P. Vemareddy1, S. Nair2, and S. Gosain3
1. Indian Institute of Astrophysics, II Block, Koramangala, Bengaluru-560 034, India
2 National Institute of Technology, Warangal-506 004, India
3 National Solar Observatory, 3665 Discovery Drive, Boulder, 80303 CO, USA
Solar flares and Coronal Mass Ejections (CMEs) are the primary drivers of space weather, capable of triggering intense geomagnetic storms on Earth. While we know these eruptions are powered by magnetic energy stored in Active Regions (ARs), the exact “tipping point” — the mechanism that transitions a stable magnetic field into an explosive eruption — remains a subject of intense debate. A recent study of AR 13500 provides a breakthrough by successfully simulating the life cycle of a “sigmoid” structure, from its slow formation to its eventual eruption.
Figure 1 | Multi-observation scenario: This figure sets the stage by showing the “S-shape” sigmoid in EUV light (AIA 131 Å) alongside the HMI magnetogram. It links the surface magnetic polarities to the massive halo-CME captured by the LASCO coronagraph. This highlights the direct connection between small-scale magnetic twisting and large-scale solar eruptions.
The Anatomy of an Eruption
On November 28, 2023, AR 13500, located near the solar disk center, produced a fast CME and an M9.8 flare. This event was particularly interesting because the AR was in its “decay phase,” with its total magnetic flux actually decreasing. Despite this, the internal motions of the sunspots were twisting the magnetic field lines like a rubber band, injecting a property called magnetic helicity [1].
Using a data-driven Magnetofrictional (MF) simulation[2], researchers were able to recreate the coronal magnetic field’s evolution starting nearly three days before the blast. The simulation showed that the initial simple magnetic arches (arcades) gradually twisted into a massive, S-shaped flux rope. This “sigmoid” structure perfectly matched the observations from the Solar Dynamics Observatory (SDO).
Figure 2 | Comparison of the simulated magnetic structure with the coronal observations. (a) Rendered magnetic structure at 45 hr of the simulated field. (b) Proxy emission map of the simulated magnetic field. (c) AIA 304 Å image at 28T07:00 UT—note the striking morphological similarity of the simulated proxy emission with the AIA 304 Å image broadly reproducing the sigmoidal feature. (d) Hα image from KSO at 28T02:20 UT—the blue arrow points to the filament and its supporting magnetic structure.
Key Research Highlights
- Helicity as a Predictor: The study found that the ratio of “current-carrying” helicity to total helicity is a vital diagnostic. When this ratio hit a threshold of approximately 0.3, the system became unstable. This is in line with MHD simulations by [3].
- Torus Instability: The eruption was likely triggered by “torus instability” [4] which occurs when the magnetic “hoop force” of the twisted flux rope overcomes the restraining tension of the overlying magnetic field.
- Morphological Agreement: The simulated “proxy emission” maps (calculated from electric currents) showed a striking similarity to the actual S-shaped structures seen in extreme ultraviolet (EUV) light.
Figure 3 | The Helicity Ratio Evolution: This plot provides the “smoking gun” for the eruption trigger. It tracks the HJ/HV ratio (current-carrying helicity vs. total relative helicity). The vertical dotted line marks the onset of the CME, exactly when the ratio climbs above 0.3, supporting the theory that this specific helicity threshold is a reliable marker for torus instability and eruptive potential.
These results demonstrate that data-driven simulations are no longer just theoretical exercises—they are becoming robust tools for assessing the “eruptive potential” of active regions and could one day significantly improve our space weather forecasting.
For more details, please refer to the full publication: Vemareddy et al. 2026 ApJ 1001 16 DOI：10.3847/1538-4357/ae4eda
References:
[1] Berger MA, Field GB. (1984) Journal of Fluid Mechanics.147, 133 doi:10.1017/S0022112084002019
[2] Yang, Sturrock, and Antiochos (1986), The Astrophysical Journal, 309, 383. doi:10.1086/164610.
[3] Zuccarello, F. P., Pariat, E., Valori, G., & Linan, L. (2018), The Astrophysical Journal, 863, 41 doi:10.3847/1538-4357/aacdfc
[4] Torok, Kliem, and Titov (2004), Astronomy and Astrophysics, 413, L27 doi:10.1051/0004-6361:20031691.