A fine-scale bright kernel captured by Hi-C 3 in the post-maximum phase of an M-class solar flare

Nugget
Number:	516
1st Author:	Sanjiv TIWARI
2nd Author:
Published:	February 9, 2026
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Previous Nugget:	On the Relationship Between Nanoflare Energy and Delay in the Closed Solar Corona

Introduction

The third successful flight (Hi-C Flare) of the High-Resolution Coronal Imager sounding rocket (Hi-C) observed NOAA AR 13645 in a 129 Å spectral band dominated by Fe XXI emission. This line forms at about 11 million K, and thus definitely results from flare activity. Hi-C has an unprecedented spatial and temporal resolution (~300 km, 1.3 s). The ~5-minute observation, obtained near disk center on 2024 April 17 during the decay phase of an M1.6 flare, captured a small, localized, transient EUV brightening at the outer edge of the flare.

While fine-scale bright kernels during the rapid rise phase of solar flares have been reported in recent observations, their occurrence during the decay phase is rarely documented (Ref. [1]) and has not previously been reported as isolated events like the one presented here.

Data and Methods

We complemented the ~5 minutes of Hi-C Flare observations with co-temporal (lower resolution) data from the HMI and AIA instruments on board SDO. Due to mirror recoating, a modest degradation in spatial resolution relative to previous Hi-C flights is expected, with the current resolution estimated at ~0.45". The Hi-C Flare full field of view was 2.2' x 4.4', and our analysis focuses on the flaring region shown in Figure 1a. The bright kernel studied here is highlighted in Figure 2 (b), its evolution in Hi-C 129 Å, and AIA 131 Å are shown in panels c and d. SDO/HMI magnetograms were used to track magnetic field distribution and evolution at the photospheric base of the kernel.

Figure 1: Overview (panels a, b) and evolution (panels c, d) of the fine-scale bright kernel. An inset in (b) presents two intensity plots along two orthogonal cuts through the kernel, with Gaussian fits over-plotted as asterisks. Panel (e) shows GOES, AIA, Hi-C, and SSAXI-Rocket light curves of the flare with cyan shade outlining the timing of Hi-C observations. The magnetic field distribution at and near the bright kernel is shown in panels (f) and (g), with contour levels of [20, 60, 150, 500] G overlaid on an AIA 131 Å image and an HMI magnetogram, respectively.

Figure 2: Comparison of Hi-C 129 Å and AIA 131 Å channel characteristics: effective area and thermal response. A vertical dashed-dotted line in panel (a) marks the wavelength location of Fe VIII (131 Å). At the cooler temperature peak, AIA is about 2.5 times more sensitive than Hi-C, whereas both are nearly equally sensitive to hotter plasma.

Results

The flare began at 21:55 UT, peaked at 22:08 UT, and lasted approximately 40 minutes. Hi-C observed the region for over 5 minutes (22:15:45-22:21:25 UT), starting about 8 minutes after the flare maximum. During this interval, a sudden, compact bright burst appeared near the footpoints of post-flare loops. The feature was 875 ± 25 km wide (Figure 1), persisted for 90 ± 1.3 s, exhibited a proper motion of ~50 km/s, and split into two toward the end of its lifetime. Its size and brightness are reminiscent of flare-ribbon kernels typically observed during the rapid rise phase of flares, making its appearance during the decay phase unexpected.

HMI magnetograms show that the kernel is rooted in unipolar positive magnetic flux, adjacent to a small embedded negative-polarity patch (Figure 1). The kernel is detected in all SDO/AIA channels, with the 131 Å channel being closest to the Hi-C 129 Å passband. The Hi-C 129 Å band is significantly less sensitive to cooler plasma than AIA 131 Å (Figure 2). The kernel's 1600 Å light curve peaks approximately 50 s earlier than the 131 Å light curve, similar to flare-ribbon kernels, though with a shorter delay (~25 s) than typically observed during the impulsive phase (Figure 3).

Figure 3: Light curves from SDO/AIA and Hi-C Flare data. Panel (a) shows light curves for the bright kernel in Figure 1(b), also shown in the inset. Panel (b) shows light curves of different AIA channels of a localized flare-ribbon kernel (shown in the inset) during the impulsive phase of the flare. The light curves of 1600 Å and 131 Å are made thicker for easy distinction. In each of two panels, AIA 1600 Å channel peaks first, and then other AIA channels and Hi-C light curves peak, with a delay of about 25-50 s between 1600 Å and 131 Å.

Discussion and Conclusions

The bright kernel most likely represents an isolated late-phase flare-ribbon kernel produced by delayed coronal magnetic reconnection. This interpretation is supported by recurrent pre-flare brightenings at the same location, the absence of a preference for mixed-polarity regions, its location in a unipolar field, flare-ribbon-like light curves, and its position at the footpoint of a loop connecting to the active-region spine, consistent with particle precipitation from coronal reconnection. Alternatively, the kernel may be a localized microflare, as suggested by its exceptional brightness and proximity to a mixed-polarity region, possibly linked by a small loop (opposite-polarity possibly unresolved by HMI), as previously seen in Hi-C 2.1 and IRIS observations (Ref. [2]). A sharp photospheric neutral line is not required, since reconnection can occur in a standing current sheet extending into the higher atmosphere. Also, an obvious conjugate foot point is not detected, likely because it was too weak or obscured by the flare emission. For full details see Ref. [3].

References

[1] "Evidence for Chromospheric Evaporation in the Late Gradual Flare Phase from SOHO/CDS Observations"

[2] "Fine-scale Explosive Energy Release at Sites of Prospective Magnetic Flux Cancellation in the Core of the Solar Active Region Observed by Hi-C 2.1, IRIS, and SDO"

[3] "A Sudden Fine-scale Bright Kernel Captured by Hi-C Flare in 11 MK Emission during an M1.6-class Solar Flare's Postmaximum Phase"