A Secondary Fan-spine Magnetic Structure in Active Region 11897

Abstract

Fan-spine is a special topology in solar atmosphere and is closely related to magnetic null point as well as circular-ribbon flares, which can provide important information for understanding the intrinsic three-dimensional (3D) nature of solar flares. However, the fine structure within the fan has rarely been investigated. In present paper, we investigate a secondary fan-spine (SFS) structure within the fan of a larger fan-spine topology. On 2013 November 18, this large fan-spine structure was traced out due to the partial eruption of a filament, which caused a circular-ribbon flare in active region 11897. The extrapolated 3D magnetic fields and squashing factor Q maps depict distinctly this fan-spine topology, its surrounding quasi-separatrix layer (QSL) halo, and a smaller quasi-circular ribbon with high Q located in the center, which implies the existence of fine structure within the fan. The imaging observations, extrapolated 3D fields, and Q maps on November 17 show that there indeed exists an SFS surrounded by a QSL, which is enveloped by another QSL-halo corresponding to the overlying larger domeshaped fan. Moreover, the material flows caused by the null-point reconnection are also detected along this SFS. After checking the evolution of the underneath magnetic fields, we suggest that the continuous emergence of magnetic flux within the central parasitic region encompassed by the opposite-polarity fields results in the formation of the SFS under the large fan.

Observation and modelling of solar jets

“…By contrast, a two-sided-loop jet appears as a pair of plasma beams ejecting in opposite directions from the eruption source region [44][45][46][47][48][49]. Recently, high-resolution observations combined with extrapolated three-dimensional (3D) coronal magnetic fields revealed the fan-spine topology magnetic system of straight anemone jets [50], which consists of a coronal nullpoint, a dome-like fan that represents the [51][52][53][54][55][56][57][58][59][60]. A fan-spine topology often arises when a parasitic magnetic polarity emerges (or carries) into a pre-existing magnetic field region with opposite polarity, and a jet occurring within it can lead to three flare ribbons as a result of the low-altitude impact of the particle beams accelerated through the nullpoint magnetic reconnection; namely, an inner bright patch surrounded by a circular ribbon relevant to the inner spine and the dome-like fan structure, and a remote elongated bright ribbon associated with the outer spine.…”

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…Emerging flux can destabilize the magnetic field and cause an eruption. In an event documented by Hou et al (2019), an embedded fan-spine structure forms via flux emergence and subsequent reconnection with the overlying flux, triggering a confined flare. In this case, high-spatial-resolution images from IRIS confirm predictions from NLFFF modeling that imply the existence of a small fan-spine system embedded in a larger one.…”

A Hot Cusp-shaped Confined Solar Flare

“…Veronig & Polanec 2015), and the formation of circular or quasi-circular ribbons (e.g. Reid et al 2012;Joshi et al 2015;Hernandez-Perez et al 2017;Li et al 2018b;Zhong et al 2019;Hou et al 2019;Chen et al 2019;Zhang et al 2019). However, although rarely reported in literature, some confined flares occasionally exhibit features similar to those observed in eruptive flares.…”