arXiv subjects

G. Del Zanna

Publications and source records attributed to G. Del Zanna.

At least 37 records · Page 2Linked to original sources

arXiv2021

The influence of photo-induced processes and charge transfer on carbon and oxygen in the lower solar atmosphere

To predict line emission in the solar atmosphere requires models which are fundamentally different depending on whether the emission is from the chromosphere or the corona. At some point between the two regions, there must be a change between the two modelling regimes. Recent extensions to the coronal modelling for carbon and oxygen lines in the solar transition region have shown improvements in the emission of singly- and doubly-charged ions, along with Li-like ions. However, discrepancies still remain, particularly for singly-charged ions and intercombination lines. The aim of this work is to explore additional atomic processes that could further alter the charge state distribution and the level populations within ions, in order to resolve some of the discrepancies. To this end, excitation and ionisation caused by both the radiation field and by atom-ion collisions have been included, along with recombination through charge transfer. The modelling is carried out using conditions which would be present in the quiet Sun, which allows an assessment of the part atomic processes play in changing coronal modelling, separately from dynamic and transient events taking place in the plasma. The effect the processes have on the fractional ion populations are presented, as well as the change in level populations brought about by the new excitation mechanisms. Contribution functions of selected lines from low charge states are also shown, to demonstrate the extent to which line emission in the lower atmosphere could be affected by the new modelling.

astro-ph.SR↗

arXiv2021

CHIANTI -- an atomic database for emission lines -- Paper XVI: Version 10, further extensions

We present version 10 of the CHIANTI package. In this release, we provide updated atomic models for several helium-like ions and for all the ions of the beryllium, carbon and magnesium isoelectronic sequences that are abundant in astrophysical plasmas. We include rates from large-scale atomic structure and scattering calculations that are in many cases a significant improvement over the previous version, especially for the Be-like sequence, which has useful line diagnostics to measure the electron density and temperature. We have also added new ions and updated several of them with new atomic rates and line identifications. Also, we have added several improvements to the IDL software, to speed up the calculations and to estimate the suppression of dielectronic recombination.

physics.atom-ph↗

arXiv2020

R-matrix electron-impact excitation data for the N-like iso-electronic sequence

Spectral lines from N-like ions can be used to measure the temperature and density of various types of astrophysical plasmas. The atomic databases of astrophysical plasma modelling codes still have room for improvement in their electron-impact excitation data sets for N-like ions, especially $R$-matrix data. This is particularly relevant for future observatories (e.g. Arcus) which will host high-resolution spectrometers. We aim to obtain level-resolved effective collision strengths for all transitions up to $nl=5d$ over a wide range of temperatures for N-like ions from O II to Zn XXIV (i.e., O$^{+}$ to Zn$^{23+}$) and to assess the accuracy of the present work. We also examine the impact of our new data on plasma diagnostics by modelling solar observations with CHIANTI. We have carried-out systematic $R$-matrix calculations for N-like ions which included 725 fine-structure target levels in both the configuration interaction target and close-coupling collision expansions. The $R$-matrix intermediate coupling frame transformation method was used to calculate the collision strengths, while the AUTOSTRUCTURE code was used for the atomic structures. We compare the present results for selected ions with those in archival databases and the literature. The comparison covers energy levels, oscillator strengths, and effective collision strengths. We show examples of improved plasma diagnostics when compared to CHIANTI models which use only distorted wave data as well as some which use previous $R$-matrix data. The electron-impact excitation data are archived according to the Atomic Data and Analysis Structure (ADAS) data class it adf04 and will be available in OPEN-ADAS. The data can be used to improve the atomic databases for astrophysical plasma diagnostics.

astro-ph.SR↗

arXiv2020

Effects of density on the oxygen ionisation equilibrium in collisional plasmas

The ion populations most frequently adopted for diagnostics in collisional plasmas are derived from the density independent, coronal approximation. In higher density, lower temperature conditions, ionisation rates are enhanced once metastable levels become populated, and recombination rates are suppressed if ions recombine into Rydberg levels. As a result, the formation temperatures of ions shift, altering the diagnostics of the plasma. To accurately model the effect of ionisation from metastable levels, new electron impact, ionisation cross sections have been calculated for oxygen, both for direct ionisation and excitation--auto-ionisation of the ground and metastable levels. The results have been incorporated into collisional radiative modelling to show how the ionisation equilibrium of oxygen changes once metastable levels become populated. Suppression of dielectronic recombination has been estimated and also included in the modelling, demonstrating the shifts with density in comparison to the coronal approximation. The final results for the ionisation equilibrium are used in differential emission measure modelling to predict line intensities for many lines emitted by O II-VI in the solar transition region. The predictions show improved agreement by 15-40% for O II, O VI and the inter-combination lines of O III-V, when compared to results from coronal approximation modelling. While there are still discrepancies with observations of these lines, this could, to a large part, be explained by variability in the observations.

astro-ph.SR↗

arXiv2020

Fe III emission in quasars: evidence for a dense turbulent medium

Recent improvements to atomic energy-level data allow, for the first time, accurate predictions to be made for the Fe III line emission strengths in the spectra of luminous, $L_\text{bol}=10^{46}-10^{48}$ erg/s, Active Galactic Nuclei. The Fe III emitting gas must be primarily photoionized, consistent with observations of line reverberation. We use CLOUDY models exploring a wide range of parameter space, together with 26,500 rest-frame ultraviolet spectra from the Sloan Digital Sky Survey, to constrain the physical conditions of the line emitting gas. The observed Fe III emission is best accounted for by dense ($n_H=10^{14}$ cm$^{-3}$) gas which is microturbulent, leading to smaller line optical depths and fluorescent excitation. Such high density gas appears to be present in the central regions of the majority of luminous quasars. Using our favoured model, we present theoretical predictions for the relative strengths of the Fe III UV34 $λλ$1895,1914,1926 multiplet. This multiplet is blended with the Si III] $λ$1892 and C III] $λ$1909 emission lines and an accurate subtraction of UV34 is essential when using these lines to infer information about the physics of the broad line region in quasars.

astro-ph.GA↗

arXiv2020

Helium line emissivities in the solar corona

We present new collisional-radiative models (CRMs) for helium in the quiescent solar corona, and predict the emissivities of the He and He$^+$ lines to be observed by DKIST, Solar Orbiter, and Proba-3. We discuss in detail the rates we selected for these models, highlighting several shortcomings we have found in previous work. As no previous complete and self-consistent coronal CRM for helium existed, we have benchmarked our largest model at a density of 10$^{6}$ cm$^{-3}$ and temperature of 20,000 K against recent CRMs developed for photoionised nebulae. We then present results for the outer solar corona, using new dielectronic recombination rates we have calculated, which increase the abundance of neutral helium by about a factor of two. We also find that all the optical triplet He I lines, and in particular the well known He I 10830 and 5876 A lines are strongly affected by both photo-excitation and photo-ionisation from the disk radiation, and that extensive CRM models are required to obtain correct estimates. Close to the Sun, at an electron density of 10$^{8}$ cm$^{-3}$ and temperature of 1 MK, we predict the emissivity of the He I 10830 A to be comparable to that of the strong Fe XIII coronal line at 10798 A. However, we expect the He I emissivity to sharply fall in the outer corona, with respect to Fe XIII. We confirm that the He$^+$ Lyman $α$ at 304 A is also significantly affected by photo-excitation and is expected to be detectable as a strong coronal line up to several solar radii.

physics.atom-ph↗

arXiv2020

Large-scale multiconfiguration Dirac-Hartree-Fock calculations for astrophysics: n=4 levels in P-like ions from Mn~XI to Ni~XIV

Using the multiconfiguration Dirac-Hartree-Fock and the relativistic configuration interaction methods, a consistent set of transition energies and radiative transition data for the lowest 546 (623, 701, 745) states of the $3p^4 3d$, $3s 3p^2 3d^2$, $3s 3p^3 4p$, $3s 3p^4$, $3s^2 3d^3$, $3s^2 3p^2 3d$, $3s^2 3p^2 4d$, $3s^2 3p^2 4s$, $3p^3 3d^2$, $3p^5$, $3s 3p 3d^3$, $3s 3p^3 3d$, $3s 3p^3 4s$, $3s^2 3p 3d^2$, %$3s^2 3p^2 4f$, $3s^2 3p^2 4p$, $3s^2 3p^3$ configurations in Mn~XI (Fe~XII, Co~XIII, Ni~XIV) is provided. The comparison between calculated excitation energies for the $n=4$ states and available experimental values for Fe XII indicate that the calculations are highly accurate, with uncertainties of only a few hundred cm$^{-1}$. Lines from these states are prominent in the soft X-rays. With the present calculations, several recent new identifications are confirmed. Other identifications involving $3p^2 4d$ levels in Fe~XII that were found questionable are discussed and a few new assignments are recommended. As some $n=4$ states of the other ions also show large discrepancies between experimental and calculated energies, we reassess their identification. The present study provides highly accurate atomic data for the $n=4$ states of P-like ions of astrophysical interest, for which experimental data are scarce.

physics.atom-ph↗

arXiv2019

R-matrix electron-impact excitation data for the C-like iso-electronic sequence

Emission and absorption features from C-like ions serve as temperature and density diagnostics of astrophysical plasmas. $R$-matrix electron-impact excitation data sets for C-like ions in the literature merely cover a few ions, and often only for the ground configuration. Our goal is to obtain level-resolved effective collision strength over a wide temperature range for C-like ions from \ion{N}{II} to \ion{Kr}{XXXI} (i.e., N$^{+}$ to Kr$^{30+}$) with a systematic set of $R$-matrix calculations. We also aim to assess their accuracy. For each ion, we included a total of 590 fine-structure levels in both the configuration interaction target and close-coupling collision expansion. These levels arise from 24 configurations $2l^3 nl^{\prime}$ with $n=2-4$, $l=0-1$, and $l^{\prime}=0-3$ plus the three configurations $2s^22p5l$ with $l=0-2$. The AUTOSTRUCTURE code was used to calculate the target structure. Additionally, the $R$-matrix intermediate coupling frame transformation method was used to calculate the collision strengths. We compare the present results of selected ions with archival databases and results in the literature. The comparison covers energy levels, transition rates, and effective collision strengths. We illustrate the impact of using the present results on an \ion{Ar}{xiii} density diagnostic for the solar corona. The electron-impact excitation data is archived according to the Atomic Data and Analysis Structure (ADAS) data class adf04 and will be available in OPEN-ADAS. The data will be incorporated into spectral codes, such as CHIANTI and SPEX, for plasma diagnostics.

astro-ph.SR↗

arXiv2019

Large-scale multiconfiguration Dirac-Hartree-Fock calculations for astrophysics: Cl-like ions from Cr~VIII to Zn~XIV

We use the multiconfiguration Dirac-Hartree-Fock (MCDHF) method combined with the relativistic configuration interaction (RCI) approach (GRASP2K) to provide a consistent set of transition energies and radiative transition data for the lower $n =3$ states in all Cl-like ions of astrophysical importance, from \ion{Cr}{8} to \ion{Zn}{14}. We also provide excitation energies calculated for \mbox{Fe X} using the many-body perturbation theory (MBPT, implemented within FAC). The comparison of the present MCDHF results with MBPT and with the available experimental energies indicates that the theoretical excitation energies are highly accurate, with uncertainties of only a few hundred cm$^{-1}$. Detailed comparisons for Fe~X and Ni~XII highlight discrepancies in the experimental energies found in the literature. Several new identifications are proposed.

physics.atom-ph↗

arXiv2019

The Solar Orbiter SPICE instrument -- An extreme UV imaging spectrometer

The Spectral Imaging of the Coronal Environment (SPICE) instrument is a high-resolution imaging spectrometer operating at extreme ultraviolet (EUV) wavelengths. In this paper, we present the concept, design, and pre-launch performance of this facility instrument on the ESA/NASA Solar Orbiter mission. The goal of this paper is to give prospective users a better understanding of the possible types of observations, the data acquisition, and the sources that contribute to the instrument's signal. The paper discusses the science objectives, with a focus on the SPICE-specific aspects, before presenting the instrument's design, including optical, mechanical, thermal, and electronics aspects. This is followed by a characterisation and calibration of the instrument's performance. The paper concludes with descriptions of the operations concept and data processing. The performance measurements of the various instrument parameters meet the requirements derived from the mission's science objectives. The SPICE instrument is ready to perform measurements that will provide vital contributions to the scientific success of the Solar Orbiter mission.

astro-ph.IM↗

arXiv2019

Exploring the damping of Alfvén waves along a long off-limb coronal loop, up to 1.4 R$_\odot$

The Alfvén wave energy flux in the corona can be explored using the electron density and velocity amplitude of the waves. The velocity amplitude of Alfvén waves can be obtained from the non-thermal velocity of the spectral line profiles. Previous calculations of the Alfvén wave energy flux with height in active regions and polar coronal holes have provided evidence for the damping of Alfvén waves with height. We present off-limb Hinode EUV imaging spectrometer (EIS) observations of a long coronal loop up to 1.4~R$_\odot$. We have obtained the electron density along the loop and found the loop to be almost in hydrostatic equilibrium. We obtained the temperature using the EM-loci method and found the loop to be isothermal across, as well as along, the loop with a temperature of about 1.37 MK. We significantly improve the estimate of non-thermal velocities over previous studies by using the estimated ion (equal to electron) temperature. Estimates of electron densities are improved using the significant updates of the CHIANTI v.8 atomic data. More accurate measurements of propagating Alfvén wave energy along the coronal loop and its damping are presented up to distances of 1.4 R$_\odot$, further than have been previously explored. The Alfvén wave energy flux obtained could contribute to a significant part of the coronal losses due to radiation along the loop.

astro-ph.SR↗

arXiv2019

Hinode EIS line widths in the quiet corona up to 1.5 Rsun

We present an analysis of several Hinode EIS observations of coronal line widths in the quiet Sun, up to 1.5 Rsun radial distances. No significant variations are found, which indicates no damping of Alfvén waves in the quiescent corona. However, the uncertainties in estimating the instrumental width mean that a firm conclusion cannot be reached. We present a discussion of various EIS instrumental issues and suggest that the strongest lines, from Fe XII at 193.5 and 195.1 A, have anomalous instrumental widths. We show how line widths in EIS are uncertain when the signal is low, and that the instrumental variation along the slit is also uncertain. We also found an anomalous decrease (up to 40%) in the intensities of these lines in many off-limb and active region observations, and suggest that this is due to opacity effects. We find that the most reliable measurements are obtained from the weaker lines.

astro-ph.SR↗

arXiv2019

Modelling ion populations in astrophysical plasmas: carbon in the solar transition region

The aim of this work is to improve the modelling of ion populations in higher density, lower temperature astrophysical plasmas, of the type commonly found in lower solar and stellar atmospheres. Ion population models for these regions frequently employ the coronal approximation, which assumes conditions more suitable to the upper solar atmosphere, where high temperatures and lower densities prevail. Using the coronal approximation for modelling the solar transition region gives theoretical lines intensities for the Li-like and Na-like isoelectronic sequences which can be factors of 2-5 times lower than observed. The works of Burgess & Summers (1969) and Nussbaumer & Storey (1975) showed the important part ions in excited levels play when included in the modelling. Their models, however, used approximations for the atomic rates to determine the ion balance. Presented here is the first stage in updating these earlier models of carbon by using rates from up-to-date atomic calculations and more recent photo-ionising radiances for the quiet Sun. Where such atomic rates are not readily available, in the case of electron-impact direct ionisation and excitation--auto-ionisation, new calculations have been made and compared to theoretical and experimental studies. The effects each atomic process has on the ion populations as density changes is demonstrated, and final results from the modelling are compared to the earlier works. Lastly, the new results for ion populations are used to predict line intensities for the solar transition region in the quiet Sun, and these are compared with predictions from coronal-approximation modelling and with observations. Significant improvements in the predicted line intensities are seen in comparison to those obtained from zero-density modelling of carbon.

astro-ph.SR↗

arXiv2019

Elemental composition in quiescent prominences

The first ionization potential (FIP) bias is currently used to trace the propagation of solar features ejected by the wind and solar eruptions (coronal mass ejections). The FIP bias also helps us to understand the formation of prominences, as it is a tracer for the solar origin of prominence plasma. This work aims to provide elemental composition and FIP bias in quiescent solar prominences. This is key information to link these features to remnants of solar eruptions measured in-situ within the heliosphere and to constrain the coronal or photospheric origin of prominence plasma. We used the differential emission measure technique to derive the FIP bias of two prominences. Quiet Sun chromospheric and transition region data were used to test the atomic data and lines formation processes. We used lines from low stage of ionization of \ion{Si}{}, \ion{S}{}, \ion{Fe}{}, \ion{C}{}, \ion{N}{}, \ion{O}{}, \ion{Ni}{}, \ion{Mg,}{} and \ion{Ne}{}, constraining the FIP bias in the range $4.2 \le \log T \le 5.8$. We adopted a density-dependent ionization equilibrium. We showed that the two prominences have photospheric composition. We confirmed a photospheric composition in the quiet Sun. We also identified opacity and/or radiative excitation contributions to the line formation of a few lines regularly observed in prominences. With our results we thus provide important elements for correctly interpreting the upcoming Solar Orbiter/SPICE spectroscopic data and to constrain prominence formation.

astro-ph.SR↗

arXiv2019

The EUV spectrum of the Sun: quiet and active Sun irradiances and chemical composition

We benchmark new atomic data against a selection of irradiances obtained from medium-resolution quiet Sun spectra in the EUV, from 60 to 1040 Å. We use as a baseline the irradiances measured during solar minimum on 2008 April 14 by the prototype (PEVE) of the Solar Dynamics Observatory Extreme ultraviolet Variability Experiment (EVE). We take into account some inconsistencies in the PEVE data, using flight EVE data and irradiances we obtained from Solar & Heliospheric Observatory (SoHO) Coronal Diagnostics Spectrometer (CDS) data. We perform a differential emission measure and find overall excellent agreement (to within the accuracy of the observations, about 20%) between predicted and measured irradiances in most cases, although we point out several problems with the currently available ion charge state distributions. We used the photospheric chemical abundances of Asplund et al. (2009). The new atomic data are nearly complete in this spectral range, for medium-resolution irradiance spectra. Finally, we use observations of the active Sun in 1969 to show that also in that case the composition of the solar corona up to 1 MK is nearly photospheric. Variations of a factor of 2 are present for higher-temperature plasma, which is emitted within active regions.These results are in excellent agreement with our previous findings.

astro-ph.SR↗

arXiv2019

Uncertainties on atomic data. A case study: N IV

We consider three recent large-scale calculations for the radiative and electron-impact excitation data of N IV, carried out with different methods and codes. The scattering calculations employed the relativistic Dirac $R$-matrix (DARC) method, the intermediate coupling frame transformation (ICFT) $R$-matrix method, and the B-spline $R$-matrix (BSR) method. These are all large-scale scattering calculations with well-tested and sophisticated codes, which use the same set of target states. One concern raised in previous literature is related to the increasingly large discrepancies in the effective collision strengths between the three sets of calculations for increasingly weak and/or high-lying transitions. We have built three model ions and calculated the intensities of all the main spectral lines in this ion. We have found that, despite such large differences, excellent agreement (to within $\pm$~20\%) exists between all the spectroscopically-relevant line intensities. This provides confidence in the reliability of the calculations for plasma diagnostics. We have used the differences in the radiative and excitation rates amongst the three sets of calculations to obtain a measure of the uncertainty in each rate. Using a Monte Carlo approach, we have shown how these uncertainties affect the main theoretical ratios which are used to measure electron densities and temperatures.

astro-ph.SR↗

arXiv2018

Energy Levels, Lifetimes and Transition rates for P-like ions from Cr X to Zn XVI from large-scale Relativistic Multiconfiguration Calculations

The fully relativistic multiconfiguration Dirac--Hartree--Fock method is used to compute excitation energies and lifetimes for the 143 lowest states of the $3s^23p^3$, $3s3p^4$, $3s^23p^23d$, $3s3p^33d$, $3p^5$, $3s^23p3d^2$ configurations in P-like ions from Cr X to Zn XVI. Multipole (E1, M1, E2, M2) transition rates, line strengths, oscillator strengths, and branching fractions among these states are also given. Valence-valence and core-valence electron correlation effects are systematically accounted for using large basis function expansions. Computed excitation energies are compared with the NIST ASD and CHIANTI compiled values and previous calculations. The mean average absolute difference, removing obvious outliers, between computed and observed energies for the 41 lowest identified levels in Fe XII is only 0.057 \%, implying that the computed energies are accurate enough to aid identification of new emission lines from the sun and other astrophysical sources. The amount of energy and transition data of high accuracy is significantly increased for several P-like ions of astrophysics interest, where experimental data are still very scarce.

physics.atom-ph↗

arXiv2016

Analysis and modelling of recurrent solar flares observed with Hinode/EIS on March 9, 2012

Three homologous C-class flares and one last M-class flare were observed by both the Solar Dynamics Observatory (SDO) and the Hinode EUV Imaging Spectrometer (EIS) in the AR 11429 on March 9, 2012. All the recurrent flares occurred within a short interval of time (less than 4 hours), showed very similar plasma morphology and were all confined, until the last one when a large-scale eruption occurred. The C-class flares are characterized by the appearance, at approximatively the same locations, of two bright and compact footpoint sources of $\approx$~3--10~MK evaporating plasma, and a semi-circular ribbon. During all the flares, the continuous brightening of a spine-like hot plasma ($\approx$~10~MK) structure is also observed. Spectroscopic observations with Hinode/EIS are used to measure and compare the blueshift velocities in the \fexxiii\ emission line and the electron number density at the flare footpoints for each flare. Similar velocities, of the order of 150--200~km~s$^{-1}$, are observed during the C2.0 and C4.7 confined flares, in agreement with the values reported by other authors in the study of the last M1.8 class flare. On the other hand, lower electron number densities and temperatures tend to be observed in flares with lower peak soft X-ray flux.In order to investigate the homologous nature of the flares, we performed a Non-Linear Force-Free Field (NLFFF) extrapolation of the 3D magnetic field configuration in the corona. The NLFFF extrapolation and the Quasi-Separatrix Layers (QSLs) provide the magnetic field context which explains the location of the kernels, spine-like and semi-circular brightenings observed in the (non-eruptive) flares. Given the absence of a coronal null point, we argue that the homologous flares were all generated by the continuous recurrence of bald patch reconnection.

astro-ph.SR↗