Resolving the Fan-Spine Reconnection Geometry of a Small-Scale Chromospheric Jet Event with the New Solar Telescope

TL;DR

This study provides high-resolution evidence of a fan-spine magnetic reconnection structure in a small-scale chromospheric jet, revealing reconnection occurring in the upper chromosphere and supporting a two-step reconnection model.

Contribution

First clear high-resolution imaging of a fan-spine reconnection structure in a small-scale chromospheric jet, confirming reconnection in the upper chromosphere.

Findings

Bi-directional flows indicate magnetic reconnection.

Fan-spine structure clearly resolved at chromospheric heights.

Reconnection occurred in the upper chromosphere, not visible in EUV or X-ray images.

Abstract

Jets present ubiquitously in both quiet and active regions on the Sun. They are widely believed to be driven by magnetic reconnection. A fan-spine structure has been frequently reported in some coronal jets and flares, regarded as a signature of ongoing magnetic reconnection in a topology consisting of a magnetic null connected by a fan-like separatrix surface and a spine. However, for small-scale chromospheric jets, clear evidence of such structures is rather rare, although they are implied in earlier works that show an inverted-Y-shaped feature. Here we report high-resolution (0$"$.16) observations of a small-scale chromospheric jet obtained by the New Solar Telescope (NST) using 10830~{\AA} filtergrams. Bi-directional flows were observed across the separatrix regions in the 10830~\AA{} images, suggesting that the jet was produced due to magnetic reconnection. At the base of the jet, a fan-spine structure was clearly resolved by the NST, including the spine and the fan-like surface, as well as the loops before and after the reconnection. A major part of this fan-spine structure, with the exception of its bright footpoints and part of the base arc, was invisible in the extreme ultraviolet and soft X-ray images (observed by the Atmosphere Imaging Assembly and the X-Ray Telescope, respectively), indicating that the reconnection occurred in the upper chromosphere. Our observations suggest that the evolution of this chromospheric jet is consistent with a two-step reconnection scenario proposed by \citet{Torok2009}