A pattern that kept returning
The important finding is a repeating pattern rather than a single flash. During observations of NOAA active region 12297 on March 11 and 12, 2015, a fishbone-like solar magnetic structure showed 18 distinct brightening episodes. The authors call the pattern sequential magnetic reconnection, using that term for magnetic-energy release that appears chronologically at nodes, or junctions, along the structure's spine. They place those signatures mainly in the chromosphere and transition region, drawing on multi-wavelength observations from NVST, SDO, IRIS and Hinode.
The brightenings had a measurable rhythm, although the gaps varied. Their average duration was about 9.0 minutes, measured by the full width at half maximum, a way of estimating the width of a peak in a light curve. The interval between adjacent peaks ranged from 13.2 to 59.2 minutes, with a mean of about 29.7 minutes and a median of 26.2 minutes. Overall, the catalogue corresponded to about 1.91 brightenings per hour.
Following the activity along the spine
Images showed the geometry behind the name: a long, narrow spine with herringbone branches connecting to regions labeled N1 and N2. Nodes sat where the branches met the spine. The authors used these branch-spine intersections as the locations in their interpretation of repeated reconnection along the structure.
During the brightenings, a compact bright feature moved from west to east along the spine. The brightenings and associated footpoint ribbons expanded in the same direction. The feature's mean speed was about 16.9 kilometers per second. In the authors' reading, that coordinated motion is an imaging signature of reconnection occurring in sequence at the spine nodes.
Four selected brightening events also showed plasma blobs being ejected from the expanding brightening region. The reported blobs moved toward the positive-polarity sunspot P. In two representative single-node cases, a bright point was followed by flows moving in both directions along the herringbone branches, together with enhanced corresponding coronal emission.
The evidence behind the count
To build the event catalogue, the researchers analyzed the AIA 193-angstrom light curve. They smoothed it with a 51-point, third-order Savitzky-Golay filter, then set the detection threshold at the baseline plus three times the root-mean-square noise measured in quiet intervals. Candidate peaks had to be at least 50 data points apart and show a prominence of 80 DN per second; closely spaced peaks were merged. The resulting list contained 18 distinct episodes.
The broader picture came from observations at several wavelengths, including NVST, SDO, IRIS and Hinode. Photospheric motion was estimated with DAVE4VM from HMI vector magnetic-field data sampled every 12 minutes over a two-hour interval. The calculation identified persistent southeastward flows in N1 and N, while feature p moved northwestward. Flux emergence continued in the mixed-polarity area, however, so the observations could not determine whether magnetic cancellation occurred.
Where the interpretation stops
The central uncertainty is what linked one episode to the next. The images did not directly show reconnection at one node triggering a neighboring node. A common external driver in the photosphere remains possible, so the observed sequence cannot by itself be called a chain reaction.
Another explanation also remains open. The authors cannot rule out slipping reconnection as an explanation for the apparent motion of coronal flare ribbons. The reported compact feature and associated ribbons nevertheless moved west to east along the structure.
Taken together, the observations support a careful conclusion. This structure hosted recurring brightenings, moving compact features, plasma-blob ejections and, in two representative cases, two-way branch flows with enhanced coronal emission. The authors interpret that combination as sequential magnetic reconnection concentrated mainly in the chromosphere and transition region. The work does not establish node-to-node triggering or magnetic cancellation, and it leaves slipping reconnection in play. The document is arXiv:2608.25556v1 in the astro-ph.SR category, dated 26 August 2026, and is a preprint.
Paper data and sources
Original title: Sequential Magnetic Reconnections in a Fishbone-like Structure Leading to Recurrent Brightenings
Authors: Bo Yang, Jiayan Yang, Junchao Hong, Yi Bi
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
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