Solar flare precursors

Abstract

We describe recent progress in the study of flare precursors and onset. We provide some new theoretical results on filament formation and eruption, including especially the dynamical effects of magnetic fields. We find ample evidence that energetic processes are already at work in the onset phase, a few minutes before the rapid rise of the hard-radiation impulse. In particular, the prevalence of soft and hard X-ray preheating (to ≲ 10 8 K) is detailed, along with a connection to the launch of coronal mass ejections. A possible interpretation of the empirical time profiles near onset is that “preheating” signifies that the flare has slowly started, and the transition to the impulsive stage then represents a change of phase in the flare-instability process. Finally, the problem of the interpretation of microwave preflare data is delineated. This represents the final report of this part of the Flare Build-up Study.