An extremely complex active region with very strong non-neutralized electric currents

Nugget
Number:	473
1st Author:	Ioannis KONTOGIANNIS
2nd Author:
Published:	October 14, 2024
Next Nugget:	Source regions of SEPs
Previous Nugget:	All microflares that accelerate electrons to high energies are rooted in sunspots

Introduction

Flares and CMEs originate in solar active regions, usually involving sunspot groups. These are basically magnetic structures, built from a skeleton of magnetism originating in the solar interior. In addition to this skeleton, some of the active-region magnetic fields originate in via Ampere's Law by currents flowing through the corona, injected from beneath the photosphere. These currents express "free energy" that can be released catastrophically, resulting in a solar flare or a coronal mass ejection.

Solar ejecta then can cause geomagnetic storms. The geomagnetic storm between 10 and 13 May 2024 was the strongest since 2003. It was a consequence of repeated, strong eruptions from the solar active region NOAA 13664, one of the most complex regions of the last two Solar Cycles (Ref. [1]).

Analysis and results

One important aspect of the complexity of the magnetic field is the associated electric current. A given flux loop in the corona will have two photospheric footpoints, each with a different polarity (field up or down). When the magnetic polarities of an active region are well separated the rather isolated magnetic flux tubes should carry, theoretically, negligible net electric current. This net current is the result of a volume electric current and an opposite directed surface current, which neutralize one another. However, when new flux is emerging or regions of close interaction appear, the deformed magnetic field polarities may develop significant non-neutralized (net) electric current.

One way to estimate the non-neutralized electric current from measurements of the magnetic field vector at the photosphere is to partition the magnetic field into unipolar patches and measure the net vertical electric current they carry, using Ampere's Law. If it is significantly high, then the partition is non-neutralized (Ref. [2]) and carries a net electric current.

Applying this method to a sample of active regions showed that the total unsigned non-neutralized electric current of active regions builds up during flux emergence and when magnetic polarities approach one another and form polarity inversion lines. The most complex active regions carry at least one order of magnitude stronger non-neutralized electric current and exhibit significantly faster electric-current increase rates (Ref. [3]).

Active region NOAA 13664 was a unique case because it formed by the merging of three subregions, two of which were of considerable individual complexity. The result was an extremely complex configuration, with many sites of interaction between opposite polarity magnetic fields (Figure 1).

Figure 1: Overview of the evolution of the photospheric magnetic field of NOAA 13664. The three subregions are indicated by R1, R2, and R3. Arrows point to regions of strong interactions between opposite magnetic fields. These locations are also associated with very strong non-neutralized electric current (see also Figure 2)

During this evolution, numerous magnetic patches developed, which carried increasingly strong non-neutralized electric current and exhibited persistent shearing motions (Figure 2). The intervals of particularly strong and repeated M- and X-class flaring between 8 and 13 May followed the local increase of the non-neutralized electric currents of the individual partitions.

Figure 2: Left: Temporal evolution of the electric current carried by the non-neutralized partitions of the active region. Red, yellow and green rectangles indicate X-, M- and C-class flares. Right: Photospheric magnetogram of NOAA 13664. Overplotted colored crosses indicate the trajectories of the non-neutralized partitions, whose electric currents are plotted on the left.

The total unsigned non-neutralized electric current of the region was enormous (Figure 3). It surpassed by far all previous flare-productive regions of Solar Cycles 24 and 25 [3], including NOAA 12192, NOAA 12673, and NOOA 11158.

Figure 3: Temporal evolution of the total unsigned non-neutralized electric current of NOAA 13664 (black) and comparison with other active regions of Solar Cycle 24.

Conclusions

Active region NOAA 13664, likely exhibited the strongest non-neutralized electric currents that have been measured since regular magnetic field measurements became available. The total unsigned non-neutralized electric currents and average injection rates reached about 6 x 10¹³ A and 1.5 x 10¹³ A/day. The analysis described here examines how non-neutralized electric currents develop both in active regions as a whole and locally, during emergence events and magnetic interactions. Future applications of the method can improve our understanding of the formation of super-active regions and the prediction of solar eruptions, by extending previous work [4,5] to include also the local evolution of electric current.

Acknowledgments

I would like to thank Manolis Georgoulis, who proposed the method to derive non-neutralized electric currents in Ref [2] and co-authored Ref. [3], and Hugh Hudson for helping with editing this nugget.