| Abstract * |
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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. |
