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

TL;DR

This study shows that compact Ca II K brightenings occur minutes before solar flares and can serve as reliable chromospheric indicators for predicting flare onset, based on high-cadence observations and magnetic topology analysis.

Contribution

It provides the first systematic analysis linking Ca II K brightenings to flare precursors, demonstrating their potential as short-lead indicators for solar flare prediction.

Findings

Ca II K brightenings peak 10-45 min before flares.

Brightenings are associated with chromospheric reconnection.

Propagation speed of brightenings is ~30-35 km/s.

Abstract

We present a uniform analysis of compact Ca II K (3934 \AA) 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 \AA\ 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.