Systematic Nonthermal Velocity Increase Preceding Soft X-Ray Flare Onset: A Large-scale Hinode/EIS Study

Abstract Nonthermal velocities, derived from spectral line broadening, can provide crucial insights into plasma dynamics before and during solar flares. To systematically study the preflare phase, we constructed a Hinode/Extreme-ultraviolet Imaging Spectrometer flare catalog of 1449 flares from 2011 to 2024. This enabled a large-scale analysis of flare loop footpoint nonthermal velocity evolution across different flare magnitudes (C, M, X-classes). Analyzing Fe viii –Fe xxiv emission lines formed at log( T /K) ∼ 5.7–7.3 with piecewise linear fits in the preflare period, we find that nonthermal velocities consistently increase 4–25 minutes before Geostationary Operational Environmental Satellite (GOES) soft X-ray start in C and M-class flares. Onset timing patterns vary with flare magnitude: smaller flares show temperature-dependent progression, while larger flares exhibit more compressed, near-simultaneous onsets across temperatures. While our limited X-class sample ( N = 18) also show onset before GOES, larger statistics are needed to confirm its behavior. M-class flares show a systematic precursor nonthermal velocity onset ∼30–60 minutes before GOES peak. In a subset of M-class flares (2011–2018), coronal-mass-ejection (CME)-associated events show earlier and more uniform precursor onsets (45–74 minutes before peak) than non-CME events, of which only some lines display a precursor, suggesting a strong link between extended preflare nonthermal broadening and successful eruptions. This large scale study establishes preflare nonthermal velocity increase at footpoints as a common precursor observable before any X-ray signature.