This paper presents measurements of the energy radiated by the lower solar atmosphere, at optical, UV, and EUV wavelengths, during an X-class solar flare (SOL2011-02-15T01:56) in response to an injection of energy assumed to be in the form of nonthermal electrons. Hard X-ray observations from RHESSI were used to track the evolution of the parameters of the nonthermal electron distribution to reveal the total power contained in flare accelerated electrons. By integrating over the duration of the impulsive phase, the total energy contained in the nonthermal electrons was found to be > 2 × 10 31 erg. The response of the lower solar atmosphere was measured in the free-bound EUV continua of H I (Lyman), He I, and He II, plus the emission lines of He II at 304Å and H I (Lyα) at 1216Å by SDO/EVE, the UV continua at 1600Å and 1700Å by SDO/AIA, and the WL continuum at 4504Å, 5550Å, and 6684Å, along with the Ca II H line at 3968Å using Hinode/SOT. The summed energy detected by these instruments amounted to ∼ 3 × 10 30 erg; about 15% of the total nonthermal energy. The Lyα line was found to dominate the measured radiative losses. Parameters of both the driving electron distribution and the resulting chromospheric response are presented in detail to encourage the numerical modelling of flare heating for this event, to determine the depth of the solar atmosphere at which these line and continuum processes originate, and the mechanism(s) responsible for their generation.

We use radiation hydrodynamic simulations to examine two models of solar flare chromospheric heating: Alfvén wave dissipation and electron beam collisional losses. Both mechanisms are capable of strong chromospheric heating, and we show that the distinctive atmospheric evolution in the mid-to-upper chromosphere results in Mg ii k-line emission that should be observably different between wave-heated and beam-heated simulations. We also present Ca ii 8542Å profiles which are formed slightly deeper in the chromosphere. The Mg ii k-line profiles from our wave-heated simulation are quite different from those from a beam-heated model and are more consistent with IRIS observations. The predicted differences between the Ca ii 8542Å in the two models are small. We conclude that careful observational and theoretical study of lines formed in the mid-toupper chromosphere holds genuine promise for distinguishing between competing models for chromospheric heating in flares.

The bulk of the radiative output of a solar flare is emitted from the chromosphere, which produces enhancements in the optical and UV continuum, and in many lines, both optically thick and thin. We have, until very recently, lacked observations of two of the strongest of these lines: the Mg ii h and k resonance lines. We present a detailed study of the response of these lines to a solar flare. The spatial and temporal behaviour of the integrated intensities, k/h line ratios, line of sight velocities, line widths and line asymmetries were investigated during an M class flare (SOL2014-02-13T01:40). Very intense, spatially localised energy input at the outer edge of the ribbon is observed, resulting in redshifts equivalent to velocities of ∼15-26 km s −1 , line broadenings, and a blue asymmetry in the most intense sources. The characteristic central reversal feature that is ubiquitous in quiet Sun observations is absent in flaring profiles, indicating that the source function increases with height during the flare. Despite the absence of the central reversal feature, the k/h line ratio indicates that the lines remain optically thick during the flare. Subordinate lines in the Mg ii passband are observed to be in emission in flaring sources, brightening and cooling with similar timescales to the resonance lines. This work represents a first analysis of potential diagnostic information of the flaring atmosphere using these lines, and provides observations to which synthetic spectra from advanced radiative transfer codes can be compared.

The Interface Region Imaging Spectrograph (IRIS) routinely observes the Si iv resonance lines. When analyzing observations of these lines it has typically been assumed they form under optically thin conditions. This is likely valid for the quiescent Sun, but this assumption has also been applied to the more extreme flaring scenario. We used 36 electron beam driven radiation hydrodynamic solar flare simulations, computed using the RADYN code, to probe the validity of this assumption. Using these simulated atmospheres we solved the radiation transfer equations to obtain the non-LTE, non-equilibrium populations, line profiles, and opacities for a model Silicon atom, including charge exchange processes. This was achieved using the 'minority species' version of RADYN. The inclusion of charge exchange resulted in a substantial fraction of Si iv at cooler temperatures than those predicted by ionisation equilibrium. All simulations with an injected energy flux F > 5 × 10 10 erg cm −2 s −1 resulted in optical depth effects on the Si iv emission, with differences in both intensity and line shape compared to the optically thin calculation. Weaker flares (down to F ≈ 5 × 10 9 erg cm −2 s −1 ) also resulted in Si iv emission forming under optically thick conditions, depending on the other beam parameters. When opacity was significant, the atmospheres generally had column masses in excess of 5×10 −6 g cm −2 over the temperature range 40 to 100 kK, and the Si iv formation temperatures were between 30 and 60 kK. We urge caution when analyzing Si iv flare observations, or when computing synthetic emission without performing a full radiation transfer calculation.