Circular-ribbon flares and the related activities

In this paper, I will present the recent progress on circular-ribbon flares (CRFs) and their related activities, including coronal jets, filaments, CMEs, radio bursts, coronal dimmings, and coronal loop oscillations. Owing to the prevalence of 3D magnetic null points and the corresponding fan-spine topology in the solar atmosphere, CRFs are regularly observed in UV, 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 QSLs may explain the sequential brightening or rapid degradation of the circular ribbons. Periodic or quasi-periodic acceleration and precipitation of nonthermal particles in the chromosphere produce observed 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.