We report the first detection of a magnetic discontinuity in the upper solar chromosphere associated with a coronal loop brightening. Our findings are based on novel observations obtained by the Chromospheric LAyer SpectroPolarimeter (CLASP2.1) suborbital space experiment. During its short 6 minute flight, CLASP2.1 successfully obtained spectropolarimetric data across the Mg ii h and k lines, which allowed us to map the longitudinal magnetic field at multiple heights in the chromosphere of a solar active region. During our observation, we found a coronal loop brightening that was intermittently and recurrently heated. The temperature of the brightening plasma is above 2.5 MK, which is higher than the ambient temperature, and no corresponding brightening is detected in the images of the photosphere and lower chromosphere. The notable finding of our study is that the coronal loop brightening of interest is observed across a region where the magnetic field polarity changes abruptly in the upper chromosphere. Our results provide observational evidence that the heating mechanism of the coronal loop brightening is consistent with magnetic reconnection in the upper solar chromosphere.

We determine magnetic fields from the photosphere to the upper chromosphere combining data from the Hinode satellite and the CLASP2.1 sounding rocket experiment. CLASP2.1 provided polarization profiles of the Mg |sc ii h and k lines, as well as of the Mn |sc i lines around 2800 |AA, across various magnetic structures in an active region, containing a plage, a pore, and the edges of a sunspot penumbra. By applying the Weak-Field Approximation (WFA) to the circular polarization profiles of these spectral lines, we obtain a longitudinal magnetic field map at three different heights in the chromosphere (lower, middle, and upper). This is complemented by data from Hinode (photospheric magnetic field), IRIS, and SDO (high-spatial-resolution observations of the chromosphere and corona). We quantify the height expansion of the plage magnetic fields and find that the magnetic fields expand significantly in the middle chromosphere, shaping the moss observed above in the transition region and corona. We identified an area with polarity reversal at the upper chromosphere around the edge of the pore, suggesting the presence of a magnetic discontinuity in the upper chromosphere. Transient and recurrent jet-like events are observed in this region, likely driven by magnetic reconnection. Around the penumbral edge, we find large-scale magnetic fields corresponding to the superpenumbral fibrils seen in the upper chromosphere. In the superpenumbral fibrils, we find Zeeman-induced linear polarization signals, suggesting the presence of a significantly inclined magnetic field, as strong as 1000 G in the upper chromosphere.

The Chromospheric LAyer SpectroPolarimeter missions, CLASP2 and CLASP2.1, demonstrated that the near-UV spectral region between 279.30 and 280.68 nm is suitable for studying the magnetism of the solar chromosphere. In particular, the spectropolarimetric observations in the Mg ii h and k resonant doublet, Mn i 279.91 and 280.19 nm resonant lines, and Fe ii 279.79 and 280.66 nm lines acquired by these suborbital space experiments have been proven useful for inferring the magnetic field stratification in the solar chromosphere. However, several lines of the CLASP2/2.1 spectral region with significant circular polarization signals had remained unexplored. After identifying two Ni i (279.95 and 280.59 nm) lines, one Mn ii (280.62 nm) line, and one Fe i (280.53 nm) line, here we apply the weak-field approximation to the spectropolarimetric observations of active region plages by CLASP2 and CLASP2.1. By comparing the results with previous studies, we are able to estimate the formation heights of these CLASP2/2.1 additional spectral lines and to demonstrate their suitability to determine the magnetic field stratification from the photosphere to the upper chromosphere.

We apply the HanleRT Tenerife Inversion Code to the spectro-polarimetric observations obtained by the Chromospheric LAyer SpectroPolarimeter. This suborbital space experiment measured the variation with wavelength of the four Stokes parameters in the near-ultraviolet spectral region of the Mg II h & k lines over a solar disk area containing part of an active region plage and the edge of a sunspot penumbra. We infer the stratification of the temperature, the electron density, the line of-sight velocity, the micro-turbulent velocity, and the longitudinal component of the magnetic field from the observed intensity and circular polarization profiles. The inferred model atmosphere shows larger temperature and electron density in the plage and the superpenumbra regions than in the quiet regions. The shape of the plage region in terms of its brightness is similar to the pattern of the inferred longitudinal component of the magnetic field in the chromosphere, as well as to that of the overlying moss observed by AIA in the 171 A band, which suggests a similar magnetic origin for the heating in both the plage and the moss region. Moreover, this heating is particularly significant in the regions with larger inferred magnetic flux. In contrast, in the superpenumbra, the regions with larger electron density and temperature are usually found in between these regions with larger magnetic flux, suggesting that the details of the heating mechanism in the chromosphere of the superpenumbra may be different to those in the plage, but with the magnetic field still playing a key role.

Nanoflares are thought to be one of the prime candidates that can heat the solar corona to its multi-million kelvin temperature. Individual nanoflares are difficult to detect with the present generation instruments, however their presence can be inferred by comparing simulated nanoflare-heated plasma emissions with the observed emission. Using HYDRAD coronal loop simulations, we model the emission from an X-ray bright point (XBP) observed by the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), along with nearest-available observations from the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO) and X-Ray Telescope (XRT) onboard Hinode observatory. The length and magnetic field strength of the coronal loops are derived from the linear-force-free extrapolation of the observed photospheric magnetogram by Helioseismic and Magnetic Imager (HMI) onboard SDO. Each loop is assumed to be heated by random nanoflares, whose magnitude and frequency are determined by the loop length and magnetic field strength. The simulation results are then compared and matched against the measured intensity from AIA, XRT, and MaGIXS. Our model results indicate the observed emissions from the XBP under study could be well matched by a distribution of nanoflares with average delay times 1500 s to 3000 s, which suggest that the heating is dominated by high-frequency events. Further, we demonstrate the high sensitivity of MaGIXS and XRT to diagnose the heating frequency using this method, while AIA passbands are found to be the least sensitive.

Regular multi-height measurements of the magnetic field of the chromosphere are crucial for understanding the solar atmosphere and the flow of energy between the photosphere and corona.The need for these measurements has been understood for years and stated in recent decadal surveys.The techniques and instrumentation necessary to obtain consistent measurements of the chromospheric magnetic field are now sufficiently mature to realize this goal in the coming decade.The tenability of multi-height chromospheric magnetic field measurements has been shown on sub-orbital rockets, such as CLASP, and a limited subset of these capabilities may become available with ALMA and DKIST.We recommend a continued focus on the study of chromospheric magnetism through modeling but also an increased investment in instrumentation that has the power to continuously observe the magnetic field of the entire chromosphere, as well as tools to interpret those observations.

COMPLETE is a flagship mission concept combining broadband spectroscopic imaging and comprehensive magnetography from multiple viewpoints around the Sun to enable tomographic reconstruction of 3D coronal magnetic fields and associated dynamic plasma properties, which provide direct diagnostics of energy release. COMPLETE re-imagines the paradigm for solar remote-sensing observations through purposefully co-optimized detectors distributed on multiple spacecraft that operate as a single observatory, linked by a comprehensive data/model assimilation strategy to unify individual observations into a single physical framework. We describe COMPLETE's science goals, instruments, and mission implementation. With targeted investment by NASA, COMPLETE is feasible for launch in 2032 to observe around the maximum of Solar Cycle 26.

Heliophysics image data largely relies on a forty-year-old ecosystem built on the venerable Flexible Image Transport System (FITS) data standard. While many in situ measurements use newer standards, they are difficult to integrate with multiple data streams required to develop global understanding. Additionally, most data users still engage with data in much the same way as they did decades ago. However, contemporary missions and models require much more complex support for 3D multi-parameter data, robust data assimilation strategies, and integration of multiple individual data streams required to derive complete physical characterizations of the Sun and Heliospheric plasma environment. In this white paper we highlight some of the 21$^\mathsf{st}$ century challenges for data frameworks in heliophysics, consider an illustrative case study, and make recommendations for important steps the field can take to modernize its data products and data usage models. Our specific recommendations include: (1) Investing in data assimilation capability to drive advanced data-constrained models, (2) Investing in new strategies for integrating data across multiple instruments to realize measurements that cannot be produced from single observations, (3) Rethinking old data use paradigms to improve user access, develop deep understanding, and decrease barrier to entry for new datasets, and (4) Investing in research on data formats better suited for multi-dimensional data and cloud-based computing.

The coronal magnetic field is the prime driver behind many as-yet unsolved mysteries: solar eruptions, coronal heating, and the solar wind, to name a few. It is, however, still poorly observed and understood. We highlight key questions related to magnetic energy storage, release, and transport in the solar corona, and their relationship to these important problems. We advocate for new and multi-point co-optimized measurements, sensitive to magnetic field and other plasma parameters, spanning from optical to $\gamma$-ray wavelengths, to bring closure to these long-standing and fundamental questions. We discuss how our approach can fully describe the 3D magnetic field, embedded plasma, particle energization, and their joint evolution to achieve these objectives.

Radiative transfer investigations of the solar Mg ii h and k resonance lines around 280 nm have shown that, while their circular polarization (Stokes V ) signals arise from the Zeeman effect, the linear polarization profiles (Stokes Q and U ) are dominated by the scattering of anisotropic radiation and the Hanle and magneto-optical (MO) effects. Using the unprecedented observations of the Mg ii and Mn i resonance lines obtained by the Chromospheric LAyer Spectro-Polarimeter (CLASP2), here we investigate how the linear polarization signals at different wavelengths (i.e., at the center, and at the near and far wings of the k line) vary with the longitudinal component of the magnetic field ( B L ) at their approximate height of formation. The B L is estimated from the V signals in the aforementioned spectral lines. Particular attention is given to the following quantities that are expected to be influenced by the presence of magnetic fields through the Hanle and MO effects: the sign of the U signals, the total linear polarization amplitude (LP) and its direction ( χ ) with respect to a reference direction. We find that at the center and near wings of the k line, the behavior of these quantities is significantly different in the observed quiet and plage regions, and that both LP and χ seem to depend on B L . These observational results are indicative of the operation of the Hanle effect at the center of the k line and of the MO effects at the near wings of the k line.

It is essential that there be coordinated and co-optimized observations in X-rays, gamma-rays, and EUV during the peak of solar cycle 26 (~2036) to significantly advance our understanding of impulsive energy release in the corona. The open questions include: What are the physical origins of space-weather events? How are particles accelerated at the Sun? How is impulsively released energy transported throughout the solar atmosphere? How is the solar corona heated? Many of the processes involved in triggering, driving, and sustaining solar eruptive events -- including magnetic reconnection, particle acceleration, plasma heating, and energy transport in magnetized plasmas -- also play important roles in phenomena throughout the Universe. This set of observations can be achieved through a single flagship mission or, with foreplanning, through a combination of major missions (e.g., the previously proposed FIERCE mission concept).

We apply the Tenerife Inversion Code (TIC) to the plage spectropolarimetric observations obtained by the Chromospheric LAyer SpectroPolarimeter (CLASP2). These unprecedented data consist of full Stokes profiles in the spectral region around the Mg ii h and k lines for a single slit position, with around two thirds of the 196″ slit crossing a plage region and the rest crossing an enhanced network. A previous analysis of these data had allowed us to infer the longitudinal component of the magnetic field by applying the weak-field approximation (WFA) to the circular polarization profiles, and to assign the inferred magnetic fields to different layers of the solar atmosphere based on the results of previous theoretical radiative transfer investigations. In this work, we apply the recently developed TIC to the same data. We obtain a stratified model atmosphere that fits the intensity and circular polarization profiles at each position along the spectrograph slit and we compare our results for the longitudinal component of the magnetic field with the previously obtained WFA results, highlighting the generally good agreement in spite of the fact that the WFA is known to produce an underestimation when applied to the outer lobes of the Mg ii h and k circular polarization profiles. Finally, we use the inverted model atmospheres to give a rough estimation of the energy that could be carried by Alfvén waves propagating in the chromosphere in the plage and network regions, showing that it is sufficient to compensate for the estimated energy losses in the chromosphere of solar active regions.

The outer atmosphere of the Sun is composed of plasma heated to temperatures well in excess of the visible surface. We investigate short cool and warm (<1 MK) loops seen in the core of an active region to address the role of field-line braiding in energizing these structures. We report observations from the High-resolution Coronal imager (Hi-C) that have been acquired in a coordinated campaign with the Interface Region Imaging Spectrograph (IRIS). In the core of the active region, the 172 Å band of Hi-C and the 1400 Å channel of IRIS show plasma loops at different temperatures that run in parallel. There is a small but detectable spatial offset of less than 1″ between the loops seen in the two bands. Most importantly, we do not see observational signatures that these loops might be twisted around each other. Considering the scenario of magnetic braiding, our observations of parallel loops imply that the stresses put into the magnetic field have to relax while the braiding is applied: the magnetic field never reaches a highly braided state on these length scales comparable to the separation of the loops. This supports recent numerical 3D models of loop braiding in which the effective dissipation is sufficiently large that it keeps the magnetic field from getting highly twisted within a loop.

The Extreme-ultraviolet Stellar Characterization for Atmospheric Physics and Evolution (ESCAPE) mission is an astrophysics Small Explorer employing ultraviolet spectroscopy (EUV: 80 - 825 Å and FUV: 1280 - 1650 Å) to explore the high-energy radiation environment in the habitable zones around nearby stars. ESCAPE provides the first comprehensive study of the stellar EUV and coronal mass ejection environments which directly impact the habitability of rocky exoplanets. In a 20 month science mission, ESCAPE will provide the essential stellar characterization to identify exoplanetary systems most conducive to habitability and provide a roadmap for NASA's future life-finder missions. ESCAPE accomplishes this goal with roughly two-order-of-magnitude gains in EUV efficiency over previous missions. ESCAPE employs a grazing incidence telescope that feeds an EUV and FUV spectrograph. The ESCAPE science instrument builds on previous ultraviolet and X-ray instrumentation, grazing incidence optical systems, and photon-counting ultraviolet detectors used on NASA astrophysics, heliophysics, and planetary science missions. The ESCAPE spacecraft bus is the versatile and high-heritage Ball Aerospace BCP Small spacecraft. Data archives will be housed at the Mikulski Archive for Space Telescopes (MAST).

We have developed an advanced UV spectropolarimeter called Chromospheric LAyer SpectroPolarimeter (CLASP2), aimed at achieving very high accuracy measurements (<0.1% at $3\sigma $ ) of the linear ( $Q/I$ and $U/I$ ) and circular ( $V/I$ ) polarizations of the Mg ii h and k lines (280 nm). CLASP2 was launched on board a NASA sounding rocket on April 11, 2019. It successfully detected the full Stokes vector in an active-region plage and in the quiet Sun near the limb across the Mg ii h and k lines for the first time. To verify the polarization characteristics of CLASP2, the response matrix is estimated by combining the results obtained from the preflight calibration on the ground, with the results of the inflight calibration acquired at the solar-disk center. We find that the response matrix of CLASP2 in the Mg ii h and k lines is notably close to an ideal response matrix, i.e., the scale factor and the crosstalk terms are close to 1 and 0, respectively. Moreover, the uncertainty of each Stokes parameter estimated by the repeatability of the measurements is verified to be within the required tolerance. Based on our investigation, we conclude that CLASP2 achieves $0.1\%$ polarization accuracy at a $3\sigma $ level.

Routine ultraviolet imaging of the Sun’s upper atmosphere shows the spectacular manifestation of solar activity; yet, we remain blind to its main driver, the magnetic field. Here, we report unprecedented spectropolarimetric observations of an active region plage and its surrounding enhanced network, showing circular polarization in ultraviolet (Mg ii h & k and Mn i ) and visible (Fe i ) lines. We infer the longitudinal magnetic field from the photosphere to the very upper chromosphere. At the top of the plage chromosphere, the field strengths reach more than 300 G, strongly correlated with the Mg ii k line core intensity and the electron pressure. This unique mapping shows how the magnetic field couples the different atmospheric layers and reveals the magnetic origin of the heating in the plage chromosphere.

Solar flares are rapid energy release phenomena that appear as bright ribbons in the chromosphere and high temperature loops in the corona, respectively. Supra-arcade Downflows (SADs) are plasma voids that first come out above the flare loops and then move quickly toward the flare loop top during the decay phase of the flare. In our work, we study 20 SADs appearing in three flares. By differential emission measure (DEM) analysis, we calculate the DEM weighted average temperature and emission measure of the front region and the main body of SADs. It is found that the temperatures of the SAD front and body tend to increase during the course of SADs flowing downwards. The relationship between the pressure and temperature fits well with the adiabatic equation for both the SAD front and body, suggesting that the heating of SADs is mainly caused by adiabatic compression. Moreover, we also estimate the velocities of SADs via the Fourier Local Correlation Tracking method and find that increase of the temperature of the SAD front presents a correlation with the decrease of the SAD kinetic energy, while the SAD body does not, implying that the viscous process may also heat the SAD front in spite of a limited role.

The National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute.

The University of Colorado led Extreme-ultraviolet Stellar Characterization for Atmospheric Physics and Evolution (ESCAPE) small explorer mission concept is designed to measure the extreme- and far-ultraviolet (EUV; 80 - 560 A, 600 - 825 A, FUV; 1280 - 1650 A) irradiance and are activity of exoplanet host stars; essential measurements for assessing the stability of rocky planet atmospheres in the liquid-water habitable zone. The ESCAPE design consists of a fixed optical configuration with a grazing incidence Gregorian, or "Hetterick- Bowyer", telescope feeding grazing and normal incidence spectroscopic channels. The telescope is provided by a joint NASA Marshall Space Flight Center and Smithsonian Astrophysics Observatory team. The grazing incidence gratings have a radial profile and are ruled into single-crystal silicon using electron-beam lithography in the nanofabrication laboratory at Pennsylvania State University. Normal incidence gratings have aberration correcting holographic solutions and are supplied by Horiba Jobin Yvon. Spectra are imaged onto a curved microchannel plate detector supplied by the University of California, Berkeley. ESCAPE utilizes the Ball Aerospace BCP spacecraft. The simple, fixed configuration design of ESCAPE is projected to exceed the effective area of the last major EUV astrophysics spectrograph, EUV E-DS/S, by more than a factor of 50, providing unprecedented sensitivity in this essential bandpass for exoplanet host-star characterization. We report on the ESCAPE design, projected performance and mission implementation plan, as well as the trade studies carried out over Phase A to scope the first NASA EUV astrophysics mission in nearly 30 years. If selected, ESCAPE will launch in Fall 2025.

Following the success of the first mission, the High-Resolution Coronal Imager (Hi-C) was launched for a third time (Hi-C 2.1) on 29th May 2018 from the White Sands Missile Range, NM, USA. On this occasion, 329 seconds of 17.2 nm data of target active region AR 12712 was captured with a cadence of ~4s, and a plate scale of 0.129''/pixel. Using data captured by Hi-C 2.1 and co-aligned observations from SDO/AIA 17.1 nm we investigate the widths of 49 coronal strands. We search for evidence of substructure within the strands that is not detected by AIA, and further consider whether these strands are fully resolved by Hi-C 2.1. With the aid of Multi-Scale Gaussian Normalization (MGN), strands from a region of low-emission that can only be visualized against the contrast of the darker, underlying moss are studied. A comparison is made between these low-emission strands with those from regions of higher emission within the target active region. It is found that Hi-C 2.1 can resolve individual strands as small as ~202km, though more typical strands widths seen are ~513km. For coronal strands within the region of low-emission, the most likely width is significantly narrower than the high-emission strands at ~388km. This places the low-emission coronal strands beneath the resolving capabilities of SDO/AIA, highlighting the need of a permanent solar observatory with the resolving power of Hi-C.