Damian J. Christian

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.

Bright points (BPs) are small-scale, dynamic features that are ubiquitous across the solar disc and are often associated with the underlying magnetic field. Using broadband photospheric images obtained with the Visible Broadband Imager at the National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST), the properties of BPs have been analyzed with DKIST for the first time at the highest spatial resolutions achievable. BPs were observed to have an average lifetime of 95 ± 29 s and a mean transverse velocity of 1.60 ± 0.41 km s ⁻¹ . The BPs had a log-normal area distribution with a peak at 2300 km ² . Transverse velocities and lifetimes across the DKIST images were comparable and consistent with previous studies. The area distribution of the DKIST data peaked in areas significantly lower than those from the literature. This was explored further and was observed to be due to an overestimation of BP areas due to the merging of close features when the spatial resolution is reduced, in tandem with possible oversplitting of features in the DKIST images. Furthermore, the effect of variable seeing in the data was determined. This showed that the average spatial resolution of the data was around 0 . ″ 034 ± 0 . ″ 007 , in comparison to the theoretical diffraction limit of 0 . ″ 022 . Accounting for the influence of seeing, the peak of the area distribution of BPs in the DKIST data was estimated as 4800 km ² , which is still significantly lower than previously observed.

We present a uniform analysis of compact Ca ii K (3934 Å) brightenings that occur near flare kernels and assess their value as short-lead indicators of solar flare onset. Using high-cadence imaging from the Rapid Oscillations in the Solar Atmosphere instrument at the Dunn Solar Telescope, we examine eight flare sequences (seven C-class and one B-class) obtained between 2021 and 2025. Fixed, detector-coordinate regions of interest are used to generate mean-intensity light curves, which are detrended and smoothed to isolate impulsive brightenings. In every event, a compact Ca ii K brightening is detected within or adjacent to the flaring region that peaks 10–45 minutes before the primary kernel and the corresponding rise in GOES 1–8 Å flux. The measured temporal offsets scale with the deprojected separation between the brightening and flare kernels, implying an apparent propagation speed of ∼30–35 km s ⁻¹ that is consistent with chromospheric reconnection. Complementary Spectropolarimeter for Infrared and Optical Regions spectropolarimetry for one event shows topological reconfiguration from closed to open or extended connectivity, supporting a reconnection-driven origin. These results demonstrate that compact Ca ii K brightenings are reproducible, physically meaningful precursors to flare onset. Their simplicity and cadence make them attractive chromospheric indicators, and future work will evaluate their predictive skill alongside established UV/EUV and magnetic diagnostics.

We present high resolution X-ray spectroscopy of the active late-type star HR 1099 obtained with the Chandra Low Energy Transmission Grating Spectrometer. A spectrum extracted from the early stages of a flare was created to search for enhanced emission due to line coincidence photopumping at X-ray wavelengths, rather than the usual optical and ultraviolet cases. Specifically, we have identified a scheme where a He-like Ne ix line at 11 Å is photopumped by He-like Na x , which predicts intensity enhancement in the Ne ix 82.76 Å transition under the high electron density and large coronal pathlength conditions found in stellar and solar flares. We previously tentatively identified a Ne ix enhancement in a solar flare spectrum, but the result was extremely uncertain. However, Ne ix 82.76 Å was detected at the 3-sigma level in the Chandra spectrum, leading to a measured Ne ix 82.76/13.45 intensity ratio of 0.21 ± 0.08. By contrast, the theoretical ratio from the CHIANTI database is at most 0.031, indicating an enhancement factor of at least 7 ± 3 for the 82.76 Å line, in agreement with the expected value of ∼7–15 from a photopumping plasma model. This confirms detection, for the first time to our knowledge, of X-ray photopumping in an astrophysical object. We note that the identification of such X-ray photopumping in other spatially unresolved sources provides in principle a new diagnostic for independently determining the sizes of their coronal regions.

Solar magnetic fields are responsible for solar coronal mass ejections and other eruptive phenomena that govern space weather. Today, most of our knowledge about the solar magnetic field topologies was derived from the measurements of the magnetic field of the solar photosphere. The solar chromosphere is dilute, and associated magnetic fields are weak, which makes them difficult to measure. To address this challenging issue, we propose the use of the He I D ₃ 5876 Å line, together with the He I 10830 Å line, to measure the off-limb weak and moderate chromospheric magnetic fields in the Hanle effect regime. We show that the current approach of using the polarization amplitude or degree cannot reliably retrieve the magnetic fields. We demonstrated that by using the Stokes linear polarization profiles simultaneously with the 5876 and 10830 Å lines, without the use of the circular polarization, chromospheric magnetic fields as low as and down to a few gauss can be measured easily and reliably with today’s instruments. We further demonstrate the potential to measure the weak chromospheric magnetic fields with the Stokes linear polarization profiles by using the He 5876 Å line alone, which has not been fully investigated yet. This work will provide a new technique for future daily synoptic observations for the weak magnetic fields in the solar chromosphere for both the active and quiescent regions, which are still difficult to measure.

The Sunspot Solar Observatory Data Archive (SSODA) stores data acquired with the suite of instruments at the Richard B. Dunn Solar Telescope (DST) from February 2018 to the present. The instrumentation at the DST continues to provide high cadence imaging, spectroscopy, and polarimetry of the solar photosphere and chromosphere across a wavelength range from 3500 Å to 11,000 Å. At the time of writing, the archive contains approximately 374 TiB of data across more than 520 observing days (starting on February 1, 2018). These numbers are approximate as the DST remains operational, and is actively adding new data to the archive. The SSODA includes both raw and calibrated data. A subset of the archive contains the results of photospheric and chromospheric spectropolarimetric inversions using the Hazel-2.0 code to obtain maps of magnetic fields, temperatures, and velocity flows. The SSODA represents a unique resource for the investigation of plasma processes throughout the solar atmosphere, the origin of space weather events, and the properties of active regions throughout the rise of Solar Cycle 25.

Context. Magnetic bright points (MBPs) are one of the smallest manifestations of the magnetic field in the solar atmosphere and are observed to extend from the photosphere up to the chromosphere. As such, they represent an excellent feature to use in searches for types of magnetohydrodynamic (MHD) waves and mode coupling in the solar atmosphere. Aims. In this work, we aim to study wave propagation in the lower solar atmosphere by comparing intensity oscillations in the photosphere with the chromosphere via a search for possible mode coupling, in order to establish the importance of these types of waves in the solar atmosphere, and their contribution to heating the chromosphere. Methods. These observations were conducted in July 2011 with the Rapid Oscillations of the Solar Atmosphere (ROSA) and the Hydrogen-Alpha Rapid Dynamics Camera (HARDCam) instruments at the Dunn Solar Telescope. Observations with good seeing were made in the G-band and Hα wave bands. Speckle reconstruction and several post facto techniques were applied to return science-ready images. The spatial sampling of the images was 0.069″/pixel (50 km/pixel). We used wavelet analysis to identify traveling MHD waves and derive frequencies in the different bandpasses. We isolated a large sample of MBPs using an automated tracking algorithm throughout our observations. Two dozen of the brightest MBPs were selected from the sample for further study. Results. We find oscillations in the G-band MBPs, with frequencies between 1.5 and 3.6 mHz. Corresponding MBPs in the lower solar chromosphere observed in Hα show a frequency range of 1.4–4.3 mHz. In about 38% of the MBPs, the ratio of Hα to G-band frequencies was near two. Thus, these oscillations show a form of mode coupling where the transverse waves in the photosphere are converted into longitudinal waves in the chromosphere. The phases of the Hα and G-band light curves show strong positive and negative correlations only 21% and 12% of the time, respectively. Conclusions. From simple estimates we find an energy flux of ≈45 × 10³ W m⁻² and show that the energy flowing through MBPs is enough to heat the chromosphere, although higher-resolution data are needed to explore this contribution further. Regardless, mode coupling is important in helping us understand the types of MHD waves in the lower solar atmosphere and the overall energy budget.

The magnetic geometry of the solar atmosphere, combined with projection effects, makes it difficult to accurately map the propagation of ubiquitous waves in fibrillar structures. These waves are of interest due to their ability to carry energy into the chromosphere and deposit it through damping and dissipation mechanisms. To this end, the Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope was employed to capture high-resolution Hα spectral scans of a sunspot, with the transverse oscillations of a prominent superpenumbral fibril examined in depth. The oscillations are reprojected from the helioprojective Cartesian frame to a new frame of reference oriented along the average fibril axis through nonlinear force-free field extrapolations. The fibril was found to be carrying an elliptically polarized, propagating kink oscillation with a period of 430 s and a phase velocity of 69 ± 4 km s⁻¹. The oscillation is damped as it propagates away from the sunspot with a damping length of approximately 9.2 Mm, resulting in the energy flux decreasing at a rate on the order of 460 W m⁻²/Mm. The Hα line width is examined and found to increase with distance from the sunspot, a potential sign of a temperature increase. Different linear and nonlinear mechanisms are investigated for the damping of the wave energy flux, but a first-order approximation of their combined effects is insufficient to recreate the observed damping length by a factor of at least 3. It is anticipated that the reprojection methodology demonstrated in this study will aid with future studies of transverse waves within fibrillar structures.