Authors: Haotian Da (University of Maryland), James Drake (University of Maryland), Marc Swisdak (University of Maryland)

A recent studies of pre-flare events revealed that there is an extended “hot onset” interval where the intensity of soft X-rays gradually increases while plasma temperatures are elevated (> 10 MK) and remain relatively stationary until the impulsive phase begins. Observations from GOES, RHESSI, and Solar Orbiter/STIX further demonstrated that the emission measure increases by two orders of magnitude during the hot onset. Since hard X-ray emission is detected only later in the flare, non-thermal electrons cannot be the dominant driver of hot thermal. The observations suggest that magnetic reconnection during the hot-onset phase drives hot thermal electrons but very few non-thermal electrons. The previous study of magnetic reconnection reveals that the strength of the guide field determines whether thermal heating or non-thermal acceleration dominates the total energization process. Additionally, recent results from both simulation and observation of an M6.9 flare showed the strong-to-weak guide field evolution in two-ribbon flares. Thus, a possible explanation of hot onset is that flares begin during reconnection with a strong guide field that produces only hot thermal plasma then accumulate the nonthermal plasma with weak guide field at flare impulsive phase.

To discover the thermal and non-thermal energization of solar plasma, we applied the variable magnetic profile for numerical reconnection simulation using our k-global model. The spatial structure of the initial magnetic field can mimic the expected temporal evolution of the guide field during hot onset. The result of the simulation shows a slowly increasing temperature and a constantly increasing emission measure within the separatrices, which highly agree with the observation. Since the k-global model is designed to explore the self-consistent production of energetic particles during magnetic reconnection in macroscale systems, we also successfully observe the power-law tail in the particle energy spectrum and significant growth of the fraction of non-thermal particle density at the end of simulation.