Alexandra Tritschler

One of the first high-cadence chromospheric image sequences recorded with the recently commissioned Daniel K. Inouye Solar Telescope (DKIST) has revealed propagating arc-shaped bright fronts of a few hundred kilometers width. These bright fronts maintain their arc shape over up to one minute, travel with apparent horizontal speeds of ∼8 km s ⁻¹ , and originate from chromospheric bright grains seen at the center of the arc. We identify photospheric G -band bright points underneath the chromospheric bright grains, which are typically the signature of small-scale magnetic flux concentrations. The G -band bright points are found to interact with colocated vortices prior to the appearance of the chromospheric bright fronts. Images synthesized from three-dimensional magnetohydrodynamic simulations show a striking similarity with the discovered phenomenon and reveal that the arc-shaped bright fronts arise from essentially acoustic, weak shock fronts that propagate in the ambient medium, seemingly triggered by the vortex dynamics of the underlying magnetic flux concentration. In this work, we provide the first hints of a hitherto unknown magnetohydrodynamic process by which vortex-driven torsional Alfvén pulses couple to predominantly acoustic waves that steepen into shocks and dissipate their energies. The detected number density and frequency of these events are at least a factor 10 too small to be a major source of solar chromospheric heating but simulations show a seven times higher number density. We therefore expect to discover many more events with tailored narrow band observations.

Context. Solar Orbiter and the Daniel K. Inouye Solar Telescope (DKIST) are two of the newest facilities available to the solar physics community. The first coordinated observations of the Sun by these two facilities occurred over the course of one week in October 2022. The returned data are open-access and will provide a valuable resource to researchers in the field. Aims. Here, we provide an overview of the datasets collected by Solar Orbiter and DKIST through this coordination and discuss their scientific potential. Our aim is to demonstrate how these unique high-resolution coordinated observations, as well as similar observations obtained through subsequent campaigns, can help tackle important science questions in the field. Methods. A decayed active region (without NOAA number during our observation but identified as AR13110 during previous solar rotation) was simultaneously observed by Solar Orbiter and DKIST at specific times between 18 and 24 October 2022. Between these dates, the Solar Orbiter spacecraft moved from a position with a separation angle of 77° with Earth to a position with a separation angle of 51°, allowing stereoscopic observations to be collected with the ground-based telescope DKIST. From Solar Orbiter, observations are provided by the Extreme Ultraviolet Imager (EUI), Polarimetric and Helioseismic Imager (PHI), and the Spectral Imaging of the Coronal Environment (SPICE) instruments. Meanwhile, DKIST observed using the Cryogenic Near Infrared Spectropolarimeter (CryoNIRSP), the Visible Broadband Imager (VBI), and the Visible Spectropolarimeter (ViSP). Results. Coordinated observations were successfully collected at several distinct times over the week. Despite the active region itself being in an advanced decayed phase, a range of interesting features are evident in the collected data. As such, a variety of research topics can be advanced using these observations. In this article, we focus on three specific topics as representative examples, namely, coronal loop physics, the formation and evolution of the small-scale active region brightenings, and coronal rain dynamics. Conclusions. The first coordinated observation campaign conducted by both Solar Orbiter and DKIST was a success. These open-access observations, and others like them, should help the solar physics community tackle key questions in the field. Such stereoscopic coordinated observations open up a new era in the analysis of the solar atmosphere.

We present the highest-resolution Hα observations of a solar flare to date, collected during the decay phase of an X1.3-class flare on 2024 August 8 at 20:12 UT. Observations with the Visible Broadband Imager at the National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) reveal dark coronal loop strands at unprecedented spatial resolution in the flare arcade above highly structured chromospheric flare ribbons. After surveying the 20 best-seeing images, we calculate a mean loop width near the top of the arcade of 48.2 km, with a minimum loop width of ∼21 km and distribution mode of ∼43 km. The distributions of loop widths observed by the DKIST in our study are often symmetric about the mean loop width. This is initial evidence that the DKIST may be capable of resolving the fundamental scale of coronal loops, although further investigation is required to confirm this result. We demonstrate that the resolving power of the DKIST represents a significant step toward advancing modern flare models and our understanding of fine structure in the coronal magnetic field.

Solar Orbiter and the Daniel K. Inouye Solar Telescope (DKIST) are two of the newest facilities available to the solar physics community. The first coordinated observations of the Sun by these two facilities occurred over the course of one week in October 2022. The returned data are open-access and will provide a valuable resource to researchers in the field. We provide an overview of the datasets collected by Solar Orbiter and DKIST through this coordination and discuss their scientific potential. Our aim is to demonstrate how these unique high-resolution coordinated observations, as well as similar observations obtained through subsequent campaigns, can help tackle important science questions in the field. Between 18 and 24 October 2022, Solar Orbiter and DKIST observed a decayed active region simultaneously. During this period, Solar Orbiter's separation angle with Earth decreased from 77° to 51°, enabling stereoscopic observations with DKIST. From Solar Orbiter, observations are provided by the Extreme Ultraviolet Imager (EUI), Polarimetric and Helioseismic Imager (PHI), and the Spectral Imaging of the Coronal Environment (SPICE) instruments. Meanwhile, DKIST observed using the Cryogenic Near Infrared Spectropolarimeter (CryoNIRSP), the Visible Broadband Imager (VBI), and the Visible Spectropolarimeter (ViSP). The first coordinated Solar Orbiter and DKIST campaign was a success, collecting observations multiple times over the week. We focus on three specific topics as representative examples, namely, coronal loop physics, the formation and evolution of the small-scale active region brightenings, and coronal rain dynamics. These open-access observations, and others like them, should help the solar physics community tackle key questions in the field. Such stereoscopic coordinated observations open up a new era in the analysis of the solar atmosphere.

Striated granular edges observed in the solar photosphere represent one of the smallest-scale phenomena on the Sun. They arise from the interaction of strongly coupled hydrodynamic, magnetic, and radiative properties of the plasma. In particular, modulations in the photospheric magnetic field strength cause variations in density and opacity along the line of sight, leading to their formation. Therefore, the striation patterns can be used as valuable diagnostics for studying the finest-scale structure of the photospheric magnetic field. The Daniel K. Inouye Solar Telescope (DKIST) allows observations of the solar atmosphere with a spatial resolution of better than 0 . ″ 03 with its current instrumentation. We analyze images acquired with the Visible Broadband Imager using the G-band channel to investigate the characteristics of fine-scale striations in the photosphere and compare them with state-of-the-art radiation-MHD simulations at similar spatial resolution. Both observed and synthetic images reveal photospheric striae with widths of approximately 20−50 km, suggesting that at least 4 m class solar telescopes are necessary to resolve this ultrafine structure. Analysis of the numerical simulations confirms that the striation observed in the filtergrams is associated with spatial variations in photospheric magnetic flux concentrations, which cause shifts in the geometrical height where the emergent intensity forms. Some fine-scale striations in the synthetic images originate from magnetic field variations of approximately 100 G, resulting in Wilson depressions as narrow as 10 km. This suggests that DKIST G-band images can trace the footprints of magnetic field variations and Wilson depressions at a similar scale.

Striated granular edges observed in the solar photosphere represent one of the smallest-scale phenomena on the Sun. They arise from the interaction of strongly coupled hydrodynamic, magnetic, and radiative properties of the plasma. In particular, modulations in the photospheric magnetic field strength cause variations in density and opacity along the line of sight, leading to their formation. Therefore, the striation patterns can be used as valuable diagnostics for studying the finest-scale structure of the photospheric magnetic field. The Daniel K. Inouye Solar Telescope allows observations of the solar atmosphere with a spatial resolution of better than 0."03 with its current instrumentation. We analyze images acquired with the Visible Broadband Imager using the G-band channel to investigate the characteristics of fine-scale striations in the photosphere and compare them with state-of-the-art radiation-MHD simulations at similar spatial resolution. Both observed and synthetic images reveal photospheric striae with widths of approximately 20$-$50 km, suggesting that at least 4-meter class solar telescopes are necessary to resolve this ultrafine structure. Analysis of the numerical simulations confirms that the striation observed in the filtergrams is associated with spatial variations in photospheric magnetic flux concentrations, which cause shifts in the geometrical height where the emergent intensity forms. Some fine-scale striation in the synthetic images originate from magnetic field variations of approximately a hundred Gauss, resulting in a Wilson depressions as narrow as 10 km. This suggests that DKIST G-band images can trace footprints of magnetic field variations and Wilson depressions at a similar scale.

Regular remote sensing of the magnetic field embedded within the million-degree solar corona is severely lacking. This reality impedes fundamental investigations of the nature of coronal heating, the generation of solar and stellar winds, and the impulsive release of energy into the solar system via flares and other eruptive phenomena. Resulting from advancements in large aperture solar coronagraphy, we report unprecedented maps of polarized spectra emitted at 1074 nm by Fe ⁺¹² atoms in the active corona. We detect clear signatures of the Zeeman effect that are produced by the coronal magnetic field along the optically thin path length of its formation. Our comparisons with global magnetohydrodynamic models highlight the valuable constraints that these measurements provide for coronal modeling efforts, which are anticipated to yield subsequent benefits for space weather research and forecasting.

We report commissioning observations of the Si x 1430 nm solar coronal line observed coronagraphically with the Cryogenic Near-Infrared Spectropolarimeter at the National Science Foundation’s Daniel K. Inouye Solar Telescope. These are the first known spatially resolved observations of this spectral line, which has strong potential as a coronal magnetic field diagnostic. The observations target a complex active region located on the solar northeast limb on 2022 March 4. We present a first analysis of these data that extracts the spectral line properties through a careful treatment of the variable atmospheric transmission that is known to impact this spectral window. Rastered images are created and compared with extreme-UV observations from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) instrument. A method for estimating the electron density from the Si x observations is then demonstrated that makes use of the forbidden line density-sensitive emissivity and an emission-measure analysis of the SDO/AIA bandpass observations. In addition, we derive an effective temperature and nonthermal line width across the region. This study informs the calibration approaches required for more routine observations of this promising diagnostic line.