Direct evidence that twisted flux tube emergence creates solar active regions

D Mactaggart,B Raphaldini,C Prior,S L Guglielmino,P Romano

doi:10.1038/s41467-021-26981-7

D Mactaggart, B Raphaldini + Show 3 more

Open Access

https://doi.org/10.1038/s41467-021-26981-7

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Abstract

The magnetic nature of the formation of solar active regions lies at the heart of understanding solar activity and, in particular, solar eruptions. A widespread model, used in many theoretical studies, simulations and the interpretation of observations, is that the basic structure of an active region is created by the emergence of a large tube of pre-twisted magnetic field. Despite plausible reasons and the availability of various proxies suggesting the accuracy of this model, there has not yet been a methodology that can clearly and directly identify the emergence of large pre-twisted magnetic flux tubes. Here, we present a clear signature of the emergence of pre-twisted magnetic flux tubes by investigating a robust topological quantity, called magnetic winding, in solar observations. This quantity detects the emerging magnetic topology despite the significant deformation experienced by the emerging magnetic field. Magnetic winding complements existing measures, such as magnetic helicity, by providing distinct information about field line topology, thus allowing for the direct identification of emerging twisted magnetic flux tubes.

Highlights

The magnetic nature of the formation of solar active regions lies at the heart of understanding solar activity and, in particular, solar eruptions
This has led to observational proxies that are indicative of twisted tube emergence
The flux of magnetic helicity through the photosphere can be measured in solar observations[18,19], so this topological quantity can potentially indicate what kind of magnetic topology is emerging

Summary

Introduction

The magnetic nature of the formation of solar active regions lies at the heart of understanding solar activity and, in particular, solar eruptions. Many works[18,19,20,21,22,23,24,25,26,27,28,29,30] have studied the injection of this quantity in active regions, but a clear-cut indication of an emerging magnetic field’s magnetic topology from magnetic helicity flux has remained elusive One reason for this is that magnetic helicity combines two distinct properties, field line topology and magnetic flux, into one quantity. The net magnetic helicity input can change sign during the emergence of twisted flux ropes due to the strength of convective down-flows, leading to a potential misinterpretation of the magnetic field’s structure, i.e. a failure to diagnose the true twisted nature of the emerging field[33] This is not to say that the magnetic helicity flux produces an erroneous signal but, rather, that it provides information based on a combination of factors and not just the global magnetic topology. Combining the information given by magnetic winding with other proxies and measures (including magnetic helicity) provides a much more detailed picture of the topological structure and evolution of an emerging solar active region

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