Non-Maxwellian Analysis of the Transition-region Line Profiles Observed by the Interface Region Imaging Spectrograph

Dudík, J.; Polito, Vanessa; Dzifčáková, E.; Zanna, G. Del; Testa, Paola

doi:10.3847/1538-4357/aa71a8

Cited by 40 publications

(53 citation statements)

References 108 publications

(171 reference statements)

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“…The intensities of the S iv and O iv transition region lines were measured by a Gaussian fit to the IRIS FUV spectra. As noted by Polito et al (2016) and Dudík et al (2017) these lines sometimes show departures from Gaussian shapes and can be well-fitted by a Lorentzian or double Gaussian function. We used a double Gaussian to fit each line and cross-checked each fit by performing an additional single Gaussian fit, finding that the line intensities are consistent with the double Gaussian method to within ∼ 15 − 20%.…”

Section: Observational Datamentioning

confidence: 75%

Quantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS. I. Non-equilibrium Ionization and Density-dependent Rates

Bradshaw

Testa

2019

ApJ

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In the work described here we investigate atomic processes leading to the formation of emission lines within the IRIS wavelength range at temperatures near 10 5 K. We focus on (1) non-equilibrium and (2) density-dependent effects influencing the formation and radiative properties of S iv and O iv. These two effects have significant impacts on spectroscopic diagnostic measurements of quantities associated with the plasma that emission lines from S iv and O iv provide. We demonstrate this by examining nanoflare-based coronal heating to determine what the detectable signatures are in transition region emission. A detailed comparison between predictions from numerical experiments and several sets of observational data is presented to show how one can ascertain when non-equilibrium ionization and/or density-dependent atomic processes are important for diagnosing nanoflare properties, the magnitude of their contribution, and what information can be reliably extracted from the spectral data. Our key findings are the following.(1) The S/O intensity ratio is a powerful diagnostic of non-equilibrium ionization.(2) Non-equilibrium ionization has a strong effect on the observed line intensities even in the case of relatively weak nanoflare heating. (3) The densitydependence of atomic rate coefficients is only important when the ion population is out of equilibrium. (4) In the sample of active regions we examined, weak nanoflares coupled with non-equilibrium ionization and density-dependent atomic rates were required to explain the observed properties (e.g. the S/O intensity ratios). (5) Enhanced S/O intensity ratios cannot be due solely to the heating strength and must depend on other processes (e.g. heating frequency, non-Maxwellian distributions).

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Section: Observational Datamentioning

confidence: 75%

Quantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS. I. Non-equilibrium Ionization and Density-dependent Rates

Bradshaw

Testa

2019

ApJ

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“…At time t = P/2s, the electron beam is switched on until t = P, when it is switched off for another P/2. The distribution of plasma with the electron beam is represented by a κ-distribution (e.g., Olbert 1968;Vasyliunas 1968a,b;Owocki & Scudder 1983;Livadiotis 2015;Dzifčáková et al 2015;Dudík et al 2017b),…”

Section: Methodsmentioning

confidence: 99%

“…A particular example is the Si iv doublet at 1393.8 and 1402.8 Å, whose peak formation temperature should lie at log(T max [K]) = 4.9 under equilibrium conditions that are characterized by a Maxwellian distribution (Dudík et al 2014a). However, their intensities are usually enhanced by a factor of five or more compared to the neighboring intercombination O iv lines that are formed at similar temperatures of log(T max [K]) = 5.15 (e.g., Doyle & Raymond 1984;Judge et al 1995;Curdt et al 2001;Yan et al 2015;Polito et al 2016;Doschek et al 2016;Dudík et al 2017b). Furthermore, the TR is known to be highly dynamic, with intensity changes on the order of several seconds or tens of seconds (e.g., Testa et al 2013;Régnier et al 2014;Hansteen et al 2014;Peter et al 2014;Tian et al 2014Tian et al , 2016Vissers et al 2015;Tajfirouze et al 2016;Martínez-Sykora et al 2016;Hou et al 2016;Warren et al 2016;Samanta et al 2017) as identified in recent observations performed by the Interface Region Imaging This dynamics represents a modeling challenge, since the intensity changes can be shorter than the change in ionization equilibration timescales (see Fig.…”

Section: Introductionmentioning

confidence: 99%

“…Some of the results were revised using newer atomic data by Del Zanna et al (2015b). Indications of the possible presence of κ-distributions in TR have been obtained by fitting the TR line profiles (Dudík et al 2017b) and the O iv intensity ratios relative to Si iv one (Dudík et al 2014a(Dudík et al , 2017b. The Si iv / O iv ratios are also strongly sensitive to the presence of non-equilibrium ionization, however, which also decrease the O iv lines relative to Si iv (Doyle et al 2013;Martínez-Sykora et al 2016).…”

Section: Introductionmentioning

confidence: 99%

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Non-equilibrium ionization by a periodic electron beam

Dzifčáková

Dudík

2018

A&A

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Context. Transition region (TR) spectra typically show the Si IV 1402.8 Å line to be enhanced by a factor of 5 or more compared to the neighboring O IV 1401.2 Å, contrary to predictions of ionization equilibrium models and the Maxwellian distribution of particle energies. Non-equilibrium effects in TR spectra are therefore expected. Aims. To investigate the combination of non-equilibrium ionization and high-energy particles, we apply the model of the periodic electron beam, represented by a κ-distribution that recurs at periods of several seconds, to plasma at chromospheric temperatures of 104 K. This simple model can approximate a burst of energy release involving accelerated particles. Methods. Instantaneous time-dependent charge states of silicon and oxygen were calculated and used to synthesize the instantaneous and period-averaged spectra of Si IV and O IV. Results. The electron beam drives the plasma out of equilibrium. At electron densities of Ne = 1010 cm−3, the plasma is out of ionization equilibrium at all times in all cases we considered, while for a higher density of Ne = 1011 cm−3, ionization equilibrium can be reached toward the end of each period, depending on the conditions. In turn, the character of the period-averaged synthetic spectra also depends on the properties of the beam. While the case of κ = 2 results in spectra with strong or even dominant O IV, higher values of κ can approximate a range of observed TR spectra. Spectra similar to typically observed spectra, with the Si IV 1402.8 Å line about a factor 5 higher than O IV 1401.2 Å, are obtained for κ = 3. An even higher value of κ = 5 results in spectra that are exclusively dominated by Si IV, with negligible O IV emission. This is a possible interpretation of the TR spectra of UV (Ellerman) bursts, although an interpretation that requires a density that is 1–3 orders of magnitude lower than for equilibrium estimates.

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“…One possible explanation is that the forward modeling erroneously assumed that the lines were optically thin, which may or may not hold true for C II (Rathore & Carlsson 2015) and O I (Lin & Carlsson 2015), depending on the particular conditions of the event. It is also possible that the line shapes may be non-Maxwellian, which can affect the opacity (Dudík et al 2017). We seek to directly test the importance of NLTE effects by redoing the simulations with the improved model, and then synthesizing the emission with a radiative transfer solver.…”

Section: Introductionmentioning

confidence: 99%

Efficient Calculation of Non-local Thermodynamic Equilibrium Effects in Multithreaded Hydrodynamic Simulations of Solar Flares

Reep

Bradshaw

Crump

et al. 2019

ApJ

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Understanding the dynamics of the solar chromosphere is crucial to understanding energy transport across the solar atmosphere. The chromosphere is optically thick at many wavelengths and described by non-local thermodynamic equilibrium (NLTE), making it difficult to interpret observations. Furthermore, there is considerable evidence that the atmosphere is filamented, and that current instruments do not sufficiently resolve small scale features. In flares, it is likely that multithreaded models are required to describe the heating. The combination of NLTE effects and multithreaded modeling requires computationally demanding calculations, which has motivated the development of a model that can efficiently treat both. We describe the implementation of a solver in a hydrodynamic code for the hydrogen level populations that approximates the NLTE solutions. We derive an accurate electron density across the atmosphere, that includes the effects of non-equilibrium ionization for helium and metals. We show the effects on hydrodynamic simulations, which are used to synthesize light curves using a post-processing radiative transfer code. We demonstrate the utility of this model on IRIS observations of a small flare. We show that the Doppler shifts in Mg II, C II, and O I can be explained with a multithreaded model of loops subjected to electron beam heating, so long as NLTE effects are treated. The intensities, however, do not match observed values very well, which is due to assumptions about the initial atmosphere. We briefly show how altering the initial atmosphere can drastically alter line profiles, and therefore derived quantities, and suggest that it should be tuned to pre-flare observations.

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Non-Maxwellian Analysis of the Transition-region Line Profiles Observed by the Interface Region Imaging Spectrograph

Cited by 40 publications

References 108 publications

Quantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS. I. Non-equilibrium Ionization and Density-dependent Rates

Quantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS. I. Non-equilibrium Ionization and Density-dependent Rates

Non-equilibrium ionization by a periodic electron beam

Efficient Calculation of Non-local Thermodynamic Equilibrium Effects in Multithreaded Hydrodynamic Simulations of Solar Flares

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