We analyze decay phase observations of the GOES class C6.7 flare SOL2022-08-19T20:31 by the Visible Spectropolarimeter (ViSP) on the National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST). The data include the first flare-time DKIST observations of the chromospheric Ca II H 396.8 nm and H ϵ 397.0 nm spectral lines. These diagnostics have rarely been studied together during the modern era of high-resolution solar flare observations, and never at the spectral and spatial resolution of the DKIST. We directly compare DKIST spectra to state-of-the-art RADYN+RH simulations, including one heated by a nonthermal electron beam and one by in-situ thermal conduction. While certain salient properties of the spectra such as the width of H ϵ are reproduced, the models severely underestimate the width of Ca II H in the red wing and fail to reproduce the exact relative intensity of Ca II H to H ϵ . The models exhibit a range of chromospheric electron densities spanning over an order of magnitude. Unlike the modeled lower-order Balmer-series lines, we find that the width of H ϵ is not solely related to the high-density upper chromosphere; the widths and intensities are also sensitive to the deeper flare layers. We outline possible avenues towards improvement of flare models, such as a comprehensive evaluation of flare heating mechanisms in the context of both impulsive and decay phase high-resolution data.

We present the highest-resolution H α 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.

We present an analysis of flare ribbon fine structure observed during a GOES C2-class flare using high spatial and spectral resolution multiline spectropolarimetric observations from the ViSP instrument at the DKIST. The ViSP recorded full Stokes spectra in three lines: Fe i 6301 & Aring; line pair, Na i D1, and Ca ii 8542 & Aring;. To infer the stratification of temperature and line-of-sight (LOS) velocity across the ribbon, we performed non-local thermodynamic equilibrium multiline inversions of ViSP spectra. In the red wing of the Ca ii line, we identified multiple compact, roundish, and quasi-equally spaced bright structures, referred to as ribbon blobs, embedded within the flare ribbon. The sizes of these blobs range from 320 to 455 km, and they are spaced roughly similar to 1100 km apart. These features exhibit complex spectral profiles with pronounced asymmetries and double peaks near the line core of the Ca ii 8542 line. The blob regions were found to be significantly hotter (by similar to 1 kK) at log tau 500=-4 compared to surrounding ribbon areas (similar to 7 kK). The LOS velocity maps revealed both upflows and downflows at log tau 500=-4 and log tau 500=-3 , respectively. We discuss the plausible origins of these fine structures in the chromosphere, which may be related to electron beam heating, plasma draining, or tearing-mode instabilities in the reconnecting current sheet.

Coronal plumes are narrow, collimated structures that are primarily viewed above the solar poles and in coronal holes in the extreme ultraviolet, but also in sunspots. Open questions remain about plume formation, including the role of small-scale transients and whether plumes embedded in different magnetic field configurations have similar formation mechanisms. We report on coordinated Solar Orbiter/Extreme Ultraviolet Imager (EUI), Interface Region Imaging Spectrograph, and Solar Dynamics Observatory observations of the formation of a plume in sunspot penumbra in 2022 March. During this observation, Solar Orbiter was positioned near the Earth–Sun line and EUI observed at a 5 s cadence with a spatial scale of 185 km pixel ^−1 in the solar corona. We observe fine-scale dots at various locations in the sunspot, but the brightest and highest density of dots is at the plume base. Space-time maps along the plume axis show parabolic and V-shaped patterns, and we conclude that some of these dots are possible signatures of magneto-acoustic shocks. Compared to other radial cuts around the sunspot, along the plume shows the longest periods (∼7 minutes) and the most distinct tracks. Bright dots at the plume base are mostly circular and do not show elongations from a fixed origin, in contrast to jetlets and previously reported penumbral dots. We do not find high-speed, repeated downflows along the plume, and the plume appears to brighten coherently along its length. Our analysis suggests that jetlets and downflows are not a necessary component of this plume’s formation, and that mechanisms for plume formation could be dependent on magnetic topology and the chromospheric wave field.

The convectively driven, weakly magnetized regions of the solar photosphere dominate the Sun’s surface at any given time, but the temporal variations of these quiet regions of the photosphere throughout the solar cycle are still not well known. To look for cycle-dependent changes in the convective properties of the quiet Sun photosphere, we use high spatial and spectral resolution spectropolarimetric observations obtained by the Hinode Solar Optical Telescope and apply the Spectropolarimetric Inversions Based on Response Functions code to infer physical conditions in the lower solar photosphere. Using a homogeneous set of 49 data sets, all taken at the disk center, we analyze the temperature stratification and the line-of-sight velocities of the granules and intergranules over a period of 15 years. We use a k-means clustering technique applied to the spectral profiles to segment the granules and intergranules based on both intensity and velocity. We also examine the profile bisectors of these different structures and compare them to past analyses. Our results show fairly constant properties over this period with no clear dependence on the solar cycle. We do, however, find a slight increase in the photospheric temperature gradient during the declining phase of the solar cycle. Our findings could have significant implications for understanding the coupling between the quiet Sun atmosphere and the global solar dynamo.

Three uniquely powerful solar and heliospheric facilities are now operational at the same time. The US National Science Foundation's Daniel K Inouye Solar Telescope, NASA's Parker Solar Probe, and ESA's Solar Orbiter each represent frontiers in space science, and each pursue richly tailored science missions. At the intersection of these missions, though, lie unparalleled opportunities for multi-vantage point science. This symbiotic relationship is especially pronounced during PSP's perihelia and Solar Orbiter remote science windows. As the most advanced solar polarimeter ever built, DKIST strengthens many of the multi-facility use cases by opening new diagnostic windows into solar magnetism – spanning the photosphere, chromosphere, and corona – at unprecedented spatial, spectral, and temporal resolution. In this article, we report recent efforts to maximize the scientific potential of coordinated DKIST, PSP, and Solar Orbiter observations. Existing DKIST data from coordinated observations with Solar Orbiter and PSP are highlighted alongside some first investigations of these data.

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 degrees with Earth to a position with a separation angle of 51 degrees, 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.

The US National Science Foundation 4m Daniel K. Inouye Solar Telescope (DKIST) on Haleakala, Maui is the largest solar telescope in the world. DKIST's superb resolution and polarimetric sensitivity will enable astronomers to explore the origins of solar magnetism, the mechanisms of coronal heating and drivers of flares and coronal mass ejections. DKIST operates as a coronagraph at infrared wavelengths, providing crucial measurements of the magnetic field in the corona. During its Operations Commissioning Phase, DKIST has already conducted a significant number of shared-risk observations for community researchers. The complex raw data are calibrated by the DKIST Data Center located in Boulder and distributed to the science community. We'll present examples of science results and discuss lessons learned. Ongoing instrument development efforts include, an upgrade of the single-conjugate adaptive optics system to a multi-conjugate AO, the implementation of image slicers for the DL-NIRSP instrument and development of infrared detectors the DL- and CRYO-NIRSP instruments.

The strongly coupled hydrodynamic, magnetic, and radiation properties of the plasma in the solar chromosphere makes it a region of the Sun's atmosphere that is poorly understood. We use data obtained with the high-resolution Visible Broadband Imager (VBI) equipped with an H$\beta$ filter and the Visible Spectro-Polarimeter (ViSP) at the Daniel K. Inouye Solar Telescope to investigate the fine-scale structure of the plage chromosphere. To aid the interpretation of the VBI imaging data, we also analyze spectra from the CHROMospheric Imaging Spectrometer on the Swedish Solar Telescope. The analysis of spectral properties, such as enhanced line widths and line depths explains the high contrast of the fibrils relative to the background atmosphere demonstrating that H$\beta$ is an excellent diagnostic for the enigmatic fine-scale structure of the chromosphere. A correlation between the parameters of the H$\beta$ line indicates that opacity broadening created by overdense fibrils could be the main reason for the spectral line broadening observed frequently in chromospheric fine-scale structures. Spectropolarimetric inversions of the ViSP data in the Ca II 8542 {\AA} and Fe I 6301/6302 {\AA} lines are used to construct semiempirical models of the plage atmosphere. Inversion outputs indicate the existence of dense fibrils in the Ca II 8542 {\AA} line. The analyses of the ViSP data show that the morphological characteristics, such as orientation, inclination and length of fibrils are defined by the topology of the magnetic field in the photosphere. Chromospheric maps reveal a prominent magnetic canopy in the area where fibrils are directed towards the observer.

We present high-resolution observations of a C4.1-class solar flare (SOL2023-05-03T20:53) in AR 13293 from the Visible Spectro-Polarimeter (ViSP) and Visible Broadband Imager (VBI) instruments at the DKIST. The fast cadence, good resolution, and high polarimetric sensitivity of ViSP data provide a unique opportunity to explore the photospheric magnetic fields before and during the flare. We infer the magnetic field vector in the photosphere from the Fe i 6302 & Aring; line using Milne-Eddington inversions. Combined analysis of the inverted data and VBI images reveals the presence of two opposite polarity pores exhibiting rotational motion both prior to and throughout the flare event. Data-driven simulations further reveal a complex magnetic field topology above the rotating pores, including a null-point-like configuration. We observed a 30% relative change in the horizontal component (delta F h ) of Lorentz force at the flare peak time and roughly no change in the radial component. We find that the changes in delta F h are the most likely driver of the observed pore rotation. These findings collectively suggest that the back reaction of magnetic field line reconfiguration in the corona may influence the magnetic morphology and rotation of pores in the photosphere on a significantly smaller scale.

The Sun's proximity offers us a unique opportunity to study in detail the physical processes on a star's surface; however, the highly dynamic nature of the stellar surface -- in particular, energetic eruptions such as flares and coronal mass ejections -- presents tremendous observational challenges. Spectroscopy probes the physical state of the solar atmosphere, but conventional scanning spectrographs and spectrometers are unable to capture the full evolutionary history of these dynamic events with a sufficiently wide field of view and high spatial, spectral, and temporal resolution. Resolving the physics of the dynamic sun requires gathering simultaneous spectra across a contiguous area over the full duration of these events, a goal now tantalizingly close to achievable with continued investment in developing powerful new Integral Field Spectrographs to serve as the foundation of both future ground- and space-based missions. This technology promises to revolutionize our ability to study solar flares and CMEs, addressing NASA's strategic objective to "understand the Sun, solar system, and universe." Since such events generate electromagnetic radiation and high-energy particles that disrupt terrestrial electric infrastructure, this investment not only advances humanity's scientific endeavors but also enhances our space weather forecasting capability to protect against threats to our technology-reliant civilization.

The National Science Foundation's Daniel K. Inouye Telescope (DKIST) has recently commenced operations, and soon data will become available to the community at large.With this whitepaper, we advocate for a continuing

Comparisons between the Atacama Large Millimeter/sub-millimeter Array (ALMA) 3 mm emission and a range of optical and UV solar observations have found the strongest correspondence between the width of the hydrogen alpha line at 656.3 nm and the 3 mm brightness temperature. Previous studies on the oscillatory power of p-modes using ALMA Band 3 and Band 6 data in the 3–5 min period bandpass have found a confusing mix of results, with many reporting a complete lack of the p-mode enhancement typically found in other chromospheric observables. We study these issues using an extensive, publicly available coordinated data set targeting a region of weak network flux near disk center at time SOL 2017-03-17T15:42-16:45. We focus on the Interferometric Bidimensional Spectropolarimeter (IBIS) H-alpha and ALMA 3 mm data series. We confirm the strong correlation between the H-alpha line width and the 3 mm brightness temperature, but find a bimodal relation between the two diagnostics, with a shallower slope of 7.4e-5 Å/K in cooler regions and steeper slope of 1.2e-4 Å/K in hotter regions. The origin of the bimodal distribution is unknown, but does hold for the duration of the observations. Both slopes are steeper than a previously reported value, but this is likely due to systematic differences in the analysis. We then calculate the oscillatory power in the H-alpha and 3 mm data. The IBIS data clearly show the p-mode oscillations in spatially averaged power spectra while the ALMA data do not. However, when we remove IBIS data at times corresponding to the ALMA calibration windows, the spatially averaged power spectra for the two data series are nearly identical, with a Pearson correlation coefficient of 0.9895. Further, the power in the two bands remains strongly correlated when the spatial information is retained but the power is integrated over different temporal frequency bands. We therefore argue that the lack of observed p-modes in the ALMA data may be predominantly due to spectral windowing induced by the timing and duration of the calibration observations. Finally, we find that spatial maps of oscillatory power at 3 mm display the pattern of magnetic shadows and halos typically displayed by other chromospheric diagnostics.

Models and preliminary studies indicate that the 300-400 nm spectral region hosts a wealth of diagnostics, fundamental for a large variety of solar studies, including abundance estimates, physics of flares, dynamo processes, atmosphere modeling and solar/stellar variability.However, due to the lower solar emission on one hand, and observational constraints on the other (seeing, scattered light and reduced optical and atmospheric transmittance), this spectral region is for the vast majority unexplored with modern instrumentation.The diagnostic capabilities of the near-ultraviolet will be demonstrated by data that will be soon acquired by the Sunrise UV Spectro-Polarimeter and Imager aboard the SUNRISE III balloon-borne telescope.However, SUNRISE III will operate in a short (~days) temporal window.On the contrary, many characteristics of DKIST, including its large aperture, i.e. its high photon collecting power, even further enhanced by its ability to accommodate instruments at its Nasmyth focus, and the exceptional sky quality of the observatory site, which is characterized by low level of scattered light, make it now possible to routinely perform ground-based observations in the near-ultraviolet region.With this White Paper we make the case for a DKIST second-generation spectro-polarimeter that operates in the 300-400 nm spectral range, and lay out its high level scientific requirements.

Plage regions are patches of concentrated magnetic field in the Sun’s atmosphere where hot coronal loops are rooted. While previous studies have shed light on the properties of plage magnetic fields in the photosphere, there are still challenges in measuring the overlying chromospheric magnetic fields, which are crucial to understanding the overall heating and dynamics. Here, we utilize high-sensitivity, spectropolarimetric data obtained by the 4 meter Daniel K. Inouye Solar Telescope to investigate the dynamic environment and magnetic field stratification of an extended, decaying plage region. The data show strong circular polarization signals in both plage cores and surrounding fibrils. Notably, weak linear polarization signals clearly differentiate between plage patches and the fibril canopy, where they are relatively stronger. Inversions of the Ca II 8542 Å spectra show an imprint of the fibrils in the chromospheric magnetic field, with typical field strength values ranging from ∼200 to 300 G in fibrils. We confirm the weak correlation between field strength and cooling rates in the lower chromosphere. Additionally, we observe supersonic downflows and strong velocity gradients in the plage periphery, indicating dynamical processes occurring in the chromosphere. These findings contribute to our understanding of the magnetic field and dynamics within plages, emphasizing the need for further research to explore the expansion of magnetic fields with height and the three-dimensional distribution of heating rates in the lower chromosphere.

The strongest known correspondence between ALMA 3mm emission and other solar observations is between the H-alpha line width and 3 mm brightness temperature, while the typical 3-5min p-mode oscillations found in many chromospheric diagnostics are often lacking from ALMA Band 3 and 6 observations. We study these issues using a publicly available data set of weak network flux near disk center at time SOL2017-03-17T15:42-16:45 that includes IBIS H-alpha and ALMA 3 mm data series. We confirm the correlation between the H-alpha line width and the 3 mm temperature, but find a different slope between the two diagnostics for hot versus cool regions, both of which are steeper than previous reports. The origin of the two slopes is unknown, but does hold for the duration of the observations. Spatially averaged power spectra of the IBIS data do show p-mode oscillations while the ALMA data do not. However, removing IBIS data at times corresponding to the ALMA calibration windows makes the averaged power spectra for the two data series nearly identical. Spatial maps of the power integrated over p-mode frequency bands agree well between the two data series and show the typical pattern of magnetic shadows and halos found in many chromospheric diagnostics. We therefore argue that the lack of observed p-modes in the ALMA data may be predominantly due to spectral windowing induced by the timing and duration of the calibration observations.

Advances in our understanding of the Sun will require the accurate and consistent measurement of the physical conditions in different regions of the solar atmosphere.The richest information, encoding the full state integrated over of the atmospheric volume in which they are formed, is contained in the detailed shapes of the numerous spectral lines present in the solar spectrum.But this integration along the line of sight is highly non-linear, which means that inferring the true physical conditions requires solving a complicated, ill-posed inverse problem.This challenge is being addressed through powerful codes that treat ever more complex aspects of the solar atmosphere -the breakdown of local thermodynamic equilibrium (or non-LTE), non-equilibrium hydrogen ionization, deviations from hydrostatic equilibrium, and multiple methods of deriving chromospheric and coronal magnetic fields.However, to reach the full potential of these techniques, the community must make a concerted, sizable effort to improve and enhance them, both in terms of their physical veracity as well as their computational tractability.These methods will be essential for fully exploiting the data from new and proposed solar facilities or missions.Reliable outputs from inversions will be a key component of more realistic extrapolations of the magnetic field of active regions and the heliosphere.These inversion methods, if accurate and robust, can become a key tool in determining the structure of stellar atmospheres, but will also become crucial for the operational prediction of the energetics of the Sun's magnetic field and the driving of space weather and changes in the solar spectral irradiance.

We review observations of solar activity, geomagnetic variation, and auroral visibility for the extreme geomagnetic storm on 1872 February 4. The extreme storm (referred to here as the Chapman–Silverman storm) apparently originated from a complex active region of moderate area (≈ 500 μ sh) that was favorably situated near disk center (S19° E05°). There is circumstantial evidence for an eruption from this region at 9–10 UT on 1872 February 3, based on the location, complexity, and evolution of the region, and on reports of prominence activations, which yields a plausible transit time of ≈29 hr to Earth. Magnetograms show that the storm began with a sudden commencement at ≈14:27 UT and allow a minimum Dst estimate of ≤ −834 nT. Overhead aurorae were credibly reported at Jacobabad (British India) and Shanghai (China), both at 19.°9 in magnetic latitude (MLAT) and 24.°2 in invariant latitude (ILAT). Auroral visibility was reported from 13 locations with MLAT below ∣20∣° for the 1872 storm (ranging from ∣10.°0∣–∣19.°9∣ MLAT) versus one each for the 1859 storm (∣17.°3∣ MLAT) and the 1921 storm (∣16.°2∣ MLAT). The auroral extension and conservative storm intensity indicate a magnetic storm of comparable strength to the extreme storms of 1859 September (25.°1 ± 0.°5 ILAT and −949 ± 31 nT) and 1921 May (27.°1 ILAT and −907 ± 132 nT), which places the 1872 storm among the three largest magnetic storms yet observed.

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