Circular-ribbon flares and the related activities

Abstract

Solar flares are an impulsive increase of emissions as a result of impetuous release of magnetic free energy. This paper will present the recent progress on circular-ribbon flares (CRFs) and their related activities, including coronal jets, filaments, coronal mass ejections (CMEs), radio bursts, coronal dimmings, and coronal loop oscillations. Owing to the prevalence of three-dimensional (3D) magnetic null points and the corresponding fan-spine topology in the solar atmosphere, CRFs are regularly observed in ultraviolet (UV), extreme-ultraviolet (EUV), and Hα passbands. Spine reconnection and fan reconnection around the null points are predominantly responsible for the energy release and subsequent particle acceleration. Slipping reconnection at quasi-separatrix layers (QSLs) may explain the sequential brightening or rapid degradation of the circular ribbons. Periodic or quasi-periodic acceleration and precipitation of non-thermal particles in the chromosphere produce observed quasi-periodic pulsations (QPPs) of CRFs in multiple altitudes as well as wavelengths. Like two-ribbon flares, the injected high-energy particles result in explosive evaporation in circular and inner ribbons, which is characterized by simultaneous blueshifts in the coronal emission lines and redshifts in the chromospheric emission lines. Homologous CRFs residing in the same active region present similar morphology, evolution, and energy partition. The peculiar topology of CRFs with closed outer spines facilitates remote brightenings and EUV late phases, which are uncommon in two-ribbon flares. Besides, CRFs are often accompanied by coronal jets, type III radio bursts, CMEs, shock waves, coronal dimmings, and kink oscillations in coronal loops and filaments. Magnetohydrodynamics numerical simulations are very helpful to understand the key problems that are still unclear up to now. Multiwavelength and multipoint observations with state-of-the-art instruments are enormously desired to make a breakthrough. The findings in CRFs are important for a comprehensive understanding of solar flares and have implication for stellar flares.