Near-continuous tracking of solar active region NOAA 13664 over three solar rotations

TL;DR

This work achieves near-continuous tracking of a highly complex solar active region (NOAA 13664/13697/13723) over 94 days by combining Solar Orbiter and SDO observations from Earth-facing and far-side perspectives. It builds a unified dataset of LOS magnetograms and EUV images, computes MPIL- and R-value–based non-potentiality metrics, and correlates these with a long flare record (GOES and STIX). The region exhibits sustained high non-potentiality and prolonged flaring across multiple rotations, driven by repeated flux emergence and intricate MPIL structures. The results demonstrate the power of multi-vantage observations to overcome solar rotation–related data gaps and highlight the potential for improved eruption forecasting through continuous, cross-instrument magnetic monitoring.

Abstract

Magnetic flux emergence and decay in the Sun span from days to months. However, their tracking is typically limited to about half a solar rotation when relying on single-vantage-point observations. Combining observations from both the Earth-facing and far side of the Sun, we monitored the magnetic and coronal evolution and characterised the non-potentiality of one of the most complex and eruptive regions of the past two decades, over more than three full solar rotations. We used photospheric magnetograms and EUV filtergrams from the Solar Orbiter and the Solar Dynamics Observatory along with flare detections from the GOES and the STIX instrument on board the Solar Orbiter. All images were deprojected into a common coordinate system and merged into a unified dataset. We tracked the evolution of magnetic flux and EUV emission and computed magnetic field parameters from the line-of-sight magnetograms to quantify the region's non-potentiality. We identified the region's initial emergence and followed its evolution through to its decay. The region developed through successive flux emergence episodes over a period of 20 days, reached its peak complexity one month after the first emergence, and gradually decayed over the subsequent two months. Unlike many complex regions, it consistently maintained high levels of non-potentiality for most of its lifetime, sustaining equally strong flaring activity. The derived time series of non-potentiality parameters, the first of their kind, far exceeded the typical 14-day window imposed by solar rotation and were remarkably consistent, exhibiting strong correlation with the flaring activity of the region. Multi-vantage-point observations can significantly improve our understanding of how magnetic flux emerges, evolves, and drives solar activity, beyond the two-week limit imposed by solar rotation on observations along the Sun-Earth line.

Figures (7)

• Figure 1: Left: The orbit of Solar Orbiter with respect to the Earth in Geocentric Solar Ecliptic (GSE) coordinates from 16 April to 18 July 2024. Right: Temporal coverage distribution of the region between the two instruments, The region was within the SO/PHI-FDT (HMI) FOV during the yellow (green) intervals. During each revolution, the region was assigned a different NOAA number, as indicated in the figure. Indicatively, we mark the observing gap between 26 and 29 April with the two vertical dotted lines. Similar gaps are also found later.
• Figure 2: Snapshots of the evolution of NOAA 13664/13697/13723 from 16 April 2024. Columns 1 and 3 show the HG-projected maps of $B_{LOS}$, while columns 2 and 4 show the corresponding maps of EUV emission at the 171-174 Å range. The magnetic field values have been scaled between $\pm$300 G. An animated version of the magnetic field evolution is available online.
• Figure 3: From top to bottom: summary evolution of the magnetic flux, magnetic area and magnetic flux density (magnetic flux over magnetic area) of the region, when it was crossing the central meridian. Black curves represent the total unsigned magnetic flux (and corresponding area), while blue and red represent positive and negative flux-related quantities, correspondingly. The blue and red diamonds in the middle panel represent the individual areas of the positive and negative magnetic partitions. Red arrows in the top panel indicate the times of flux emergence episodes in the FOV.
• Figure 4: The daily flare index (top) and the individual flares (bottom) of of the NOAA 13664/13697/13723 activity complex, as detected by GOES (red) and STIX (green). Overplotted are also gray arrows that indicate the conspicuous flux emergence episodes (see text.)
• Figure 5: a) The total unsigned magnetic flux calculated from the collated data set, for both $\mu$-angle corrected and uncorrected LOS magnetograms (dotted and dashed lines respectively). The red line represents the average between the two curves. The blue arrows indicate the onset times of flux emergence events, also shown in Fig. \ref{['fig:flux']}. b) The total unsigned magnetic flux provided from the SHARP and calculated using the radial component, $B_{r}$, of the magnetic field. c) the total unsigned vertical electric current provided from the SHARP (blue crosses) and the total unsigned non-neutralized electric current calculated for the same data (magenta crosses). Dashed, vertical lines mark the first day of each month, to facilitate comparison.