANTUM INTERFERENCE SIGNATURES OF THE Ba II D24554 Å LINE IN THE SECOND SOLAR SPECTRUM

Quantum interference effects play a vital role in shaping the linear polarization profiles of solar spectral lines. The Ba II D2 line at 4554 A is a prominent example, where the F-state interference effects due to the odd isotopes produce polarization profiles, which are very different from those of the even isotopes that have no F-state interference. It is therefore necessary to account for the contributions from the different isotopes to understand the observed linear polarization profiles of this line. Here we do radiative transfer modeling with partial frequency redistribution (PRD) of such observations while accounting for the interference effects and isotope composition. The Ba II D2 polarization profile is found to be strongly governed by the PRD mechanism. We show how a full PRD treatment succeeds in reproducing the observations, while complete frequency redistribution (CRD) alone fails to produce polarization profiles that have any resemblance with the observed ones. However, we also find that the line center polarization is sensitive to the temperature structure of the model atmosphere. To obtain a good fit to the line center peak of the observed Stokes Q/I profile, a small modification of the FALX model atmosphere is needed, by lowering the temperature in the line-forming layers. Because of the pronounced temperature sensitivity of the Ba II D2 line it may not be a suitable tool for Hanle magnetic-field diagnostics of the solar chromosphere, because there is currently no straightforward way to separate the temperature and magnetic-field effects from each other.