Are Solar Active Regions Formed By Large Twisted Flux Tubes? (Planetary Science)

By investigating a fundamental topological quantity called “magnetic winding”, MacTaggart and colleagues found first direct evidence that, solar active regions are formed by large twisted flux tubes. Their study recently appeared in Arxiv.

Solar active region is an area with an especially strong magnetic field. These regions frequently spawn various types of solar activity, including explosive “solar storms” such as solar flares and coronal mass ejections (CME). Several studies theoretically suggested that the basic structure of an active region is created by the emergence of a large tube of pre-twisted magnetic field. But, there has not yet been any direct observational evidence of the emergence of large magnetic flux tubes. Thus, the question arise, whether the solar active regions are formed by large twisted flux tubes, or not?

But, MacTaggart and colleagues now answered this question and provided evidence to support this by investigating a fundamental topological quantity called “magnetic winding” in solar observations.

They showed that, when magnetic winding is combined with other measurable quantities such as magnetic helicity, it provides a powerful analysis tool which can give direct information about magnetic topology or provide direct evidence that large twisted flux tubes emerge to create solar active regions.

“Despite its close connection to magnetic helicity, magnetic winding can behave very differently in an evolving magnetic field and, hence, provide new and distinct information.”

They have also given examples of active region observations where the magnetic winding gives a clear indication that the emerging magnetic field is composed of pre-twisted magnetic field. This confirms the prediction made by theoretical studies that, pre-twisted flux tubes play a fundamental role in active region formation.

Figure 1: Winding accumulation for AR11318. It is clear from this figure that the emergence accumulation (Lemerge) dominates strongly over the braiding accumulation (Lbraid) , and so the winding input is due primarily to the emergence of a pre-twisted structure rather than an untwisted structure whose twist develops in the solar atmosphere due to photospheric motions. © MacTaggart et al.

Finally, they suggested that the pre-twisted magnetic field that emerges into the photosphere represents a source to explain, self-consistently, the shearing, rotational and compressional motions invoked in models of coronal mass ejection formation. These motions can develop due to the transportation of twist into the higher atmosphere as the magnetic field expands into the corona.

“Although we have presented evidence that twisted flux tubes can create active regions, it is likely that other magnetic topologies also emerge to create active regions. This is an important area of research, for which magnetic winding will play a pivotal role.”

— concluded authors of the study

Featured image: Simulation of the initial emergence of a twisted magnetic tube: It shows the emerged flux tube, at 𝑡=2500 seconds, which has been deformed significantly by convection and developed a “serpentine” field line structure. © MacTaggart et al.


Reference: David MacTaggart, Chris Prior, Breno Raphaldini, Paolo Romano, Salvatore Guglielmino, “Direct evidence: twisted flux tube emergence creates solar active regions”, Arxiv, pp. 1-20, 2021. https://arxiv.org/abs/2106.11638


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