Velocity Response of the Observed Explosive Events in the Lower Solar Atmosphere. I. Formation of the Flowing Cool-loop System

We observe plasma flows in cool loops using the Slit-Jaw Imager on board the Interface Region Imaging Spectrometer (IRIS). Huang et al. observed unusually broadened Si iv 1403 Å line profiles at the footpoints of such loops that were attributed to signatures of explosive events (EEs). We have chosen one such unidirectional flowing cool-loop system observed by IRIS where one of the footpoints is associated with significantly broadened Si iv line profiles. The line-profile broadening indirectly indicates the occurrence of numerous EEs below the transition region (TR), while it directly infers a large velocity enhancement/perturbation, further causing the plasma flows in the observed loop system. The observed features are implemented in a model atmosphere in which a low-lying bipolar magnetic field system is perturbed in the chromosphere by a velocity pulse with a maximum amplitude of 200 km s−1. The data-driven 2D numerical simulation shows that the plasma motions evolve in a similar manner as observed by IRIS in the form of flowing plasma filling the skeleton of a cool-loop system. We compare the spatio-temporal evolution of the cool-loop system in the framework of our model with the observations, and conclude that their formation is mostly associated with the velocity response of the transient energy release above their footpoints in the chromosphere/TR. Our observations and modeling results suggest that the velocity responses most likely associated to the EEs could be one of the main candidates for the dynamics and energetics of the flowing cool-loop systems in the lower solar atmosphere.