arXiv subjects

R. P. Dufresne

Publications and source records attributed to R. P. Dufresne.

9 recordsLinked to original sources

arXiv2026

Diagnostics for the solar chromosphere using neutral carbon Rydberg lines

Diagnostics for the solar chromosphere are relatively few compared to other parts of the atmosphere. Despite this, hundreds of Rydberg lines emitted by neutrals in this region have been observed at UV wavelengths. Here, we investigate their diagnostic potential by modelling the lines emitted by neutral carbon using recent atomic data. We use the radiative transfer code Lightweaver to explore how they form and how they respond to temperature, density and micro-turbulent velocity perturbations in the atmosphere. To simplify the modelling, we investigate lines emitted from levels with principal quantum number $n\geq10$, which are expected to be in Saha-Boltzmann equilibrium with the ground state of the singly-charged ion. Optical depth effects are apparent in the lines and their response to atmospheric perturbations suggest that they will be useful in reconstructions of the atmosphere using inversions. The study opens the way for using many such lines emitted by multiple elements over a range of heights, a large number of which will be observed by the forthcoming Solar-C EUV High-throughput Spectroscopic Telescope (EUVST).

astro-ph.SR↗

arXiv2025

Improving the atomic modelling for solar UV radiative transfer calculations

Radiative transfer calculations have been produced over the years for many lines and continua in the UV wavelength range of solar and cool stellar atmospheres for a variety of conditions. Despite significant improvements in computing power and availability of atomic data over time, atomic models are often still limited in size and rely on approximations for data. There have also been inconsistencies in the way photo-ionisation and radiative recombination have been treated. Here, we incorporate into the Lightweaver radiative transfer code new data and updated modelling of atomic processes for the low charge states of C, Si and S. Data are taken from the CHIANTI database and other widely-available sources for the relevant elements. We show the significant impact this has on the UV continua in the 1100-1700Å region, especially for Si. The results are in much better agreement with averaged, quiet Sun observations, and remove the need to invoke "missing opacity" to resolve discrepancies. The present treatment has important implications for radiative transfer calculations and the model atmospheres used as inputs.

astro-ph.SR↗

arXiv2024

CHIANTI -- an atomic database for emission lines -- Paper XVIII. Version 11, advanced ionization equilibrium models: density and charge transfer effects

Version 11 of the CHIANTI database and software package is presented. Advanced ionization equilibrium models have been added for low charge states of seven elements (C, N, O, Ne, Mg, Si and S), and represent a significant improvement especially when modelling the solar transition region. The models include the effects of higher electron density and charge transfer on ionization and recombination rates. As an illustration of the difference these models make, a synthetic spectrum is calculated for an electron pressure of 7$\times 10^{15}$ cm$^{-3}$ K and compared with an active region observation from HRTS. Increases are seen of factors of two to five in the predicted radiances of the strongest lines in the UV from Si IV, C IV, and N V, compared to the previous modelling using the coronal approximation. Much better agreement (within 20\%) with the observation is found for the majority of the lines. The new atomic models better equip both those who are studying the transition region and those who are interpreting emission from higher density astrophysical and laboratory plasma. In addition to the advanced models, several ion datasets have been added or updated, and data for the radiative recombination energy loss rate have been updated.

astro-ph.SR↗

arXiv2023

New atomic data for C I Rydberg states compared with solar UV spectra

We use the Breit Pauli $R$-matrix method to calculate accurate energies and radiative data for states in C I up to $n$=30 and with $l\le 3$. We provide the full dataset of decays to the five 2s$^2$2p$^2$ ground configuration states $^3$P$_{0,1,2}$, $^1$D$_2$, $^1$S$_0$. This is the first complete set of data for transitions from $n\ge 5$. We compare oscillator strengths and transition probabilities with the few previously calculated values for such transitions, finding generally good agreement (within 10%) with the exception of values recently recommended by NIST, where significant discrepancies are found. We then calculate spectral line intensities originating from the Rydberg states using typical chromospheric conditions and assuming LTE, and compare them with well-calibrated SOHO SUMER UV spectra of the quiet Sun. The relative intensities of the Rydberg series are in excellent agreement with observation, which provides firm evidence for the identifications and blends of nearly 200 UV lines. Such comparison also resulted in a large number of new identifications of C I lines in the spectra. We also estimate optical depth effects and find that these can account for much of the absorption noted in the observations. The atomic data can be applied to model a wide range of solar and astrophysical observations.

astro-ph.SR↗

arXiv2021

Modelling low charge ions in the solar atmosphere

Extensions have been made recently to the coronal approximation for the purpose of modelling line emission from carbon and oxygen in the lower solar atmosphere. The same modelling is used here for other elements routinely observed in the solar transition region: N, Ne, Mg, Si and S. The modelling includes the effects of higher densities suppressing dielectronic recombination and populating long-lived, metastable levels; the presence of metastable levels typically causes effective ionisation rates to increase and recombination rates to decrease. Processes induced by the radiation field, namely photo-ionisation and photo-excitation, have been included, along with charge transfer, which occurs when electrons are exchanged during atom-ion and ion-ion collisions. The resulting ion balances are shown, and indicate significant changes compared to the frequently-employed coronal approximation. The effect on level populations within ions caused by photo-excitation is also assessed. To give an illustration of how line emission could be altered by these processes, selected line contribution functions are presented at the end.

astro-ph.SR↗

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↗

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↗

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↗