A prolific flare factory: Nearly continuous monitoring of an active region nest with Solar Orbiter

(Solar Orbiter Nugget #61 by Adam J. Finley¹, A. Sacha Brun¹, Antoine Strugarek¹, Barbara Perri¹)

1. Introduction

ESA's Solar Orbiter mission provides a unique opportunity to study the Sun's magnetic activity across its entire surface as it spends a few months each year observing the far side from Earth [1]. This vantage point complements Earth-based observations allowing for nearly continuous monitoring of solar activity. Magnetic activity on the Sun’s far side can have significant consequences for predicting space weather [2]. In this study, we used observations from Solar Orbiter along with data from the Solar Dynamics Observatory and GOES satellites to investigate the distribution of magnetic activity on the Sun. We focused on regions where intense magnetic fields frequently emerge in close proximity to one another, forming so-called active region (AR) nests.

2. Active Region Nesting 

During the solar cycle, magnetic flux emerges in latitudinal bands that progress towards the equator in each hemisphere [3,4]. Longitudinal patterns in flux emergence are also present, called active longitudes or nests, but their long-term evolution is obscured from Earth alone. AR nests can remain active over several solar rotations due to localised and repeated flux emergence events. Figure 1 shows an AR nest from 2022, where the clustering of magnetic activity was evident in extreme-ultraviolet observations. Until recently, we have lacked the observations required to constrain their properties. Now, during favourable alignments with Earth, Solar Orbiter facilitates nearly continuous monitoring of the magnetic activity and flaring from AR nests. 

Figure 1: Active region nesting. Left: Averaged extreme-ultraviolet from SDO in 2022, with persistent hot spots of activity highlighted. Right: Time-evolution of EUV activity in the northern hemisphere. Activity in the “region of interest” is continuous throughout 2022.

3. Flare Factory

In this study, we focused on the AR nest from Figure 1. This region was continuously observed by Earth and Solar Orbiter from April to October 2022. Observations from Solar Orbiter/EUI [5] were combined with SDO/AIA, and magnetic field measurements from Solar Orbiter/PHI [6] with SDO/HMI. An example of this is shown in Figure 2. X-ray flares statistics were taken from GOES and Solar Orbiter/STIX [7].

Figure 2: Combined map of the solar surface in extreme-ultraviolet from SDO, STEREO-A, and Solar Orbiter (on the far side) from May 2022. The AR nest is highlighted with a white contour.

Figure 3 shows two examples of solar flares observed during this period, highlighting the advantage of Solar Orbiter's far side position in capturing events not visible from Earth. Panel a) shows a flare seen by both GOES (near-Earth) and STIX (Solar Orbiter), while the flare in panel b) was only observed by STIX on the far side.

Figure 3: Example of two large solar flares from the AR nest during 2022. Panel (a) shows a solar eruption visible to both Earth (SDO/AIA image with GOES lightcurve) and Solar Orbiter (SolO/EUI image with STIX lightcurve). Panel (b) shows an eruption from the nest captured by Solar Orbiter on the far side to Earth. 

Using these observations, we were able to study the distribution of solar flares from this AR nest, summarised in Figure 4. The two right panels show the peak flux distribution of solar flares from the AR nest during the eight Carrington rotations with nearly continuous observations (CR 2255 to 2262). We fit a power-law distribution with the exponent α, ranging from -1.6 to -2.1 with an average value of −1.86±0.18. This is consistent with previously derived exponents for soft x-ray flares [8]. During this time, the AR nest produced between 50-70% of all eruptions over the entire solar surface, including some of the strongest solar flares (X-class events). The repeated emergence of magnetic flux created complex active regions (as defined by the Hale classification system) that were more likely to produce strong solar flares [9]. In total, the AR nest contained 10 of the 17 complex flaring ARs recorded in 2022. Our work suggests that AR nests may act like assembly lines for the production of complex ARs, with new magnetic flux emerging into pre-existing flux [10]. 

Figure 4: Combined solar flare statistics for the AR nest using GOES and STIX. Left: Histogram of solar flare class (peak x-ray flux) versus time in 2022. From April to October the AR nest was near-continuously monitored. Right: Frequency of solar flares as a function of peak x-ray flux over the entire Sun for each solar rotation (28 days). 

4. Conclusions

In 2022, an AR nest appeared in the Sun’s northern hemisphere and dominated solar activity over the entire solar surface for several months. A better understanding of the formation, evolution, and flaring characteristics of AR nests, facilitated by missions like Solar Orbiter, is crucial for improving space weather forecasts in the short to medium term. By providing a more complete picture of solar magnetic activity, including events on the far side, this research helps to predict and mitigate the potential impacts of solar eruptions on Earth.

This nugget is based on the following paper: Finley, A. J., et al. 2025, A&A, 697, A217

Affiliations
(1) Department of Astrophysics, Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France

References

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