Testing CaII K chromospheric flare precursors with spectropolarimetric inversions and data-constrained LARE2D simulations

Introduction Compact CaII K brightenings have been identified as short-lead chromospheric precursors to solar flare onset, but their physical role remains uncertain. In particular, it is unclear whether these brightenings are passive early emission signatures or low-atmosphere perturbations capable of helping trigger reconnection in flare-ready magnetic fields. Here we test this question for the 2025 June 13 GOES C3.1 flare. Methods We combine high-cadence ROSA CaII K imaging, SPINOR full-Stokes spectropolarimetry, HAZEL/SIR inversions, and data-constrained LARE2D resistive-MHD simulations. The observations sample the pre-flare, impulsive, and post-flare evolution of five flare footpoints using simultaneous Fei 5,250 Å and CaII 8,542 Å St profiles. The MHD experiment uses the observed chromospheric perturbation as a constraint to test whether a disturbance with comparable properties can form current sheets and initiate reconnection in a flare-ready arcade. Results The event shows a compact CaII K brightening at the jet-trigger site approximately 20 min before the GOES soft-X-ray rise. The pre-flare inversions reveal a non-uniform atmosphere, including a persistent but low-significance blueshift at FP1 and a localised NLTE chromospheric temperature enhancement of order Δ T ≈ 1000 K near the precursor region. The post-flare changes are small in the photosphere and non-uniform in the chromosphere. We find no coherent polarity-dependent magnetic or thermal response and no consistent footpoint-dependent condensation pattern. In the LARE2D experiments, all tested thermal-amplitude and injection-height configurations form a PIL current sheet and cross the reconnection threshold within 3.6–4.4 min. The resulting reconnection is bursty, launches upward outflows that reach coronal heights, and continues after the driver weakens. A no-thermal control further shows that the converging shear is sufficient to initiate reconnection, while the thermal perturbation advances its onset by approximately one snapshot interval. Discussion Together, the observations and simulations support the interpretation that compact CaII K precursors can be physically meaningful perturbations in flare-ready active regions, rather than only passive early brightenings. This does not imply that every CaII K brightening is a flare trigger. Rather, such brightenings may become important when they occur at magnetically connected footpoints in an already stressed arcade. A larger sample with matched spectropolarimetric coverage, including both precursor-positive and precursor-negative events, will be required to determine whether compact CaII K brightenings are necessary, sufficient, or conditional indicators of flare onset.