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Register a new userQuantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS: I. Non-Equilibrium Ionization and Density-Dependent Rates
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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 density-dependence 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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The intensity of the oiv~2s$^{2}$ 2p $^{2}$P-2s2p$^{2}$ $^{4}$P and siv~3 s$^{2}$ 3p $^{2}$P- 3s 3p$^{2}$ $^{4}$ P intercombination lines around 1400~AA~observed with the textit{Interface Region Imaging Spectrograph} (IRIS) provide a useful tool to diagnose the electron number density ($N_textrm{e}$) in the solar transition region plasma. We measure the electron number density in a variety of solar features observed by IRIS, including an active region (AR) loop, plage and brightening, and the ribbon of the 22-June-2015 M 6.5 class flare. By using the emissivity ratios of oiv and siv lines, we find that our observations are consistent with the emitting plasma being near isothermal (log$T$[K] $approx$ 5) and iso-density ($N_textrm{e}$ $approx$~10$^{10.6}$ cm$^{-3}$) in the AR loop. Moreover, high electron number densities ($N_textrm{e}$ $approx$~10$^{13}$ cm$^{-3}$) are obtained during the impulsive phase of the flare by using the siv line ratio. We note that the siv lines provide a higher range of density sensitivity than the oiv lines. Finally, we investigate the effects of high densities ($N_textrm{e}$ $gtrsim$ 10$^{11}$ cm$^{-3}$) on the ionization balance. In particular, the fractional ion abundances are found to be shifted towards lower temperatures for high densities compared to the low density case. We also explored the effects of a non-Maxwellian electron distribution on our diagnostic method.
The polytropic (adiabatic) index for pure hydrogen plasma is analytically calculated as function of reciprocal temperature and degree of ionization. Additionally, the polytropic index is graphically represented as a function of temperature and density. It is concluded that the partially ionized hydrogen plasma cannot be exactly polytropic. The calculated deviations from the mono-atomic value 5/3 are measurable. The analytical result for pure hydrogen plasma is a test example how this approach can be extended for arbitrary gas cocktail.
This study on plasma heating considers the time-dependent ionization process during a large solar flare on September 10, 2017, observed by Hinode/EIS. The observed FeXXIV / FeXXIII ratios increase downstream of the reconnection outflow, and they are consistent with the time-dependent ionization effect at a constant electron temperature Te = 25 MK. Moreover, this study also shows that the non-thermal velocity, which can be related to the turbulent velocity, reduces significantly along the downstream of the reconnection outflow, even when considering the time-dependent ionization process.
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.
Aims. We present the first high-resolution non-equilibrium ionization simulation of the joint evolution of the Local Bubble (LB) and Loop I superbubbles in the turbulent supernova-driven interstellar medium (ISM). The time variation and spatial distribution of the Li-like ions Civ, Nv, and Ovi inside the LB are studied in detail. Methods. This work uses the parallel adaptive mesh refinement code EAF-PAMR coupled to the newly developed atomic and molecular plasma emission module E(A+M)PEC, featuring the time-dependent calculation of the ionization structure of H through Fe, using the latest revision of solar abundances. The finest AMR resolution is 1 pc within a grid that covers a representative patch of the Galactic disk (with an area of 1 kpc^2 in the midplane) and halo (extending up to 10 kpc above and below the midplane). Results. The evolution age of the LB is derived by the match between the simulated and observed absorption features of the Li-like ions Civ, Nv, and Ovi . The modeled LB current evolution time is bracketed between 0.5 and 0.8 Myr since the last supernova reheated the cavity in order to have N(Ovi) < 8 times 10^12 cm-2, log[N(Civ) /N(Ovi) ] < -0.9 and log[N(Nv) /N(Ovi) ] < -1 inside the simulated LB cavity, as found in Copernicus, IUE, GHRS-IST and FUSE observations.
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