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During solar flares, a large flux of energetic electrons propagate from the tops of reconnecting magnetic flux tubes toward the lower atmosphere. Over the course of the electrons' transport, a co-spatial counterstreaming return current is induced, thereby balancing the current density. In response to the return-current electric field, a fraction of the ambient electrons will be accelerated into the runaway regime. However, models describing the accelerated electron beam/return-current system have not previously taken these suprathermal runaway electrons into account self-consistently. We develop a model in which an accelerated electron beam drives a steady-state, sub-Dreicer co-spatial return-current electric field, which locally balances the direct beam current and freely accelerates a fraction of background (return-current) electrons. The model is self-consistent, i.e., the electric field induced by the coevolution of the direct beam and the runaway current is considered. We find that (1) the return-current electric field can return a significant number of suprathermal electrons to the acceleration region, where they can be further accelerated to higher energies, runaway electrons can be a few tens of percent of the return-current flux returning to the nonthermal beam's acceleration region, (2) the energy gain of the suprathermal electrons can be up to 10–35 keV, (3) the heating rate in the corona can be reduced by a factor of 3 for medium range injected fluxes in comparison to models which neglect the runaway component. The results depend on the injected beam flux density, the temperature, and density of the background plasma. [ABSTRACT FROM AUTHOR]

While solar flares are predominantly characterized by an intense broadband enhancement to the solar radiative output, certain spectral lines and continua will, in theory, exhibit flare-induced dimmings. Observations of transitions of orthohelium He i λλ 10830 Å and the He i D3 lines have shown evidence of such dimming, usually followed by enhanced emission. It has been suggested that nonthermal collisional ionization of helium by an electron beam, followed by recombinations to orthohelium, is responsible for overpopulating those levels, leading to stronger absorption. However, it has not been possible observationally to preclude the possibility of overpopulating orthohelium via enhanced photoionization of He i by EUV irradiance from the flaring corona followed by recombinations. Here we present radiation hydrodynamics simulations of nonthermal electron-beam-driven flares where (1) both nonthermal collisional ionization of helium and coronal irradiance are included, and (2) only coronal irradiance is included. A grid of simulations covering a range of total energies deposited by the electron beam and a range of nonthermal electron-beam low-energy cutoff values were simulated. In order to obtain flare-induced dimming of the He i 10830 Å line, it was necessary for nonthermal collisional ionization to be present. The effect was more prominent in flares with larger low-energy cutoff values and longer lived in weaker flares and flares with a more gradual energy deposition timescale. These results demonstrate the usefulness of orthohelium line emission as a diagnostic of flare energy transport. [ABSTRACT FROM AUTHOR]

We report on multi-wavelength monitoring observations of an M-dwarf flare star AD Leonis with the Seimei Telescope (6150–7930 Å), SCAT (Spectroscopic Chuo-university Astronomical Telescope; 3700–7500 Å), and NICER (Neutron Star Interior Composition Explorer; 0.2–12.0 keV), with the collaboration of the OISTER (Optical and Infrared Synergetic Telescopes for Education and Research) program. Twelve flares are detected in total, including ten Hα, four X-ray, and four optical-continuum flares; one of them is a superflare with a total energy of ∼2.0 × 1033 erg. We found that: (1) during the superflare, the Hα emission line full width at 1/8 maximum dramatically increases to 14 Å from 8 Å in the low-resolution spectra (R ∼ 2000) accompanied by large white-light flares, (2) some weak Hα/X-ray flares are not accompanied by white-light emissions, and (3) the non-flaring emissions show clear rotational modulations in X-ray and Hα intensity in the same phase. To understand these observational features, one-dimensional hydrodynamic flare simulations are performed using the RADYN code. We find the simulated Hα line profiles with hard and high-energy non-thermal electron beams to be consistent with the initial phase line profiles of the superflares, while those with a softer and/or weak-energy beam are consistent with those in decay phases, indicating the changes in the energy fluxes injected to the lower atmosphere. Also, we find that the relation between the optical continuum and Hα intensity is nonlinear, which can be one cause of the non-white-light flares. The flare energy budget exhibits diversity in the observations and models, and more observations of stellar flares are necessary for constraining the occurrence of various emission line phenomena in stellar flares. [ABSTRACT FROM AUTHOR]

Flare ribbons are bright manifestations of flare energy dissipation in the lower solar atmosphere. For the first time, we report on high-resolution imaging spectroscopy observations of flare ribbons situated off limb in the Hβ and Ca ii 8542 Å lines and make a detailed comparison with radiative hydrodynamic simulations. Observations of the X8.2 class solar flare SOL 2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright horizontal emission layers in Hβ line-wing images located near the footpoints of the flare loops. The apparent separation between the ribbon observed in the Hβ wing and the nominal photospheric limb is about 300–500 km. The Ca ii 8542 Å line-wing images show much fainter ribbon emissions located right on the edge of the limb, without clear separation from the limb. RADYN models are used to investigate synthetic spectral line profiles for the flaring atmosphere, and good agreement is found with the observations. The simulations show that, toward the limb, where the line of sight is substantially oblique with respect to the vertical direction, the flaring atmosphere model reproduces the high contrast of the off-limb Hβ ribbons and their significant elevation above the photosphere. The ribbons in the Ca ii 8542 Å line-wing images are located deeper in the lower solar atmosphere with a lower contrast. A comparison of the height deposition of electron beam energy and the intensity contribution function shows that the Hβ line-wing intensities can be a useful tracer of flare energy deposition in the lower solar atmosphere. [ABSTRACT FROM AUTHOR]

We study the evolution of chromospheric line and continuum emission during the impulsive phase of the X-class SOL2014-09-10T17:45 solar flare. We extend previous analyses of this flare to multiple chromospheric lines of Fe i , Fe ii , Mg ii , C i , and Si ii observed with the Interface Region Imaging Spectrograph, combined with radiative-hydrodynamical (RHD) modeling. For multiple flaring kernels, the lines all show a rapidly evolving double-component structure: an enhanced emission component at rest, and a broad, highly redshifted component of comparable intensity. The redshifted components migrate from 25 to 50 km s−1 toward the rest wavelength within ∼30 s. Using Fermi hard X-ray observations, we derive the parameters of an accelerated electron beam impacting the dense chromosphere, using them to drive an RHD simulation with the RADYN code. As in Kowalski et al. (2017), our simulations show that the most energetic electrons penetrate into the deep chromosphere, heating it to T ∼ 10,000 K, while the bulk of the electrons dissipate their energy higher, driving an explosive evaporation, and its counterpart condensation—a very dense (ne ∼ 2 × 1014 cm−3), thin layer (30–40 km thickness), heated to 8–12,000 K, moving toward the stationary chromosphere at up to 50 km s−1. The synthetic Fe ii 2814.45 Å profiles closely resemble the observational data, including a continuum enhancement, and both a stationary and a highly redshifted component, rapidly moving toward the rest wavelength. Importantly, the absolute continuum intensity, ratio of component intensities, relative time of appearance, and redshift amplitude are sensitive to the model input parameters, showing great potential as diagnostics. [ABSTRACT FROM AUTHOR]