Reconciling Observational Challenges to the Impulsive-Piston Shock-Excitation Scenario. II. Shock Waves Produced in CME-Less Events with a Null-Point Topology

Abstract

Continuing Article I, we revisit challenging events previously identified by different authors, whose analysis led to conclusions about various mechanisms of the shock-wave excitation. Here, we reconsider four events that involved fan-spine coronal configurations with a null-point topology (NPT). The presence of Type-II radio bursts in all events as well as extreme-ultraviolet disturbances (EUV waves) observed in three events evidence the presence of shock waves, whereas no coronal mass ejections (CMEs) were detected in most events. One idea proposed to explain the observations was the shock-wave excitation by the straightening of a postreconnection kinked loop. The Type-II burst in another event appeared in association with a compact flare with a high thermal pressure that looked to be in favor of a flare-generated blast wave. One event was associated with a possible pseudo-CME. All of these challenging events have been reconciled in terms of an impulsively excited piston shock. CME-less filament eruptions in NPT configurations appear to represent a distinct category of events responsible for some of the observed shock waves.