Compact Ca II K Brightenings Precede Solar Flares: A Dunn Solar Telescope Pilot Study

Abstract

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 (ROSA) instrument at the Dunn Solar Telescope (DST), 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 min 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 $\sim$30--35 km s$^{-1}$ that is consistent with chromospheric reconnection. Complementary Spectropolarimeter for Infrared and Optical Regions (SPINOR) 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.

Figures (8)

• Figure 1: Context Ca ii K images (ROSA/DST) for the eight analyzed events. Orange rectangles mark fixed ROIs. The ROI labels are standardized across all events: ROI 1 marks the flare kernel, ROI 2 the precursor brightening, ROI 0 the quiescent reference region, and ROIs 3 and higher denote additional contextual locations.
• Figure 2: Multi--wavelength timing of precursor and flare kernels. For each representative event, the left panel shows the H$\alpha$ context image from DST/HARDcam near the time of onset, the right upper panel shows the detrended Ca ii K light curve of the compact brightening site, and the right lower panel shows the corresponding flare kernel. In both light curves, dashed green and red lines mark the brightening and flare peak times, respectively, while the vermilion curve shows the co-temporal GOES 1--8 Å soft X-ray flux (right $y$-axis). The temporal ordering shows that the compact Ca ii K brightening leads the main flare kernel and the soft X-ray rise, identifying it as the earliest detectable chromospheric energy release in these events. Panels (a)--(c) correspond to the events on 2025-06-13, 2024-06-12, and 2021-09-14. In all panels ROI 1 labels the flare kernel, ROI 2 the precursor brightening, and ROI 0 a quiescent reference patch; this convention matches the Ca ii K ROI maps.
• Figure 3: Pre–flare connectivity traced on the background image. Streamlines from seed FPs are shown; trajectories that do not terminate at a sink FP are drawn as open.
• Figure 4: Post–flare connectivity with routing constraints (allow–map, quotas, gate). Open trajectories use a single color; captured paths are colored by source$–$sink pair (color online).
• Figure 6: Example of detrending and ambiguity in precursor identification for the 2021-09-14 event. Left column: raw Ca ii K light curves with quadratic background fits (orange). Right column: detrended intensity series $\Delta I$ used for timing analysis. Several ROIs brighten before the main flare kernel, producing potential false positives that are currently resolved by visual inspection.