Abstract:The physical processes resulting in energy exchange between the Sun's hot corona and its cool lower atmosphere are still poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (~20-60s) of intensity and velocity on small spatial scales (≲500km) at the footpoints of hot and dynamic coronal loops. Comparison with numerical simulations reveal that the observations are consistent with heating by beams of non-thermal electrons and that these beams are generated even in small impulsive (≲30s) heating events called "coronal nanoflares". The accelerated electrons deposit a significant fraction of their energy (≲10 25 erg) in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the non-flaring corona. Main Text:Though it is established that the magnetic field plays a major role in the energetics of the bright corona, determining the details of the physical mechanisms that heat the solar corona remains one of the outstanding open issues in astrophysics. There are several candidate physical processes for heating the corona, including dissipation of magnetic stresses via reconnection, and dissipation of magnetohydrodynamic waves (1,2,3). In many heating models, the energy release is characterized by small spatial and temporal scales. For instance, in the "nanoflare" model, random photospheric motions lead to braiding or shearing of magnetic field lines and to reconnection which yields impulsive heating of the coronal plasma (4,5). Several statistical studies of large numbers of solar flares (6-8) have suggested that the mechanisms producing flares are likely similar within a large range from micro-to X-class flares. If nanoflares behave as a scaled down version of larger flares, particles accelerated in the corona by reconnection processes could play a significant role in the heating of plasma even in absence of large flares. Hard X-ray observations of microflares (E~10 27 erg) in active regions reveal the presence of non-thermal particles (8,9), but nanoflare size events (E~10 24 erg) are not currently accessible to hard X-ray studies due to limited sensitivity. As a result, the properties and generation of non-thermal particles in the solar atmosphere and their role in quiescent coronal heating remain poorly known.The observational tracers of the coronal heating are elusive because the corona is highly conductive, washing out the signatures of heating release. However, the emission of the TR, where the temperature steeply increases to MK values in a narrow layer (~1-3 ×10 8 cm), is instead highly responsive to heating since its density, temperature gradients and spatial dimensions change rapidly during heating events (10)(11)(12). This is the also the case for coronal heating events where ...

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.

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.

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