WhiFlash: White-Light Flare Dynamics

Updated 5 July 2026

WhiFlash is a white-light flare characterized by enhanced optical continuum emission, chromospheric line changes, and co-spatial hard-X-ray sources.
Researchers use multi-instrument methods such as difference imaging, Gaussian fits, and moment analysis to quantify spectral line shifts and continuum enhancements.
WhiFlash events provide practical insights into electron-driven energy deposition, showcasing chromospheric evaporation and condensation in varied flare magnitudes.

Searching arXiv for papers relevant to “WhiFlash” and the associated white-light flare literature. {"query":"all:\"white-light flare\" chromospheric evaporation electron-driven", "max_results": 10} WhiFlash, in the usage reflected by the cited literature, denotes a white-light flare (WLF): a solar flare characterized by an enhancement in the optical continuum. The term is represented in recent case studies spanning an M5.7 event associated with a filament eruption, a C9.6 event showing electron-driven explosive chromospheric evaporation, and a C2.3 event with coordinated spectral, imaging, hard-X-ray, and magnetic diagnostics. Across these studies, WhiFlash is defined operationally through the co-occurrence of white-light brightening with lower-atmosphere line-profile changes, nonthermal hard-X-ray footpoint sources, and flare-ribbon or footpoint structuring in the impulsive phase (Song et al., 2018, Li et al., 2023, Li et al., 2024).

1. Defining characteristics

A WhiFlash event is identified by enhanced optical continuum emission. In the 2022 August 27 flare, the radiation enhancement at Fe I 6569.2 Å and 6173 Å was taken as evidence that the event was a WLF. In the 2022 December 20 flare, measured enhancements reached ≈6.4% in the photospheric Fe I 6569.2 Å line, ≈3.2% in the nearby CHASE continuum, and ≈4.7% in the WST 3600 Å continuum. In the 2012 May 10 event, the relative enhancement in HMI 6173 Å continuum was reported as $0.26 \pm 0.02$ , using the standard difference-image definition $\frac{I-I_0}{I_0}$ (Li et al., 2023, Li et al., 2024, Song et al., 2018).

The cited cases also show that WhiFlash is not restricted to major flares. One study emphasizes that a GOES C2.3 flare can produce measurable white-light emission, and frames such small WLFs as especially useful for understanding lower-atmosphere energy deposition because white-light brightenings are uncommon in C-class events. Another study presents a C9.6 WLF near the west limb. A third analyzes an M5.7 WLF in a complex $\beta\gamma\delta$ sunspot group (Li et al., 2024, Li et al., 2023, Song et al., 2018).

A recurrent observational property is footpoint localization. The 2012 May 10 event showed white-light enhancement at the two footpoints of the erupting filament. The 2022 December 20 event displayed two main white-light brightening kernels, K1 and K2, co-spatial with the H $\alpha$ flare ribbons and aligned with nonthermal hard-X-ray sources. The 2022 August 27 event showed a white-light kernel co-spatial with a nonthermal hard-X-ray source and with the site of Fe XXI blueshift (Song et al., 2018, Li et al., 2024, Li et al., 2023).

2. Observational and diagnostic framework

The WhiFlash literature is strongly multi-instrumental. The 2022 August 27 event combined CHASE/HIS for H $\alpha$ and Fe I diagnostics, IRIS spectroscopy in Fe XXI, C I, and Si IV, SDO/AIA and SDO/HMI for EUV, continuum, and magnetograms, STIX and GECAM for hard X-rays, GOES for soft X-rays, and SWAVES for radio context, with no type III escape signature reported. The 2012 May 10 event used SDO/HMI, ONSET 3600 Å and 4250 Å, SDO/AIA, RHESSI, NoRH, NoRP, and GOES. The 2022 December 20 event used ASO-S, CHASE, SDO/HMI, SDO/AIA, GOES, ASO-S/HXI, and Solar Orbiter/STIX (Li et al., 2023, Song et al., 2018, Li et al., 2024).

Several complementary spectral-analysis methods are used to isolate lower-atmosphere and coronal signatures. For the 2022 August 27 flare, the authors applied moment analysis to CHASE H $\alpha$ spectra to derive line intensity, Doppler velocity, and width from the zeroth, first, and second moments; a bisector technique on H $\alpha$ contrast profiles to verify redshifts independently; and Gaussian fitting of IRIS spectra, including multi-Gaussian fits for blended Fe XXI and C I windows and a single-Gaussian fit for isolated Si IV. For the 2022 December 20 flare, the Fe I 6569.2 Å line was fitted as an absorption profile with a good symmetric Gaussian shape, while H $\alpha$ asymmetry was quantified through

$RA = \frac{I_{\rm rp}-I_{\rm bp}}{I_{\rm rp}+I_{\rm bp}},$

where $I_{\rm rp}$ and $\frac{I-I_0}{I_0}$ 0 are the red- and blue-wing peak intensities, respectively (Li et al., 2023, Li et al., 2024).

Difference imaging is central to white-light identification. In the 2012 May 10 study, both the white-light enhancement and the difference images used

$\frac{I-I_0}{I_0}$ 1

with $\frac{I-I_0}{I_0}$ 2 as the pre-flare intensity and $\frac{I-I_0}{I_0}$ 3 as the intensity near the white-light peak. The same event coupled continuum diagnostics with RHESSI spectral fitting using a thermal plus non-thermal thick-target bremsstrahlung model (Song et al., 2018).

3. Representative events

Event	White-light signature	Coupled diagnostics
2022 Aug 27, C9.6, NOAA 13088, S26W66 (Li et al., 2023)	Enhancement at Fe I 6569.2 Å and 6173 Å	H $\frac{I-I_0}{I_0}$ 4, C I, Si IV redshifts $\frac{I-I_0}{I_0}$ 5; Fe XXI blueshifts 30–40 km s $\frac{I-I_0}{I_0}$ 6; co-spatial nonthermal HXR source; energy flux $\frac{I-I_0}{I_0}$ 7
2012 May 10, M5.7, NOAA 11476 (Song et al., 2018)	WL enhancement at the two footpoints of the erupting filament; HMI relative enhancement $\frac{I-I_0}{I_0}$ 8	Circular flare ribbon; remote brightening; HXR and microwave co-spatiality; WL peak lag of about 1–2 minutes behind HXR and microwave
2022 Dec 20, C2.3, NOAA 13171, N26E59 (Li et al., 2024)	≈6.4% in Fe I 6569.2 Å, ≈3.2% in CHASE continuum, ≈4.7% at 3600 Å	Two kernels K1/K2 co-spatial with H $\frac{I-I_0}{I_0}$ 9 ribbons and HXR sources; Fe I redshift up to ≈1.7 km s $\beta\gamma\delta$ 0; opposite H $\beta\gamma\delta$ 1 asymmetry at conjugate footpoints

The 2022 August 27 event is presented as a compact, multi-instrument case for electron-driven explosive chromospheric evaporation in a WLF. Its most distinctive feature is the combination, at the same compact footpoint region and in the same impulsive phase, of white-light enhancement, redshifted cool lines, blueshifted Fe XXI, and a co-spatial nonthermal hard-X-ray source (Li et al., 2023).

The 2012 May 10 event is notable for its association with a small filament eruption, a circular flare ribbon, and a remote brightening. The white-light kernels were concentrated at the filament footpoints rather than distributed broadly across the flare ribbons. The event is interpreted as a fan-spine eruption above a small flux rope/filament system (Song et al., 2018).

The 2022 December 20 event shows that a small C-class flare can still exhibit several diagnostics usually associated with larger WLFs: measurable optical continuum enhancement, nonthermal HXR footpoints, ribbon-associated line asymmetries, chromospheric flows, and localized magnetic-field changes. This suggests that small WhiFlash events can serve as compact laboratories for flare energy-release studies (Li et al., 2024).

4. Atmospheric dynamics and energy transport

A central WhiFlash signature is the coupled presence of chromospheric condensation and chromospheric evaporation. In the 2022 August 27 flare, the low-temperature lines H $\beta\gamma\delta$ 2, C I, and Si IV showed redshifts of less than $\beta\gamma\delta$ 3 at the flare kernels, interpreted as downflows caused by chromospheric condensation. At the same time, the hot coronal line Fe XXI showed blueshifts of about 30–40 km s $\beta\gamma\delta$ 4, interpreted as upflows driven by chromospheric evaporation. Because the flare was near the limb, the reported values are likely projection-reduced, so the true flow speeds could be larger (Li et al., 2023).

The energetic driver in that event was inferred from STIX thick-target fitting. The nonthermal electron beam was estimated to deliver at least

$\beta\gamma\delta$ 5

which the authors describe as at or above the classic threshold for explosive evaporation $\beta\gamma\delta$ 6. They further emphasize that this is probably a lower limit, because the low-energy cutoff is conservatively constrained and the HXR source may be unresolved, implying a smaller true footpoint area and therefore a larger flux (Li et al., 2023).

The 2012 May 10 event supports a related but temporally distinct scenario. There, HXR and microwave peaks occurred around 04:16:30 UT, whereas the WL peak occurred around 04:18:00 UT. Given the 45 s cadence of HMI continuum images, the resulting 1–2 minute lag was interpreted as favoring the back-warming mechanism: non-thermal electrons heat the chromosphere, the chromosphere emits intense radiation, and that radiation then back-warms the photosphere. The same study notes that if very high-energy electrons directly heated the lower atmosphere, the WL and HXR peaks should be nearly simultaneous. It also reports no evidence for Alfvén waves in that event (Song et al., 2018).

The 2022 December 20 event also supports nonthermal electron-beam heating, while leaving radiative backwarming open. Its HXR spectra showed a nonthermal component above ≈20 keV, and reconstructed HXR images at 20–35 keV with HXI and 16–28 keV with STIX placed the strongest nonthermal sources directly on the flare ribbons, well matched to K1 and K2. The authors caution, however, that the timing relationship is not fully conclusive because of the relatively low cadence of WST and CHASE (Li et al., 2024).

5. Magnetic topology and footpoint structuring

Magnetic topology is a major organizing principle in WhiFlash events. The 2012 May 10 flare was analyzed with forced field extrapolation (FFE) for the low atmosphere and potential field extrapolation for the overlying coronal field. The resulting topology contained a flux rope rooted in the two main sunspots, a dome-like magnetic structure above the flux rope, a larger-scale fan-spine configuration, and a magnetic null point at approximately

$\beta\gamma\delta$ 7

in the potential-field grid, corresponding to a height of about 5.6 Mm above the photosphere. The computed squashing factor $\beta\gamma\delta$ 8 showed a dome-like QSL, a spine-like QSL, and a circular QSL on a low horizontal plane; the observed circular flare ribbon matched the circular QSL, while the remote brightening fell on the outer QSL (Song et al., 2018).

This topological analysis motivated a staged eruption scenario. A small twisted flux rope / filament lay under a dome-like coronal magnetic field; the filament began to rise; a brightening below the filament appeared first; then fan-spine reconnection near the null produced the circular ribbon and remote brightening around 04:15 UT; finally, the white-light enhancement appeared around 04:16 UT, especially at the two filament footpoints (Song et al., 2018).

The 2022 December 20 flare showed a more compact footpoint organization. In the HMI magnetogram, K1 lay in negative polarity with relatively strong field, whereas K2 was in a mixed-polarity, weaker-field region. In AIA 131 Å, both kernels appeared to connect the same set of flare loops, suggesting a pair of conjugate footpoints. A localized photospheric magnetic-field change was detected near the kernels: K2 showed an evident $\beta\gamma\delta$ 9 change of 40.5 G, whereas K1 did not show a clear stepwise change (Li et al., 2024).

The same study fitted $\alpha$ 0 with a stepwise function and reported that the CHASE continuum increase correlates moderately with $\alpha$ 1 at flare ribbons, with Pearson correlation coefficient $\alpha$ 2, while the CHASE continuum has essentially no correlation with $\alpha$ 3 itself $\alpha$ 4 and only weak relation with H $\alpha$ 5 asymmetry $\alpha$ 6. By contrast, the WST 3600 Å continuum showed no linear relationship with $\alpha$ 7, $\alpha$ 8, or H $\alpha$ 9 asymmetry. The authors interpret this as a possible indication that the CHASE continuum near 6569.2 Å forms lower in the atmosphere than the 3600 Å Balmer-continuum emission, but they also stress that the relation between continuum emission and magnetic-field change is still not fully understood (Li et al., 2024).

6. Interpretation, points of debate, and nomenclature

The dominant interpretation across the cited solar papers is that WhiFlash is driven primarily by nonthermal electrons. In the 2022 August 27 event, the proposed chain is explicit: magnetic reconnection accelerates nonthermal electrons; the electrons precipitate into the chromosphere at the flare footpoint; their energy deposition produces white-light continuum enhancement, chromospheric condensation seen as redshifted H $\alpha$ 0, C I, and Si IV, and chromospheric evaporation seen as blueshifted Fe XXI; and the co-spatial HXR source confirms the particle beam as the driver (Li et al., 2023).

At the same time, the cited literature does not reduce all WhiFlash events to a single timing pattern or a single radiative pathway. The 2012 May 10 event favors back-warming because the white-light peak lagged the HXR and microwave peaks by about 1–2 minutes; the 2022 December 20 event supports nonthermal electron-beam heating but does not exclude radiative backwarming; and the 2022 August 27 event instead emphasizes close temporal and spatial correlation between white-light enhancement, Fe XXI upflow, and HXR burst (Song et al., 2018, Li et al., 2024, Li et al., 2023). A common oversimplification is therefore that white-light and HXR emission must always be nearly simultaneous if electrons are involved. The event-by-event results summarized here indicate a more differentiated picture.

The line-profile diagnostics likewise resist a one-to-one mapping onto simple flow labels. In the 2022 December 20 event, K1 showed red asymmetry and K2 showed blue asymmetry in H $\alpha$ 1, with reported asymmetry values of 0.079 at K1 and $\alpha$ 2 at K2. The authors note that such asymmetry reversal at conjugate footpoints may reflect different plasma flows, viewing geometry, or asymmetric energy deposition, and caution that red or blue asymmetry and measured Doppler shifts do not map one-to-one onto simple downflow or upflow interpretations (Li et al., 2024).

In nomenclature, the cited descriptions use “WhiFlash” as a label for white-light flare cases. A similar string appears independently in a different field as HiFlash, a communication-efficient hierarchical federated learning system that combines synchronous client-edge aggregation, asynchronous edge-cloud aggregation, adaptive staleness control, and heterogeneity-aware client-edge association (Wu et al., 2023). The two usages are unrelated. Within the solar-flare literature summarized here, WhiFlash refers to WLF phenomenology rather than to a standardized subfield taxonomy.