arXiv subjects

Bart De Pontieu

Publications and source records attributed to Bart De Pontieu.

At least 37 records · Page 2Linked to original sources

arXiv2025

Determining the Magnetic Field in the Atmosphere of a Solar Active Region Observed by the CLASP2.1 Sounding Rocket Experiment

We determine magnetic fields from the photosphere to the upper chromosphere combining data from the Hinode satellite and the CLASP2.1 sounding rocket experiment. CLASP2.1 provided polarization profiles of the Mg~{|sc ii} $h$ and $k$ lines, as well as of the Mn~{|sc i} lines around 2800~{|AA}, across various magnetic structures in an active region, containing a plage, a pore, and the edges of a sunspot penumbra. By applying the Weak-Field Approximation (WFA) to the circular polarization profiles of these spectral lines, we obtain a longitudinal magnetic field map at three different heights in the chromosphere (lower, middle, and upper). This is complemented by data from Hinode (photospheric magnetic field), IRIS, and SDO (high-spatial-resolution observations of the chromosphere and corona). We quantify the height expansion of the plage magnetic fields and find that the magnetic fields expand significantly in the middle chromosphere, shaping the moss observed above in the transition region and corona. We identified an area with polarity reversal at the upper chromosphere around the edge of the pore, suggesting the presence of a magnetic discontinuity in the upper chromosphere. Transient and recurrent jet-like events are observed in this region, likely driven by magnetic reconnection. Around the penumbral edge, we find large-scale magnetic fields corresponding to the superpenumbral fibrils seen in the upper chromosphere. In the superpenumbral fibrils, we find Zeeman-induced linear polarization signals, suggesting the presence of a significantly inclined magnetic field, as strong as 1000~G in the upper chromosphere.

astro-ph.SR↗

arXiv2025

Quantifying Suppression of Solar Surface Magnetic Flux Advection with Increasing Field Strength

One of the main theories for heating of the solar corona is based on the idea that solar convection shuffles and tangles magnetic field lines to make many small-scale current sheets that, via reconnection, heat coronal loops. Tiwari et al 2017 present evidence that, besides depending on loop length and other factors, the brightness of a coronal loop depends on the field strength in the loop feet and the freedom of convection in the feet. While it is known that strong solar magnetic fields suppress convection, the decrease in the speed of horizontal advection of magnetic flux with increasing field strength has not been quantified before. We quantify that trend by analyzing 24hours of HMI SHARP vector magnetograms of each of six sunspot active regions and their surroundings. Using Fourier Local Correlation Tracking, we estimate the horizontal advection speed of the magnetic flux at each pixel in which the vertical component of the magnetic field strength (Bz) is well above (greater than or equal to 150 G) noise level. We find that the average horizontal advection speed of magnetic flux steadily decreases as Bz increases, from 110 pm 3 meters per sec for 150 G (in network and plage) to 10 pm 4 meters per sec for 2500 G (in sunspot umbra). The trend is well fit by a fourth degree polynomial. These results quantitatively confirm the expectation that magnetic flux advection is suppressed by increasing magnetic field strength. The presented quantitative relation should be useful for future MHD simulations of coronal heating.

astro-ph.SR↗

arXiv2025

On the million-degree signature of spicules

Spicules have often been proposed as substantial contributors toward the mass and energy balance of the solar corona. While their transition region (TR) counterpart has unequivocally been established over the past decade, the observations concerning the coronal contribution of spicules have often been contested. This is mainly attributed to the lack of adequate coordinated observations, their small spatial scales, highly dynamic nature, and complex multi-thermal evolution, which are often observed at the limit of our current observational facilities. Therefore, it remains unclear how much heating occurs in association with spicules to coronal temperatures. In this study, we use coordinated high-resolution observations of the solar chromosphere, TR, and corona of a quiet Sun region and a coronal hole with the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) to investigate the (lower) coronal ($\sim$1MK) emission associated with spicules. We perform differential emission measure (DEM) analysis on the AIA passbands using basis pursuit and a newly developed technique based on Tikhonov regularization to probe the thermal structure of the spicular environment at coronal temperatures. We find that the EM maps at 1 MK reveal the presence of ubiquitous, small-scale jets with a clear spatio-temporal coherence with the spicules observed in the IRIS/TR passband. Detailed space-time analysis of the chromospheric, TR, and EM maps show unambiguous evidence of rapidly outward propagating spicules with strong emission (2--3 times higher than the background) at 1 MK. Our findings are consistent with previously reported MHD simulations that show heating to coronal temperatures associated with spicules.

astro-ph.SR↗

arXiv2024

On the Nature of Nonthermal Broadening of Spectral Lines Observed by IRIS

The origin of nonthermal broadening in solar spectra is one of the long-standing questions in solar physics. Various processes have been invoked - including unresolved flows, waves, and turbulent processes - but definitive answers are lacking. To investigate the physical processes responsible for nonthermal broadening, we examine its relation with the angle between the magnetic field and the line of sight in three different closed-field regions above plage regions at different locations on the solar disk. We obtained the nonthermal width of transition-region Si IV 1403 Å spectra observed in active regions by the Interface Region Imaging Spectrograph, after subtraction of the thermal and instrumental line broadening. To investigate the dependence of the measured broadening on the viewing angle between the line of sight and magnetic field direction, we determined the magnetic field direction at transition-region heights using nonlinear force-free extrapolations based on the observed photospheric vector magnetic field taken by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We found that the nonthermal broadening shows a correlation with downward motion (redshifts)and alignment between the magnetic field and the observer's line-of-sight direction. Based on the observed correlations, we suggest that velocity gradients within plasma flowing down along the magnetic field may lead to a significant portion of the observed nonthermal broadening of transition-region spectral lines in closed fields above plage regions.

astro-ph.SR↗

arXiv2023

High Resolution Observations of the Low Atmospheric Response to Small Coronal Heating Events in an Active Region Core

High resolution spectral observations of the lower solar atmosphere (chromosphere and transition region) during coronal heating events, in combination with predictions from models of impulsively heated loops, provide powerful diagnostics of the properties of the heating in active region cores. Here we analyze the first coordinated observations of such events with the Interface Region Imaging Spectrograph (IRIS) and the CHROMospheric Imaging Spectrometer (CHROMIS), at the Swedish 1-m Solar Telescope (SST), which provided extremely high spatial resolution and revealed chromospheric brightenings with spatial dimensions down to ~150km. We use machine learning methods (k-means clustering) and find significant coherence in the spatial and temporal properties of the chromospheric spectra, suggesting, in turn, coherence in the spatial and temporal distribution of the coronal heating. The comparison of IRIS and CHROMIS spectra with simulations suggest that both non-thermal electrons with low energy (low-energy cutoff ~5keV) and direct heating in the corona transported by thermal conduction contribute to the heating of the low atmosphere. This is consistent with growing evidence that non-thermal electrons are not uncommon in small heating events (nano- to micro-flares), and that their properties can be constrained by chromospheric and transition region spectral observations.

astro-ph.SR↗

arXiv2023

An optically thin view of the solar chromosphere from observations of the O I 1355Å spectral line

The O I 1355Å spectral line is one of the only optically thin lines that are both routinely observed and thought to be formed in the chromosphere. We present analysis of a variety of observations of this line with the Interface Region Imaging Spectrograph (IRIS), and compare it with other IRIS diagnostics as well as diagnostics of the photospheric magnetic field. We utilize special deep exposure modes on IRIS and provide an overview of the statistical properties of this spectral line for several different regions on the Sun. We analyze the spatio-temporal variations of the line intensity, and find that it is often significantly enhanced when and where magnetic flux of opposite polarities cancel. Significant emission occurs in association with chromospheric spicules. Because of the optically thin nature of the O I line, the non-thermal broadening can provide insight into unresolved small-scale motions. We find that the non-thermal broadening is modest, with typical values of 5-10 km/s, and shows some center-to-limb variation, with a modest increase towards the limb. The dependence with height of the intensity and line broadening off-limb is compatible with the line broadening being dominated by the superposition of Alfvén waves on different structures. The non-thermal broadening shows a modest but significant enhancement above locations that are in between photospheric magnetic flux concentrations in plage, i.e., where the magnetic field is likely to be more inclined with respect to the line-of-sight. Our measurements provide strict constraints on future theoretical models of the chromosphere.

astro-ph.SR↗

arXiv2023

Slow Solar Wind Connection Science during Solar Orbiter's First Close Perihelion Passage

The Slow Solar Wind Connection Solar Orbiter Observing Plan (Slow Wind SOOP) was developed to utilise the extensive suite of remote sensing and in situ instruments on board the ESA/NASA Solar Orbiter mission to answer significant outstanding questions regarding the origin and formation of the slow solar wind. The Slow Wind SOOP was designed to link remote sensing and in situ measurements of slow wind originating at open-closed field boundaries. The SOOP ran just prior to Solar Orbiter's first close perihelion passage during two remote sensing windows (RSW1 and RSW2) between 2022 March 3-6 and 2022 March 17-22, while Solar Orbiter was at a heliocentric distance of 0.55-0.51 and 0.38-0.34 au from the Sun, respectively. Coordinated observation campaigns were also conducted by Hinode and IRIS. The magnetic connectivity tool was used, along with low latency in situ data, and full-disk remote sensing observations, to guide the target pointing of Solar Orbiter. Solar Orbiter targeted an active region complex during RSW1, the boundary of a coronal hole, and the periphery of a decayed active region during RSW2. Post-observation analysis using the magnetic connectivity tool along with in situ measurements from MAG and SWA/PAS, show that slow solar wind, with velocities between 210 and 600 km/s, arrived at the spacecraft originating from two out of the three of the target regions. The Slow Wind SOOP, despite presenting many challenges, was very successful, providing a blueprint for planning future observation campaigns that rely on the magnetic connectivity of Solar Orbiter.

astro-ph.SR↗

arXiv2023

A Statistical Study of IRIS Observational Signatures of Nanoflares and Non-thermal Particles

Nanoflares are regarded as one of the major mechanisms of magnetic energy release and coronal heating in the solar outer atmosphere. We conduct a statistical study on the response of the chromosphere and transition region to nanoflares, as observed by the Interface Region Imaging Spectrograph (IRIS), by using an algorithm for the automatic detection of these events. The initial atmospheric response to these small heating events is observed, with IRIS, as transient brightening at the footpoints of coronal loops heated to high temperatures (>4 MK). For four active regions, observed over 143 hours, we detected 1082 footpoint brightenings under the IRIS slit, and for those we extracted physical parameters from the IRIS Mg II and Si IV spectra that are formed in the chromosphere and transition region, respectively. We investigate the distribution of the spectral parameters, and the relationship between the parameters, also comparing them with predictions from RADYN numerical simulations of nanoflare-heated loops. We find that these events, and the presence of non-thermal particles, tend to be more frequent in flare productive active regions, and where the hot Atmospheric Imaging Assembly 94 Å emission is higher. We find evidence for highly dynamic motions characterized by strong Si IV non-thermal velocity (not dependent on the heliocentric x coordinate, i.e., on the angle between the magnetic field and the line-of-sight) and asymmetric Mg II spectra. These findings provide tight new constraints on the properties of nanoflares, and non-thermal particles, in active regions, and their effects on the lower atmosphere.

astro-ph.SR↗

arXiv2023

The chromosphere underneath a Coronal Bright Point

Coronal Bright Points (CBPs) are sets of small-scale coronal loops, connecting opposite magnetic polarities, primarily characterized by their enhanced extreme-ultraviolet (EUV) and X-ray emission. Being ubiquitous, they are thought to play an important role in heating the solar corona. We aim at characterizing the barely-explored chromosphere underneath CBPs, focusing on the related spicular activity and on the effects of small-scale magnetic flux emergence on CBPs. We used high-resolution observations of a CBP in H$β$ and Fe I 617.3 nm from the Swedish 1-m Solar Telescope (SST) in coordination with the Solar Dynamics Observatory (SDO). This work presents the first high-resolution observation of spicules imaged in H$β$. The spicules were automatically detected using advanced image processing techniques, which were applied to the Dopplergrams derived from H$β$. Here we report their abundant occurrence close to the CBP ``footpoints", and find that the orientation of such spicules is aligned along the EUV loops, indicating that they constitute a fundamental part of the whole CBP magnetic structure. Spatio-temporal analysis across multiple channels indicates that there are coronal propagating disturbances associated with the studied spicules, producing transient EUV intensity variations of the individual CBP loops. Two small-scale flux emergence episodes appearing below the CBP were analyzed; one of them leading to quiet-sun Ellerman bombs and enhancing the nearby spicular activity. This paper presents unique evidence of the tight coupling between the lower and upper atmosphere of a CBP, thus helping to unravel the dynamic phenomena underneath CBPs and their impact on the latter.

astro-ph.SR↗

arXiv2022

Numerical simulations and observations of Mg II in the solar chromosphere

The Mg II h&k lines are amongst the best diagnostic tools of the upper solar chromosphere. This region of the atmosphere is of particular interest as it is the lowest region of the Sun's atmosphere where the magnetic field is dominant in the energetics and dynamics, defining its structure. While highly successful in the photosphere and lower to mid chromosphere, numerical models have produced syntheticMg II lines that do not match the observations well. We present a number of large scale models with magnetic field topologies representative of the quiet Sun, ephemeral flux regions and plage, and also models where the numerical resolution is high and where we go beyond the MHD paradigm. The results of this study show models with a much improved correspondence with \iris\ observations both in terms of intensities and widths, especially underscoring the importance of chromospheric mass loading and of capturing the magnetic field topology and evolution in simulations. This comes in addition to the importance of capturing the generation of small scale velocity fields and including non-equilibrium ionization and ion-neutral interaction effects. Understanding and modeling all these effects and their relative importance is necessary in order to reproduce observed spectral features.

astro-ph.SR↗

arXiv2022

The impact of multi-fluid effects in the solar chromosphere on the ponderomotive force under LTE and NEQ ionization conditions

The ponderomotive force is suggested to be the main mechanism to produce the so-called first ionization potential (FIP) effect - the enrichment of low FIP elements observed in the outer solar atmosphere. It is well known that the ionization of these elements occurs within the chromosphere. Therefore, this phenomenon is intimately tied to the plasma state in the chromosphere and the corona. In addition, the chromosphere is a highly complex region with a large variation in the ion-neutral collision frequencies, and hydrogen and helium ionization is largely out of equilibrium. For this study, we combine IRIS observations, a single fluid 2.5D radiative magnetohydrodynamics (MHD) model of the solar atmosphere, including ion-neutral interaction effects and non-equilibrium (NEQ) ionization effects, and a novel multi-fluid multi-species (MFMS) numerical code (Ebysus). Nonthermal velocities of Si IV measured from IRIS spectra can provide an upper limit for the strength of any high-frequency Alfven waves. With the single-fluid model, we investigate the possible impact of NEQ ionization within the region where the FIP may occur and the plasma properties in those regions. These models suggest that regions with strong enhanced network and type II spicules are associated with the presence of large ponderomotive forces. From the plasma properties from the single-fluid MHD model and IRIS observations, we initialize our multi-fluid models to investigate the multi-fluid effects on the ponderomotive force associated with Alfven waves. Our multi-fluid analysis reveals that collisions and NEQ ionization effects dramatically impact the behavior of the ponderomotive force in the chromosphere, and existing theories may need to be revisited.

astro-ph.SR↗

arXiv2022

$IRIS^{2+}$: A comprehensive database of stratified thermodynamic models in the low solar atmosphere

Our knowledge of the low solar atmosphere, i.e., the photosphere and chromosphere, is based on the knowledge gained from the observations and the theoretical and numerical modeling of these layers. In this sense, the thermodynamical and magnetic semi-empirical models of the solar atmosphere have significantly contributed to the advance in the understanding of the physics of the Sun. In the past, many of these models have been used as a reference that helps us to, e.g., constrain the theoretical and numerical modeling, or to verify the goodness of physical parameters obtained from the inversion of the spectral lines. Nevertheless, semi-empirical models are quite limited by the assumptions that are inherent to the approach and do not necessarily provide an accurate view of the instantaneous and local thermodynamic conditions in the solar atmosphere. In this work, we provide an extensive collection of thermodynamic model atmospheres for active regions (ARs) obtained from the simultaneous inversion of 6 lines sensitive to changes in the thermodynamical conditions in the chromosphere, and another 6 lines sensitive to changes in the thermodynamical conditions in the photospere. These inversions were made using 320 representative profiles (RP) obtained by clustering the profiles in the umbra, penumbra, pore-like, plage, and surrounding quiet-sun in 126 active regions. Due to the simultaneous inversion of the selected lines, the resulting representative model atmosphere (RMA) samples the thermodynamics from the bottom of the photosphere to the top of the chromosphere. As a result, this database, named $IRIS^{2+}$ and formed by $40,320$ RP-RMA pairs, represents the most comprehensive collection of stratified-in-optical-depth thermodynamic models of the low solar atmosphere.

astro-ph.SR↗

arXiv2022

Small-scale chromospheric spectropolarimetric observables in the internetwork from a 3D radiative MHD model

The presence of the magnetic field is critical to transport energy through the solar atmosphere. The new generation of telescopes will provide new insight into how the magnetic field arrives into the chromosphere and its role in the energy balance of the solar atmosphere. We have used a 3D radiative MHD numerical model of the solar atmosphere with high spatial resolution (4~km) calculated with the Bifrost code. This code solves the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along magnetic field lines. The model shows how the lower chromosphere in the internetwork, a region dominated by magneto-acoustic shocks and where plasma beta is greater than 1, is able to generate magnetic field in-situ Martinez-Sykora et al 2019. We have synthesized full-polarimetric Stokes profiles from this model for several spectral lines formed in the photosphere and the chromosphere. These synthetic profiles illustrate the types of observables expected from DKIST and IRIS. Our work provides insight into how to interpret observations from these observatories. We find that in order to discern the chromospheric magnetic observables it is crucial to compensate for Doppler shift rather than use fix wavelength range.

astro-ph.SR↗

arXiv2022

Chromospheric and Coronal heating in active region plage by dissipation of currents from braiding

It remains unclear which physical processes are responsible for the dramatic increase with height of the temperature in stellar atmospheres, known as the chromospheric ($\sim$10,000 K) and coronal (several million K) heating problems. Statistical studies of sun-like stars reveal that chromospheric and coronal emissions are correlated on a global scale, constraining, in principle, theoretical models of potential heating mechanisms. However, so far, spatially resolved observations of the Sun have surprisingly failed to show a similar correlation on small spatial scales, leaving models poorly constrained. Here we use unique coordinated high-resolution observations of the chromosphere (from the Interface Region Imaging Spectrograph or IRIS satellite) and low corona (from the Hi-C 2.1 sounding rocket), and machine-learning based inversion techniques to show a strong correlation on spatial scales of a few hundred km between heating in the chromosphere and low corona for regions with strong magnetic field ("plage"). These results are compatible with recent advanced 3D radiative magnetohydrodynamic simulations in which dissipation of current sheets formed due to the braiding of the magnetic field lines deep in the atmosphere is responsible for heating the plasma simultaneously to chromospheric and coronal temperatures. Our results provide deep insight into the nature of the heating mechanism in solar active regions.

astro-ph.SR↗

arXiv2022

Chromospheric Heating from Local Magnetic Growth and Ambipolar Diffusion Under Non-Equilibrium Conditions

The heating of the chromosphere in internetwork regions remains one of the foremost open questions in solar physics. In the present study we tackle this old problem by using a very high spatial-resolution simulation of quiet-Sun conditions performed with radiative MHD numerical models and IRIS observations. We have expanded a previously existing 3D radiative MHD numerical model of the solar atmosphere, which included self-consistently locally driven magnetic amplification in the chromosphere, by adding ambipolar diffusion and time-dependent non-equilibrium hydrogen ionization to the model. The energy of the magnetic field is dissipated in the upper chromosphere, providing a large temperature increase due to ambipolar diffusion and the non-equilibrium ionization (NEQI). At the same time, we find that adding the ambipolar diffusion and NEQI in the simulation has a minor impact on the local growth of the magnetic field in the lower chromosphere and its dynamics. Our comparison between synthesized Mg II profiles from these high spatial resolution models, with and without ambipolar diffusion and NEQI, and quiet Sun and coronal hole observations from IRIS now reveal a better correspondence. The intensity of profiles is increased and the line cores are slightly broader when ambipolar diffusion and NEQI effects are included. Therefore, the Mg II profiles are closer to those observed than in previous models, but some differences still remain.

astro-ph.SR↗

arXiv2022

Coronal Abundances in an Active Region: Evolution and Underlying Chromospheric and Transition Region Properties

The element abundances in the solar corona and solar wind are often different from those of the solar photosphere, typically with a relative enrichment of elements with low first ionization potential (FIP effect). Here we study the spatial distribution and temporal evolution of the coronal chemical composition in an active region (AR) over about 10 days, using Hinode/EIS spectra, and we also analyze coordinated IRIS observations of the chromospheric and transition region emission to investigate any evidence of the footprints of the FIP effect in the lower atmosphere. To derive the coronal abundances we use a spectral inversion method recently developed for the MUSE investigation (Cheung et al. 2019, De Pontieu et al. 2020). We find that in the studied active region (AR 12738) the coronal FIP bias presents significant spatial variations, with its highest values (~ 2.5-3.5) in the outflow regions at the boundary of the AR, but typically modest temporal variability. Some moss regions and some regions around the AR sunspot show enhanced FIP (~ 2-2.5) with respect to the AR core, which has only a small FIP bias of ~1.5. The FIP bias appears most variable in some of these moss regions. The IRIS observations reveal that the chromospheric turbulence, as derived from IRIS2 inversions of the Mg II spectra, is enhanced in the outflow regions characterized by the high FIP bias, providing significant new constraints to both models aimed at explaining the formation of AR outflows and models of chemical fractionation.

astro-ph.SR↗

arXiv2022

Chromospheric thermodynamic conditions from inversions of complex Mg II h&k profiles observed in flares

The flare activity of the Sun has been studied for decades, using both space- and ground-based telescopes. In particular, the Interface Region Imaging Spectrograph (IRIS) provides unique diagnostics to investigate the thermodynamics of flares in the solar atmosphere. The Mg II h&k and Mg II UV triple lines provide key information about the thermodynamics of low to upper chromosphere, while the C II 1334 & 1335 AA lines cover the upper-chromosphere and low transition region. The Mg II h&k and Mg II UV triplet lines show a peculiar, pointy shape before and during the flare activity. The physical interpretation that can explain these profiles has remained elusive. In this paper, we show the results of a non-LTE inversion of such peculiar profiles. To better constrain the atmospheric conditions, the Mg II h&k and Mg II UV triple lines are simultaneously inverted with the C II 1334 & 1335 AA lines. This combined inversion leads to more accurate derived thermodynamic parameters, especially the temperature and the turbulent motions (micro-turbulence velocity). We use the inversion code STiC to look for the best fit between the observed profile and a synthetic profile obtained by solving the radiative transfer problem considering non-local thermodynamic equilibrium and partial frequency redistribution of the radiation due to scattered photons. We are able to conclude that these unique, pointy profiles are associated with a simultaneous increase of the temperature and the electron density in the chromosphere, while the micro-turbulence velocity has values between 5-15 km/s, which seem to be more realistic values than the ones suggested in previous work. More importantly, the line-of-sight velocity shows a large gradient along the optical depth in the high chromosphere. This seems to be the parameter that gives the pointy aspect to these profiles.

astro-ph.SR↗

arXiv2022

Recovering thermodynamics from spectral profiles observed by IRIS (II): improved calculation of the uncertainties based on Monte Carlo experiments

Observations by the Interface Region Imaging Spectrograph (IRIS) in the Mg II h & k spectral lines have provided a new diagnostic window towards the knowledge of the complex physical conditions in the solar chromosphere. Theoretical efforts focused on understanding the behavior of these lines have allowed us to obtain a better and more accurate vision of the chromosphere. These efforts include forward modeling, numerical simulations, and inversions. In this paper, we focus our attention on the uncertainties associated with the thermodynamic model atmosphere obtained after the inversion of the Mg II h & k lines. We have used ~ 50;000 synthetic representative profiles of the IRIS2 database to characterize the most important source of uncertainties in the inversion process, viz.: the inherent noise of the observations, the random initialization of process, and the selection criteria in a high-dimensional space. We have applied a Monte Carlo approach to this problem. Thus, for a given synthetic representative profile, we have created five randomized noise realizations (representative of the most popular exposure times in the IRIS observations), and inverted these profiles five times with different inversion initializations. The resulting 25 inverted profiles, fits to noisy data, and model atmospheres are then used to determine the uncertainty in the model atmosphere, based on the standard deviation and empirical selection criteria for the goodness of fit. With this approach, the new uncertainties of the models available in the IRIS2 database are more reliable at the optical depths where the Mg II h & k lines are sensitive to changes in the thermodynamics.

astro-ph.SR↗