Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres

Abstract

The origin of the near-ultraviolet and optical continuum radiation in flares is critical for understanding particle acceleration and impulsive heating in stellar atmospheres. Radiative-hydrodynamic simulations in 1D have shown that high energy deposition rates from electron beams produce two flaring layers at T ∼ 10 4 K that develop in the chromosphere: a cooling condensation (downflowing compression) and heated non-moving (stationary) flare layers just below the condensation. These atmospheres reproduce several observed phenomena in flare spectra, such as the red wing asymmetry of the emission lines in solar flares and a small Balmer jump ratio in M dwarf flares. The high beam flux simulations are computationally expensive in 1D, and the (human) timescales for completing NLTE models with adaptive grids in 3D will likely be unwieldy for a time to come. We have developed a prescription for predicting the approximate evolved states, continuum optical depth, and the emergent continuum flux spectra of radiative-hydrodynamic model flare atmospheres. These approximate prescriptions are based on an important atmospheric parameter: the column mass (m ref ) at which hydrogen becomes nearly completely ionized at the depths that are approximately in steady state with the electron beam heating. Using this new modeling approach, we find that high energy flux density (>F11) electron beams are needed to reproduce the brightest observed continuum intensity in IRIS data of the 2014-Mar-29 X1 solar flare and that variation in m ref from 0.001 to 0.02 g cm −2 reproduces most of the observed range of the optical continuum flux ratios at the peaks of M dwarf flares.

Modeling Mg ii h, k and Triplet Lines at Solar Flare Ribbons

“…As a consequence, though we find a few Mg II profiles with a component that is redshifted by 0.6 − 0.7Å between t = 7 s and t = 8.5 s, the redshifted component in the 5F11 model is incapable of producing enough far wing emission up to +1.0Å. Such high-speed downflows were found by Rubio da Costa & Kleint (2017), and they can be triggered by electron beams with a higher energy flux like in the F13 models (e.g., Kowalski et al 2015;Kowalski, & Allred 2018). Panos et al (2018) used machine learning to characterize different types of Mg II line profiles observed at flare ribbons.…”

The Near-ultraviolet Continuum Radiation in the Impulsive Phase of HF/GF-type dMe Flares. I. Data

“…Models of Fe II will presented in a future work to constrain flaring temperatures and densities implied by this value of E upper . We note that Fe II lines with E upper /hc ∼ 60, 000 cm −1 are prominent in IRIS spectra (De Pontieu et al 2014) of solar flares in the NUV and these lines (in LTE) are sensitive to temperatures around T ∼ 8, 000 − 18, 000 K for a range of densities (Kowalski et al 2017a, Kowalski et al 2018.…”

“…A blackbody-like spectrum with a color temperature of T ∼ 10, 000 K in the impulsive phase of some dMe flares means that there is significant heating at high column mass (m 0.01 g cm −2 ; Kowalski & Allred 2018), which is not possible to reproduce in RHD simulations of low-to-moderately high flux electron beam energy deposition rates (Allred et al 2006). RHD models with very high beam energy deposition rates (Kowalski et al 2015b) suggest that the broadband appearance of a hot blackbody can be explained by hydrogen recombination emissivity that escapes over regions of the atmosphere at T ∼ 10, 000 K with wavelength dependent continuum optical depth of τ λ,continuum between ∼ 0.4 − 5.…”

The Response of the Lyα Line in Different Flare Heating Models

“…3). This region is referred to as chromospheric condensation in momentum balance to the chromospheric evaporation (Fisher et al 1985b;Abbett & Hawley 1999;Kowalski & Allred 2018). The condensation has a downward velocity of around 20 km s −1 , while the evaporation velocity has increased to 100 km s −1 at 1.9 Mm.…”