White-Light and Lyman-alpha Emissions in Solar Flares: Timing, Timescale, Energy, and Scaling

Nugget
Number:	528
1st Author:	Dechao SONG et al.
2nd Author:
Published:	June 8, 2026
Next Nugget:	TBD
Previous Nugget:	Space Weather Impact of Three Solar Flares Observed at Millimeter Wavelengths

Introduction

Solar flares are among the most energetic eruptive phenomena in the solar system, producing enhanced radiation across nearly the entire electromagnetic spectrum. Among flare emissions, white-light (WL) continuum and hydrogen Lyα emission at 1216 Å provide particularly important diagnostics of energy deposition in the lower solar atmosphere. White-light flares (WLFs) are intense brightenings in the WL continuum, indicating substantial energy deposition in the lower chromosphere and possibly even deeper layers. Lyα, the strongest line in the solar vacuum ultraviolet spectrum, traces the chromospheric and transition-region response. Although WL and Lyα emissions are thought to share a common flare-energy driver, their relationship remains poorly understood. In this Nugget, we present a statistical study of 69 WLFs observed during 2010-2015 (Figure 1), combining GOES Lyα and SXR irradiance with WL continuum intensity near 6173 Å from SDO/HMI (Ref. [1]).

Figure 1: Spatial distribution of the 69 WLFs on the solar disk. The sample includes 22 X-class (magenta), 44 M-class (cyan), and 3 C-class (gray) flares.

Stronger Lyα Enhancement in WLFs

We define the Lyα contrast as (F_peak-F_bkg)/F_bkg x 100%. For our 69 WLFs, this contrast ranges from 0.8% to 28.5% (mean 7.0%), with a 95th percentile (P95) of ~20%. In comparison, Ref. [2]) reported a P95 of only ~10% for 477 M- and X-class flares not restricted to WLFs, indicating that WLFs produce a notably stronger Lyα respone.

Peak-time Ordering: A Nonthermal Imprint

For most WLFs, the Lyα peak is nearly cotemporal with the SXR time-derivative peak - a hallmark of the Neupert effect - underscoring the nonthermal origin of flare Lyα emission (Figure 2 (b)). The WL peak is cotemporal with or slightly lags the Lyα and SXR derivative peaks by a median of ~30-40 s (Figures 2 (a) and (c)), likely due to hydrogen recombination and/or radiative backwarming processes.

Figure 2: Distributions of the peak-time differences among Ly, WL, and the SXR time derivative, highlighting the temporal ordering of impulsive-phase emissions. The green background is the uncertainty introduced by the different temporal resolutions of the observations.

Correlated Rise Phases and Timescale Comparisons

The Lyα and WL emissions exhibit positive power-law correlations in rise time (Kendall's tau correlation coefficient (KCC) = 0.41), and in the peak-enhancement growth rate ((F_peak-F_bkg)/t_rise, KCC = 0.49), with remarkably similar rise times of ~3-4 minutes, suggesting a common impulsive driver. However, their decay times (e-folding) and durations (rise plus decay times) show only low-to-medium correlations (KCCs < 0.34), indicating distinct post-peak cooling mechanisms.

Radiated Energy: What Shapes It?

For the Lyα band, all four parameters peak enhancement, rise time, decay time, and duration - exhibit positive power-law correlations with Lyα energy (KCCs ≥ 0.40), with the strongest dependencies on the decay time and duration (KCCs ≥ 0.53), implying that the Lyα energy budget accumulates significantly during the gradual phase (Figure 3).

Figure 3: Relationships of the radiated energy with the peak enhancement, rise time, decay time, and duration for the Lyα (top) and WL (bottom) bands.

In contrast, WL energy shows the strongest correlation with its peak enhancement (KCC = 0.67), whereas its correlations with the rise time, decay time, and duration are all weaker. This indicates that it is more closely associated with the magnitude of the impulsive peak enhancement than with flare timescales. The WL energy-duration relation τ ~ E^{0.32 ± 0.06} closely matches the theoretical value 1/3 predicted by simplified magnetic reconnection models and is comparable to values found for stellar superflares (Ref. [3]), consistent with a common reconnection-driven scaling across solar and stellar flare energy ranges.

Conclusions and Outlook

We have established quantitative power-law scaling relationships between Lyα and WL emissions for 69 solar WLFs. These empirical solar scaling laws provide a useful bridge for estimating the Lyα emission properties of solar-like stellar flares from routinely observed WL emissions. We also note that HMI's narrowband pseudocontinuum near 6173 Å differs from broadband stellar photometry, which introduces some limitations.

== Acknowledgments ==

Ying LI, Qiao Li, and Xiaofeng LIU are co-authors of this Nugget and of Ref. [1].

References

[1] "The Lyα Emission in Solar Flares. II. A Statistical Study on Its Relationship with the White-light plus Soft X-Ray Emission"

[2] "Lyman-alpha Variability During Solar Flares Over Solar Cycle 24 Using GOES-15/EUVS-E"

[3] "Statistical Studies of Solar White-light Flares and Comparisons with Superflares on Solar-type Stars"