Predicted white-light solar flare emission from the F-CHROMA grid of models

Abstract

Much of a solar flare's energy is thought to be released in the continuum. The optical continuum (white light) is of special interest due to the ability to observe it from the ground. We aim to investigate the prevalence of white-light (WL) emissions in simulations of purely electron beam-driven solar flares, what determines the occurrence of these enhancements, and the underlying causes. We utilized the F-CHROMA grid of flare simulations created using the radiative hydrodynamics code RADYN. We probed the spectral index, total energy, and low-energy cutoff to draw conclusions about their relationships to the white-light intensity. Furthermore, we calculated the 6684 Å continuum intensities, the Balmer, and the Paschen ratios. Finally, we analyzed two particular cases, one with high 6684 Å intensity and one with a large Balmer ratio, to determine the dominant mechanisms in these simulations. 33 of the 84 flares included in the F-CHROMA grid show white-light intensity enhancements that exceed 0.1% relative to the pre-flare level. We conclude that, with the parameters presented in the F-CHROMA grid, purely electron beam-driven simulations of solar flares are not able to reproduce observed WL enhancements, as the maximum enhancements in the grid are below 4%. The total energy (which is correlated with the maximum beam flux) is the main factor for deciding whether excess white-light emissions will be detectable. There is a linear relationship between the Balmer (and Paschen) ratio and the relative continuum increase. Both case studies show that during the time of maximum WL excess, hydrogen ionization and subsequent recombination in an optically thin medium is the dominant mechanism for WL continuum emission enhancements. Increased H$^-$ emission in the photosphere as a result of radiative backwarming becomes dominant during the declining phase of WL emissions in both case studies.

Figures (14)

• Figure 1: Light curves of $I_{c,rel}$, as well as intensities left and right of the Balmer jump for a WL and a non-WL case. Left: $I_{c,rel}$ as a function of time for the WL case $\mathrm{d3\_1.0e12\_20}$ and the non-WL case $\mathrm{d3\_1.0e11\_10}$. Right: Intensities blueward (at 3646.9 Å; solid lines) and redward (at 3647.1 Å; dashed lines) of the Balmer jump. In both panels, red indicates the WL case, whereas gray specifies the non-WL case.
• Figure 2: $I_{c,max}$ as a function of $E_c$, $d$, and $E_{tot}$ for all simulations included in the F-CHROMA grid. The values for $d$ have been divided into four distinct locations to avoid overlap and to show $E_{tot}$ corresponding to each simulation (indicated at the top). Dashed vertical lines indicate the border between the different spectral index values. The size corresponds to $I_{c,max}$, as indicated in the legend below the x-axis. Red indicates WL cases, whereas gray specifies non-WL cases. The cases used for the case studies are indicated with numbers 1 and 2.
• Figure 3: Scatter plots of the $R_{B,max}$ (top panel) and $R_{P,max}$ (middle panel) as a function of $I_{c,max}$. The bottom panel shows $R_{P,max}$ as a function of $R_{B,max}$. In all panels, red indicates WL cases, whereas gray specifies non-WL cases.
• Figure 4: $I_{c,rel}$ (top) and $R_B$ (bottom) as a function of time for the two cases $\mathrm{d4\_1.0e12\_20}$ ("case 1," dashed blue line) and $\mathrm{d3\_1.0e12\_25}$ ("case 2," dashed-triple-dotted magenta line).
• Figure 5: Beam heating term in the internal energy equation for case 1 (top panel) and case 2 (bottom panel) as a function of height and time. The scale is displayed as a color bar on the right side of each panel. The bin size in the space domain is 10 km, and 0.1 s in the time domain.