A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

Abstract

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.

The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. II. Hydrogen Broadening Predictions for Solar Flare Observations with the Daniel K. Inouye Solar Telescope

“…Solar flares are one such manifestation of magnetic activity and space weather that will increase in regularity. We are continuing to learn much about the solar fares that occurred in Sunspot Cycle 24, due in part to missions such as RHESSI, Hinode, the Goode Solar Telescope, and the Interface Region Imaging Spectrograph (IRIS) (Melrose 2018;Kerr et al 2020a,b;De Pontieu et al 2021). In particular, powerlaw (hereafter, "beam") electrons with order-of-magnitude greater inferred heating fluxes than were typically reported in the past (e.g., Neidig et al 1994) have become commonplace (Neidig et al 1993;Krucker et al 2011;Milligan et al 2014;Alaoui & Holman 2017;Graham et al 2020).…”

Probing the physics of the solar atmosphere with the Multi-slit Solar Explorer (MUSE): I. Coronal Heating

“…The detailed nature of the processes that power the corona and solar wind remains poorly constrained even though access to both new observations and numerical models has advanced the field rapidly the last ten years (Carlsson et al 2019;Reale 2014;Van Doorsselaere et al 2020;De Pontieu et al 2021). We know that mechanical driving in the lower atmosphere transports sufficient energy flux to sustain the million-degree corona (De Pontieu et al 2007;Rast et al 2021), and that waves, cur-rents and reconnection may carry or release substantial energy, but it remains unclear how important each is for the local energy balance, how this depends on the ambient environment (e.g., active regions, quiet Sun), and how the conversion of non-thermal to thermal energy works in detail.…”

“…We know that mechanical driving in the lower atmosphere transports sufficient energy flux to sustain the million-degree corona (De Pontieu et al 2007;Rast et al 2021), and that waves, cur-rents and reconnection may carry or release substantial energy, but it remains unclear how important each is for the local energy balance, how this depends on the ambient environment (e.g., active regions, quiet Sun), and how the conversion of non-thermal to thermal energy works in detail. While previous missions like Hinode, SDO, and IRIS have shown tantalizing glimpses of how magneto-convective energy generated in the interior of the Sun drives solar activity and energizes the low solar atmosphere and corona, currently available observations lack coronal coverage and/or spatio-temporal resolution and are unable to arbitrate between competing theories of coronal heating (e.g., Hinode Review Team et al 2019;De Pontieu et al 2021). These theories invoke processes like wave propagation, mode conversion, and dissipation through turbulence or resonant absorption, field-line braiding and magnetic reconnection, or nonthermal particle acceleration.…”

“…The physical processes at work in the solar outer atmosphere, leading to the heating of coronal plasma to millions of degrees, to the acceleration of the solar wind, and to dynamic events such as flares and coronal mass ejections (CMEs) are still poorly understood. Although significant progress has been made recently, thanks to the ever increasing quality of solar observations as well as continuous advances in numerical modeling (e.g., Reale 2014;Van Doorsselaere et al 2020;De Pontieu et al 2021), the lack of spectroscopic measurements with sufficient spatial and temporal resolution and spatial coverage has hampered progress in understanding these phenomena, because they involve many spatial and temporal scales at once. The Multi-slit Solar Explorer (MUSE, De Pontieu et al 2020;Cheung et al 2019a), a mission proposed to NASA as a Medium-class Explorer and now in a Phase A study, aims to overcome these shortcomings of single-slit spectrometers by using an innovative approach using multiple slits and several different and narrow EUV spectral bands.…”

Simultaneous Observations of Chromospheric Evaporation and Condensation during a C-class Flare

“…The mergence/move speed could be as high as about 200 km s −1 , which is roughly equal to the upflow speed of the hot evaporated materials. Thanks to the spectroscopic observations, the chromospheric evaporation in solar flares can be well diagnosed by the line profile (e.g., Milligan & Dennis 2009;Doschek et al 2013;Tian et al 2014;Li et al 2015;Polito et al 2017;De Pontieu et al 2021). In the typical chromospheric evaporation, the hot coronal lines always appear high-velocity blue shifts, which are attributed to the fast upward mass motions.…”