National Solar Observatory

Abstract We have examined how the characteristics of the tachocline—i.e., the change in rotation rate δ Ω, or the “jump,” the position of the midpoint of the tachocline, r d , and the width of the tachocline, w d —change as a function of time at different latitudes using 30 yr of helioseismic data obtained by the GONG network. We find a statistically significant change in the jump; however, these changes do not have a simple correlation with solar activity. The dependence is different for Solar Cycles 23 and 24, and for Cycle 25, it is more similar to that of Cycle 24. While our measured changes of the tachocline’s width with time are marginally statistically significant, the cross correlation is statistically significant and implies that the width is larger when the solar activity is smaller, suggesting that magnetic fields play a role in confining the tachocline. The position of the tachocline shows a significant secular change at low latitudes (≲50°). At these latitudes, the tachocline has been moving steadily closer to the base of the convection zone. This is consistent with other measurements that have shown that the overall complexity of solar activity has been decreasing over the last few decades. It leads us to speculate that strong magnetic fields tend to push the tachocline deeper into the radiative zone.

Abstract Recent three-dimensional flare models suggest that flare ribbon substructure is linked to the fragmentation of the reconnecting current sheet in the corona. Flare ribbon substructure can therefore potentially serve as a unique diagnostic tool for physical processes in the flare current sheet. In this paper, we describe a new method to quantify the evolution of ribbon substructure that first extracts the ribbon’s bright leading edge and then quantifies its morphology using the box-counting dimension and correlation dimension mapping (CDM). We first test our method using synthetic observations. We then apply it to an M6.5-class solar flare on 2015 June 22 observed by the Interface Region Imaging Spectrograph (IRIS) 1330 Å slit-jaw imager. We find that when the flare ribbon boundary has more multiple-spatial-scale features (a higher box-counting dimension), hard X-ray emission and magnetic reconnection rates are the strongest. We also find that the flare ribbon complexity characterized by CDM has a moderate correlation with the IRIS Si IV 1402.77 Å nonthermal velocity (in the negative polarity ribbon) and reconnection flux rates (in ribbons of both magnetic polarities). We conclude that the buildup of the spatial complexity of the ribbons at multiple spatial scales can serve as an observational proxy for current-sheet fragmentation in the corona.

We performed high-spatial resolution spectropolarimetric observations of active region NOAA 13363 during a C-class flare with the Gregor Infrared Spectrograph (GRIS) on 16 July 2023. We examine the coupling between the photosphere and the chromosphere, studying the polarimetric signals during a period that encompasses the decaying phase of a C-class flare and the appearance of a new C-class flare at the same location. We focus on the analysis of various spectral lines. In particular, we study the Si I 10827 Å, Ca I 10833.4 Å, Na I 10834.9 Å, and Ca I 10838.9 Å photospheric lines, as well as the He I 10830 Å triplet. GRIS data revealed the presence of flare-related red- and blueshifted spectral line components, reaching Doppler velocities up to 90 km/s, and complex Si I profiles where the He i spectral line contribution is blueshifted. In contrast, the photospheric Ca i and Na i transitions remained unchanged, indicating that the flare did not modify the physical conditions of the lower photosphere. We combined that information with simultaneous imaging in the Ca ii H line and TiO band with the improved High-resolution Fast Imager (HiFI+), finding that the flare emission did not affect the inverse granulation or nearby plage, in agreement with the results from GRIS. We also complement the previous studies with a forward modelling computation, concluding that the He I spectral line emission reflects a complex response of the flaring chromosphere. Radiative excitation from coronal EUV irradiation, energy deposition by flare-accelerated electrons, and dynamic field-aligned plasma flows likely act together to produce the observed supersonic downflows and upflows. We plan to expand these findings through inversions of the He I 10830 Å triplet signals in the future.

Abstract A demonstrated failure mode for operational solar flare forecasting is the inability to forecast flares that occur near, or just beyond, the solar limb. To address this shortcoming, we develop a “” full‐heliosphere event forecasting framework and evaluate its statistical classification ability against this specific challenge. A magnetic surface flux transport model is used to generate full‐sun maps of the photospheric radial magnetic field from which active regions (ARs) are identified and tracked using a new labeling scheme that is observer‐location agnostic and allows for post‐facto modifications. Flare‐relevant magnetic parameters couple to a “visibility” index that specifies AR location relative to the visible solar limb and expected flare detection. Flare labels are assigned according to peak Soft X‐ray flux, and a statistical classification is performed using nonparametric discriminant analysis. A version where new or emerging ARs on the far (“invisible”) side of the Sun are incorporated into the model by way of far‐side helioseismology, is also tested. We evaluate the new framework by its performance specifically including the limb areas using Brier Skill Score and ROC Skill Score, finding improvement at the level or less. However, we do find that the number of False Negatives, or “missed” forecasts decreases, and find strong evidence that the additional information provided by the far‐side helioseismology can help predict near‐ and just‐beyond‐limb flares, particularly for East‐limb events. While individual components of this framework could be improved, we demonstrate that a known failure mode for solar flare forecasting can be mitigated with available resources.

Context. Stellar flares can significantly influence the atmospheres and habitability of orbiting exoplanets, especially around young and active M dwarfs. Understanding the temporally and spectrally resolved activity of such stars is essential for assessing their impact on planetary environments. Aims. We aim to examine in detail state-of-the-art concepts of flare models to identify what is missing in our understanding of energy deposition during the flare event. By comparing synthetic and observed flare spectra, we seek to determine the modelling frameworks best suited for representing flare energetics and spectral far-ultraviolet features while providing a foundation for investigating flare impacts on exoplanet atmospheres. Methods. In this work, we built the Young M dwarf flare (YMDF) model utilising the combination of radiative-hydrodynamic (RHD) stellar atmosphere models with a high- and low-energy electron beam and corresponding synthetic observables. These models are based on physical principles and were validated with solar and stellar observations. Results. The newly developed YMDF model reproduces the observed continuum rise in both the TESS photometric band and the FUV-A spectral range. Furthermore, the flare distributions generated within this framework show consistency with those observed in our sample of stars. Conclusions. We have developed the YMDF model as a tool to reproduce the time-dependent spectra of flaring young M dwarfs, providing a physically motivated description of their spectral and temporal evolution during flare events.

Abstract We present a physically motivated analytic framework for estimating solar wind mass-loading rates from ion sources using a simplified “particles in a box” (PIB) model. By applying conservation of energy and conservation of magnetic flux, we derive a general equation for the global pickup ion rate that explicitly accounts for both kinetic and electromagnetic (Poynting) energy fluxes supplied by the upstream wind and distributed to downstream particles and fields. Our analysis reveals that the global pickup rate may be fundamentally constrained by upstream wind parameters and the geometry of the interaction in a nontrivial manner. We further derive a theoretical upper limit to the maximum mass flux that a super-Alfvénic wind can transport. We call this limit the carrying capacity; it is an inherent property of the wind. We apply this framework to the Giotto flyby of Grigg–Skjellerup, and to unmagnetized and weakly magnetized terrestrial planets. The results suggest that the PIB framework may provide a useful big-picture approach for understanding and estimating stellar wind ion pickup in cometary and planetary environments inside and out of the solar system.

Scattering polarization signals offer unique diagnostics of the physical conditions in the solar atmosphere, in particular magnetic fields via the Hanle effect. However, their spatial structure remains poorly constrained due to the difficulty of achieving a high spatial resolution and polarimetric sensitivity simultaneously. We present the first direct observation of sub-arcsecond structuring in the linear scattering polarization of the photospheric Sr i 4607,Å line near the solar disk center (μ = 0.74), obtained with the Visible Spectro-Polarimeter (ViSP) at the Daniel K. Inouye Solar Telescope (DKIST). The data achieve about ∼0 resolution with 30,s integration and sufficient sensitivity to detect fine-scale patterns in the total linear polarization, which are evident in Sr i but absent in a nearby Fe i line that is simultaneously observed. Since this Fe i line is more Zeeman-sensitive than the Sr i 4607,Å,, this disparity confirms that the signals in the Sr i 4607,Å line arise from scattering. These data provide the first spatially resolved two-dimensional maps of photospheric scattering polarization at sub-arcsecond scales, enabled by the capabilities of a 4-meter solar telescope.

Abstract Radiative losses play a critical role in the cooling of plasmas. When chromospheric plasma is sufficiently heated, it can flow into coronal loops, which subsequently cool down due to radiation. From observations, we infer that this cooling does not occur uniformly, often resulting in coronal condensations such as coronal rain. To date, coronal condensations have only been found in simulations of steadily heated loops, and never in impulsively heated ones. We implement spatiotemporally variable elemental abundances in a radiative hydrodynamic code. Flows, including chromospheric evaporation, directly cause a shift in the local elemental abundances, which then affects the local radiative loss rate. As a consequence, we find that incorporating spatiotemporal low first ionization potential elemental abundances into coronal loop simulations directly causes coronal condensations, which are otherwise absent in impulsively heated loop or flare models. We conclude that spatiotemporal variations in elemental abundances are a fundamental feature of the solar corona and are therefore necessary to accurately model radiation.

Abstract Solar extreme-ultraviolet (EUV) irradiance plays a crucial role in heating the Earth’s ionosphere, thermosphere, and mesosphere, affecting atmospheric dynamics over varying time scales. Although significant effort has been spent studying short-term EUV variations from solar transient events, there is little work to explore the long-term evolution of the EUV flux over multiple solar cycles. Continuous EUV flux measurements have only been available since 1995, leaving significant gaps in earlier data. In this study, we propose a Bayesian deep learning model, named SEMNet, to fill the gaps. We validate our approach by applying SEMNet to construct Solar and Heliospheric Observatory/Solar EUV Monitor EUV flux measurements in the period between 1998 and 2014 using Ca II K images from the Precision Solar Photometric Telescope. We then extend SEMNet through transfer learning to reconstruct solar EUV irradiance in the period between 1950 and 1960 using Ca II K images from the Kodaikanal Solar Observatory. Experimental results show that SEMNet provides reliable predictions along with uncertainty bounds, demonstrating the feasibility of Ca II K images as a robust proxy for long-term EUV fluxes. These findings contribute to a better understanding of solar influences on Earth’s climate over extended periods.