Fine structures in solar flare ribbons
| Nugget | |
|---|---|
| Number: | 514 |
| 1st Author: | Jonas THOEN FABER |
| 2nd Author: | |
| Published: | January 12, 2026 |
| Next Nugget: | TBD |
| Previous Nugget: | The M- and X-class White-light Flares in Super Active Region NOAA 13664/13697 |
Introduction and flare observations
As a working model, flare ribbons show the presence of accelerated particles in the chromosphere as a consequence of magnetic reconnection in the corona.
In Ref. [1] we analysed chromospheric ribbons of the GOES C2.4 class flare (SOL2022-06-26T08:12) in Ca II 8542 Å, H and H. During the impulsive phase, the ribbon consisted of short-lived and spatially periodic fine-scale structures as was seen in Ca II 8542 +0.5 Å and Hβ +0.8 Å wings that we termed "blobs." They had widths in the range 140-200 km and separation distances in the range 330-550 km. We argued that the blobs are evidence of a direct link between the formation of these features and fragmented or patchy reconnections in the corona, consistent with the theory of tearing-mode reconnection.
These results motivated the project reported in Ref. [2], for the analysis of three different flares observed with CHROMIS@SST with similar observing programs, i.e. covering the Hβ line with a cadence of about 11 seconds. The details about each flare is given in Table 1. These flares, which are of various GOES class and viewing angles, provide different perspectives of the fine-scale structures in flare ribbons. An example of the fine-scale structures are shown in Fig. 1 in the H&\beta +0.8Å channel, where these features extend some height above the main body of the ribbon.
Methods
We applied k-means clustering on all three datasets to automatically fetch pixels associated with flare ribbons. By iteratively mapping each cluster onto the H +0.80.8Å images, we flagged the clusters that overlapped prominent fine-scale structure in the ribbons. Each cluster is represented by a representative profile (RP), which is the average of all spectral lines within the cluster. The flagged RPs were categorised based on the shape of the profile, i.e. if they were single-peaked or double-peaked.
All pixels that were flagged by the k-means clustering were fitted into a Gaussian function. The pixels associated with a single-peaked RP were fitted to a single Gaussian function exhibiting an emission profile. The pixels associated with double-peaked RP were fitted to two Gaussian functions, where one exhibits an emission profile and the other an absorption feature. The latter would mimic material in the line of sight of the fine-scale structures, as they are considered to be in emission. We extracted the amplitudes, Doppler shifts and line widths from the emission component from both the single-peaked and double-peaked fitted functions.
Results
We found fine-scale structures in all three flares. In addition to being short-lived and coherent along the ribbons, they are also recognised as plasma columns when viewed from the side, i.e. as when observed close to the limb. See Figure 1, which shows plasma columns extend from the ribbon main body. We identify fine-scale structures as "riblets" (Ref. [3]) and think that they are likely present in most, if not all flare ribbons. The riblets suggest that the ribbons are not a continuous structure, but rather a composition of many coherent features, particularly during the impulsive phase. We measure the widths of the riblets in all three flares to range from 110-310 km, consistent with the results from our previous analysis. The vertical lengths of the riblets span 620-1220 km with a potential maximum of 2000 km when projection effects are accounted for.
Figure 2 shows the results of the k-means clustering and the obtained parameters from fitting the pixels to a Gaussian function of the M1.8 disc-centre flare. It is evident that the riblets are located at the ribbon fronts, as the ribbon's leading edge is moving to the right in all panels. A trend of red-shifted profiles are evident between all three flares, and the distribution of the computed red-shifts overlaps in the range 16-21 km/s independently of viewing angle. Figure 2c shows the subtraction between the ±0.8Å offsets from the core, where red indicates a stronger red wing and provides a rough indication of evident red-shifts.
Conclusions
The analysed riblets detected in the three flares supplement observations of fine-scale structure in flare ribbons in the literature. We suggest that riblet formation is a common feature in most flares of GOES class C or stronger, but a comprehensive statistical analysis is needed to support this claim. We believe that flare ribbons are not a continuous structure, but rather a composite of many features as a response to fragmented reconnections in the current sheet in the corona.
The spectral profiles associated with the riblets tend to always be in emission and red-shifted, but can be attributed with an absorption component that is in the line-of-sight of the riblet. The red-shifts suggest downflows in the riblets.
References
[1] "High-resolution observational analysis of flare ribbon fine structures"; see also the SolarNugget
[3] "Multi-wavelength observations of substructures in solar flare ribbons"