Fan-spine topology formation through two-step reconnection driven by twisted flux emergence

TL;DR

This study combines observations and simulations to reveal a two-step magnetic reconnection process that forms fan-spine topologies and anemone regions in the solar corona, advancing understanding of magnetic field restructuring during flux emergence.

Contribution

It introduces a new model for fan-spine topology formation through a detailed two-step reconnection process driven by twisted flux emergence, supported by both observations and 3D MHD simulations.

Findings

Reconnection at the nullpoint leads to fan-spine topology formation.

A torsional MHD wave propagates along arcade fields during reconnection.

The model reproduces observed features of anemone regions in the solar corona.

Abstract

We address the formation of 3D nullpoint topologies in the solar corona by combining Hinode/XRT observations of a small dynamic limb event, which occurred beside a non-erupting prominence cavity, with a 3D zero-beta MHD simulation. To this end, we model the boundary-driven kinematic emergence of a compact, intense, and uniformly twisted flux tube into a potential field arcade that overlies a weakly twisted coronal flux rope. The expansion of the emerging flux in the corona gives rise to the formation of a nullpoint at the interface of the emerging and the pre-existing fields. We unveil a two-step reconnection process at the nullpoint that eventually yields the formation of a broad 3D fan-spine configuration above the emerging bipole. The first reconnection involves emerging fields and a set of large-scale arcade field lines. It results in the launch of a torsional MHD wave that propagates along the arcades, and in the formation of a sheared loop system on one side of the emerging flux. The second reconnection occurs between these newly formed loops and remote arcade fields, and yields the formation of a second loop system on the opposite side of the emerging flux. The two loop systems collectively display an anenome pattern that is located below the fan surface. The flux that surrounds the inner spine field line of the nullpoint retains a fraction of the emerged twist, while the remaining twist is evacuated along the reconnected arcades. The nature and timing of the features which occur in the simulation do qualititatively reproduce those observed by XRT in the particular event studied in this paper. Moreover, the two-step reconnection process suggests a new consistent and generic model for the formation of anemone regions in the solar corona.