Tracing field lines that are reconnecting, or expanding, or both

Explosive energy release in the solar atmosphere is driven magnetically, but mechanisms triggering the onset of the eruption remain in debate. In the case of flares and CMEs, ideal or non-ideal instabilities usually occur in the corona, but direct observations and diagnostics there are difficult to obtain. To overcome this difficulty, we analyze observational signatures in the upper chromosphere or transition region, in particular, brightenings and dimmings at the feet of coronal magnetic structures. In this paper, we examine the time evolution of spatially resolved light curves in two eruptive flares, and identify a variety of tempo-spatial sequences of brightenings and dimmings, such as dimming followed by brightening, and dimming preceded by brightening. These brightening-dimming sequences are indicative of the configuration of energy release in the form of plasma heating or bulk motion. We demonstrate the potential of using these analyses to diagnose properties of magnetic reconnection and plasma expansion in the corona during the early stage of the eruption.