[en] We present radiative-hydrodynamic simulations of solar flares generated by the RADYN and RH codes to study the perturbations induced in photospheric Fe i lines by electron beam heating. We investigate how variations in the beam parameters result in discernible differences in the induced photospheric velocities. Line synthesis revealed a significant chromospheric contribution to the line profiles resulting in an apparent red asymmetry by as much as 40 m s⁻¹ close to the time of maximum beam heating, which was not reflective of the upflow velocities that arose from the radiative-hydrodynamic simulations at those times. The apparent redshift to the overall line profile was produced by significant chromospheric emission that was blueshifted by as much as 400 m s⁻¹ and fills in the blue side of the near-stationary photospheric absorption profile. The velocity information that can be retrieved from photospheric line profiles during flares must therefore be treated with care to mitigate the effects of higher parts of the atmosphere providing an erroneous velocity signal.

[en] We present an analysis of off-limb cool flare loops observed by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) during the gradual phase of SOL2017-09-10T16:06 X8.2-class flare. In the extreme-ultraviolet (EUV) channels starting from the 335 Å one, cool loops appear as dark structures against the bright loop arcade. These dark structures were precisely coaligned (spatially and temporally) with loops observed by Swedish Solar Telescope (SST) in emission lines of hydrogen and ionized calcium. A recently published semi-empirical model of cool loops based on SST observations serves to predict the level of hydrogen and helium recombination continua. The continua were synthesized using an approximate non-LTE (i.e., departures from local thermodynamic equilibrium) approach and theoretical spectra were then transformed to AIA signals. Comparison with signals detected inside the dark loops shows that only in AIA 211 Å channel the computed level of recombination continua is consistent with observations for some models, while in all other channels that are more distant from the continua edges the synthetic continuum is far too low. In analogy with on-disk observations of flares we interpret the surplus emission as due to numerous EUV lines emitted from hot but faint loops in front of the cool ones. Finally we briefly comment on failure of the standard absorption model when used for analysis of the dark-loop brightness.

[en] High-resolution observations of dynamic phenomena give insights into the properties and processes that govern the low solar atmosphere. We present an analysis of jet-like phenomena emanating from a penumbral footpoint in active region (AR) 12192 using imaging and spectral observations from the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. These jets are associated with line-of-sight Doppler speeds of ±10–22 km s⁻¹ and bright fronts that seem to move across the plane-of-sky at speeds of 23–130 km s⁻¹. Such speeds are considerably higher than the expected sound speed in the chromosphere. The jets have signatures that are visible both in the cool and hot channels of IRIS and AIA. Each jet lasts on average 15 minutes and occurs 5–7 times over a period of 2 hr. Possible mechanisms to explain this phenomenon are suggested, the most likely of which involve p-mode or Alfvén wave shock trains impinging on the transition region and corona as a result of steepening photospheric wavefronts or gravity waves.

[en] We study a solar spicule observed off-limb using high-resolution imaging spectroscopy in the Ca ii 8542 Å line obtained with the CRisp Imaging SpectroPolarimeter (CRISP) on the Swedish 1 m Solar Telescope. Using a new version of the non-LTE code NICOLE specifically developed for this problem we invert the spicule single- and double-peak line profiles. This new version considers off-limb geometry and computes atomic populations by solving the 1D radiative transfer assuming a vertical stratification. The inversion proceeds by fitting the observed spectral profiles at 14 different heights with synthetic profiles computed in the model by solving the radiative transfer problem along its length. Motivated by the appearance of double-peak Ca ii 8542 Å spicule profiles, which exhibit two distinct emission features well separated in wavelength, we adopt a double-component scenario. We start from the ansatz that the spicule parameters are practically constant along the spicule axis for each component, except for a density drop. Our results support this ansatz by attaining very good fits to the entire set of 14 × 4 profiles (14 heights and 4 times). We show that the double-component model with uniform temperature of 9560 K, exponential decrease of density with a height scale of 1000–2000 km, and the counter-oriented line-of-sight velocities of components reproduce the double-peak line profiles at all spicule segments well. Analyses of the numerical response function reveals the necessity of the inversions of spectra at multiple height positions to obtain height-dependent, degeneracy-free reliable models with a limited number of free parameters.

[en] We present multi-wavelength observations of an M-class flare (M3.9) that occurred on 2014 June 11. Our observations were conducted with the Dunn Solar Telescope (DST), employing adaptive optics, the multi-camera system Rapid Oscillations in Solar Atmosphere (ROSA), the new Hydrogen-Alpha Rapid Dynamics camera (HARDcam) in various wavelengths, such as Ca II K, Mg I b₂ (at 5172.7 Å), and Hα narrow band and G-band continuum filters. Images were re-constructed using the Kiepenheuer-Institut Speckle Interferometry Package (KISIP) code, to improve our image resolution. We observed intensity increases of ≈120%–150% in the Mg, Ca K and Hα narrow band filters during the flare. Intensity increases for the flare observed in the SDO EUV channels were several times larger, and the X-rays, as recorded by GOES, increased over a factor of 30 for the harder band. Only a modest delay was found between the onset of flare ribbons of a nearby sympathetic flare and the main flare ribbons observed in these narrow band filters. The peak flare emission occurred within a few seconds for the Ca K, Mg and Hα bands. Time-distance techniques indicate propagation velocities of ≈60 km s⁻¹ for the main flare ribbon and as high as 300 km s⁻¹ for smaller regions, which we attribute to filament eruptions. This result and delays and velocities observed with SDO (≈100 km s⁻¹) for different coronal heights agree well with the simple model of energy propagation versus height, although a more detailed model for the flaring solar atmosphere is needed. Finally, we detected marginal quasi-periodic pulsations (QPPs) in the 40–60 s range for the Ca K, Mg and Hα bands, and such measurements are important for disentangling the detailed flare-physics. (paper)

[en] 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.