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Near Continuous Tracking of a Super Active Region for Three Solar Rotations

(Solar Orbiter Nugget #76 by I. Kontogiannis^1,2, Y. Zhu¹, K. Barczynski³, M. Z. Stiefel^1,4, H. Collier^1,4, J. McKevitt^5,6, J.S. Castellanos Durán⁷, S. Berdyugina^2,8, and L.K. Harra^1,3)

Introduction

Active regions are regions on the Sun where extended, strong, and often complex magnetic fields are measured. They are formed when strongly magnetized plasma emerges from the interior of the Sun. They often host strong eruptive events like flares and coronal mass ejections, during which the stored magnetic energy is converted into electromagnetic emission, plasma, and particle acceleration. These can harm space missions and cause geomagnetic storms, impacting on various technological activities and infrastructure. Aiming to understand the causes of solar eruptions, improve our ability to predict them and alleviate their potential impact, we monitor the emergence and evolution of active regions, particularly the ones that exhibit the stronger and more complex magnetic field. In this endeavor, the Solar Orbiter mission [1, 2] with its orbit around the Sun offers new possibilities, particularly in synergy with other space-borne observatories [3].

During the first two weeks of May 2024, one of the most eruptive active regions of the past two solar cycles appeared on the Earth-facing side of the Sun. This was active region NOAA 13664, source of the events that led to the strongest geomagnetic storm since 2003. Super active regions like NOAA 13664 are extremely complex and evolve over several weeks or even months, during which they shape solar magnetism and space environment drastically. when it rotated into the Earth-facing side of the Sun, the region itself was already big and complex enough to produce moderate flares, but soon evolved into an extended and extremely complex region [4]. Due to its large size and complexity it persisted for several weeks and it reappeared on the visible solar side twice, as NOAA 13697 and 13723, which also exhibited considerable flaring activity.

Single-vantage-point monitoring of active region evolution is limited by solar rotation, allowing their continuous observations at most for 14 days. With the Solar Orbiter mission, we gain access to solar observations from varying vantage points, which under certain conditions can complement our perspective, allowing us to observe regions continuously for longer durations. 
 

Figure 1: The orbit of Solar Orbiter with respect to the Earth from 16 April to 18 July 2024 (left) and the temporal coverage distribution of the region between the two instruments(right). The region was assigned a different NOAA number, during its passage on the Earth-facing side of the Sun. 

In this work, we utilize such a favorable configuration (Figure 1) to produce the first, near-continuous, 94-day long dataset for super active region NOAA 13664. For this we used full-disk line-of-sight magnetograms and EUV images taken between 16 April and 18 July by the Extreme Ultraviolet Imager (EUI) [5] and Photospheric and Helioseismic Imager (PHI) [6] onboard the Solar Orbiter and Helioseismic and Magnetic Imager (HMI) [7] and Atmospheric Imaging Assemly (AIA) [8] onboard the Solar Dynamics Observatory. All images were projected on a common Carrington heliographic coordinate system. The region cut-outs had a six hour cadence, with a spatial scale equal to 2500 Mm and 1450 Mm for magnetograms and EUV images correspondingly (Figure 2). 

Results 

Evolution from emergence to decay

We were able to locate the earliest stages of emergence of NOAA 13664 with Solar Orbiter on the far solar side, before the region rotated into view for the first time. A small bipole appeared initially, which was followed by repeated magnetic flux emergence episodes, gradually building a complex magnetic configuration. In total, at least 16 flux emergence events were detected over a period of 94 days. The region’s complex morphology persisted for at least two months, maintaining an elevated flaring output. Particularly between 5 and 20 May, the magnetic field of the region exhibited a braided morphology, with many closely-neighboring magnetic polarities, indicative of huge amounts of stored free energy. During this period, the region exhibited its highest flaring activity, culminating to a X16.5 flare, on 20 May 2024, when the region was on the far side of the Sun.

Figure 2: The evolution of the photospheric magnetic field and the coronal morphology of active region NOAA13664/13697/13723, from 16 April 2024 to 18 July 2024.

Flare output over three solar rotations

To monitor the flaring activity of the region we put together flare detections by the Geostationary Operational Environmental Satellite (GOES) and the Spectrometer/Telescope for Imaging X-rays [9].

In total, 969 flares were detected within the outlined FOV, including 38 X-, 146 M-, 527 C-, and 258 B-class flares (Figure 3). More than half of the strongest events took place during the first month, but intense flaring activity persisted for two months. The strongest events were seen between 10 – 20 May, including the repeated X-class flares and CMEs between 10-13 May, which caused the strongest geomagnetic storm since 2003. The region produced some of the strongest flares of Solar Cycle 25, that is, the X8.7 on 14 May (Earth-facing side) and the X16.5 flare on 20 May 2024 (far side).
 

Figure 3. The daily flare index [10] (top) and individual flares (bottom) produced by the region, from 16 April to 18 July 2024. Red and green lines mark events detected on the front side by GOES and on the far side by STIX, respectively.

Nonpotentiality evolution over three solar rotations

Using the line-of-sight magnetic field we can monitor the evolution of the total unsigned magnetic flux as well as calculate parameters that serve as proxies for the amount of magnetic energy stored in the region [11, 12]. Specifically for this study we calculated the R-parameter ([13], i.e. the total unsigned flux within about 15 Mm of strong magnetic polarity inversion lines), the total length of the magnetic polarity inversion lines (MPIL), MPIL_tot, the length of the main polarity inversion line, MPIL_max, and the associated magnetic flux, MPIL_flux [14]. Such parameters, along with the total unsigned magnetic flux, are used routinely for flare prediction purposes but they can be calculated at most over a 14-day window per active region, due to solar rotation. Using the combined Solar Orbiter – SDO data set we calculated 94-day long time series, allowing for the first time a long-term monitoring of an active region over more than three solar rotations. 
The time series exhibit periodic gaps when the region is located near the visible limb. The total unsigned magnetic flux also exhibits a periodic trend due to foreshortening/instrumental effects [15,16], but the instrumental effects are not so pronounced in the time series of the other parameters. Despite having been produced by two instruments, with varying relative positions and spatial scales, the extended time series are consistent with the ones derived from HMI original magnetograms and Cylindrical Equal Area projected ones. Additionally, the parameters themselves exhibit strong correlation with the flare index.

Figure 4. Time series of the total unsigned magnetic flux, the R-parameter, the total length of the MPIL, the length of the main MPIL and the magnetic flux around MPILs, calculated from the combined data set. For comparison we also include the same parameters calculated for the HMI magnetograms.

Conclusions

Active region NOAA 13664 exhibited persistent complexity and consistently high eruptive activity for nearly three rotations. With two-vantage point observations we can monitor nearly continuously all the evolutionary stages of such complex, super-active regions, from their emergence to their decay. Additionally, under certain conditions we can construct long time series of magnetic parameters, which can be used to widen the prediction window of solar eruptions. However, our study shows that even two-vantage-point observations suffer from gaps, due to geometric foreshortening and/or instrumental effects. These shortcomings can be effectively treated in the future with missions relying on clusters of spacecraft.

Affiliations

(1) ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093, Zürich, Switzerland
(2) Istituto Ricerche Solari Aldo e Cele Daccò (IRSOL), Faculty of Informatics, Università della Svizzera Italiana, CH-6605 Locarno,
Switzerland
(3) PMOD/WRC, Dorfstrasse 33, 7260, Davos Dorf, Switzerland
(4) University of Applied Sciences and Arts Northwestern Switzerland, Bahnhofstrasse 6, 5210, Windisch, Switzerland
(5) University College London, Mullard Space Science Laboratory Holmbury St Mary, Dorking Surrey, RH5 6NT, UK
(6) University of Vienna, Institute of Astrophysics Türkenschanzstrasse 17 Vienna A-1180, Austria
(7) Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
(8) Euler Institute, Faculty of Informatics, Università Svizzera Italiana, Lugano, Switzerland

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