HAO Colloquium - Stephen Bradshaw, Rice University

Learning to Diagnose Coronal Heating Signatures in Transition Region Spectral Lines

In the work reported here we have investigated signatures of nanoflare-based coronal heating in transition region radiative emission. The transition region is brighter, but very strongly coupled to the corona, and responds quickly to changing coronal conditions. Direct signatures of coronal nanoflares are expected to be extremely difficult to detect and perhaps only through such indirect observations may we find sufficient evidence to pin-down the coronal heating mechanism.

We have conducted numerical and forward modeling to predict and examine the properties of key transition region spectral lines that are observed by IRIS and encode information concerning coronal heating. However, predicting and interpreting transition region spectra are beset with their own challenges. The dynamic nature of the plasma, the steep local gradients, and the overlying hot corona all combine to produce conditions that invalidate assumptions such as thermal equilibrium of the electron and ion populations in the transition region, and we expect the emission spectrum to be strongly decoupled from the local temperature. In our modeling study we focus on three processes that may each play a key role in forming the emission lines observed by IRIS, to determine which have the greatest influence: (1) non-equilibrium ionization; (2) density-dependence of collisional processes, especially the quenching of dielectronic recombination with increasing density; and (3) the formation of high-energy tails on the local electron distribution due to a streaming component from the hot corona.

Among our findings are that line intensities and plasma properties such as the electron density, as derived using spectroscopic diagnostic methods, are grossly underestimated (densities by up to 1000% in the case of strong heating) when the aforementioned atomic processes are ignored; density-dependence of collisional processes, particularly dielectronic recombination, is most significant when coupled to non-equilibrium ionization; the range of temperatures over which emission lines are formed can be increased by more than a factor of 2 (in log space) than is predicted in equilibrium; the distribution of data points produced by plotting a S IV / O IV line ratio against electron density is sensitive to the prominence of non-equilibrium ionization and may provide an important diagnostic of this process.


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Thursday, June 21, 2018 - 2:00am to 3:00am

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