We present new results from the Interface Region Imaging Spectrograph showing the dynamic evolution of chromospheric evaporation and condensation in a flare ribbon, with the highest temporal and spatial resolution to date. IRIS observed the entire impulsive phase of the X-class flare SOL2014-09-10T17:45 using a 9.4 second cadence 'sit-and-stare' mode. As the ribbon brightened successively at new positions along the slit, a unique impulsive phase evolution was observed for many tens of individual pixels in both coronal and chromospheric lines. Each activation of a new footpoint displays the same initial coronal up-flows of up to ∼ 300 km s −1 , and chromospheric downflows up to 40 km s −1 . Although the coronal flows can be delayed by over 1 minute with respect to those in the chromosphere, the temporal evolution of flows is strikingly similar between all pixels, and consistent with predictions from hydrodynamic flare models. Given the large sample of independent footpoints, we conclude that each flaring pixel can be considered a prototypical, 'elementary' flare kernel.

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.

Aims. We investigate the oscillatory properties of the quiet solar chromosphere in relation to the underlying photosphere, with particular regard to the effects of the magnetic topology. Methods. For the first time we perform a Fourier analysis on a sequence of line-of-sight velocities measured simultaneously in a photospheric (Fe I 709.0 nm) and a chromospheric line (Ca II 854.2 nm). The velocities were obtained from full spectroscopic data acquired at high spatial resolution with the Interferometric BIdimensional Spectrometer (IBIS). The field of view encompasses a full supergranular cell, allowing us to discriminate between areas with different magnetic characteristics. Results. We show that waves with frequencies above the acoustic cut-off propagate from the photosphere to upper layers only in restricted areas of the quiet Sun. A large fraction of the quiet chromosphere is in fact occupied by "magnetic shadows", surrounding network regions, that we identify as originating from fibril-like structures observed in the core intensity of the Ca II line. We show that a large fraction of the chromospheric acoustic power at frequencies below the acoustic cut-off, residing in the proximity of the magnetic network elements, directly propagates from the underlying photosphere. This supports recent results arguing that network magnetic elements can channel low-frequency photospheric oscillations into the chromosphere, thus providing a way to input mechanical energy in the upper layers.

Context. The chromosphere remains a poorly understood part of the solar atmosphere, as current modeling and observing capabilities are still ill-suited to investigate in depth its fully 3-dimensional nature. In particular, chromospheric observations that can preserve high spatial and temporal resolution while providing spectral information over extended fields of view are still very scarce. Aims. In this paper, we seek to establish the suitability of imaging spectroscopy performed in the Ca II 854.2 nm line as a means to investigate the solar chromosphere at high resolution. Methods. We utilize monochromatic images obtained with the Interferometric BIdimensional Spectrometer (IBIS) at multiple wavelengths within the Ca II 854.2 nm line and over several quiet areas. We analyze both the morphological properties derived from narrow-band monochromatic images and the average spectral properties of distinct solar features such as network points, internetwork areas and fibrils. Results. The spectral properties derived over quiet-Sun targets are in full agreement with earlier results obtained with fixed-slit spectrographic observations, highlighting the reliability of the spectral information obtained with IBIS. Furthermore, the very narrowband IBIS imaging reveals with much clarity the dual nature of the Ca II 854.2 nm line: its outer wings gradually sample the solar photosphere, while the core is a purely chromospheric indicator. The latter displays a wealth of fine structures including bright points, akin to the Ca II H 2V and K 2V grains, as well as fibrils originating from even the smallest magnetic elements. The fibrils occupy a large fraction of the observed field of view even in the quiet regions, and clearly outline atmospheric volumes with different dynamical properties, strongly dependent on the local magnetic topology. This highlights the fact that 1-D models stratified along the vertical direction can provide only a very limited representation of the actual chromospheric physics. Conclusions. Imaging spectroscopy in the Ca II 854.2 nm line currently represents one of the best observational tools to investigate the highly structured and highly dynamical chromospheric environment. A high performance instrument such as IBIS is crucial in order to achieve the necessary spectral purity and stability, spatial resolution, and temporal cadence.

Stably stratified fluids, such as stellar and planetary atmospheres, can support and propagate gravity waves. On Earth these waves, which can transport energy and momentum over large distances and can trigger convection, contribute to the formation of our weather and global climate. Gravity waves also play a pivotal role in planetary sciences (e.g. Young et al. 1997) and modern stellar physics (Charbonnel & Talon 2007). They have also been proposed as an agent for the heating of stellar atmospheres and coronae (Mihalas & Toomre 1981), the exact mechanism behind which is one of the outstanding puzzles in solar