The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29

Abstract

The 2014 March 29 X1 solar flare (SOL20140329T17:48) produced bright continuum emission in the far-and near-ultraviolet (NUV) and highly asymmetric chromospheric emission lines, providing long-sought constraints on the heating mechanisms of the lower atmosphere in solar flares. We analyze the continuum and emission line data from the Interface Region Imaging Spectrograph (IRIS) of the brightest flaring magnetic footpoints in this flare. We compare the NUV spectra of the brightest pixels to new radiative-hydrodynamic predictions calculated with the RADYN code using constraints on a nonthermal electron beam inferred from the collisional thick-target modeling of hard X-ray data from Reuven Ramaty High Energy Solar Spectroscopic Imager. We show that the atmospheric response to a high beam flux density satisfactorily achieves the observed continuum brightness in the NUV. The NUV continuum emission in this flare is consistent with hydrogen (Balmer) recombination radiation that originates from low optical depth in a dense chromospheric condensation and from the stationary beam-heated layers just below the condensation. A model producing two flaring regions (a condensation and stationary layers) in the lower atmosphere is also consistent with the asymmetric Fe II chromospheric emission line profiles observed in the impulsive phase.

OBSERVATIONS AND SIMULATIONS OF THE Na i D1 LINE PROFILES IN AN M-CLASS SOLAR FLARE

“…This suggests that the actual footpoint size of the flaring loops could be smaller, and hence the energy flux could be higher than the estimated 10 11 erg s −1 cm −2 . To assess the effects of a higher energy input on the synthesized Na I D 1 line profile we performed RADYN simulations for a stronger (5× F11) energy flux (Kowalski et al 2016) and used the resulting atmospheric snapshots in RH for synthesizing Na I D 1 line profiles. The resulting line profiles show a similar evolution pattern.…”

Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres

“…These RWAs exhibit spectrally resolved peaks with redshifts of λ − λ rest = 15 − 140 km s −1 . The brightness of the RWA in NUV Fe II lines relative to the intensity of the line component at the rest wavelength has been reproduced with a high flux electron beam of 5x10 11 erg cm −2 s −1 (hereafter, 5F11; Kowalski et al 2017a). In magnetically active M dwarf (dMe) flares, the observed NUV and optical flare continuum (sometimes referred to as white-light radiation if detected in broadband optical radiation on the Sun or in the Johnson U -band in dMe stars) distribution can be reproduced in 1D model snapshots of a very dense, evolved CC that results from the extremely high energy flux density (∼ 10 13 erg cm −2 s −1 , hereafter F13) in nonthermal electron beams lasting several seconds (Kowalski et al , 2017b.…”

“…Thus, these high energy particles are likely the source of powering most of the chromospheric heating and radiative response. Moreover, recent high spatial resolution imagery of NUV and optical footpoints in solar flares suggest a very high electron beam flux density (Fletcher et al 2007;Krucker et al 2011;Kleint et al 2016;Jing et al 2016;Kowalski et al 2017a;Sharykin et al 2017) which may be difficult to sustain due to plasma instabilities (Lee et al 2008;Li et al 2014).…”

“…The small time steps (10 −7 − 10 −8 s) in these calculations are caused by the accuracy of the helium population convergence at these steep gradients and are exacerbated by a radiative instability in a very narrow (∼ 5 km), cool region between the flare corona and the large temperature gradient at the chromospheric explosion (see Kennedy et al 2015). The onset threshold of explosive evaporation and condensation in the chromosphere depends upon all of the parameters that characterize electron beam heating (Fisher 1989) but generally occurs at high energy flux densities, typically exceeding 10 11 erg cm −2 s −1 (F11) with a moderate low-energy cutoff E c value (E c ∼ 25 keV; Kowalski et al 2017a). …”

“…The spectrally resolved red-wing asymmetry (often referred to as "RWA") in chromospheric lines such as Hα, Mg II, and Fe II is frequently observed in the impulsive phase of flares (Ichimoto & Kurokawa 1984;Graham & Cauzzi 2015;Kowalski et al 2017a). These RWAs exhibit spectrally resolved peaks with redshifts of λ − λ rest = 15 − 140 km s −1 .…”