The origin and early evolution of a bipolar magnetic region in the solar photosphere

TL;DR

This study investigates the early development of a bipolar magnetic region on the Sun, revealing asymmetrical growth and challenging the flux tube emergence model, suggesting in situ magnetic amplification.

Contribution

It provides observational evidence that the early evolution of a bipolar magnetic region does not follow the flux tube emergence model, indicating in situ magnetic field amplification.

Findings

Asymmetrical growth of magnetic polarities in flux and amplitude.

Absence of strong horizontal magnetic fields between poles.

No evidence of large-scale divergent flows during BMR formation.

Abstract

Finding the formation mechanisms for bipolar configurations of strong local magnetic field under control of the relatively weak global magnetic field of the Sun is a key problem of the physics of solar activity. This study is aimed at discriminating whether the magnetic field or fluid motion plays a primary, active role in this process. The very origin and early development stage of Active Region 12548 are investigated based on SDO/HMI observations of 2016 May 20--25. Full-vector magnetic and velocity fields are analyzed in parallel. The leading and trailing magnetic polarities are found to grow asymmetrically in terms of their amplitude, magnetic flux, and the time variation of these quantities. The leading-polarity magnetic element originates as a compact feature against the background of a distributed trailing-polarity field, with an already existing trailing-polarity magnetic element. No signs of strong horizontal magnetic fields are detected between the two magnetic poles. No predominant upflow between their future locations precedes the origin of this bipolar magnetic region (BMR); instead, upflows and downflows are mixed, with some prevalence of downflows. Any signs of a large-scale horizontal divergent flow from the area where the BMR develops are missing; in contrast, a normal supergranulation and mesogranulation pattern is preserved. This scenario of early BMR evolution is in strong contradiction with the expectations based on the model of a rising $\Omega$-shaped loop of a flux tube of strong magnetic field, and an \emph{in situ} mechanism of magnetic-field amplification and structuring should operate in this case.