Quantifying the Influence of Key Physical Processes on the Formation of Emission Lines Observed by IRIS: I. Non-Equilibrium Ionization and Density-Dependent Rates. (arXiv:1901.03935v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bradshaw_S/0/1/0/all/0/1">Stephen J. Bradshaw</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Testa_P/0/1/0/all/0/1">Paola Testa</a>
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).
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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