Cool and dense ejections, typically Hα surges, often appear alongside EUV or X-Ray coronal jets as a result of the emergence of magnetized plasma from the solar interior. Idealized numerical experiments explain those ejections as being indirectly associated with the magnetic reconnection taking place between the emerging and preexisting systems. However, those experiments miss basic elements that can importantly affect the surge phenomenon. In this paper we study the cool surges using a realistic treatment of the radiation transfer and material plasma properties. To that end, the Bifrost code is used, which has advanced modules for the equation of state of the plasma, photospheric and chromospheric radiation transfer, heat conduction and optically thin radiative cooling. We carry out a 2.5D experiment of the emergence of magnetized plasma through (meso)granular convection cells and the low atmosphere to the corona. Through detailed Lagrange tracing, we study the formation and evolution of the cool ejection and, in particular, the role of the entropy sources: this allows us to discern families of evolutionary patterns for the plasma elements. In the launch phase many elements suffer accelerations well in excess of gravity; when nearing the apex of their individual trajectories, instead, the plasma elements follow quasi-parabolic trajectories with acceleration close to g . We show how the formation of the cool ejection is mediated by a wedge-like structure composed of two shocks, one of which leads to the detachment of the surge from the original emerged plasma dome.

The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst other dynamical phenomena, Ellerman bombs (EBs), which are observed in the wings of Hα and are due to magnetic reconnection in the photosphere below the chromospheric canopy. Signs of magnetic reconnection are also observed in other spectral lines, typical of the chromosphere or the transition region. An example are the UV bursts observed in the transition region lines of Si iv and the upper chromospheric lines of Mg ii. In this work we analyze high cadence, high resolution coordinated observations between the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft. Hα images from the SST provide us with the positions, timings and trajectories of EBs in an emerging flux region. Simultaneous, co-aligned IRIS slit-jaw images at 133 (C ii, transition region), 140 (Si iv, transition region) and 279.6 (Mg ii k, core, upper chromosphere) nm, as well as spectroscopy in the far and near ultraviolet from the fast spectrograph raster, allow us to study the possible chromospheric/transition region counterparts of those EBs. Our main goal is to study the possible temporal and spatial relationship between several reconnection events at different layers in the atmosphere (namely EBs and UV bursts), the timing history between them, and the connection of these dynamical phenomena to the ejection of surges in the chromosphere. We also investigate the properties of an extended UV burst and their variations across the burst domain. Our results suggest a scenario where simultaneous and co-spatial EBs and UV bursts are part of the same reconnection system occurring sequentially along a vertical or nearly vertical current sheet. Heating and bidirectional jets trace the location where reconnection takes place. These results support and expand those obtained from recent numerical simulations of magnetic flux emergence.

Surges often appear as a result of the emergence of magnetized plasma from the solar interior. Traditionally, they are observed in chromospheric lines such as Hα 6563 Å and Ca II 8542 Å. However, whether there is a response to the surge appearance and evolution in the Si IV lines or, in fact, in many other transition region lines has not been studied. In this paper, we analyze a simultaneous episode of an Hα surge and a Si IV burst that occurred on 2016 September 03 in active region AR 12585. To that end, we use coordinated observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. For the first time, we report emission of Si IV within the surge, finding profiles that are brighter and broader than the average. Furthermore, the brightest Si IV patches within the domain of the surge are located mainly near its footpoints. To understand the relation between the surges and the emission in transition region lines like Si IV, we have carried out 2.5D radiative MHD (RMHD) experiments of magnetic flux emergence episodes using the Bifrost code and including the nonequilibrium ionization of silicon. Through spectral synthesis, we explain several features of the observations. We show that the presence of Si IV emission patches within the surge, their location near the surge footpoints and various observed spectral features are a natural consequence of the emergence of magnetized plasma from the interior to the atmosphere and the ensuing reconnection processes.

The Interface Region Imaging Spectrograph (IRIS) has been obtaining near- and far-ultraviolet images and spectra of the solar atmosphere since July 2013. IRIS is the highest resolution observatory to provide seamless coverage of spectra and images from the photosphere into the low corona. The unique combination of near- and far-ultraviolet spectra and images at sub-arcsecond resolution and high cadence allows the tracing of mass and energy through the critical interface between the surface and the corona or solar wind. IRIS has enabled research into the fundamental physical processes thought to play a role in the low solar atmosphere such as ion–neutral interactions, magnetic reconnection, the generation, propagation, and dissipation of waves, the acceleration of non-thermal particles, and various small-scale instabilities. IRIS has provided insights into a wide range of phenomena including the discovery of non-thermal particles in coronal nano-flares, the formation and impact of spicules and other jets, resonant absorption and dissipation of Alfvénic waves, energy release and jet-like dynamics associated with braiding of magnetic-field lines, the role of turbulence and the tearing-mode instability in reconnection, the contribution of waves, turbulence, and non-thermal particles in the energy deposition during flares and smaller-scale events such as UV bursts, and the role of flux ropes and various other mechanisms in triggering and driving CMEs. IRIS observations have also been used to elucidate the physical mechanisms driving the solar irradiance that impacts Earth’s upper atmosphere, and the connections between solar and stellar physics. Advances in numerical modeling, inversion codes, and machine-learning techniques have played a key role. With the advent of exciting new instrumentation both on the ground, e.g. the Daniel K. Inouye Solar Telescope (DKIST) and the Atacama Large Millimeter/submillimeter Array (ALMA), and space-based, e.g. the Parker Solar Probe and the Solar Orbiter, we aim to review new insights based on IRIS observations or related modeling, and highlight some of the outstanding challenges.