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

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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 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.
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  • National Science Foundation

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