Transient dark ribbons at the outer boundaries of sunspot superpenumbrae in the chromosphere

Abstract

Context. Although the flow of material in sunspot superpenumbrae has been studied intensively, detailed observations about the evolution of material flow at the outer boundaries of sunspot superpenumbrae are rare. Aims. We study the evolution of the flow of material at the outer boundaries of superpenumbrae. Methods. Based on Hα observations from the New Vacuum Solar Telescope, we tracked the evolution of the flow of material at the outer boundaries of superpenumbrae in the chromosphere. Data from the Solar Dynamics Observatory Helioseismic Magnetic Imager were employed to examine the photospheric magnetic field environment relevant to the material flow. Results. We observed transient dark ribbons (TDRs) at the outer boundary of a sunspot superpenumbra in the chromosphere on 2018 May 12. The TDRs are located in the middle of the sunspot and a flocculus, and both of them have negative magnetic fields. While the ratio of the length and width of a TDR is greater than 8, we tracked the TDR from its formation to disappearance, so that six TDRs were studied in detail. Five TDRs were formed by the connection of several small dark sticks from end to end, and the sixth TDR was formed by the merging of two thin sticks. In the later stage of the TDR evolution, five TDRs were broken down into several small dark sticks, and the sixth TDR dissipated with oscillation. During the evolution process, no obvious flow of material along the TDRs is detected. Conclusions. We suggest that these TDRs are an observational representation of the regular aggregation of superpenumbral fibril footpoints and adjacent solar flocculus fibril footpoints at the outer boundaries of the sunspot superpenumbrae.