Successive occurrences of quasi-circular ribbon flares in a fan-spine-like configuration involving hyperbolic flux tube

TL;DR

This paper analyzes the formation and evolution of a fan-spine magnetic configuration with an hyperbolic flux tube, leading to four homologous quasi-circular ribbon flares driven by flux cancellation and flux rope eruptions.

Contribution

It provides a detailed observational study linking magnetic topology, flux cancellation, and filament eruptions to homologous flares in a complex fan-spine structure.

Findings

Identification of an hyperbolic flux tube (HFT) via Q-maps.

Observation of flux cancellation and flux rope eruptions.

Correlation between magnetic decay index and eruption conditions.

Abstract

We present a comprehensive analysis of the formation and evolution of a fan-spine-like configuration that developed over a complex photospheric configuration where dispersed negative polarity regions were surrounded by positive polarity regions. This unique photospheric configuration, analogous to the geological "atoll" shape, hosted four homologous flares within its boundary. Computation of the degree of squashing factor (Q) maps clearly revealed an elongated region of high Q-values between the inner and outer spine-like lines, implying the presence of an hyperbolic flux tube (HFT). The coronal region associated with the photospheric atoll configuration was distinctly identified in the form of a diffused dome-shaped bright structure directly observed in EUV images. A filament channel resided near the boundary of the atoll region. The activation and eruption of flux ropes from the filament channel led to the onset of four eruptive homologous quasi-circular ribbon flares within an interval of $\approx$11 hours. During the interval of the four flares, we observed continuous decay and cancellation of negative polarity flux within the atoll region. Accordingly, the apparent length of the HFT gradually reduced to a null-point-like configuration before the fourth flare. Prior to each flare, we observed localised brightening beneath the filaments which, together with flux cancellation, provided support for the tether-cutting model of solar eruption. The analysis of magnetic decay index revealed favourable conditions for the eruption, once the pre-activated flux ropes attained the critical heights for torus instability.