Fast Reconnection in a Coronal Torn Plasma Sheet

Abstract Tearing instability, also known as plasmoid instability, is an effective mechanism for accelerating the magnetic reconnection process, and works in a wide range of magnetized plasma systems with different spatial scales, ionization degrees, and collisionalities. However, due to observational limitations, observations of plasma-sheet tearing and the resulting plasmoids remain rather scarce. This scarcity significantly hinders our understanding of the role of plasmoids in the reconnection process from an observational perspective. Using high-spatiotemporal, multiwavelength observations from the Solar Dynamics Observatory, we trace the entire evolution of a coronal plasma sheet. Its formation is driven by the emergence of photospheric magnetic flux, followed by tearing and eventual decay. The evolution of the plasma sheet exhibits two distinct stages. Initially, it rises rapidly, lengthens, and undergoes tearing at a low frequency. Subsequently, its ascent slows, it begins to shorten, and tearing occurs more frequently. A detailed analysis of the reconnecting plasma sheet focuses on heating, plasmoid dynamics (formation and ejection), and the resulting changes in the reconnection rate. Two key heating processes are identified: plasma-sheet tearing and coalescence involving plasmoids and magnetic cusps. More importantly, combining observations with analytical studies suggests that plasmoids act as key carriers of magnetic flux, rapidly transporting it within the observed torn plasma sheet, and their formation and ejection significantly enhance the reconnection rate and facilitate the onset of fast reconnection.