Plasma Heating and Energization in Hot-Onset Flare Precursor Events

RESUMO

Recent observations of solar flares have revealed a distinct 'hot onset' phase preceding the impulsive phase, characterized by elevated plasma temperature (>10 MK), a steady increase in emission measure, and the absence of detectable nonthermal emission. The standard flare model attributes the formation of hot flare plasma to chromospheric evaporation driven by energy deposition from nonthermal electrons, and therefore does not naturally explain why thermal heating precedes nonthermal emission. Motivated by recent observations of guide-field evolution in two-ribbon flares and by large-scale MHD simulations, we investigate hot-onset energization using simulations of magnetic reconnection with the kinetic macroscale model kglobal initialized with a spatially non-uniform guide field. Because the efficiency of electron non-thermal energy production from Fermi acceleration decreases with guide-field strength, the evolving guide field produces two distinct energization regimes. During the initial high-guide-field phase, the bulk electron temperature rises to approximately twice its initial value, while non-thermal particle production remains weak. Upon entering the low-guide-field regime, energetic particle production is significantly enhanced, with the population of high-energy electrons growing rapidly. These findings provide a self-consistent physical explanation for the hot onset, demonstrating that the macroscopic evolution of the guide field naturally bridges the hot-onset and impulsive phases of solar flares without requiring a separate trigger mechanism.