Astronomers See Braided Magnetic Fields Above a Sunspot

Revealed in high resolution, researchers have spotted complex twisting and braiding motions in fine threads of plasma suspended above a sunspot. The flow of material that followed this braiding provides evidence for magnetic reconnection.

Heating Up the Corona

One of the leading challenges for the field of solar physics is explaining how the temperature of the Sun skyrockets from roughly 6000K at its surface, or photosphere, to more than 10⁶K in its wispy corona. The top candidates for the resolution to this mystery are magnetohydrodynamic waves and magnetic reconnection, in which nearby field lines reorganize into a lower-energy configuration and release magnetic energy.

One way for magnetic reconnection to heat the corona is along magnetic fields that loop up into the corona and are anchored to the solar photosphere. The footprints of these field lines — the spots where they land in the photosphere — wiggle around in random ways. As this happens, the arcing magnetic field lines become braided together, and the twisted magnetic field can then undergo magnetic reconnection, heating the corona.

Braiding and Unbraiding

This process has been explored theoretically, but it’s been challenging to observe directly. As increasingly capable Sun-studying instruments have come online, though, researchers have been able to attain the high resolution needed to see this process at work.

Recently, Hechao Chen (Yunnan University; Peking University; Yunnan Key Laboratory of Solar Physics and Space Science) used data from the New Vacuum Solar Telescope, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory to study the braiding and unbraiding of fine magnetic structures above a sunspot.

The images show a bundle of narrow strands of plasma stretching about 17,000–22,000 miles (28,000–36,000 kilometers). These plasma threads trace magnetic field lines as they emerge from the sunspot before diving down again in a network region, where the magnetic field is slightly enhanced at the border of a giant convective cell.

Driven by Reconnection

The plasma threads appear to lie nearly parallel to one another at the beginning of the observation. Images from all three facilities showed the plasma threads intertwining, providing evidence for the twisting of magnetic field lines predicted to happen when their footprints move around. (As further evidence for this footprint motion, the team also observed complicated flow patterns near one of the footprints.) This intertwining was followed by a sudden brightening where the threads were most tightly knotted, creating two bright “blobs” that traveled outward toward the footprints. Afterward, the threads resumed their nearly parallel state.

The team witnessed these bright points form and travel outward repeatedly, moving with projected velocities of 20–230 km/s. They posited that the motion and heating of these blobs of plasma was driven by the release of magnetic tension, and that the threads returned to a parallel configuration once reconnection was complete. They estimated that each of these events produced 10¹⁷ Joules, roughly the energy expected for small-scale reconnection events.

This work beautifully illustrates the process of magnetic reconnection on the Sun, showing how random motions can set the stage for the buildup of magnetic tension. The heating and acceleration of plasma that follows demonstrates how small-scale reconnection events like the ones shown here can provide the energy needed to heat the solar corona.

Citation

“Witnessing Magnetic Reconnection in Tangled Superpenumbral Fibrils Around a Sunspot,” Hechao Chen et al 2025 ApJ 995 94. doi:10.3847/1538-4357/ae12e9