Tiago M. D. Pereira

The Formation of IRIS Diagnostics. II. The Formation of the Mg II h&k Lines in the Solar Atmosphere

NASAs Interface Region Imaging Spectrograph (IRIS) small explorer mission will study how the solar atmosphere is energized. IRIS contains an imaging spectrograph that covers the Mg II h&k lines as well as a slit-jaw imager centered at Mg II k. Understanding the observations requires forward modeling of Mg II h&k line formation from 3D radiation-MHD models. We compute the vertically emergent h&k intensity from a snapshot of a dynamic 3D radiation-MHD model of the solar atmosphere, and investigate which diagnostic information about the atmosphere is contained in the synthetic line profiles. We find that the Doppler shift of the central line depression correlates strongly with the vertical velocity at optical depth unity, which is typically located less than 200 km below the transition region (TR). By combining the Doppler shifts of the h and the k line we can retrieve the sign of the velocity gradient just below the TR. The intensity in the central line depression is anticorrelated with the formation height, especially in subfields of a few square Mm. This intensity could thus be used to measure the spatial variation of the height of the transition region. The intensity in the line-core emission peaks correlates with the temperature at its formation height, especially for strong emission peaks. The peaks can thus be exploited as a temperature diagnostic. The wavelength difference between the blue and red peaks provides a diagnostic of the velocity gradients in the upper chromosphere. The intensity ratio of the blue and red peaks correlates strongly with the average velocity in the upper chromosphere. We conclude that the Mg II h&k lines are excellent probes of the very upper chromosphere just below the transition region, a height regime that is impossible to probe with other spectral lines.

The Formation of IRIS Diagnostics. I. A Quintessential Model Atom of Mg II and General Formation Properties of the Mg II h&k Lines

NASAs Interface Region Imaging Spectrograph (IRIS) space mission will study how the solar atmosphere is energized. IRIS contains an imaging spectrograph that covers the Mg II h&k lines as well as a slit-jaw imager centered at Mg II k. Understanding the observations will require forward modeling of Mg II h&k line formation from 3D radiation-MHD models. This paper is the first in a series where we undertake this forward modeling. We discuss the atomic physics pertinent to h&k line formation, present a quintessential model atom that can be used in radiative transfer computations and discuss the effect of partial redistribution (PRD) and 3D radiative transfer on the emergent line profiles. We conclude that Mg II h&k can be modeled accurately with a 4-level plus continuum Mg II model atom. Ideally radiative transfer computations should be done in 3D including PRD effects. In practice this is currently not possible. A reasonable compromise is to use 1D PRD computations to model the line profile up to and including the central emission peaks, and use 3D transfer assuming complete redistribution to model the central depression.

Oxygen lines in solar granulation - II. Centre-to-limb variation, NLTE line formation, blends, and the solar oxygen abundance

Context. There is a lively debate about the solar oxygen abundance and the role of 3D models in its recent downward revision. These models have been tested using high-resolution solar atlases of flux and disk-centre intensity. Further testing can be done using centreto-limb variations. Aims. Using high-resolution and high S/N observations of neutral oxygen lines across the solar surface, we seek to test that the 3D and 1D models reproduce their observed centre-to-limb variation. In particular we seek to assess whether the latest generation of 3D hydrodynamical solar model atmospheres and NLTE line formation calculations are appropriate to derive the solar oxygen abundance. Methods. We use our recent observations of O i 777 nm, O i 615.81 nm, [O i] 630.03 nm, and nine lines of other elements for five viewing angles 0.2 ≤ μ ≤ 1 of the quiet solar disk. We compared them with the predicted line profiles from the 3D and 1D models computed with the most up-to-date line formation codes and line data and allowing for departures of LTE. The centre-to-limb variation of the O i 777 nm lines is also used to obtain an empirical correction for the poorly known efficiency of the inelastic collisions with H i. Results. The 3D model generally reproduces the centre-to-limb observations of the lines very well, particularly the oxygen lines. From the O i 777 nm lines we find that the classical Drawin recipe slightly overestimates H i collisions (S H ≈ 0.85 agrees best with the observations). The limb observations of the O i 615.82 nm line allow us to identify a previously unknown contribution of molecules for this line, prevalent at the solar limb. A detailed treatment of the [O i] 630.03 nm line that includes the recent nickel abundance shows that the 3D modelling closely agrees with the observations. The derived oxygen abundances with the 3D model are 8.68 (777 nm lines), 8.66 (630.03 nm line), and 8.62 (615.82 nm line). Conclusions. These additional tests have reinforced the trustworthiness of the 3D model and line formation for abundance analyses.

How realistic are solar model atmospheres

Context. Recently, new solar model atmospheres have been developed to replace classical 1D local thermodynamical equilibrium (LTE) hydrostatic models and used to for example derive the solar chemical composition. Aims. We aim to test various models against key observational constraints. In particular, a 3D model used to derive the solar abundances, a 3D magnetohydrodynamical (MHD) model (with an imposed 10 mT vertical magnetic field), 1D NLTE and LTE models from the PHOENIX project, the 1D MARCS model, and the 1D semi-empirical model of Holweger & Muller. Methods. We confronted the models with observational diagnostics of the temperature profile: continuum centre-to-limb variations (CLVs), absolute continuum fluxes, and the wings of hydrogen lines. We also tested the 3D models for the intensity distribution of the granulation and spectral line shapes. Results. The predictions from the 3D model are in excellent agreement with the continuum CLV observations, performing even better than the Holweger & Muller model (constructed largely to fulfil such observations). The predictions of the 1D theoretical models are worse, given their steeper temperature gradients. For the continuum fluxes, predictions for most models agree well with the observations. No model fits all hydrogen lines perfectly, but again the 3D model comes ahead. The 3D model also reproduces the observed continuum intensity fluctuations and spectral line shapes very well. Conclusions. The excellent agreement of the 3D model with the observables reinforces the view that its temperature structure is realistic. It outperforms the MHD simulation in all diagnostics, implying that recent claims for revised abundances based on MHD modelling are premature. Several weaknesses in the 1D hydrostatic models (theoretical and semi-empirical) are exposed. The di erences between the PHOENIX LTE and NLTE models are small. We conclude that the 3D hydrodynamical model is superior to any of the tested 1D models, which gives further confidence in the solar abundance analyses based on it.

On the prevalence of small-scale twist in the solar chromosphere and transition region

The solar chromosphere and transition region (TR) form an interface between the Sun’s surface and its hot outer atmosphere. There, most of the nonthermal energy that powers the solar atmosphere is transformed into heat, although the detailed mechanism remains elusive. High-resolution (0.33–arc second) observations with NASA’s Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere and TR that are replete with twist or torsional motions on sub–arc second scales, occurring in active regions, quiet Sun regions, and coronal holes alike. We coordinated observations with the Swedish 1-meter Solar Telescope (SST) to quantify these twisting motions and their association with rapid heating to at least TR temperatures. This view of the interface region provides insight into what heats the low solar atmosphere.

The Formation of IRIS Diagnostics. III. Near-ultraviolet Spectra and Images

The Mg II hk the relations between the spectral features and atmospheric properties are mostly unchanged. The peak separation is the most affected diagnostic, but mainly due to limitations of the simulation. The effects of noise start to be noticeable at a signal-to-noise ratio (S/N) of 20, but we show that with noise filtering one can obtain reliable diagnostics at least down to a S/N of 5. The many photospheric lines present in the NUV window provide velocity information for at least eight distinct photospheric heights. Using line-free regions in the h&k far wings, we derive good estimates of photospheric temperature for at least three heights. Both of these diagnostics, in particular the latter, can be obtained even at S/Ns as low as 5.

The Unresolved Fine Structure Resolved: IRIS Observations of the Solar Transition Region

The heating of the outer solar atmospheric layers, i.e., the transition region and corona, to high temperatures is a long-standing problem in solar (and stellar) physics. Solutions have been hampered by an incomplete understanding of the magnetically controlled structure of these regions. The high spatial and temporal resolution observations with the Interface Region Imaging Spectrograph (IRIS) at the solar limb reveal a plethora of short, low-lying loops or loop segments at transition-region temperatures that vary rapidly, on the time scales of minutes. We argue that the existence of these loops solves a long-standing observational mystery. At the same time, based on comparison with numerical models, this detection sheds light on a critical piece of the coronal heating puzzle.The heating of the outer solar atmospheric layers, i.e., the transition region and corona, to high temperatures is a long-standing problem in solar (and stellar) physics. Solutions have been hampered by an incomplete understanding of the magnetically controlled structure of these regions. The high spatial and temporal resolution observations with the Interface Region Imaging Spectrograph (IRIS) at the solar limb reveal a plethora of short, low-lying loops or loop segments at transition-region temperatures that vary rapidly, on the time scales of minutes. We argue that the existence of these loops solves a long-standing observational mystery. At the same time, based on comparison with numerical models, this detection sheds light on a critical piece of the coronal heating puzzle.

THE FORMATION OF IRIS DIAGNOSTICS. IV. THE Mg II TRIPLET LINES AS A NEW DIAGNOSTIC FOR LOWER CHROMOSPHERIC HEATING

A triplet of subordinate lines of Mg II exists in the region around the h&k lines. In solar spectra these lines are seen mostly in absorption, but in some cases can become emission lines. The aim of this work is to study the formation of this triplet, and investigate any diagnostic value they can bring. Using 3D radiative magnetohydrodynamic simulations of quiet Sun and flaring flux emergence, we synthesize spectra and investigate how spectral features respond to the underlying atmosphere. We find that emission in the lines is rare and is typically caused by a steep temperature increase in the lower chromosphere (above 1500 K, with electron densities above 10

Oxygen lines in solar granulation I. Testing 3D models against new observations with high spatial and spectral resolution

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ON THE TEMPORAL EVOLUTION OF SPICULES OBSERVED WITH IRIS, SDO, AND HINODE

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