White-light Solar Telescope Overview

• White-light Solar Telescope is an observational instrument engineered for full-disk imaging in the near-UV and visible range to capture white-light flares and photospheric events.
• It employs advanced calibration techniques, including flat-field correction and attenuation filters, to achieve high radiometric accuracy and mitigate stray-light interference.
• The instrument’s high spatial and temporal resolution facilitates rigorous statistical, morphological, and energy diagnostics of solar flares, deepening our understanding of energy deposition in the lower solar atmosphere.

A White-light Solar Telescope (WST) is an observational instrument dedicated to imaging the Sun in broad-band optical continuum, typically in the near-ultraviolet or visible region, with a primary objective to detect and analyze white-light flares (WLFs) and photospheric phenomena such as sunspots, prominences, and coronal mass ejections (CMEs). WSTs such as the instrument onboard the Advanced Space-based Solar Observatory (ASO-S) operate at wavelengths (e.g., 3600 Å, in the Balmer continuum) where solar continuum emission is most responsive to energetic processes in the lower solar atmosphere. These telescopes provide high-cadence, full-disk solar imaging, enabling rigorous statistical, morphological, and energy diagnostics of solar flares—especially those with white-light emission, which is a key tracer of impulsive energy deposition and lower-atmosphere heating in transient solar events.

1. Instrumental Design, Passbands, and Calibration

WSTs are engineered for full-disk solar coverage, with wide field of view (FOV) and moderate-to-high spatial and temporal resolution. For example, the WST on ASO-S achieves a FOV of 38.43 arcminutes and a spatial resolution of 3.0″ in the 360 nm channel, as confirmed by sunspot bridge analyses and hardware specifications (Chen et al., 4 Aug 2024). This FOV ensures uninterrupted disk coverage for large eruptive events.

The 3600 Å channel—within the Balmer continuum—offers heightened sensitivity to flare heating in the lower chromosphere, and is complementary to other white-light observing bands (e.g., 6173 Å Paschen continuum; 4250 Å and 3600 Å dual-channel systems in ONSET (Fang et al., 2013); multi-band continuum imaging in SOT (Watanabe et al., 2013)).

Radiometric calibration is achieved by referencing standard solar spectral irradiance (e.g., ASTM G173-03), and by utilizing attenuation filters for dynamic range control, yielding calibration factors such as (1.0362 ± 0.0013) × 10⁻⁹ erg cm⁻² DN⁻¹ (Chen et al., 4 Aug 2024). Flat-field response is iteratively derived using the in-flight SAT-KLL algorithm (based on Kuhn, Lin, and Loranz, 1991), with offset pointing strategies providing pixel-level correction at the 0.5% precision level (Li et al., 2020). Stray-light mitigation is handled by entrance filters and attenuation elements that block >80% of out-of-band radiation, with additional hardware features for dark-current suppression.

2. Observational Properties of White-Light Flares

WSTs are foundational for the systematic quantification of WLF occurrence, morphology, duration, and energetics. In a long-term WST survey (Oct 2022–May 2023), the occurrence rate of WLFs among major (M1.0+) solar flares reached 23.9%, with a sharp dependence on flare magnitude: 17.2% for M1–M4, 61.1% for M5–M9, and 100% for flares above X1 class (Jing et al., 14 Jan 2024). In super active regions (SARs) such as NOAA 13664/13697, the rate can increase to 53.9%, tightly correlated with sunspot number—a proxy for solar cycle phase (Jing et al., 14 Sep 2025).

White-light enhancement metrics, defined by $r = (I - I_0)/I_0$, typically average 19.4% at 360 nm, with >90% of events below 30%. Flares often have short durations at 360 nm (mean 10.3 min, >80% below 20 min) and small brightening areas (median 225 arcsec², >75% below 500 arcsec²) (Jing et al., 14 Jan 2024). Limb events feature larger enhancements due to elevated continuum formation height and radiative transfer effects (Jing et al., 14 Jan 2024, Li et al., 11 Aug 2024).

Correlation studies reveal strong relationships between white-light parameters and the peak soft X-ray flux (correlation coefficients of 0.68 to 0.80 for enhancement, duration, and area), as well as nearly synchronous timing of white-light maxima with both the hard X-ray peak and the SXR derivative, confirming the Neupert effect (Jing et al., 14 Jan 2024, Li et al., 12 Feb 2024).

Table: Key White-Light Flare Parameters from WST Surveys

Parameter	Typical Value / Range	Correlation (SXR/HXR)
Occurrence rate	23.9–53.9% (M1–X class)	Pcc ~0.64 (sunspot)
Enhancement (360 nm)	mean ~19.4%, <30% event	0.68–0.74 (SXR)
Duration	10.3 min mean, <20 min	0.58
Brightening area	median ~225 arcsec²	0.80

3. Multi-Wavelength and Multi-Instrument Synergy

WSTs are most informative when operated in synergy with spectral and magnetic diagnostics. Observations in the Balmer continuum (3600 Å) are often compared against the Paschen continuum (6173 Å, e.g., SDO/HMI), chromospheric lines (e.g., Hα, Ca II H; CHASE, SOT), and X-ray/hard X-ray detectors (HXI, RHESSI, Fermi/GBM) (Li et al., 12 Feb 2024, Song et al., 2023).

Morphologically, white-light emission is observed in diverse forms: point, ribbon, loop, and ejecta-like sources, localized to flare ribbons, footpoints, loops, or within sunspot umbra/penumbra (Li et al., 11 Aug 2024). Simultaneous Balmer/Paschen continuum imaging demonstrates higher relative enhancement and longer duration for the Balmer continuum, reflecting the deeper/hotter formation and stronger sensitivity to impulsive heating (Li et al., 11 Aug 2024, Li et al., 12 Feb 2024).

The co-spatial relationship between white-light kernels and HXR sources is robust in on-disk flares but spatially offset in off-limb events, where loop and plasma ejecta features dominate (Li et al., 11 Aug 2024). WST continuum maxima temporally coincide with HXR pulses, especially in the 20–100 keV band, supporting a nonthermal electron-beam heating scenario (Jing et al., 14 Jan 2024, Song et al., 3 Apr 2025).

4. Mechanisms of White-Light Emission: Energetic and Radiative Processes

White-light emission in flares arises from a combination of direct energy deposition, secondary radiative transport, and wave-driven mechanisms. Statistical studies (1710.09531) indicate that impulsive energy release (short SXR derivative durations, rapid electron precipitation) and compact flare geometry (short ribbon separation, strong coronal magnetic fields) are critical for WL enhancement. The electron energy flux is modeled as a power law:

$F(E) = A E^{-\delta}, (E \geq E_c)$

$$\dot{E} \approx \frac{A}{\delta - 2} E_c^{2 - \delta}$$

Direct heating by nonthermal electrons drives continuum enhancements (especially in the impulsive phase), often modulated by radiative backwarming and recombination continuum processes (Paschen/Balmer) (Jurcak et al., 2018, Song et al., 2023, Li et al., 12 Feb 2024). Chromospheric contributions can dominate continuum brightening, as shown by inversion analyses indicating that upper photospheric or chromospheric heating produces much of the observed WL intensity increase (Jurcak et al., 2018, Li et al., 12 Feb 2024).

High-resolution ground-based studies (e.g., NVST TiO imaging) have shown that electron-beam heating alone may not fully account for observed photospheric impacts; flare-generated Alfvén wave pulses may efficiently deposit energy at depth, leading to magnetic field amplification and vortex flows (Xu et al., 10 Jun 2025). The associated Poynting flux for wave energy transport is estimated as:

$S \approx V_A \frac{\delta B^2}{8\pi}$

This can greatly exceed the magnetic energy increase during the flare, further implicating wave dissipation as a key agent in WLF energy transport.

5. Statistical Scaling Laws and Cycle Dependence

Energy-duration scaling relationships elucidated through WST surveys provide crucial insights into flare physics and stellar analogs. The flare duration τ scales with radiated energy E as $\tau \propto E^{0.35}$ in SARs (Jing et al., 14 Sep 2025), in line with earlier solar and stellar superflare results ($\tau \propto E^{0.38-0.39}$) (Namekata et al., 2017). This scaling is predicted by magnetic reconnection theory under the assumption:

$\tau \propto E^{1/3} B^{-5/3}$

with stronger magnetic fields leading to shorter durations for a given energy—explaining the more impulsive nature of superflares observed on Sun-like stars (Namekata et al., 2017, Jing et al., 14 Sep 2025). Solar cycle dependence is manifest: WLF occurrence rates increase synchronously with sunspot number and major flare frequency (Jing et al., 14 Sep 2025).

6. Temporal Dynamics and Quasi-Periodic Pulsations

High-cadence WST imaging (1–2 s) has enabled discovery and mapping of quasi-periodic pulsations (QPPs) in the Balmer continuum during flare impulsive phases, with fundamental and harmonic periodicities (e.g., 20 and 11 s) spatially localized to flare ribbons (Song et al., 3 Apr 2025). These QPPs are synchronous with HXR pulses (near-zero lag), while SXR and EUV lag by ~2–3 s, indicating direct connection to nonthermal electron precipitation. Wavelet and Fourier analyses reinforce the interpretation of MHD kink-mode or reconnection-modulation mechanisms over slow-mode scenarios, with relevant periods given by:

$$P_{kink} = \frac{2L}{j c_{Ai} \sqrt{2/(1 + n_e/n_i)}}$$

$P_{slow} = \frac{2L}{j c_T}$

The occurrence of QPPs is coincident with dynamic ribbon motions and rapid elongation/separation.

7. Scientific Prospects and Future Directions

Advancements in WST design, including improved temporal resolution, enhanced calibration accuracy (flat-field and dark current modeling), multi-wavelength operational synergy, and coordinated ground-space campaigns, are necessary for resolving fine timing differences and disentangling flare layer contributions (Chen et al., 4 Aug 2024, Li et al., 2020, Xu et al., 10 Jun 2025). Expanded statistical studies across solar cycles will clarify cycle dependence and flare scaling laws.

Numerical modeling incorporating both electron-beam and Alfvén wave heating, radiative transfer, and magnetic reconnection continues to be essential for interpreting WL data and for bridging solar WLFs and stellar superflares. Further optical/UV continuum surveys will inform the formation height, energetics, and spectral dependence of flare continuum emission.

In conclusion, the White-light Solar Telescope is a cornerstone of modern solar observation, providing direct diagnostics of lower-atmosphere flare heating, particle acceleration, and magnetic energy conversion, with far-reaching implications for solar and stellar flare physics.