The ultraviolet and optical (white-light) continuum emission during the impulsive phase of solar flares often represents the majority of the radiated flare energy, yet its broad-wavelength…

This is a federal grant award to The Regents of the University of Colorado (University of Colorado) by the National Aeronautics and Space Administration (NASA) for research on the characteristics of white-light emission during solar flares. The $235,421.01 grant will fund the investigation of ultraviolet and optical emission from solar flares using data from NASA's Interface Region Imaging Spectrograph (IRIS) mission. The research aims to improve understanding of the role of nonthermal electron energy release and mass/energy flow in the solar chromosphere during flares. This grant is not associated with a set-aside program and is part of NASA's efforts to advance solar physics research through collaboration with academic institutions.

The ultraviolet and optical (white-light) continuum emission during the impulsive phase of solar flares often represents the majority of the radiated flare energy, yet its broad-wavelength characteristics have remained largely unconstrained ever since the first solar flare was observed in 1859. Some spectroscopic observations in the blue/optical were obtained several decades ago, but they rarely sampled the brightest white-light emitting areas, thus providing only a partial view of the impulsive phase continuum emission. Early static, isothermal slab models were used at the time to interpret these data. We now have access to much more sophisticated codes, such as the radiative-hydrodynamic radyn, to better understand the physics of nonthermal energy release into the lower atmosphere, ionization and recombination timescales, the detailed radiative response as a function of height, and the mass and energy flow into the corona and lower atmosphere as a result of hydrodynamic shocks. Unfortunately, high-resolution ground-based solar instrumentation is rarely capable of obtaining broad-wavelength coverage spectra at blue/optical wavelengths for a detailed comparison to models that aim to reproduce the white-light emission properties. Hydrogen recombination model spectra have been invoked to explain the range of observed continuum properties in the optical, but a recent sun-as-a-star superposed flare analysis has revealed the first evidence for a stronger, 9000 k blackbody-like component that appears to be present in small and large solar flares alike. IT is interesting that this continuum component is nearly ubiquitous during flares on younger, more active STARS, but cannot be reproduced by flare models that use a typical nonthermal electron beam energy deposition rate. The interface region imaging spectrograph (irjs) provides a timely, high-spatial and high-spectral resolution dataset that is very well suited to provide long-sought constraints on the white-light continuum emission. We will rigorously search all flares observed with iris for continuum emission and use the intensity level to discriminate between a hot blackbody component and hydrogen recombination emission in the brightest kernels. Furthermore, we will also be able to determine the in situ velocity field of the white-lightemitting plasma, thus placing important constraints on models of chromospheric radiation and hydrodynamics. We will use rhessi data and iris slit-jaw images to provide improved estimates of nonthermal heating fluxes into the lower atmosphere and use these as new constraints for a suite of radiative-hydrodynamic flare models with an updated treatment of electron beams. The high spatial and spectral resolution of iris will allow US to rigorously explore the connection between the type of white-light emission, nonthermal particle heating rates, and the magnitude of chromospheric downflows. The production of updated models combined with the new observational constraints will make excellent use of the combined capabilities of three NASA flagship missions. The proposed combination of data analysis and model synthesis will provide diagnostics that will open up exploration in new areas of flare physics. Our investigation is central to the goals of the iris mission in particular: we will improve upon the understanding of the role of nonthermal electron energy release in the chromospher~ and the flow of mass and energy into the corona via :::hromospheric evaporation, which is physically coupled to the magnitude of chromospheric downflows.