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Flares dramatically enhance emission in numerous atomic lines and continua. If properly
interpreted, this emission can reveal vital information about the Sun's atmospheric state during
flares. However, due to the complexity of the radiative transfer in the Sun's atmosphere,
extracting this information can be quite difficult. Here we propose to perform radiative
hydrodynamic simulations of the response of the solar atmosphere to flare heating. The
simulations model radiative transfer coupled with hydrodynamics and make predictions of
how emission will increase during flares. We will correlate our predictions with data from the
RHESSI and SDO space craft. We will use spectral fitting of RHESSI hard X-ray observations to
deduce details of electron beams impacting on the chromosphere. A model of the electron beam
deduced from RHESSI observations will be injected as a heating term into our code to simulate
the radiative hydrodynamic response to flare heating. With this heating as input, the model
will solve in detail the dynamics of the flaring atmosphere and can be used to predict emission,
densities, velocities, temperatures, and emission measures as a function of time during the flare
simulation. These will be compared with observations.
It is important to note that spectral fitting to RHESSI data provides information about
the electron beam and the state of the plasma where the beam impacts i.e., primarily the
chromosphere. The non-uniform ionization in the chromosphere produces a break in the X-ray
energy spectrum. Several flares have been observed that have breaks in the X-ray spectrum that
are consistent with partial ionization. We will investigate to what extent this break is caused
by partial ionization in the chromosphere. To do this we will compare emission measure and
ionization fractions obtained from the models with those obtained directly from spectral fitting.
Since the model has as its input an electron beam obtained from the spectral fit, the comparison
of the output of the model with the spectral fit essentially closes the "loop" between the two and
provides checks on both the models and spectral fits.
Accelerated ions are known to be a source of flare heating in the lower solar atmosphere. We are
enhancing our model to include the effects of ion beam heating. We will use this to investigate
the origins of white light emission, sunquakes and the broad 511 keV emission line.
The positron-electron annihilation line at 511 keV has been observed in several flares to be
unexpectedly broad. The line may be broadened by thermal mechanisms, but that would require
a significant amount of material to be heated to ~0.5 MK. In addition, the material stays hot even
after the impulsive has ended, suggesting an energy source separate from the impulsive flare
heating. One possibility is low-energy ions (< 1 MeV). Our models are ideally suited to explore
this issue. In this proposal, we will investigate low energy ion heating as a potential explanation
of the broad 511 keV line.
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Current Version: 3.1 (04/16/2014) 
