Is null-point reconnection important for solar flux emergence?

TL;DR

This study investigates the role of null-point reconnection in solar flux emergence using 3D MHD simulations, finding that nulls are prevalent but not the primary reconnection mechanism involved.

Contribution

It provides detailed analysis of null-point dynamics and suggests that null reconnection is not dominant in flux emergence, highlighting the complexity of magnetic topology.

Findings

Null points are numerous and long-lived during flux emergence.

Null-point reconnection is not the main reconnection process involved.

Magnetic topology is highly complex with many nulls present.

Abstract

The role of null-point reconnection in a 3D numerical MHD model of solar emerging flux is investigated. The model consists of a twisted magnetic flux tube rising through a stratified convection zone and atmosphere to interact and reconnect with a horizontal overlying magnetic field in the atmosphere. Null points appear as the reconnection begins and persist throughout the rest of the emergence, where they can be found mostly in the model photosphere and transition region, forming two loose clusters on either side of the emerging flux tube. Up to 26 nulls are present at any one time, and tracking in time shows that there is a total of 305 overall, despite the initial simplicity of the magnetic field configuration. We find evidence for the reality of the nulls in terms of their methods of creation and destruction, their balance of signs, their long lifetimes, and their geometrical stability. We then show that due to the low parallel electric fields associated with the nulls, null-point reconnection is not the main type of magnetic reconnection involved in the interaction of the newly emerged flux with the overlying field. However, the large number of nulls implies that the topological structure of the magnetic field must be very complex and the importance of reconnection along separators or separatrix surfaces for flux emergence cannot be ruled out.